HE AT
CONSIDERED AS
A MODE OF MOTION:
BEING
A COURSE OF TWELVE LECTURES
DELIVERED AT
THE ROYAL INSTITUTION OF GREAT BRITAIN IN THE SEASON
OF 1862.
BY
JOHN TYNDALL, F.R.S., &c.
PROFESSOR OF NATURAL PHILOSOPHY IN THE ROYAL INSTITUTION.
WITH ILLUSTRATIONS.
NEW YORK:
D. APPLETON AND COMPANY,
443 & 445 BROADWAY.
1864.








PRE F A C E.
IN the following Lectures I have endeavoured to
bring the rudiments of a new philosophy within the
reach of a person of ordinary intelligence and culture.
The first seven Lectures of the course deal with
thermometric heat; its generation and consumption in
mechanical processes; the determination of the mechanical equivalent of heat; the conception of heat as
molecular motion; the application of this conception
to the solid, liquid, and gaseous forms of matter; to
expansion and combustion; to specific and latent heat;
and to calorific conduction.
The remaining five Lectures treat of radiant heat;
the interstellar medium, and the propagation of motion
through this medium; the relations of radiant heat to
ordinary matter in its several states of aggregation;
terrestrial, lunar, and solar radiation; the constitution
of the sun; the possible sources of his energy; the relation of this energy to terrestrial forces, and to vegetable and animal life.
My aim has been to rise to the level of these questions from a basis so elementary, that a person possessing any imaginative faculty and power of concentration,
might accompany me.




iv                  PREFACE.
Wherever additional remarks, or extracts, seemed
likely to render the reader's knowledge of the subjects
referred to in any Lecture more accurate or complete,
I have introduced such extracts, or remarks, as an Appendix to the Lecture.
For the use of the Plate at the end of the volume,
I am indebted to the Council of the Royal Society; it
was engraved to illustrate some of my own memoirs in
the'Philosophical Transactions.'  For some of the
Woodcuts I am also indebted to the same learned body.
To the scientific public, the names of the builders
of this new philosophy are already familiar. As experimental contributors, Rumford, Davy, Faraday, and
Joule, stand prominently forward. As theoretic writers
(placing them alphabetically), we have Clausius, Helmholtz, Kirchoff, Mayer, Rankine, Thomson; and in the
memoirs of these eminent men the student who desires
it, must seek a deeper acquaintance with the subject.
MM. Regnault and Seguin also stand in honourable relationship to the Dynamical Theory of Heat, and M.
Verdet has recently published two lectures on it,
marked by the learning for which he is conspicuous.
To the English reader it is superfluous to mention the
well-known and highly-prized work of /IMr. Grove.
I have called the philosophy of Heat a new philosophy, without, however, restricting the term to the subject of Heat. The fact is, it cannot be so restricted;
for the connection of this agent with the general energies of the universe is such, that if we master it perfectly, we master all. Even now we can discern, though
but darkly, the greatness of the issues which connect
themselves with the progress we have made —issues
which were probably beyond the contemplation of




PREFACE.                   V
those, by whose industry and genius the foundations
of our present knowledge were laid.
In a Lecture on the' Influence of the History of
Science on Intellectual Education,' delivered at the
Royal Institution, Dr. Whewell has shown'that every
advance in intellectual education has been the effect of
some considerable scientific discovery, or group of discoveries.' If the association here indicated be invariable, then, assuredly, the views of the connection and
interaction of natural forces-organic as well as inorganic-vital as well as physical —which have grown,
and which are to grow, out of the investigation of the
laws and relations of Heat, will profoundly affect the
intellectual discipline of the coming age.
In the study of Nature two elements come into
play, which belong respectively to the world of sense
and to the world of thought. We observe a fact and
seek to refer it to its laws,-we apprehend the law, and
seek to make it good in fact. The one is Theory, the
other is Experiment; which, when applied to the ordinary purposes of life, becomes Practical Science. Nothing could illustrate more forcibly the wholesome interaction of these two elements, than the history of our
present subject. If the steam-engine had not been invented, we should assuredly stand below the theoretic
level which we now occupy. The achievements of
Heat through the steam-engine have forced, with augmented emphasis, the question upon thinking minds-'What is this agent, by means of which we can supersede the force of winds and rivers-of horses and of
men?  Heat can produce mechanical force, and mechanical force can produce Heat; some common quality
must therefore unite this agent and the ordinary forms




vi                    PREFACE.
of mechanical power.' This relationship established,
the generalising intellect could pass at once to the other
energies of the universe, and it now perceives the principle which unites them all. Thus the triumphs of
practical skill have promoted the developement of philosophy. Thus, by the interaction of thought and fact,
of truth conceived and truth executed, we have made
our science what it is,-the noblest growth of modern
times, though as yet but partially appealed to as a
source of individual and national might.
As a means of intellectual education its claims are
still disputed, though, once properly organised, greater
and more beneficent revolutions await its employment
here, than those which have already marked its applications in the material world. Surely the men whose
noble vocation it is to systemize the culture of England,
can never allow this giant power to grow up in their
midst without endeavouring to turn it to practical account. Science does not need their protection, but it
desires their friendship on honourable terms: it wishes
to work with them towards the great end of all education,-the bettering of man's estate. By continuing to
decline the offered hand, they invoke a contest which
can have but one result. Science must grow. Its developement is as necessary and as irresistible as the
motion of the tides, or the flowing of the Gulf Stream.
It is a phase of the energy of Nature, and as such is
sure, in due time, to compel the recognition, if not to
win the alliance, of those who now decry its influence
and discourage its advance.
ROYAL INSTITUTION, February 1863.




CONTENTS.
LECTURE I.
Introduction-Description of Instruments-The Thermo-electric Pile and the Galvanometer-Heat and Cold indicated by the Deflection of a Magnetic Needle
-Heat generated by Friction, Compression, and Percussion-Waterfalls generate Heat-Friction of Railway Axles-The Force necessary to heat the Axles
is withdrawn from the urging Force of the Engine-Meteorites probably rendered incandescent by Friction against Air-Rumford's Experiments on the
Heat excited by Friction-Water boiled by Friction-Consumption of Heat
when compressed Air is suffered to expand-Action of a Current of Air when
urged by a bellows against the Face of the Thermo-electric Pile  PACr 13
APPENDIX TO LECTURE I.
Mode of constructing a Thermo-electric Pile-Mode of constructing a Galvanometer-Mode of rendering Needles astatic-Experiments on the Magnetism of
Galvanometer Coils, and Mode of avoiding this Magnetism           30
LECTURE II.
The Nature of Heat-the Material Theory supposes it to be a subtle Fluid stored
up in the inter-atomic Spaces of Bodies-The idea of'Capacity' for Heat
originated in this way-The Dynamical Theory supposes Heat to be a Motion
of the ultimate Particles of Bodies-Rumford's and Davy's Views-Davy's
Fusion of Ice by Friction-Bearing of the Experiment on the Material Theory
-Heat and Light developed by the Compression of Air-Ignition of Bisulphide
of Carbon Vapour in Fire Syringe-Thermal Effects of Air in Motion-Condensation of Aqueous Vapour by the Rarefaction of Air-Machine of Schemnitz-Deportment of a Conductor between the Poles of a Magnet-Apparent
Viscosity of the Magnetic Field-The Conductor encounters Resistance to its
Motion-A Conductor swiftly rotating is struck motionless when the Magnet
is excited-When the Conductor is compelled to rotate Heat is generatedFusion of an Alloy by this Heat-Measurement of the Amount of Heat generated by a given Expenditure of Force-Dr. Mayer and Mr. Joule —The Mechanical Equivalent of Heat-Definition of the Term'Foot-pound' - Heat
developed increases as the height of the fall, and is proportional to the Square
of the Velocity-Calculation of Heat generated by the impact of Projectiles



Viii                           CONTENTS.
Heat equivalent to the Stoppage of the Earth in its Orbit-H-eat equivalent to
the Falling of the Earth into the Sun-Preliminary Statement of Meteoric
Theory of the Sun's Heat-Analysis of Combustion-Ignitiou of DiamondIts Combustion in Oxygen due to the Showering of the Atoms of the Gas
against the Surface of the Diamond-Structure of Flame-Candle and Gas
Flames-Combustion on Mont Blanc-The Light of Flames is materially
diminished by the Rarefaction of the Air, though the Quantity of Combustible
Matter consumed remains the same —Frankland's Experiments-All Cases of
Combustion are due to the Collision of Atoms which have been urged together
by their mutual Attractions.PAGE 37
APPENDIX TO LECTURE II.
Extracts from the Twentieth Aphorism of the'Novum Organum'-Abstract of
Count Rumford's Essay entitled' An Enquiry concerning the Source of the
Heat which is excited by Friction'-Note on the Compression of Bisulphide
of Carbon Vapour.66
LECTURE III.
Expansion of Bodies by Heat-Liberation of Particles from the thrall of Cohesion
-The Liquid and Gaseous States of Matter defined-Illustrations of the Expansion of Air by Heat-Ascent of Fire Balloon-Gases expand by a constant
Increment for every Degree above 32' Fahr.-Coefficient of Expansion-Heating of Gas at a constant Pressure-Heating of Gas at a constant Volume-In
the former case Work is done by the Gas —In the latter case no Work is done
-In the former case an Excess of Heat equivalent to the Work done must be
imparted-Calculation of the Mechanical Equivalent of Heat-Mayer and
Joule's Determinations-Absolute Zero of Temperature-Expansion without
Refrigeration-Expansion of Liquids-Exceptional Deportment of Water and
Bismuth-Energy of Atomic Forces-Pyrometers-Strains and Pressures
superinduced by sudden Cooling —Chilling of Metallic Wires by StretchingHeating of India-rubber by Stretching-Contraction of stretched India-rubber
by Heat.............       73
APPENDIX TO LECTURE III.
Further Remarks on Dilatation-Linear, Superficial, and Cubic Coefficients of
Expansion-The Thermometer - Extracts from Sir Humphry Davy's First
Memoir, entitled'Heat, Light, and the Combinations of Light'   -.  104
LECTURE IV.
Vibrations and Tones produced by the contact of Bodies of different Temperatures
-The Trevelyan Instrument-Rotation of hollow Spheres by ElectricityEffect of Pressure on Fusing Point-The Fusing Point of Bodies which contract on solidifying is raised by Pressure-The Fusing Point of Bodies which
expand on solidifying is lowered by Pressure-Liquefaction of Ice by Pressure
-Dissection of Ice by Calorific Beam-Negative Crystallisation, Ice reduced
internally to Liquid Flowers, having six Petals each-Central Spot a Vacuum
-Sound heard when the Spot is formed-Physical Properties of Water from
which Air has been removed-Its Cohesion enormously augmented-It can be




CONTENTS.                              ix
heated far above its boiling Point-Its Ebullition becomes Explosion-Application of this Property to explain the Sound heard, when the central Spot is
formed in Ice-Possible bearing of this Property of Water on Boiler explosions
-The Boiling Point of Liquids-Resistance to Ebullition-Cohesion of Particles,
adhesion to Vessel, external Pressure-Boiling Points on various Alpine Summits-The law of Conservation illustrated in the Steam Engine-The Geysers
of Iceland-Description of the Geysers and their Phenomena-Bunsen's Theory
-Experimental Illustration.                                 PAGE 112
APPENDIX TO LECTURE IV.
Abstract of a Lecture on the Vibrations and Tones produced by the contact of
Bodies having different Temperatures-Extracts from a Paper on the Physical
Properties of Ice.142
LECTURE V.
Application of the Dynamical Theory to the Phenomena of Specific and Latent
Heat-Definition of Energy-Potential and Dynamic Energy, illustrated by
the Raising and Falling of a Weight-Convertibility of Potential into Dynamic Energy and the reverse-Constancy of the Sum of both Energies-Application of the Ideas of Potential and Dynamic Energy to Atoms and Molecules
-Magnitude of Molecular Forces-The separation of a Body's Particles by Heat
is an Act the same in kind as the separation of a Weight from the EarthWork is here done within the heated Body-Interior Work-The Heat communicated to a Body divides itself into Potential and Dynamic Energy-All
Single Atoms, whatever be their weight, possess the same amount of Dynamic
Energy-Specific Heat or Capacity for Heat explained by Reference to Interior
Work and to Atomic Number-Experimental Illustrations of Specific HeatTable of Specific Heats-Influence of high Specific Heat of Water on Climnate
-Heat consumed in Change of Aggregation-Latent Heat of Liquids and Vapours-It is Heat consumed in conferring Potential Energy on the ultimate
Particles-By Condensation and Liquefaction this Potential Energy is converted into Heat-Mechanical Value of the Union of Oxygen and HydrogenMechanical Value of the Change from Steam to liquid XVater-Mechanical
Value of the Change from liquid Water to sol.d Ice-Experimental lllustrations-Consumption and Generation of Heat by Changes of AggregationWater frozen by its OWvn Evaporation-The Cryophorus-Solid Carbonic Acid
-The Spheroidal State of Liquids-In this State the Liquid is supported on a
Bed of its own Vapour-Proofs that the Spheroidal Drop is not in Contact with
the hot Surface underneath it-Experimental Illustrations of the Spheroidal
Condition-Possible Bearing of these Facts on the Fiery Ordeal, and on Boiler
iExplosions-Freezing of Water and Mercury in red-hot Vessels.. 150
LECTURE VI.
Convection in Air-Larger physical Phenomena-Winds caused by the heating
Action of the Sun-The upper and lower Trade Winds-Effect of the Earth's
Rotation in changing the apparent Direction of Winds-The Existence of the
upper Current proved by the Discharge of Ashes into it by Volcanoes-Effects
which accompany the apparent Motion of the Sun from side to side of the
Equator-Aqueous Vapour-Tropical Rains-Region of Calms-Europe, for
the most Part in the upper Trade-Europe the Condenser of the Western At



X                            CONTENTS.
lantic-This is the Cause of the Mildness of European Temperature-Rainfall
in Ireland-Effect of Mountain Ranges on Rainfall-Convection in LiquidsExperimental Illustrations-The Gulf Stream: its influence on the Climate
of Britain-Formation of Snowv-The Molecules aggregate to form Frozen
Stars with Rays sixty degrees apart-Figures of Snow Crystals-Collection of
Snow on Mountains-The Snow Line-Squeezing of this Snow to Ice-Formation of Glaciers-The Motion of a Glacier resembles that of a iiver-Theories
of Glacier Motion-The Regelation of Ice-The Moulding of Ice by PressureAncient Glaciers-Their Traces in Switzerland, England, Ireland, and WTales
-The Cedars of Lebanon grow on Glacier Moraines-Theories of the Glacial
Epoch-Not due to a Diminution of Solar Power, or to the passage of the Solar
System through cold Regions of Space....            PAGE 181
APPENDIX TO LECTURE VI.
Abstract of a Lecture on the Mer de Glace..                        208
LECTURE VII.
The Conduction of Heat a Transmission of Molecular Motion-Different Bodies
possess different Powers of Transmission-Good Conductors and bad Conductors-Experimental Illustrations-Experiments of Ingenhausz, Despretz,
Wiedemann, and Franz-Table of Conductivities-Parallelism of Conduction of
Heat and Conduction of Electricity —Good Conductors of the one are also good
Conductors of the other, and vice versA-Influence of Heat on Electric Conduction-The Motion of Heat interferes with the Motion of Electricity-Conduction of Cold-Constancy of Temperature of Animal Body-Capacity to
bear high Temperatures-Diversion of Heat from the Purposes of Temperature to the Performance of Work-Influence of Molecular Structure-Some
Bodies conduct differently in different Directions-Conduction in Crystals and
in Wood-Feeble Conductivity of Organic Substances-This secures them
from sudden Alternations of Temperature-Influence of Specific Heat on the
Speed of Conduction-Anomalous Case of Bismuth as compared with IronBismuth, though the worst Conductor, apparently transmits Heat most
speedily-Action of Clothing-Rumford's Experiments-Influence of mechanical Texture on Conduction-A Powder conducts ill, on account of the incessant
Break in the Continuity of the Mass along which the Motion of Heat is transmitted-Non-conductivity of Gypsum-Effect of Boiler encrustations-WVithdrawal of Heat from Flames by good Conductors-The Motion of Flame,
though intense, is much lowered by being transferred from so light a Body to
a heavy one-Effect of Wire Gauze-The Safety Lamp-Conduction of Liquids
denied by Rumford, but proved by M. Despretz-Conduction of Gases denied
by Rumford, but affirmed in the case of Hydrogen by Prof. Magnus-Cooling
Effect of Air and Hydrogen-Experiments on Gaseous Conduction doubtful..                                                            218
LECTURE VIII.
Radiant Heat-Cooling a loss of Motion-To what is this Motion imparted?-Preliminary Experiments on Sound-Communication of Vibrations through the
Air to Membranes and to Flames-The Vibrations of a Body propagated
through the Air and striking on the Drum of the Ear produce Sound-Analogous Phenomena of Light-Theories of Emission and Undulation-Discussions




CONTENTS.                               X
on the Sutbject-Newton-H-uyghens-Euler-Young-Fresnel-Space filled
with an elastic Medium called Ether-The Motion of a hot or of a luminous
body communicated to this Ether is propagated through it in waves-In this
form Heat is called Radiant Heat-The Thermo-electric Pile in relation to
Radiant Heat-Distribution of Heat in the Electric Spectrum examined experimentally-Low Calorific Power of blue End of Spectruml-The most
luminous Part not the hottest Part-Of the visible rays Red is the hottestThe maximum Calorific Action is beyond the Red, and is due to Rays which are
incompetent to excite the Sense of Vision-Extra Violet Rays-Physical Cause
of Colour-The Spectrum is to the Eye what the Gamut is to the Ear-The
Colour of Light corresponds to the Pitch of Sound-Number of Impulses involved in the Perception of Light-Theory of Exchanges-Reflection of Radiant
Heat from Plane Surfaces-Angle of Incidence equal to the Angle of Reflection-Experimental Proof-The obscure Rays of the Electric Lamp pursue the
same Track as the luminous ones-Angular Velocity of reflected Ray twice
that of rotating Mirror-Experiments with radiant Heat of Fire and of obscure
Bodies-Reflection from curved Surfaces-Parabolic Mirrors —Explosion of
Chlorine and Hydrogen in Focus of Mirror by Light-Explosion of Oxygen
and Hydrogen by Radiant Heat-Reflection of Cold...   PAGE 257
APPENDIX TO LECTURE VIII.
On the Sounds produced by the Combustion of Gases in Tubes..    284
LECTURE IX.
The Intensity of Radiant Heat diminishes as the Square of the Distance from the
radiant Point increases-Experimental Proof-Undulations of Sound longitudinal, of Light transversal-The ultimate Particles of different Bodies possess
different Powers of Communicating Motion to the Ether-Experimental Illustrations of good and bad Radiators-Reciprocity of Radiation and Absorption
-Protection by Gilding against Radiant Heat-Transmission of Radiant Heat
through Solids and Liquids-Diathermancy-Absorption occurs within the
Body-Absorption of Light by Water-Radiant Heat passes through Diathermic Bodies without heating them —Athermic Bodies are heated-Concentration of Beam on bulb of Air Thermometer —Penetrative Power of Sunbeams-Sifting of Radiant Heat-Ratio of Obscure to Luminous Radiation in various
Flames.297
APPENDIX TO LECTURE IX.
On some physical Properties of Ice.. 824
LECTURE X.
Absorption of Heat by Gases and Vapours-First Apparatus-Rocksalt PlatesPeculiarities of the Galvanometer-The higher Degrees of greater Value than
the lower ones-Improved Apparatus-Principle of Compensation permits the
use of a powerful Source of Heat while it preserves the Needle in a sensitive
Position-Air, Oxygen, Hydrogen, and Nitrogen are practical Vacua to Radiant
Heat-Opacity of Olefiant Gas and Sulphuric Ether-Radiation through other
Gases and Vapours-Great difference of Absorbing Power-Radiation of Heat
by Gases-The atom which Absorbs powerfully Radiates powerfully-Absorption by Gases at a Tension of an Atmosphere-Absorptions at smaller Tensions




xii                            CONTENTS.
-Comparison of Elementary and Compound Gases and Vapours-Radiation
through Lampblack, &c.                                      PAGE 337
APPENDIX TO LECTURE X.
Calibration of the Galvanometer.                                    370
LECTURE XI.
Action of Odorous Substances on Radiant Heat-List of Perfumes examined-Action of Ozone on Radiant Heat-Influence of the Size of the Electrodes on the
Quantity of Ozone generated-Constitution of Ozone-Radiation and Absorption of Gases and Vapours determined without any Source of Heat external to
the gaseous Body itself-Dynamic Radiation and Absorption-Varnishing a
metal Surface by a Gas-Varnishing of a Gas by a Vapour-Tenuity of Boracic Ether shown in Experiments on Dynamic Radiation-Influence of Length
of radiating Column-In a long Tube, a Vapour at a small Tension may exceed a Gas at a high Tension, while in a short Tube the Gas exceeds the
Vapour-Radiation through Humid Air —Action of the Vapour of the Atmosphere on Terrestial and Solar Radiation —Objections answered-Applicability
of Rocksalt Plates-Experiments in Tube without Plates-Experiments without either Tube or Plates-Examination of Air from various localities-Influence of the Results on the Science of Meteorology-Application to tropical
Rain Torrents-To the Formation of Cumuli-To the Condensation of a Mountainous Region-To Radiation Experiments at high and low Elevations-To
the Cold of Central Asia-To the Thermometric Range in Australia-To the
Meteorology of Sahara-To Leslie's Experiments-To Wells's Theory of Iceformation in India-To Melloni's Theory of Serein.373
APPENDIX TO LECTURE XI.
Further Details regarding the Action of aqueous Vapour on Radiant Heat. 409
LECTURE XII.
Dew —a clear Sky and calm but damp Atmosphere necessary for its copious Formation-Dewed Substances colder than undewed ones-Dewed Substances
better Radiators than undewed ones-Theory of Wells-Dew is the Condensation of the atmospheric Vapour on Substances which have been chilled by
Radiation-Lunar Radiation-Constitution of the Sun-The bright Lines in
the Spectra of the Metals-An incandescent Vapour absorbs the Rays which
it can itself emit-Kirchhofifs Generalisation-Fraunhofer's Lines, caused by
the Absorption of such Rays by the luminous Solar Atmosphere as that Atmosphere itself could emit-Solar Chemistry-Emission by the Sun-Herschell and Pouillet's Experiments —Mayer's Meteoric Theory-Effect of the
Tides on the Earth's Rotation-Energies of the Solar System-Helmholtz,
Thomson, Waterston-Relation of the Sun to Vegetable and Animal Life 414
APPENDIX TO LECTURE XII.
Lecture on Force; Letters of Dr. Joule and Prof. Tyndall..     450




HEAT
CONSIDERED AS
A MODE OF MOTION.
LECTURE I.
[January 23, 1862.]
INSTRUMENTS —-GENERATION OF HEAT BY MECHANICAL ACTIONCONSUMPTION OF HEAT IN WORK.
APPENDIX:-NOTES ON THE THERMO-ELECTRIC PILE AND GALVANOMETER.
THE chief characteristic of Natural Knowledge is its
growth; each fact is vital, and every new discovery
forms a starting point for fresh investigation. Thus it
seems destined to advance, until the phenomena and laws
of the material universe are entirely subdued by the intellect of man. But though each department of natural
knowledge has been adding to its store, at a rate unknown
in former times, no branch of it has expanded so rapidly,
of late, as that which, in these lectures, is to occupy our
attention. In scientific manuals but scanty reference has
as yet been made to the modern ideas of Heat, and thus
the public knowledge regarding it is left below the attain



14                   LECTURE I.
able level. But the reserve is natural, for the subject is
still an entangled one, and, in entering upon it, we must be
prepared to encounter difficulties. In the whole range of
Natural Science, however, there are none more worthy of
being overcome, —none whose subjugation secures a greater
reward to the worker. For by mastering the laws and relations of Heat, we make clear to our minds the interdependence of natural forces generally. Let us, then, commence our labours with heart and hope; let us familiarise
ourselves with the latest facts and conceptions regarding
this all-pervading agent, and seek diligently the links of
law which underlie the facts and give unity to their most
diverse appearances. If we succeed here we shall satisfy,
to an extent unknown before, that love of order and of
beauty which, I am persuaded, is implanted in the mind
of every person here present. From the heights at which
we aim, we shall have nobler glimpses of the system of
Nature than could possibly be obtained, if I, while acting
as your guide in the region which we are now about to enter, were to confine myself to its lower levels and already
trodden roads.
It is my first duty to make you acquainted with some
of the instruments which I intend to employ in the examination of this question. I must devise some means of
making the indications of heat and cold visible to you all,
and for this purpose an ordinary thermometer would be
useless. You could not see its action; and I am anxious
that you should see, with your own eyes, the facts on
which our subsequent philosophy is to be based. I wish to
give you the material on which an independent judgment
may be founded; to enable you to reason as I reason if you
deem me right, to correct me if I go astray, and to censure
me if you find me dealing unfairly with my subject. To
secure these ends, I have been obliged to abandon the use
of a common thermometer, and to resort to the little in



THE THERMO-ELECTRIC PILE AND GALVANOMETER. 15
strument A B (fig. 1), which you see before me on the table,
and which is called a thermo-electric pile.*
By means of this instrument I cause the heat which it
receives to generate an electric current. You know, or
ought to know, that such a current has the power of deflecting a freely suspended magnetic needle, to which it
flows parallel. Before you I have placed such a needle
m n (fig. 1), surrounded by a covered copper wire, the free
Fig. 1.
ends of which, w wto are connected with the thermo-electric
pile. The needle is suspended by a fibre, s s, of unspun
silk, and protected by a glass shade, G, from any disturbance by currents of air. To one end of the needle I have
fixed a piece of red paper, and to the other end a piece of
blue. All of you see these pieces of paper, and when the
needle moves, its motion will be clearly visible to the most
distant person in this room.I
* A brief description of the thermo-electric pile is given in the
Appendix to this Lecture.
f In the actual arrangement the galvanometer here described stood on
a stool in front of the lecture table, the wires w w, being sufficiently long




16                   LECTURE I.
At the present moment the needle is quite at rest, and
points to the zero mark on the graduated disc underneath
it. This shows that there is no current passing. I now
breathe for an instant against the naked face A of the pile
-a single puff of breath is sufficient for my purposeobserve the effect. The needle starts off and passes through
an arc of 900. It would go further, did I not limit its
swing by fixing, edgewise, a thin plate of mica at 90~,
Take notice of the direction of the deflection; the red end
of the needle moved from me towards you, as if it disliked
me, and had been inspired by a sudden affection for you.
This action of the needle is produced by the small amount
of warmth communicated by my breath to the face of the
pile, and no ordinary thermometer could give so large and
prompt an indication. We will let the heat thus communicated waste itself; it will do so in a very short time, and
you notice, as the pile cools, that the needle returns to its
first position. Observe, now, the effect of cold on the face
of the pile. I have here some ice, but I do not wish to wet
my instrument by touching it with ice. Instead of doing
so, I will cool this plate of metal by placing it on the ice;
then wipe the chilled metal, and touch with it the face of
the pile. You see the effect; a moment's contact suffices
to produce a prompt and energetic deflection of the needle.
But mark the direction of the deflection. When the pile
was warmed, the red end of the needle moved from me
towards you; now its likings are reversed, and the red end
moves from you towards me. Thus you see that cold and
heat cause the needle to move in opposite directions. The
important point here established is, that from the direction
in which the needle moves, we can, with certainty, infer
whether cold or heat has been communicated to the pile;
to reach from the table to the stool; for a further description of the galvanometer, see the Appendix to this Lecture.




HEAT OF FRICTION.               17
and the energy with which the needle moves —the promptness with which it is driven aside from its position of rest
-gives us some idea of the comparative quantity of heat
or cold imparted to it in different cases. In a future lecture
I shall explain how we may express the relative quantities
of heat with numerical accuracy; but for the present a general knowledge of the action of our instruments will be
sufficient.
My desire now is to connect heat with the more familiar forms of force, and I will, therefore, in the first place,
try to furnish you with a store of facts illustrative of the
generation of heat by mechanical processes. I have placed
some pieces of wood in the next room, which my assistant
will now hand to me. Why have I placed them there?
Simply that I may perform my experiments with that sincerity of mind and act which science demands from her
cultivators. I know that the temperature of that'room is
slightly lower than the temperature of this one, and that
hence the wood which is now before me must be slightly
colder than the face of the pile with which I intend to test
the temperature of the wood. Let us prove this. I place
the face of the pile against this piece of wood; the red end
of the needle moves from you towards me, thus showing
that the contact has chilled the pile. I now carefully rub
the face of the pile along the surface of the wood; I say' carefully,' because the pile is a brittle instrument, and
rough usage would destroy it;-mark what occurs. The
prompt and energetic motion of the needle towards you
declares that the face of the pile has been heated by this
small amount of friction. The needle, you observe, goes
quite up to 90~ on the side opposite to that towards which
it moved before the friction was applied.
Now these experiments, which illustrate the developement of heat by mechanical means, must be to us what a
boy's school exercises are to him. In order to fix them on




18                   LECTURE I.
our minds, and obtain due mastery over them, we must repeat and vary them in many ways. In this task I ask you
to accompany me. Here is a flat piece of brass with a stem
attached to it; I take the stem in my fingers, preserving
the brass from all contact with my warm hand, by enveloping the stem in cold flannel. I place the brass in contact
with the face of my pile; the needle moves, showing that
the brass is cold. I now rub the brass against the surface
of this cold piece of wood, and lay it once more against
my pile. I withdraw it instantly, for it is so hot that if I
allowed it to remain in contact with the instrument, the
current generated would dash my needle violently against
its stops, and probably derange its magnetism. You see
the strong deflection which even an instant's contact can
produce. Indeed, when a boy at school, I have often blistered my hand by the contact of a brass button, which I
had rubbed energetically against a form. Here, also, is a
razor, cooled by contact with ice; and here is a hone, without oil, along which I rub my cool razor, as if to sharpen
it. I now place the razor against the face of the pile,
and you see that the steel, which a minute ago was cold,
is now hot. Similarly, I take this knife and knife-board,
which are both cold, and rub the knife along the board. I
place the knife against the pile, and you observe the result;
a powerful deflection, which declares the knife to be hot.
I pass this cold saw through this cold piece of wood, and
place, in the first instance, the surface of the wood against
which the saw has rubbed, in contact with the pile. The
needle instantly moves in a direction which shows the
wood to be heated. I allow the needle to return to zero,
and now apply the saw to the pile. It also is hot. These
are the simplest and most common-place examples of the
generation of heat by friction, and I choose them for this
reason. Mean as they appear, they will lead us by degrees




HEAT OF COMPRESSION AND PERCUSSION.       19
into the secret recesses of Nature, and lay open to our
view the policy of the material universe.
Let me now make an experiment to illustrate the developement of heat by compression. I have here a piece
of deal, cooled below the temperature of the room, and
giving, when placed in contact with our pile, the deflection
which indicates cold. I place this wood between the
plates of a small hydraulic press, and squeeze it forcibly.
The plates of the press are also, you will observe, cooler
than the air of the room. After compression, I bring the
wood into contact with the pile; see the effect. The galvanometer declares that heat has been developed by the
act of compression. Precisely the same occurs when I
place this lead bullet between the plates of the press and
squeeze it thus to flatness.
And now for the effect of percussion. I have here a
cold lead bullet, which I place upon this cold anvil, and
strike it with a cold sledge hammer. The sledge descends
with a certain mechanical force, and its motion is suddenly
destroyed by the bullet and anvil; apparently the force of
the sledge is lost. But let us examine the lead; you see it
is heated, and could we gather up all the heat generated by
the shock of the sledge, and apply it without loss mechanically, we should be able, by means of it, to lift this hammer to the height from which it fell.
I have here arranged another experiment, which is almost
too delicate to be performed by the coarse apparatus necessary in a lecture, but which I have made several times before entering this room to-day. Into this small basin I
pour a quantity of mercury which has been cooled in the
next room. I have coated one of the faces of my thermo.
electric pile with varnish, so as to defend it from the mercury, which would otherwise destroy the pile; and, thus
protected, I can, as you observe, plunge the pile into the
liquid metal. The deflection of the needle shows you that




20                   LECTURE I.
the mercury is cold. Here are two glasses A and B (fig.
2), swathed thickly round by listing, which will effectually
prevent the warmth of my hands from reaching the mercury. Well, I pour the cold mercury from the one glass
into the other, and back. It falls with a certain mechanical force, its motion is destroyed, but heat is developed.
The amount of heat generated by a single pouring out is
extremely small; I could tell you the exact amount, but
shall defer quantitative considerations till our next lecture;
so I pour the mercury from glass to glass ten or fifteen
times. Now mark the result, when the pile is plunged into
the mercury.  The
needle moves, and its
motion declares that
the mercury, which
at the beginning of
the experiment was
cooler than the pile,
is now warmer than
the pile.  We here
lilt ~  ~      i    introduce  into  the
lecture-room an effect
-Pa~i-. which occurs in nature at the base of every waterfall. There
are friends before me
who have stood amid the foam of Niagara. Had they,
when there, dipped sufficiently sensitive thermometers into
the water at the top and bottom of the cataract, they would
have found the latter a little warmer than the former. The
sailor's tradition, also, is theoretically correct; the sea is
rendered warmer through the agitation produced by a
storm, the mechanical dash of its billows being ultimately
converted into heat.
Whenever friction is overcome, heat is produced, and




HEAT OF WATERFALLS, ETC., 4ILINT AND STEEL.  21
the heat produced is the measure of the force expended in
overcoming the friction. The heat is simply the primitive
force in another form, and if we wish to avoid this conversion, we must abolish the friction. We usually put oil
upon the surface of a hone, we grease a saw, and are careful to lubricate the axles of our railway carriages. What
are we really doing in these cases? Let us get general
notions first; we shall come to particulars afterwards. It is
tile object of a railway engineer to urge his train bodily
from one place to another; say from London to Edinburgh,
or from London to Oxford, as the case may be; he wishes
to apply the force of his steam, or of his furnace, which
gives tension to the steam, to this particular purpose.
It is not his interest to allow any portion of that force to
be converted into another form of force which would not
further the attainment of his object. He does not want
his axles heated, and hence he avoids as much as possible
expending his power in heating them. In fact, he has obtained his force from heat, and it is not his object to reconvert the force thus obtained into its primitive form. For,
for every degree of temperature generated by the friction
of his axles, a definite amount would be withdrawn from
the urging force of his engine. There is no force lost absolutely. Could we gather up all the heat generated by
the friction, and could we apply it all mechanically,  we
should, by it, be able to impart to the train the precise
amount of speed which it had lost by the friction. Thus
every one of those railway porters whom you see moving
about with his can of yellow grease, and opening the little
boxes which surround the carriage axles, is, without knowing it, illustrating a principle which forms the very solder
of Nature. In so doing, he is unconsciously affirming both
the convertibility and the indestructibility of force. He is
practically asserting that mechanical energy may be converted into heat, and that, when so converted, it cannot




22                   LECTURE I.
still exist as mechanical energy, but that, for every degree
of heat developed, a strict and proportional equivalent of
the locomot'iveforce of the engine disappears. A station is
approached, say at the rate of thirty or forty miles an hour;
the brake is applied, and smoke and sparks issue from the
wheel on which it presses. The train is brought to restHow? Simply by converting the entire moving force which
it possessed, at the moment the brake was applied, into heat.
So, also, with regard to the greasing of a saw by a carpenter. Ile applies the muscular force of his arm with the
express object of getting through the wood. He wishes to
tear the wood asunder, to overcome its mechanical cohesion
by the teeth of his saw. When the saw moves stiffly, on
account of the friction against its flat surface, the same
amount of force may produce a much smaller effect than
when the implement moves without friction. But in what
sense smaller? Not absolutely so, but smaller as regards
the act of sawing. The force not expended in the sawing
is not lost; it is converted into heat, and I gave you an
example of this a few minutes ago. Here again, if we
could collect the heat engendered by the friction, and apply
it to urge the saw, we should make good the precise amount
of work which the carpenter, by neglecting the lubrication
of his implement, had simply converted into another form
of power.
We warm our hands by rubbing, and in the case of
frostbite we thus restore the necessary heat to the injured
parts. Savages have the art of producing fire by the skilful friction of well-chosen pieces of wood. It is easy to
char wood in a lathe by friction. From the feet of the
labourers on the roads of Hampshire sparks issue copiously
on a dark night, the collision of their iron-shod shoes
against the flints producing the effect. In the common
flint and steel the particles of the metal struck off are so
much heated by the collision that they take fire and burn in




HEAT OF METEORITES.                  23
the air. But the heat precedes the combustion. Davy
found that when a gunlock, with a flint, was discharged in
vacuo, no sparks were produced, but the particles of steel
struck off, when examined under the microscope, showed
signs of fusion.* Here is a large rock-crystal; I have only
to draw this small one briskly along it, to produce a stream
of light; here are two quartz pebbles, I have only to rub
them together to make them luminous.
A bullet, in passing through the air, is warmed by the
friction, and the most probable theory of shooting stars is
that they are small planetary bodies, revolving round the
sun, which are caused to swerve from their orbits by the
attraction of the earth, and are raised to incandescence by
friction against our atmosphere. Mr. Joule has shown
that the atmospheric friction is competent to produce the
effect; and he may be correct in believing that the greater
portion of the aerolites are dissipated by heat, and the
earth thus spared a terrible bombardment.f These bodies
move with planetary velocity; the orbital velocities of the
four interior planets are as follows:Miles per second.
Mercury,...... 3040
Venus,. 22'24
Earth,...... 18'91
Mars,.... 1532
while the velocity of the aerolites varies from  18 to 36
miles a second.t The friction engendered by this enormous speed is certainly competent to produce the effects
ascribed to it.
More than sixty-four years ago Count Rumford, who
was one of the founders of the Royal Institution, executed a series of experiments on the generation of heat
* Works of Sir HI. Davy, vol. ii. p. 8.
t Philosophical Magazine, 4th Series, vol. xxxii. p. 349.
t Galbraith and Houghton's Manual of Astronomy, p. 18.




24                   LECTURE I.
by friction, which, viewed by the light of to-day are of
the highest interest and importance. Indeed, the services
which this Institution has rendered, in connection with
this question of the brotherhood of natural forces, can
never be forgotten. Thomas Young, a former professor of this Institution, laid the foundations of the undulatory theory of light, which, in its fullest application, embraces our present theory of heat. Davy entertained substantially the same views regarding heat as those which I
am now endeavouring to approach and elucidate. Faraday
established the laws of equivalence between chemistry and
electricity, and his magneto-electric discoveries were the
very first seized upon by Mr. Joule in illustration of the
mutual convertibility of heat and mechanical action.* Rumford, in a paper of great power both as regards reasoning
and experiment, advocated in 1798f the doctrine regarding
the nature of heat which the recent experiments of eminent
men have placed upon a secure basis. While engaged in
the boring of cannon at Munich, he was so forcibly struck
by the large amount of heat developed in the process of
boring, that he was induced to devise an apparatus for
the special examination of the generation of heat by friction. He had constructed a hollow cylinder of iron, into
which he fitted a solid plunger, which was caused to press
against the bottom of the cylinder. A box which surrounded the cylinder contained 183 lbs. of water, in which
a thermometer was placed. The original temperature of the
water was 60~. The cylinder was turned by horse-labour,
and an hour after the friction had commenced the temperature of the water was 107~, having been raised 47~. Half
an hour afterwards he found the temperature to be 142~.
The action was continued, and at the end of two hours the
* Philosophical Magazine, 4th Series, vol. xxiii. pp. 265, 34', 435.
t An abstract of this paper is given in the Appendix to Lecture II.




WATER BOILED BY FRICTION.            25
temperature as 178~. At the end of two hours and twenty
minutes it was 200~, and at two hours and thirty minutes
from the commencement the water actually boiled! Rumford's description of the effect of this experiment on those
who witnessed it, is quite delightful.'It would be difficult,' he says,' to describe the surprise and astonishment
expressed in the countenances of the bystanders on seeing
so large a quantity of water heated, and actually made to
boil, without any fire. Though there was nothing that
could be considered very surprising in this matter, yet I
acknowledge fairly that it afforded me a degree of childish
pleasure which, were I ambitious of the reputation of a
grave philosopher, I ought most certainly rather to hide
than to discover.'* I am sure that both you and I can dispense with the application of any philosophy which would
stifle such emotion as Rumford here avowed. In connection with this striking experiment, Mr. Joulet has estimated
the amount of mechanical force expended in producing the
heat, and obtained a result which' is not very widely different' from that which greater knowledge and more refined
experiments enabled Mr. Joule himself to obtain, as regards
the numerical equivalence of heat and work.
It would be absurd on my part to attempt here a repetition of the experiment of Count Rumford with all its
conditions. I cannot devote two hours and a half to a single experiment, but I hope to be able to show you substantially the same effect in two minutes and a half. I have
here a brass tube, four inches long, and three quarters of
an inch in interior diameter. It is stopped at the bottom,
and I thus screw it on to a whirling table, by means of
which I can cause the upright tube to rotate very rapidly.
I have here two pieces of oak wood, united by a hinge, and
* Rumford's Essays, vol. ii. p. 484,
f Philosophical Transactions, vol. cxl, p. 62.
2




26                   LEOCTURE I.
in which are two semicircular grooves, which are intended
to embrace the brass tube. Thus the pieces of wood form
a kind of tongs, T (fig. 3), by gently squeezing which I can
produce friction between the wood and the brass tube;
when the latter rotates. I almost fill the tube with cold
Fig. 8.
water, and stop it with a cork, to prevent the splashing out
of the liquid, and now I put the machine in motion. As
the action continues, the temperature of the water rises,
and though the two minutes and a half have not yet elapsed,
those near the apparatus will see steam escaping from the
cork. Three or four times to-day I have projected the cork
by the force of the steam to a height of twenty feet in the
air. There it goes again, and the steam follows it, producing by its precipitation this small cloud in the atmosphere.
In all the cases hitherto introduced to your notice, heat
has been generated by the expenditure of mechanical force.
Our experiments have gone to show that where mechanical
force is expended heat is produced, and I wish now to
bring before you the converse experiment, that is, the consumption of heat in mechanical work. And should you at
present find it difficult to form distinct conceptions as to
the bearing of these experiments, I exhort you to be patient. We are engaged on a difficult and entangled sub



COLD OF DILATATION.                27
ject, which, I hope, we shall disentangle as we go along.
I have here a strong vessel, filled, at the present moment,
with compressed air. It has been now compressed for some
hours, so that the temperature of the air within the vessel
is the same as that of the air of the room without it. At
the present moment, then, this inner air is pressing against
the sides of the vessel, and if I open this cock a portion of
the air will rush violently out of the vessel. The word'rush,' however, but vaguely expresses the true state of
things; the air which rushes out is driven out by the air
behind it; this latter accomplishes the work of urging forward the stream of air. And what will be the condition
of the working air during this process? It will be chilled.
It performs mechanical work, and the only agent which it
can call upon to perform it is the heat which it possesses,
and to which the elastic force with which it presses against
the sides of the vessel, is entirely due. A portion of this
heat will be consumed and the air will be chilled. Observe
the experiment which I am about to make. I will turn
the cock c, and allow the current of air from the vessel v
(fig. 4), to strike against the face of the pile P. See how
Fig. 4.,,,,/       1




28                   LECTURE I.
the magnetic needle responds to the act; its red end is
driven towards me, thus declaring that the pile has been
chilled by the current of air.
The effect is different when a current of air is urged'
from the nozzle of a common bellows against the thermoelectric pile. In the last experiment the mechanical work
of urging the air forward was performed by the air itself,
and a portion of its heat was consumed in the effort. In
the case of the bellows, it is my muscles which perform
the work. I raise the upper board of the bellows and the
air rushes in; I press the boards with a certain force, and
the air rushes out. The expelled air strikes the face of the
pile, has its motion stopped, and an amount of heat equivalent to the destruction of this motion is instantly generated.
Thus you observe that when I urge with the bellows B
Fig. 5.
(fig. 5), a current of air against the pile, the red end of the
needle moves towards you, thereby showing that the face
of the pile has been, in this instance, warmed by the air. I
have here a bottle of soda water; at present the bottle is
slightly warmer than the pile, as you see by the deflection
it produces; I cut the strings which holds the cork, and it




COLD OF DILATATION.                29
is it driven out by the elastic force of the carbonic acid
gas; the gas performs work, in so doing consumes heat,
and now the deflection it produces is that of cold. The
truest romance is to be found in the details of daily life,
and here, in operations with which every child is familiar,
we shall gradually discern the illustration of principles
from which all material phenomena flow.




APPENDIX TO LECTURE I.
NOTE ON THE CONSTRUCTION OF THE THERMO-ELECTRIC PILE.
LET A B (fig. 6) be a bar of antimony, and B c a bar of bismuth, and let both bars be soldered together at B. Let the free
ends A and c be united by a piece of wire, A D C.
On warming the place of junction, B, an electric  Fig. 6.
current is generated, the direction of which is
from bismuth to antimony (B to A, or against
the alphabet), across the junction, and from antimony to bismuth (A to B, or with the alphabet), through the connecting wire, A D c. The  A      C
arrow indicates the direction of the current.
If the junction B be chilled, a current is generated opposed in direction to the former. The
figure represents what is called a thermo-electric
pair or couple.
By the union of several thermo-electric pairs,
a more powerful current can be generated than    
would be obtained from a single pair. Fig. 7,
for example, represents such an arrangement, in which the shaded
bars are supposed to be all of bismuth, and the unshaded ones of
antimony; on warming all the junctions, B, B, &c., a current is
generated in each, and the sum of these currents, all of which flow
in the same direction, will produce a stronger resultant current
than that obtained from a single pair.
The V formed by each pair need not be so wide as it is shown
in fig. 7; it may be contracted without prejudice to the couple.
And if it is desired to pack several pairs into a small compass,




THERMO-ELECTRTCITY.                    31
each separate couple may be arranged as in fig. 8, where the black
lines represent small bismuth bars, and the white ones small bars
of antimony. They are soldered together at the ends, and throughout the length are usually separated by strips of paper merely. A
Fig. 7.
~A    A     A.    A
13    B3 13        3     3
collection of pairs thus compactly set together constitutes a thermo-electric pile, a drawing of which is given in fig. 9.
The current produced by heat being always from bismuth to
antimony across the heated junction, a moment's inspection of
fig. 7 will show that when any one of the junctions A, A, is heated,
a current is generated, opposed in direction to that generated
when the heat is applied to the junctions B, B. Hence, in the case
of the thermo-electric pile, the effect of heat falling upon its two
Fig. 8.                              Fig. 9.
opposite faces is to produce currents in opposite directions. If
the temperature of the two faces be alike, they neutralize each
other, no matter how high they may be heated absolutely, but if
one of them be warmer than the other, a current is produced. The
current is thus due to a difference of temperature between the two
faces of the pile, and within certain limits the strength of the current is exactly proportioned to this difference.




32                APPENDIX TO LECTURE I.
From the junction of almost any other two metals, thermoelectric currents may be obtained, but they are most copiously
generated by the union of bismuth and antimony.*
NOTE ON THE CONSTRUCTION OF THE GALVANOMETER.
The existence and direction of aj electric current are shown
by its action upon a freely suspended magnetic needle.
But such a needle is held in the magnetic meridian by the
magnetic force of the earth. Hence, to move a single needle, the
current must overcome the magnetic force of the earth.
Very feeble currents are incompetent to do this in a sufficiently
sensible degree. The following two expedients are, therefore,
combined to render sensible the action of such feeble currents:The wire through which the current flows is coiled so as to
surround the needle several times; the needle must swing freely
within the coil. The action of the single current is thus multiplied.
The second device is to neutralize the directive force of the
earth, without prejudice to the magnetism of the needle. This is
accomplished by using two needles instead of one, attaching Fig 10,
them to a common vertical stem,
and bringing their opposite poles
over each other, the north end of
the one needle, and the south end  S  
of the other, being thus turned in
the same direction. The double
needle is represented in fig. 10.' n.;  
It must be so arranged that
one of the needles shall be within the coil through which the cur* The discovery of thermo-electricity is due to Thomas Seebeck, Professor in the University of Berlin. Nobili constructed the first thermoelectric pile; but in Melloni's hands it became an instrument so important
as to supersede all others in researches on radiant heat. To this purpose
it will be applied in future lectures.




THE ASTATIC NEEDLE.                  33
rent flows, while the other needle swings freely above the coil, the
vertical connecting piece passing through an appropriate slit in
the coil. Were both the needles within, the same current would
urge them in opposite directions, and thus one needle would neutralize the other. But when one is within and the other without,
the current urges both needles in the same direction.
The way to prepare such a pair of needles is this. Magnetize
both of them to saturation; then suspend them in a vessel, or under a shade, so as to protect them from air-currents. The system
will probably set in the magnetic meridian, one needle being in
almost all cases stronger than the other; weaken the stronger
needle carefully by the touch of a second smaller magnet. When
the needles are precisely equal in strength, they will set at right
angles to the magnetic meridian.
It might be supposed that when the needles are equal in
strength, the directive force of the earth would be completely annulled, that the double needle would be perfectly astatic, and perFig. 11.
2 _
fectly neutral as regards direction; obeying simply the torsion of
its suspending fibre. This would be the case if the magnetic axes
of both needles could be caused to lie with mathematical accuracy
in the same vertical plane. In practice, this is next to impos2*




34               APPENDIX TO LECTURE I.
sible; the axes always cross each other. Let n s, n' s' (fig. 11)
represent the axes of two needles thus crossing, the magnetic
meridian being parallel to M E; let the pole n be drawn by the
earth's attractive force in the direction n m; the pole s' being
urged by the repulsion of the earth in a precisely opposite direction. When the poles n and s' are of exactly equal strength, it is
manifest that the force acting on the pole s', in the case here supposed, would have the advantage as regards leverage, and would
therefore overcome the force acting on n. The crossed needles
would therefore turn away still further from the magnetic meridian, and a little reflection will show that they cannot come to rest
until the line which bisects the angle enclosed by the needles is at
right angles to the magnetic meridian.
This is the test of perfect equality as regards the magnetism
of the needles; but in bringing the needles to this state of perfectioi, we have often to pass through various stages of obliquity to
the magnetic meridian. In these cases the superior strength of
one needle is compensated by an advantage, as regards leverage,
possessed by the other. By a happy accident a touch is sometimes sufficient to make the needles perfectly equal; but many
hours are often expended in securing this result. It is only, of
course, in very delicate experiments that this perfect equality is
needed; but in such experiments it is essential.
Another grave difficulty has beset experimenters, even after the
perfect magnetization of their needles has been accomplished.
Such needles are sensitive to the slightest magnetic action, and
the covered copper wire, of which the galvanometer coils are
formed, usually contains a trace of iron sufficient to deflect the
prepared needle from its true position. I have had coils in which
this deflection amounted to 30 degrees; and in the splendid instruments used by Professor Du Bois Raymond, in his researches
on animal electricity, the deflection by the coil is sometimes even
greater than this.  Melloni encountered this difficulty, and
proposed that the wires should be drawn through agate holes,
thus avoiding all contact with iron or steel. The disturbance has always been ascribed to a trace of iron contained
in the copper wire. Pure silver has also been proposed instead
of copper.




THE ASTATIC NEEDLE.                   35
To pursue his beautiful thermo-electric researches in a satisfactory manner, Professor Magnus, of Berlin, obtained pure copper,
by a most laborious electrolytic process, and after the metal had
been obtained, it required to be melted eight times in succession
before it could be drawn into wire. In fact, the impurity of the
coil entirely vitiated the accuracy of the instrument, and almost
any amount of labour would be well expended in removing this
great defect.
My own experience of this subject is instructive. I had a beautiful instrument constructed a few years ago by Sauerwald, of Berlin, the coil of which, when no current flowed through it, deflected
my double needle full 30 degrees from the zero line. It was impossible to attain quantitative accuracy with this instrument.
I had the wire removed by Mr. Becker, and English wire used
in its stead; the deflection fell to 3 degrees.
This was a great improvement, but not sufficient for my purpose. I commenced to make inquiries about the possibility of
obtaining pure copper, but the result was very discouraging,
Wvhen, almost despairing, the following thought occurred to me:
The action of the coil must be due to the admixture of iron with
the copper, for pure copper is diamagnetic, it is feebly repelled by
a strong magnet. The magnet therefore occurred to me as a
means of instant analysis; I could tell by it, in a moment, whether
any wire was free from the magnetic metal or not.
The wire of M. Sauerwald's coil was strongly sttracted by the
magnet. The wire of Mr. Becker's coil was also attracted, though
in a much feebler degree.
Both wires had been covered by green silk; I removed this,
but the Berlin wire was still attracted; the English wire, on the
contrary, when presented naked to the magnet was feebly
repelled; it was truly diamagnetic, and contained no sensible
trace of iron. Thus the whole annoyance was fixed upon the
green silk; some iron compound had been used in the dyeing
of it, and to this the deviation of the needle from zero was manifestly due.
I had the green coating removed and the wire overspun with
white silk, clean hands being used in the process. A perfect galvanometer is the result; the needle, when released from the action




36              APPENDIX TO LECTURE I.
of the current, returns accurately to zero, and is perfectly free from
all magnetic action on the part of the coil. In fact, while we
have been devising agate plates and other learned methods to get
rid of the nuisance of a magnetic coil, the means of doing so are
at hand. Let the copper wire be selected by the magnet, and no
difficulty will be experienced in obtaining specimens magnetically
pure.




LECTUR;E II.
[January 30, 1862.]
THE NATURE OF HEAT-THE MATERIAL THEORY-THE DYNAMICAL THEORY
-THERMAL EFFECTS OF AIR IN MOTION-GENERATION OF HEAT BY
ROTATION BETWEEN THE POLES OF A MAGNET —EXPERIMENTS OF RUMFORD, DAVY, AND JOULE-THE MECHANICAL EQUIVALENT OF HEATHEAT GENERATED BY PROJECTILES-HEAT WHICH WOULD BE GENERATED
BY STOPPING THE EARTH'S MOTION-METEORIC THEORY OF THE SUN'S
HEAT-FLAME IN ITS RELATION TO THE DYNAMICAL THEORY.
APPENDIX: —EXTRACTS FROM BACON AND RUMFORD.
N our last lecture the developement of heat by mechanical action was illustrated by a series of experiments,
which showed that heat was easily produced by friction,
by compression, and by percussion. But facts alone can
not satisfy the human mind; we desire to know the inner
and invisible cause of the fact; we search after the principle by the operation of which the phenomena are produced. Why should heat be generated by mechanical action, and what is the real nature of the agent thus generated? Two rival theories have been offered in answer to
these questions.  Till very lately, however, one of thesethe material theory-had the greater number of adherents,
being opposed by only a few eminent men. Within certain limits this theory involved conceptions of a very simple kind, and this simplicity secured its general acceptance.
The material theory supposes heat to be a kind of matter
-a subtle fluid-stored up in the inter-atomic spaces of




38                   LECTICE II.
bodies. The laborious Gmelin, for example, in his Handbook of Chemistry, defines heat to be'that substance
whose entrance into our bodies causes the sensation of
warmth, and its egress the sensation of cold.'* He also
speaks of heat combining with bodies as one ponderable
substance does with another; and many other eminent
chemists treat the subject from the same point of view.
The developement of heat by mechanical means, inasmuch as its generation seemed unlimited, was a great difficulty with the materialists; but they were acquainted with
the fact (which I shall amply elucidate in a future lecture)
that different bodies possessed different powers of holding
heat, if I may use such a term. Take, for example, the
two liquids water and mercury, and warm up a pound of
each of them, say from fifty degrees to sixty. The absolute quantity of heat required by the water to raise its
temperature 10~ is fully thirty times the quantity required
by the mercury. Technically speaking, the water is said
to have a greater capacity for heat than the mercury has,
and this term'capacity' is sufficient to suggest the views
of those who invented it. The water was supposed to
possess the power of storing up the caloric or matter of
heat; of hiding it, in fact, to such an extent that it required
thirty measures of this caloric to produce the same sensible
effect on it, that one measure would produce upon mercury.
All substances possess, in a greater or less degree, this
apparent power of storing up heat. Lead, for example,
possesses it; and the experiment with the lead bullet, in
which you saw heat generated by compression, was explained by those who held the material theory in the following way. The uncompressed lead, they said, has a higher
capacity for heat than the compressed substance; the size
of its atomic storehouse is diminished by compression, and
* English Translation, vol. i. p. 22.




MATERIAL AND DYNAMICAL THEORIES OF HEAT.  39
hence, when the lead is squeezed, a portion of that heat
which, previous to compression, was hidden, must make its
appearance, for the compressed substance can no longer
hold it all. In some similar way the experiments on friction and percussion were accounted for. The idea of calling new heat into existence was rejected by the believers
in the material theory.  According to their views, the
quantity of heat in the universe is as constant as the
quantity of ordinary matter, and the utmost we can do by
mechanical and chemical means, is to store up this heat or
to drive it from its lurking place into open light of day.
The dynamical theory, or, as it is sometimes called, the
mechanical theory of heat, discards the idea of materiality
as applied to heat. The supporters of this theory do not
believe heat to be matter, but an accident or condition of
matter; namely, a motion of its ultimate particles.  From
the direct contemplation of some of the phenomena of
heat, a profound mind is led almost instinctively to conclude that heat is a kind of motion. Bacon held a view of
this kind,* and Locke stated a similar view with singular
felicity.' Heat' he says,' is a very brisk agitation of the
insensible parts of the object, which produce in us that
sensation from whence we denominate the object hot; so
what in our sensation is heat, in the object is nothing but
motion.' In our last lecture I referred to the experiments
of Count Rumford t on the boring of cannon; he showed
that the hot chips cut from his cannon did not change their
capacity for heat; he collected the scales and powder produced by the abrasion of his metal, and holding them up
* See Appendix to this Lecture.
f I have particular pleasure in directing the reader's attention to an
abstract of Count Rumford's memoir on the Generation of Heat by Friction, contained in the Appendix to this lecture. Rumford, in this memoir, annihilates the material theory of heat. Nothing more powerful on
the subject has since been written.




40                   LECTURE II.
before his opponents, demanded whether they believed
that the vast amount of heat which he had generated had
been all squeezed out of that modicum of crushed metal?' You have not,' he might have added,' given yourselves
the trouble to enquire whether any change whatever has
occurred in the capacity of the metal for heat by the act
of friction. You are quick in inventing reasons to save
your theory from destruction, but slow to enquire whether
these reasons are not merely the finespun fancies of your
own brains.' Theories are indispensable, but they sometimes act like drugs upon the mind. Men grow fond of
them as they do of dram-drinking, and often feel discontented and irascible when the stimulant to the imagination
is taken away.
At this point an experiment of Davy comes forth in its
true significance.* Ice is solid water, and the solid has
only one half the capacity for heat that liquid water possesses. A quantity of heat which would raise a pound of
ice ten degrees in temperature, would raise a pound of water only five degrees. Further, to simply liquefy a mass
of ice, an enormous amount of heat is necessary, this heat
being so utterly absorbed or rendered'latent' as to make
no impression upon the thermometer. The question of'latent heat' shall be fully discussed in a future lecture;
what I am desirous of impressing on you at present is, that
liquid water, at its freezing temperature, possesses a vastly
greater amount of heat than ice at the same temperature.
Davy reasoned thus:'If I, by friction, liquefy ice, I
produce a substance which contains a far greater absolute
amount of heat than the ice; and, in this case, it cannot,
with any show of reason, be affirmed that I merely render
sensible the heat hidden in the ice, for that quantity is only
a small fraction of the heat contained in the water.' He
* Works of Sir H. Davy, vol. ii, p. 11.




FUSION OF ICE BY FRICTION.                 41
made the experiment, and liquefied the ice by pure friction;
and the result has been regarded as the first which proved
the immateriality of heat.
When a hammer strikes a bell, the motion of the hammer is arrested, but its force is not destroyed; it has
thrown the bell into vibrations, which affect the auditory
nerves as sound. So, also, when our sledge hammer descended upon our lead bullet, the descending motion of the
sledge was arrested: but it was not destroyed. Its motion
was transferred to the atoms of the lead, and announced itself to the proper nerves as heat. The theory, then, which
Rumford so powerfully advocated, and Davy so ably supported,* was, that heat is a kind of molecular motion; and
that, by friction, percussion, or compression, this motion
may be generated, as well as by combustion.  This is the
theory which must gradually develope itself during these
lectures, until your minds attain to perfect clearness regarding it. And, remember, we are entering a jungle, and
must not expect to find our way clear. We are striking
into the brambles in a random fashion at first; but we shall
thus become acquainted with the general character of our
work, and, with due persistence, shall, I trust, cut through
all entanglement at last.
In our first lecture I showed you the effect of projecting
a current of compressed air against the face of the thermo-'
electric pile. You saw that the instrument was chilled by
the current of air. Now, heat is known to be developed
when air is compressed; and, since last Thursday, I have
* In Davy's first scientific memoir, he calls heat a repulsive motion,
which he says may be augmented in various ways.'First, by the transmutation of mechanical into repulsive motion; that is, by friction or percussion. In this case the mechanical motion lost by the masses of matter in friction is the repulsive motion gained by their corpuscles:' an
extremely remarkable passage. I have given further extracts from this
paper in the Appendix to Lecture III.




42                  LECTURE II.
been asked how this heat was disposed of in the case of the
condensed air. Pray listen to my reply. Supposing the
vessel which contained the compressed air to be formed of
a substance perfectly impervious to heat, and supposing all
the heat developed by my arm, in compressing the air, to be
retained within the vessel, that quantity of heat would be
exactly competent to undo what I had done and to restore
the compressed air to its original volume and temperature.
But this vessel v (fig. 12), is not impervious to heat, and it
was not my object to draw upon the heat developed by my
Fig. 12.
arm; I therefore, after condensing the air, allowed the
vessel to rest, till all the heat generated by the condensation had been dissipated, and the temperature of the air
within and without the vessel was the same. When, therefore, the air rushed out, it had not the heat to draw upon,
which had been developed during compression. The heat
from which it derived its elastic force was only sufficient
to keep it at the temperature of the surrounding air. In
doing its work a portion of this heat, equivalent to the




FIRE SYRINGE.                    43
work done, was consumed, and the issuing air was consequently chilled. Do not be disheartened if this reasoning
should not appear quite clear to you. We are now in comparative darkness, but as we proceed light will gradually
appear, and irradiate retrospectively our present gloom.
I wish now to make evident to you that heat is developed by the compression of air. Here is a strong cylinder
of glass T V (fig. 13), accurately bored, and quite smooth
within. Into it this piston fits air-tight, so that, by driving
the piston down, I can forcibly compress the air
underneath it; and when the air is thus com- Fig. 13.
pressed, heat is suddenly generated.  Let me
prove this. I take a morsel of cotton wool, and
wet it with this volatile liquid, the bisulphide of
carbon. I throw this bit of wetted cotton into
the glass syringe, and instantly eject it. It has
left behind it a small residue of vapour. I compress the air suddenly, and you see a flash of light  
within the syringe. The heat developed by the
compression has been sufficient to  ignite  the
vapour. It is not necessary to eject the wetted
cotton; I replace it in the tube, and urge the piston downwards; you see the flash as before. If,
with this narrow glass tube, I blow out the fumes
generated by the combustion of the vapour, I can,
without once removing the cotton from the syringe, repeat the experiment twenty times.*
I have here arranged an experiment intended
to give you another illustration of the thermal  
effect produced in air by its own mechanical action. Here is a tin tube, stopped at both ends, and connected with this air-pump. The tin tube is at present full
of air, and I bring the face of my pile up against the
* The accident which led to this form of the experiment is referred to
in the Appendix to this Lecture.




44                     LECTURE II.
curved surface of the tube. The instrument declares that
the face of the pile in contact with the tin tube has been
warmed by the latter. I was quite prepared for this result,
having reason to know  that the air within the tube is
slightly warmer than that without. Now, what you are to
observe is this:-My assistant shall work the pump; the
cylinders of the machine will be emptied of air, and the air
within this tin tube will be driven into the exhausted cylinders by its own elastic force. I have already demonstrated the chilling effect of a current of compressed air on
the thermo-electric pile. In the present experiment I will
not examine the thermal condition of the current at all, but
of the vessel in which the work has been performed. As
this tube is exhausted I expect to see the needle, which is
now deflected so considerably in the direction of heat,
descend to zero, and pass quite up to 90~ in the direction
of cold. The pump is now in action, and observe the result. The needle falls as predicted, and its advance in the
direction of cold is only arrested by its concussion against
the stops.
Three strokes of the pump suffice to chill the tube so
as to send the needle up to 90~; * let it now come to rest.
It would require more time than we can afford to allow the
tube to assume the temperature of the air around it; but
the needle is now sensibly at rest at a good distance on
the cold side of zero. I will now allow a quantity of air
to enter the tube, equal to that which was removed from
it a moment ago by the air-pump. I can turn on this cock,
the air will enter, and each of its atoms will hit the inner
surface of the tube like a projectile. The mechanical motion of the atom will be thereby annihilated, but an amount
* The galvanometer used in this experiment was that which I employ
in my original researches: it is an exceedingly delicate one. When introduced in the lectures its dial was illuminated by the electric light; and an
image of it, two feet in diameter, was projected on the screen.




CONDENSATION OF AQUEOUS VAPOUR.                   45
of heat equivalent to this motion will be generated. Thus
as the air enters it will develope an amount of heat sufficient to re-warm the tube, to undo the present deflection,
and to send the needle up on the heat side of zero. The
air is now entering, and you see the effect: the needle
moves, and goes quite up to 90~ on that side which indicates the heating the pile.*
I have now  to direct your attention to an interesting
effect connected with this chilling of the air by rarefaction.
I place over the plate of the air pump a large glass receiver,
which is now filled with the air of this room. This air,
and, indeed, all air, unless it be dried artificially, contains
a quantity of aqueous vapour which, as vapour, is perfectly
invisible. A certain temperature is requisite to maintain
the vapour in this invisible state, and if the air be chilled
so as to bring it below this temperature, the vapour will
instantly condense, and form a visible cloud. Such a cloud,
which you will remember is not vapour, but liquid water
in a state of fine division, will form within this glass vessel
R (fig. 14), when the air is pumped out of it; and to make
this effect visible to everybody present, to those right and
left of me, as well as to those in front, these-six little gas
jets are arranged in a semicircle, which half surrounds the
receiver. Each person present sees one or more of these
* In this experiment a mere line along the surface of the tube was in
contact with the face of the pile, and the heat had to propagate itself
through the tin envelope to reach the instrument. Previous to adopting
this arrangement I had the tube pierced, and a separate pile, with its naked
face turned inwards, cemented air-tight into the orifice. The pile came
thus into direct contact with the air, and its entire face was exposed to the
action. The effects thus obtained were very large; sufficient, indeed, to
swing the needle quite round. My desire to complicate the subject as little
as possible induced me to abandon the cemented pile, and to make rse
of the instrument with which my audience had already become familiar.
With the arrangement actually adopted the effects were, moreover, so
large, that I drew only on a portion of my power to produce them.




46                   LECTURE II.
jets on looking through the receiver, and when the cloud
forms, the dimness which it produces will at once declare
its presence. The pump is now quickly worked; a very few
strokes suffice to precipitate the vapour; there it spreads
throughout the entire receiver, and many of you see a colFig. 14.
ouring of the cloud, as the light shines through it, similar
to that observed sometimes, on a large scale, around the
moon. When I allow the air to re-enter the vessel, it is
heated, exactly as in the experiment with our tin tube;
the cloud melts away, and the perfect transparency of the
air within the receiver is restored. Again I exhaust and
again the cloud forms; once more the air enters and the
cloud disappears; the heat developed being more than sufflcient to preserve it in the state of pure vapour.
Sir Humphry Davy refers, in his' Chemical Philosophy,' to a machine at Schemnitz, in Hungary, in which
air was compressed by a column of water 260 feet in
height. When a stopcock was opened, so as to allow the




FRICTION AGAINST SPACE.           47
air to escape, a degree of cold was produced which not
only precipitated the aqueous vapour diffused in the air,
but caused it to congeal in a shower of snow, while the
pipe from which the air issued became bearded with icicles.' Dr. Darwin,' writes Davy,' has ingeniously explained the
production of snow on the tops of the highest mountains,
by the precipitation of vapour from the rarefied air which
ascends from plains and valleys. The Andes, placed almost
under the line, rise in the midst of burning sands; about
the middle height is a pleasant and mild climate; the summits are covered with unchanging snows.'
I would now request your attention to another experiment, in which heat will be developed by what must appear to many of you a very mysterious agency, and, indeed,
the most instructed amongst us know, in reality, very little
about the subject. I wish to develope heat by what might
be regarded as friction against pure space. And indeed it
may be, and probably is, due to a kind of friction against
that inter-stellar medium, to which we shall have occasion
to refer more fully by and by.
I have here a mass of iron-part of a link of a huge
chain cable-which is surrounded by these multiple coils
of copper wire c c (fig. 15), and which I can instantly convert into a powerful magnet by sending an electric current
through the wire. You see, when thus excited, how powerful it is. This poker clings to it, and these chisels,
screws, and nails cling to the poker. Turned upside down,
this magnet will hold a half hundred weight attached to
eacn of its poles, and probably a score of the heaviest people in this room, if suspended from the weights. At the
proper signal my assistant will interrupt the electric current:-' Break!' The iron falls, and all the magic disappears: the magnet now is mere common iron. At the
ends of the magnet I place two pieces of iron P — movable poles, as they are called-which, when the magnet is




'G.'.




APPARENT VISCOSITY OF MAGNETIC FIELD.    49
unexcited, I can bring within any required distance of each
other. When the current passes, these pieces of iron virtually form parts of the magnet. Between them I will
place a substance which the magnet, even when exerting
its utmost power, is incompetent to attract. This substance
is simply a piece of silver-in fact, a silver medal. I bring
it close to the excited magnet; no attraction ensues. Indeed, what little force-and it is so little as to be utterly
insensible in these experiments-the magnet really exerts
upon the silver, is repulsive instead of attractive.
Well, I suspend this medal between the poles P P of
the magnet, and excite the latter. The medal hangs there;
it is neither attracted nor repelled, but if I seek to move it
I encounter resistance. To turn the medal round I must
overcome this resistance; the silver moves as if it were
surrounded by a viscous fluid. This curious effect may also
be rendered manifest, thus: I have here a rectangular plate
of copper, and if I cause it to pass quickly to and fro like
a saw between the poles, when their points are turned
towards it, I seem, though I can see nothing, to be sawing
through a mass of cheese or butter.* Nothing of this kind
is noticed when the magnet is not active: the copper saw
then encounters nothing but the infinitesimal resistance of
the air. Thus far you have been compelled to take my
statements for granted, but I have arranged an experiment
which will make this strange action of the magnet on the
silver medal, strikingly manifest to everybody present.
Above the suspended medal, and attached to it by a bit
of wire, I have a little reflecting pyramid M, formed of four
triangular pieces of looking-glass; both the medal and the
reflector are suspended by a thread which was twisted in
its manufacture, and which will untwist itself when the
weight it sustains is set free. I place our electric lamp so
* An experiment of Faraday's.
3




50                   LECTURE In
as to cast a strong beam of light on this little pyramid:
you see these long spokes of light passing through the
dusty air of the room as the mirror turns.
Let us start it from a state of rest. You now see the
beam passing through the room and striking against the
white wall. As the mirror commences to rotate, the patch
of light moves, at first slowly, over the wall and ceiling.
But the motion quickens, and now you can no longer see
the distinct patches of light, but instead of them you have
this splendid luminous band fully twenty feet in diameter
drawn upon the wall by the quick rotation of the reflected
beams. At the word of command the magnet will be excited, and the motion of the medal will be instantly
stopped.'Make!'  See the effect: the medal seems
struck dead by the excitement of the magnet, the band
suddenly disappears, and there you have the single patch
of light upon the wall. This strange mechanical effect is
produced without any visible change in the space between
the two poles. Observe the slight motion of the image on
the wall: the tension of the string is struggling with an
unseen antagonist and producing that slight motion. It is
such as would be produced if the medal, instead of being
surrounded by air, were immersed in a pot of thick treacle. I destroy the magnetic power, and the viscous character of the space between the poles instantly disappears; the
medal begins to twirl as before; there are the revolving
beams, and there is now the luminous band. I again excite the magnet: the beams are struck motionless, and the
band disappears.
By the force of my hand I can overcome this resistance
and turn the medal round; but to turn it I must expend
force. Where does that force go? It is converted into
heat. The medal, if forcibly compelled to turn, will become heated. Many of you are acquainted with the grand
discovery of Faraday, that electric currents are developed




HEAT GENERATED IN MAGNETIC FIELD.        51
where a conductor of electricity is set in motion between
the poles of a magnet. We have these currents doubtless
here, and they are competent to heat the medal. But what
are these currents? how are they related to the space between the magnetic poles-how to the force of my arm
which is expended in their generation? We do not yet
know, but we shall know by and by. It does not in the
least lessen the interest of the experiment if the force of
my arm, previous to appearing as heat, appears in another
form-in the form of electricity. The ultimate result is
the same: the heat developed ultimately is the exact equivalent of the quantity of strength required to move the
medal in the excited magnetic field.
I wish now to show you the developement of heat by
this action. I have here a solid metal cylinder, the core
of which is, however, composed of a metal more easily
melted than its outer case. The outer case is copper, and
this is filled by a hard but fusible alloy. I set this cylinder
upright between the conical poles p P (fig. 16) of the magFi,. 16.
net. A string s s passes from the cylinder to a whirling
table, and by turning the latter the cylinder is caused to
spin round. It might turn till doomsday, as long as the
magnet remains unexcited, without producing the effect
sought; but when the magnet is in action, I hope to be
able to develope an amount of heat sufficient to melt the
core of that cylinder, and, if successful, I will pour the
liquid metal out before you.  Two minutes will suffice for'
this experiment. The cylinder is now rotating, and its




52                     LECTURE II.
upper end is open. I shall leave it thus open until the
liquid metal is seen spattering over the poles of the magnet. I already see the metallic spray, though a minute has
scarcely elapsed since the commencement of the experiment. I now stop the motion for a moment, and cork up
the end of the cylinder, so as to prevent the scattering
about of the metal. Let the action continue for half a minute longer; the entire mass of the core is, I am persuaded,
now melted. I withdraw the cylinder, remove the cork,
and here is the liquefied mass, which I thus pour out before
you.*
It is now time to consider more closely than we have
hitherto done, the relation of the heat developed by mechanical action to the force which produces it. Doubtless
this relation floated in many minds before it received either
distinct enunciation or experimental proof. Those who
reflect on vital processes-on the changes which occur in
the animal body-and the relation of the forces involved in
food, to muscular force, are led naturally to entertain the
idea of interdependence between these forces. It is, therefore, not a matter of surprise that the man who first raised
the idea of the equivalence between heat and mechanical
energy to philosophic clearness in his own mind, was a
physician. Dr. Mayer, of Heilbronn, in Germany, enunciated the exact relation which subsists between heat and
work, giving the number which is now known as the'mechanical equivalent of heat,' and following up the statement of the principle by its fearless application.t It is,
* The developement of heat by causing a conductor to revolve between
the poles of a magnet was first effected by Mr. Joule (Phil. Mag. vol. xxiii.
3rd Series, year 1843, pp. 355 and 439), and his experiment was afterwards revived in a striking form by M. Foucault. The artifice above
described, of fusing the core out of the cylinder, renders the experiment
very effective in the lecture-room.
t See Lectures III. and XII.




MAYER AND JOULE.                53
however, to Mr. Joule, of Manchester, that we are almost
wholly indebted for the experimental treatment of this important subject. Entirely independent of Mayer, with his
mind firmly fixed upon a principle, and undismayed by the
coolness with which his first labours appear to have been
received, he persisted for years in his attempts to prove
the invariability of the relation which subsists between
heat and ordinary mechanical force. He placed water in a
suitable vessel, and agitated that water by paddles, driven
by measurable forces, and determined both the amount of
heat, developed by the stirring of the liquid, and the
amount of labour expended in the process. He did the
same with mercury and with sperm oil. He also caused
disks of cast iron to rub against each other, and measured
the heat produced by their friction, and the force expended
in overcoming it. He also urged water through capillary
tubes, and determined the amount of heat generated by the
friction of the liquid against the sides of the tubes. And
the results of his experiments leave no shadow of doubt
upon the mind that, under all circumstances, the quantity
of heat generated by the same amount of force is fixed and
invariable. A given amount of force, in causing the iron
disks to rotate against each other, produced precisely the
same amount of heat, as when it was applied to agitate
water, mercury, or sperm oil. Of course, at the end of an
experiment, the temperatures in the respective cases would
be very different; that of the water, for example, would
be -Jth of the temperature of the mercury, because, as we
already know, the capacity of.water for heat is 30 times
that of mercury. Mr. Joule took this into account in reducing his experiments, and found, as I have stated, that,
however the temperatures might differ, in consequence of
the different capacity of heat for the substances employed,
the absolute amount of heat generated by the same expenditure of power, was in all cases the same.




54                  LECTURE IH.
In this way it was found that the quantity of heat
which would raise one pound of water one degree Fahr. in
temperature, is exactly equal to what would be generated
if a pound weight, after having fallen through a height of
772 feet, has its moving force destroyed by collision with
the earth. Conversely, the amount of heat necessary to
raise a pound of water one degree in temperature, would,
if all applied mechanically, be competent to raise a pound
weight 772 feet high, or it would raise 772 lbs. one foot
high. The term' foot-pound' has been introduced to express, in a convenient way, the lifting of one pound to the
height of a foot. Thus the quantity of heat necessary to
raise the temperature of a pound of water one degree being
taken as a standard, 772 foot-pounds constitute what is
called the mechanical equivalent of heat.
In order to imprint upon your minds the thermal effect
produced by a body falling from a height, I will go through
the experiment of allowing a lead ball to fall from our
ceiling upon this floor. The lead ball is at the present moment slightly colder than the air of this room. I prove
this by bringing it in contact with the thermo-electric pile,
and showing.you that the deflection of the needle indicates
cold. Here on the floor I have placed a slab of iron, on
which I intend the lead to fall, and which, you observe, is
also cooler than the air of the room. At the top of the
house I have an assistant, who will heave up the ball after
I have attached it to this string. He will not touch the
ball, nor will he allow it to touch anything else. He will
now let it go; it falls, and is received upon the plate of
iron. The height is too small to get much heat by a single
fall; I will therefore have the ball drawn up and dropped
three or four times in succession. Observe, there is a
length of covered wirt attached to the ball, by which I lift
it, so that my hand never comes near the ball. There is
the fourth collision, and I think I may now examine the




MECHANICAL EQUIVALENT' OF HEAT.        55
temperature of the lead. I place the ball, which at the
commencement was cold, again upon the pile, and the immediate deflection of the needle in the opposite direction,
declares that now the ball is heated; this heat is due entirely to the destruction of the moving energy which the
ball possessed when it struck the plate of iron. According
to our theory, the common mechanical motion of the ball
as a mass, has been transferred to the atoms of the mass,
producing among them the agitation which we call heat.
What was the total amount of heat thus generated?
The space fallen through by the ball in each experiment is
twenty-six feet. The heat generated is proportional to the
height through which the body falls. Now a ball of lead,
in falling through 772 feet, would generate heat sufficient
to raise its own temperature 30~, its' capacity' being -j-th
of that of water: hence, in falling through 26 feet, which is
in round numbers J- of 772, the heat generated would, if
all concentrated in the lead, raise its temperature one degree. This is the amount of heat generated by a single
descent of the ball, and four times this amount would, of
course, be generated by four descents. The heat generated
is not, however, all concentrated in the ball; it is divided
between the ball and the iron on which it falls.
It is needless to say, that if motion be imparted to a
body by other means than gravity, the destruction of this
motion also produces heat. A rifle bullet, when it strikes
a target, is intensely heated. The mechanical equivalent
of heat enables us to calculate with the utmost accuracy
the amount of heat generated by the bullet, when its velocity is known. This is a point worthy of our attention,
and in dealing with it I will address myself to those of my
audience who are unacquainted even with the elements of
mechanics. Everybody knows that the greater the height
is from which a body falls, the greater is the force with
which it strikes the earth, and that this is entirely due to




56                   LECTURE nI.
the greater velocity imparted to the ball, in falling from the
greater height. The velocity imparted to the body is not,
however, proportional to the height from which it falls.
If the height be augmented four-fold, the velocity is augmented only two-fold; if the height be augmented ninefold, the velocity is augmented only three-fold; if the
height be augmented sixteen-fold, the velocity is augmented
only four-fold; or, expressed generally, the height augments in the same proportion as the square of the velocity.
But the heat generated by the collision of the falling
body increases simply as the height; consequently, the
heat generated increases as the square of the velocity.
If, therefore, we double the velocity of a projectile, we
augment the heat generated, when its moving force is destroyed, four-fold; if we treble its velocity, we augment the
heat nine-fold; if we quadruple the velocity, we augment
the heat sixteen-fold; and so on.
The velocity imparted to a body by gravity in falling
through 772 feet is, in round numbers, 223 feet a second,
that is to say, immediately before the body strikes the
earth, this is its velocity. Six times this quantity or 1,338
feet a second, would not be an inordinate velocity for a
rifle bullet.
But a rifle bullet, if formed of lead, moving at a velocity of 223 feet a second, would generate, on striking a
target an amount of heat which, if concentrated in the bullet, would raise its temperature 30~; with 6 times this velocity it will generate 36 times this amount of heat; hence
36 times 30, or 1,0800, would represent the augmentation
of temperature of a rifle ball on striking a target with a
velocity of 1,338 feet a second, if all the heat generated
were confined to the bullet itself. This amount of heat
would be far more than sufficient to fuse the lead; but in
reality a portion only of the heat generated is lodged in the
ball, the total amount being divided between it and the




RELATION OF HEAT TO VELOCITY.         57
target. Were the ball iron instead of lead, the heat generated, under the conditions supposed, would be competent
to raise the temperature of the ball only by about i-rd of
1,0800, because the capacity of iron for heat is about three
times that of lead.
From these considerations I think it is manifest that if
we. know the velocity and weight of any projectile, we can
calculate, with ease, the amount of heat developed by the
destruction of its moving force. For example, knowing,
as we do, the weight of the earth, and the velocity with
which it moves through space, a simple calculation would
enable us to determine the exact amount of heat which
would be developed, supposing the earth to be stopped in
her orbit. We could tell, for example, the number of degrees which this amount of heat would impart to a globe
of water equal to the earth in size. Mayer and Helmholtz
have made this calculation, and found that the quantity of
heat generated by this colossal shock would be quite suflicient, not only to fuse the entire earth, but to reduce it, in
great part, to vapour. Thus, by the simple stoppage of
the earth in its orbit'the elements' might be caused' to
melt with fervent heat.' The amount of heat thus developed would be equal to that derived from the combustion
of fourteen globes of coal, each equal to the earth in magnitude. And if, after the stoppage of its motion, the earth
should fall into the sun, as it assuredly would, the amount
of heat generated by the blow would be equal to that developed by the combustion of 5,600 worlds of solid carbon.
Knowledge, such as that which you now possess, has
caused philosophers, in speculating on the mode in which
the sun is nourished, and his supply of light and heat kept
up, to suppose the heat and light to be caused by the
showering down of meteoric matter upon the sun's surface.* Some philosophers suppose the Zodiacal Light to
* Mayer propounded this hypothesis in 1848, and worked it fully out.
8*




58                   LECTURE II.
be a cloud of meteorites, and from it, it is imagined, the
showering meteoric matter may be derived. Now, whatever be the value of this speculation, it is to be borne in
mind that the pouring down of meteoric matter, in the
way indicated, would be competent to produce the light
and heat of the sun. With regard to the probable truth or
fallacy of the theory, it is not necessary that I should offer
an opinion; I would only say that the theory deals with a
cause which, if in sufficient Qperation, would be competent
to produce the effects ascribed to it.
Let me now pass from the sun to something less, —in
fact, to the opposite pole of nature. And here that divine
power of the human intellect which annihilates mere magnitude in its dealings with law, comes conspicuously into
play. Our reasoning applies not only to suns and planets,
but equally so to the very ultimate atoms of which matter
is composed. Most of you know the scientific history of
the diamond, that Newton, antedating intellectually the
discoveries of modern chemistry, pronounced it to be an
unctuous or combustible substance. Everybody now
knows that this brilliant gem is composed of the same
substance as common charcoal, graphite, or plumbago. A
diamond is pure carbon, and carbon burns in oxygen. I
have here a diamond, held fast in a loop of platinum wire;
I will heat the gem to redness in this flame, and then
plunge it into this jar, which contains oxygen gas. See
how it brightens on entering the jar of oxygen, and now it
glows, like a little terrestrial star, with a pure white light.
How are we to figure the action here going on? Exactly
as you would present to your minds the conception of meteorites showering down upon the sun. The conceptions
It was afterwards enunciated independently by Mr. Waterston, and developed by Professor William Thomson (Transactions of the Royal Soc. of
Edinb., 1853). See Lecture XII.




THEORY OF COMBUSTION.              59
are, in quality, the same, and to the intellect the one is
not more difficult than the other. You are to figure the
atoms of oxygen showering against this diamond on all
sides. They are urged towards it by what is called chemical affinity, but this force, made clear, presents itself to the
mind as pure attraction, of the same mechanical' quality, if
I may use the term, as gravity. Every oxygen atom, as it
strikes the surface, and has its motion of translation destroyed by its collision with the carbon, assumes the motion
which we call heat: and this heat is so intense, the attractions exerted at these molecular distances are so mighty,
that the crystal is kept white-hot, and the compound,
formed by the union of its atoms with those of the oxygen, flies away as carbonic acid gas.
Let us now pass on from the diamond to ordinary
flame. I have here a burner from which I can obtain an
ignited jet of gas. Here is the flame: what is its constitution? Within the flame we have a core of pure unburnt
gas, and outside the flame we have the oxygen of the air.
The external surface of the core of gas is in contact with
the air, and here it is that the atoms clash together and
produce light and heat by their collision. But the exact
constitution of the flame is worthy of our special attention,
and for our knowledge of this we are indebted to one of
Davy's most beautiful investigations. Coal-gas is what we
call a hydro-carbon; it consists of carbon and hydrogen in
a state of chemical union. From this transparent gas escape the soot and lampblack which we notice when the
combustion of the gas is incomplete. Soot and lampblack
are there now, but they are compounded with other substances to a transparent form. Here, then, we have a surface of this compound gas, in presence of the oxygen of
our air; we apply heat, and the attractions are instantly
so intensified that the gas bursts into flame. The oxygen
has a choice of two partners, or, if you like, it is in the




60                   LECTURE II.
presence of two foes; it closes with that which it likes
best, or hates most heartily, as the case may be. It first
closes with the hydrogen, and sets the carbon free. Solid
particles of carbon thus scattered in numbers innumerable
in the midst of burning matter, are raised to a state of intense incandescence; they become white-hot, and mainly to
them the light of our lamps is due. The carbon, however,
in due time, closes with the oxygen, and becomes, or ought
to become, carbonic acid; but in passing from the hydrogen with which it'was first combined, to the oxygen, with
which it enters into final union,
Fig. 17.       it exists, for a time, in the single state, and, as a bachelor, it
gives us all the splendour of its
light.
The combustion of a candle
is in principle the same as that
of a jet of gas. Here you have
a rod of wax or tallow (fig. 17),
through which is passed the
cotton wick. You ignite the
wick; it burns, melts the tallow at its base, the liquid ascends through the wick by capillary attraction, it is converted by the heat into vapour,
and this vapour is a hydro-carbon, which burns exactly like
the gas. Here also you have unburnt vapour within, common air without, while between both is a shell which
forms the battle-ground of the clashing atoms, where they
develope their light and heat. There is hardly anything in
nature more beautiful than a burning candle; the hollow
basin partially filled with melted matter at the base of the
wick, the creeping up of the liquid; its vaporisation; the
structure of the flame; its shape, tapering to a point, while
converging air-currents rush in to supply its needs. Its




STRUCTURE OF FLAME.                61
beauty, its brightness, its mobility, have made it a favourite type of spiritual essences, and its dissection by Davy,
far from diminishing the pleasure with which we look
upon a flame, has rendered it more than ever a miracle of
beauty to the enlightened mind.
You ought now to be able to picture clearly before
your minds the structure of a candle-flame. You ought to
see the unburnt core within and the burning shell which
envelopes this core. From the core, through this shell,
the constituents of the candle are incessantly passing and
escaping to the surrounding air. In the case of a candle
you have a hollow cone of burning matter. Imagine this
cone cut across horizontally; you would then expose a
burning ring. I will practically cut the flame of a candle
thus across. I have here a piece of white paper, which I
will bring down upon the candle; pressing it down upon
the flame until it almost touches the wick. Observe the
upper surface of that paper; it becomes charred, but how?
Exactly in correspondence with the burning ring of the
candle, we have a charred ring upon the paper (fig. 18).
Fig. 18.
I might operate in the same manner with a jet of gas. I
will do so. Here is the ring which it produces. Within
the ring, you see, there is no charring of the paper, for at
this place the unburnt vapour of the candle, or the unburnt
gas of the jet, impinges against the surface, and no charring
can be produced.
To the existence, then, of solid carbon particles the
light of our lamps is mainly due. But the existence of
these particles, in the single state, implies the absence of




62                  LETIURE II.
oxygen to seize hold of them. If, at the moment of their
liberation from the hydrogen with which they are first
combined, oxygen were present to seize upon them, their
state of bachelorhood would be extinguished, and we should
no longer have their light. Thus, when we mix a sufficient
quantity of air with the gas issuing from a jet, when we
mix it so that the oxygen penetrates to the very heart of
the jet, we find the light destroyed. Here is a burner, invented by Prof. Bunsen, for the express purpose of destroying the light by causing the quick combustion of the carbon particles. The burner from which the gas escapes is
introduced into a tube; this tube is perforated nearly on a
level with the gas orifice, and through these perforations
the air enters, mingles with the gas, and the mixture issues
from the top of the tube. Fig. 19 repreFig. 19.    sents a form of this burner; the gas is discharged into the perforated chamber a,.~    \     where air mingles with it, and both ascend
the tube a b together: d is a rose-burner,
which may be used to vary the shape of
the flame. I ignite the mixture, but the
"_ ~flame produces hardly any light. Heat is
the thing here aimed at, and this lightless
flame is much hotter than the ordinary
flame, because the combustion is much quicker, and therefore more intense. If I stop the orifices in a I cut off the
supply of air, and the flame at once becomes luminous:
we have now the ordinary case of a core of unburnt gassurrounded by a burning shell. The illuminating power of
a gas may, in fact, be estimated by the quantity of air
necessary to prevent the precipitation of the solid carbon
particles; the richer the gas, the more air will be required
to produce this effect.
An interesting observation may be made on almost any
windy Saturday evening in the streets of London, on the




COMBUSTION ON MONT BLANC.           63
sudden, and almost total extinction of the light of the huge
gas jets, exposed chiefly in butchers' shops. When the
wind blows, the oxygen is carried mechanically to the very
heart of the flame, and the white light instantly vanishes
to a pale and ghastly blue. During festive illuminations
the same effect may be observed; the absence of the light
being due, as in the case of Bunsen's burner, to the presence of a sufficient amount of oxygen to consume, instantly, the carbon of the flame.
To determine the influence of height upon the rate of
combustion, was one of the problems which I had set before me, in my journey to the Alps in 1859. Fortunately
for science, I invited Dr. Frankland to accompany me on
the occasion, and to undertake the experiments on combustion, while I proposed devoting myself to observations on
solar radiation. The plan pursued was this: six candles
were purchased at Chamouni and carefully weighed; they
were then allowed to burn for an hour in the Hotel de
1'Union, and the loss of weight was determined. The same
candles were taken to the summit of Mont Blanc, and on
the morning of Aug. 21, were allowed to burn for an hour
in a tent, which perfectly sheltered them from the action
of the wind. The aspect of the six flames at the summit
surprised us both. They seemed the mere ghosts of the
flames which the same candles were competent to produce
in the valley of Chamouni-pale, small, feeble, and suggesting to us a greatly diminished energy of combustion.
The candles being carefully weighed on our return, the unexpected fact was revealed, that the quantity of stearine
consumed above was almost precisely the same as that
consumed below. Thus, though the light-giving power of
the flame was diminished in an extraordinary degree by
the elevation, the energy of the combustion was the same
above as it was below. This curious result is to be ascribed
mainly to the mobility of the air at this great height. The




64                   LECTURE II.
particles of oxygen could penetrate the flame with comparative freedom, thus destroying its light, and making
atonement for the smallness of their number by the promptness of their action.
Dr. Frankland has made these experiments the basis
of a most interesting memoir.* He shows that the quantity of a candle consumed in a given time is, within wide
limits, independent of the density of the air; and the reason is, that although by compressing the air we augment
the number of active particles in contact with the flame,
we almost, in the same degree, diminish their mobility, and
retard their combustion. When an excess of air, moreover,
surrounds the flame, its chilling effect will tend to prolong
the existence of the carbon particles in a solid form, and
even to prevent their final combustion. One of the beautiful experimental results of Dr. Frankland's investigation
is, that by condensing the air around it, the pale and
smokeless flame of a spirit lamp may be rendered as bright
as that of coal gas, and, by pushing the condensation sufficiently far, the flame may actually be rendered smoky, the
sluggish oxygen present being incompetent to effect the
complete combustion of the carbon.
But to return to our theory of combustion: it is to the
clashing together of the oxygen of the air and the constituents of our gas and candles, that the light and heat of
our flames are due. I scatter steel filings in this flame, and
you see the star-like scintillations produced by the combustion of the steel. Here the steel is first heated, till the
attraction between it and the oxygen becomes sufficiently
strong to cause them to combine, and these rocket-like
flashes are the result of their collision. It is the impact
of the atoms of oxygen against the atoms of sulphur which
produces the flame observed when sulphur is burned in
* Philosophical Transactions for 1861.




CLASHIING OF ATOMS.              65
oxygen or in air; to the collision of the same atoms
against phosphorus are due the intense heat and dazzling
light which result from the combustion of phosphorus in
oxygen gas. It is the collision of chlorine and antimony
which produces the light and heat observed where these
bodies are mixed together; and it is the clashing of sulphur and copper which causes the incandescence of the
mass when these substances are heated together in a Florence flask. In short, all cases of combustion are to be ascribed to the collision of atoms which have been urged together by their mutual attractions.




APPENDIX TO LECTURE II.
EXTRACTS FROM THE TWENTIETH APHORISM OF THE SECOND
BOOK OF THE'NOVUM ORGANUM.'
WHEN I say of motion that it is the genus of which heat is a
species, I would be understood to mean, not that heat generates
motion, or that motion generates heat (though both are true in
certain cases), but that heat itself, its essence and quiddity, is
motion, and nothing else; limited, however, by the specific differences which I will presently subjoin, as soon as I have added a
few cautions, for the sake of avoiding ambiguity.
Nor, again, must the communication of heat, or its transitive
nature, by means of which a body becomes hot when a hot body
is applied to it, be confounded with the form of heat. For heat
is one thing, and heating is another. Heat is produced by the
motion of attrition without any preceding heat.
Heat is an expansive motion, whereby a body strives to dilate
and stretch itself to a larger sphere or dimension than it had previously occupied. This difference is most observable in flame,
where the smoke or thick vapour manifestly dilates and expands
into flame.
It is shown also in all boiling liquid, which manifestly swells,
rises, and bubbles, and carries on the process of self-expansion,
till it turns into a body far more extended and dilated than the
liquid itself, namely, into vapour, smoke, or air.
*      *       *      *       *      *      *
The third specific difference is this, that heat is a motion of
expansion, not uniformly of the whole body together, but in the
smaller parts of it; and at the same time checked, repelled, and
beaten back, so that the body acquires a motion alternative, per



EXTRACTS FROM BACON.                     67
petually quivering, striving and struggling, and irritated by repercussion, whence springs the fury of fire and heat:
Again, it is shown in this that when the air is expanded in a
calender glass, without impediment or repulsion, that is to say,
uniformly and equably, there is no perceptible heat. Also, when
wind escapes from confinement, although it bursts forth with the
greatest violence, there is no very great heat perceptible; because
the motion is of the whole, without a motion alternating in the
particles.
And this specific difference is common also to the nature of
cold; for in cold contractive motion is checked by a resisting
tendency to expand, just as in heat the expansive action is checked
by a resisting tendency to contract. Thus whether the particles
of a body work inward or outward, the mode of action is the
same.
Now from this our first vintage it follows, that the form or
true definition of heat (heat that is in relation to the universe, not
simply in relation to man) is in a few words as follows: Heat is
a motion, expansive, restrained, and acting in its strife upon the
smaller particles of bodies. But the expansion is thus modified:
while it expands all ways, it has at the same time an inclination upwards. And the struggle in the particles is modified also; it is
not sluggish, but hurried and with violence.*
ABSTRACT OF COUNT RUMFORD'S ESSAY, ENTITLED'AN ENQUIRY
CONCERNING THE SOURCE OF THE HEAT WHICH IS EXCITED BY
FRICTION.'
[Read before the Royal Society, January 25, 1798.]
Being engaged in superintending the boring of cannon in the
workshops of the military arsenal at Munich, Count Rumford was
struck with the very considerable degree of heat which a brass
gun acquires, in a short time, in being bored, and with the still
more intense heat (much greater than that of boiling water) of
* Bacon's Works, vol. iv.: Spedding's Translation.




68              APPENDIX TO LECTURE II.
the metallic chips separated from it by the borer, he proposed to
himself the following questions:'Whence comes the heat actually produced in the mechanical
operation above mentioned?'Is it furnished by the metallic chips which are separated from
the metal?'
If this were the case, then the capacity for heat of the parts of
the metal so reduced to chips ought not only to be changed, but
the change undergone by them should be sufficiently great to account for all the heat produced. No such change, however, had
taken place; for the chips were found to have the same capacity
as slices of the same metal cut by a fine saw, where heating was
avoided. Hence, it is evident that the heat produced could not
possibly have been furnished at the expense of the latent heat of
the metallic chips. Rumford describes those experiments at length,
and they are conclusive.
He then designed a cylinder for the express purpose of generating heat by friction, by having a blunt borer forced against its
solid bottom, while the cylinder was turned round its axis by the
force of horses. To measure the heat developed, a small round
hole was bored in the cylinder for the purpose of introducing a
small mercurial thermometer. The weight of the cylinder was
113.13 lbs. avoirdupois.
The borer was a flat piece of hardened steel, 0-63 of an inch
thick, 4 inches long, and nearly as wide as the cavity of the bore
of the cylinder, namely, 3~ inches. The area of the surface by
which its end was in contact with the bottom of the bore was
nearly 21 inches. At the beginning of the experiment the temperature of the air in the shade and also that of the cylinder
was 60 degrees Fahr. At the end of 30 minutes, and after the
cylinder had made 960 revolutions round its axis, the temperature
was found to be 130 degrees.
Having taken away the borer, he now removed the metallic
dust, or rather scaly matter, which had been detached from the
bottom of the cylinder by the blunt steel borer, and found its
weight to be 837 grains troy.' Is it possible,' he exclaims,' that
the very considerable quantity of heat produced in this experiment-a quantity which actually raised the temperature of above
113 pounds of gun metal at least 70 degrees of Fahrenheit's ther



RUIMFORD'S EXPERIMENTS.                69
mometer-could have been furnished by so inconsiderable a quantity of metallic dust, and this merely in consequence of a change
in its capacity for heat?'But without insisting on the improbability of this supposition, we have only to recollect that from the results of actual and
decisive experiments, made for the express purpose of ascertaining
that fact, the capacity for heat of the metal of which great guns
are cast is not sensibly changed by being reduced to the form of
metallic chips, and there does not seem to be any reason to think
that it can be much changed, if it be changed at all, in being reduced to much smaller pieces by a borer which is less sharp.'
He next surrounded his cylinder by an oblong deal box, in
such a manner that the cylinder could turn water-tight in the centre of the box, while the borer was pressed against the bottom of
the cylinder. The box was filled with water until the entire cylinder was covered, and then the apparatus was set in action.
The temperature of the water on commencing was 60 degrees.'The result of this beautiful experiment,' writes Rumford,'was very striking, and the pleasure it afforded me amply repaid
me for all the trouble I had had in contriving and arranging the
complicated machinery used in making it.  The cylinder had
been in motion but a short time, when I perceived, by putting my
hand into the water, and touching the outside of the cylinder,
that heat was generated.
At the end of an hour the fluid, which weighed 18'77 lbs., or
21 gallons, had its temperature raised 47 degrees, being now 107
degrees.' In thirty minutes more, or one hour and thirty minutes after
the machinery had been set in motion, the heat of the water was
142 degrees.' At the end of two hours from the beginning, the temperature
was 178 degrees.' At two hours and twenty minutes it was 200 degrees, and at
two hours and thirty minutes it ACTUALLY BOILED!'
It is in reference to this experiment that Rumford made the
remarks regarding the surprise of the bystanders, which I have
quoted in Lecture I.
He then carefully estimates the quantity of heat possessed by
each portion of his apparatus at the conclusion of the experiment,




70              APPENDIX TO LECTURE II.
and adding all together, finds a total sufficient to raise 26-58 lbs.
of ice-cold water to its boiling point, or through 180 degrees Fahrenheit. By careful calculation, he finds this heat equal to that
given out by the combustion of 2303-8 grains (= 41 oz. troy)
of wax.
He then determines the'celerity' with which the heat was
generated, summing up his computations thus:' From the results
of these computations, it appears that the quantity of heat produced equably, or in a continuous stream, if I may use the expression, by the friction of the blunt steel borer against the bottom
of the hollow metallic cylinder, was greater than that produced in
the combustion of nine oax candles, each i of an inch in diameter,
all burning together with clear bright flames.''One horse would have been equal to the work performed,
though two were actually employed. Heat may thus be produced
merely by the strength of a horse, and, in a case of necessity, this
heat might be used in cooking victuals. But no circumstances
could be imagined in which this method of procuring heat would
be advantageous; for more heat might be obtained by using the
fodder necessary for the support of a horse as fuel.'
[This is an extremely significant passage, intimating as it
does, that Rumford saw clearly that the force of animals was derived from the food; no creation of force taking place in the animal body.]'By meditating on the results of all these experiments we are
naturally brought to that great question which has so often been
the subject of speculation among philosophers, namely, What is
heat-is there any such thing as an igneous fluid?  Is there any
thing that, with propriety, can be called caloric?''We have seen that a very considerable quantity of heat may
be excited by the friction of two metallic surfaces, and given off
in a constant stream or flux in all directions, without interruption
or intermission, and without any signs of diminution or exhaustion.
In reasoning on this subject we must not forget that most remar7kable circumstance, that the source of the heat generated by friction
in these experiments appeared evidently to be inexhaustible. (The
italics are Rumford's.) It is hardly necessary to add, that anything which any insulated body or system of bodies can continue
to furnish without limitation cannot possibly be a material sub



COMPRESSION OF BISULPHIDE OF CARBON VAPOUR. 71
stance; and it appears to me to be extremely difficult, if not
quite impossible, to form any distinct idea of anything capable
of being excited and communicated in those experiments, except
it be MOTION.
When the history of the dynamical theory of heat is written,
the man who, in opposition to the scientific belief of his time,
could experiment and reason upon experiment, as Rumford did in
the investigation here referred to, cannot be lightly passed over.
hardly anything more powerful against the materiality of heat
has been since adduced, hardly anything more conclusive in the
wary of establishing that heat is what Rumford considered it to
be, Motion.
ON THE COMPRESSION OF AIR CONTAINING BISULPHIDE
OF CARBON VAPOUR.
A very singular phenomenon was repeatedly observed during
the experiments with bisulphide of carbon. After determining
the absorption of the vapour, the tube was exhausted as perfectly
as possible, the trace of vapour left behind being exceedingly
minute. Dry air was then admitted to cleanse the tube. On
again exhausting, after the first few strokes of the pump, a jar
was felt and a kind of explosion heard, while dense volumes of
blue smoke immediately issued from the pump cylinders. The
action was confined to the latter, and never propagated itself
backwards into the experimental tube.'It is only with bisulphide of carbon that this effect has been
observed. It may, I think, be explained in the following manner:-To open the valve of the piston, the gas beneath it must
have a certain tension, and the compression necessary to produce
this appears sufficient to cause the combination of the constituents
of the bisulphide of carbon with the oxygen of the air. Such a
combination certainly takes place, for the odour of sulphurous
acid is unmistakeable amid the fumes.'To test this idea I tried the effect of compression in the air
syringe. A bit of tow or cotton wool moistened with bisulphide
of carbon, and placed in the syringe, emitted a bright flash when




72             "APPENDIX TO LECTURE II.
the air was compressed. By blowing out the fumes with a glass
tube, this experiment may be repeated twenty times with the same
bit of cotton.' It is not necessary even to let the moistened cotton remain in
the syringe. If the bit of tow or cotton be thrown into it, and
out again as quickly as it can be ejected, on compressing the air
the luminous flash is seen. Pure oxygen produces a brighter flash
than atmospheric air. These facts are in harmony with the above
explanation.' *
* Phil. Trans., 1861; Phil. Mag., Sept. 1861.




LECTURE III.
[February 6, 1862.]
EXPANSION: THE SOLID, LIQUID, AND GASEOUS FORMS OF MATTER-HYPOTHESES REGARDING THE CONSTITUTION OF GASES-COEFFICIENT OF EXPANSION-HEAT IMPARTED TO A GAS UNDER CONSTANT PRESSURE-HEAT
IMPARTED TO A GAS AT CONSTANT VOLUME —MAYER'S CALCULATION OF
THE MECHANICAL EQUIVALENT OF HEAT-DILATATION OF GASES WITHOUT
REFRIGERATION-ABSOLUTE ZERO OF TEMPERATURE —EXPANSION OF LIQUIDS AND SOLIDS: ANOMALOUS DEPORTMENT OF WATER AND BISMUTH
-ENERGY OF -THE FORCE OF CRYSTALLIZATION-THERMAL EFFECT OF
STRETCHING WIRES-ANOMALOUS DEPORTMENT OF INDIA-RUBBER.
APPENDIX:-ADDITIONAL DATA CONCERNING EXPANSION-EXTRACTS FROM
SIR H. DAVY'S FIRST SCIENTIFIC MEMOIR: FUSION OF ICE BY FRICTION, &C.
YOUR reappearance here to-day, after the strain which
has already been put upon your attention, encourages
me to hope that our present experiment will not be entirely
unsuccessful. I need not tell an audience like this that
nothing intellectually great is either accomplished or appropriated without effort. Newton ascribed the difference
between himself and other men to his patience in steadily
looking at a question, until light dawned upon it, and if we
have firmness to imitate his example, we shall, no doubt,
reap a commensurate reward.
In our first lecture I permitted a sledge-hammer to descend upon a mass of lead, and we found that the lead became heated, as soon as the mechanical motion of the hammer was arrested. Formerly it was assumed that the force
4




74    L             LECTURE III.
Of the hammer was simply lost by the concussion. In elas,
tic bodies it was supposed that a portion of the force was
restored by the elasticity of the body, which caused the
descending mass to rebound; but in the collision of inelastic bodies it was taken for granted that the force of impact
was lost. This, according to our present notions, was a
fundamental mistake; we now admit no loss, but assume,
that when the motion of the descending hammer ceases, it
is simply a case of transference, instead of annihilation.
The motion of the mass, as a whole, has been transformed
into a motion of the molecules of the mass. This motion
of heat, however, though intense, is executed within limits
too minute, and the moving particles are too-small, to be
visible. To discern these processes we must make use of
a finer eye and higher powers, namely, the eye and powers
of the mind. In the case of solid bodies, then, while the
force of cohesion still holds the particles together, you
must conceive a power of vibration, within certain limits,
to be possessed by the particles. You must suppose them
oscillating to and fro across their positions of rest; and
the greater the amount of heat we impart to the body, or
the greater the amount of mechanical action which we invest in it by percussion, compression, or friction, the more
intense will be the molecular vibration, and the wider the
amplitude of the atomic oscillations.
Now, nothing is more natural than that particles thus
vibrating, and ever as it were seeking wider room, should
urge each other apart, and thus cause the body of which
they are the constituents, to expand in volume. This, in
general, is the consequence of imparting heat to bodies —
expansion of volume. We shall closely consider the few
apparent exceptions to this law by and by. By the force
of cohesion, then, the particles are held together; by the
force of heat they are pushed asunder: here are the two
antagonist principles on which the molecular aggregation




THE LIQUID AND GASEOUS FORMS OF MATTER.   75
of the body depends. Let us suppose the communication
of heat to continue; every increment of heat pushes the
particles more widely apart; but the force of cohesion, like
all other known forces, acts more and more feebly, as the
distance between the particles which are the seat of the
force is augmented. As, therefore, the heat strengthens,
its opponent grows weak, until, finally, the particles are so
far loosed from the rigid thrall of cohesion, that they are
at liberty, not only to vibrate to and fro across a fixed position, but also to roll or glide around each other. Cohesion
is not yet destroyed, but it is so far modified, that while the
particles still offer resistance to being torn directly asunder,
their lateral-mobility over each other's surfaces is secured.
This is the liquid condition of matter.
In the interior of a mass of liquid the motion of every
atom is controlled by the atoms which surround it. But
suppose you develope heat of sufficient power within the
body of a liquid, what occurs?  Why, the particles break
the last fetters of cohesion, and fly asunder to form bubbles
of vapour. If one of the surfaces of the liquid be quite
free, that is to say, uncontrolled either by a liquid or solid;
it is quite easy to conceive that some of the vibrating superficial particles will be jerked quite away from the liquid,
and will fly with a certain velocity through space. Thus
fireed from the inence of cohesion, we have matter in the
vaporous or gaseous form.
My object here is to familiarize your minds with the
general conception of atomic motion. I have spoken of the
vibration of the particles of a solid as causing its expansion; the particles have been thought by some to revolve
round each other, and the communication of heat, by augmenting the centrifugal force of the particles, was supposed
to push them more widely asunder. I have here a weight
attached to a spiral spring; if I twirl the weight round in
the air it tends to fly away from me, the spring stretches to




76                   LECTURE III.
a certain extent, and as I augment the speed of revolution,
the spring stretches still more, the distance between my
hand and the weight being thus augmented. It has been
thought that the augmentation of the distance between a
body's atoms by heat, may be also due to a revolution of
its particles. And imagine the motion to continue till the
spring snaps; the ball attached to it would fly off along a
tangent to its former orbit, and thus represent an atom
freed, by heat, from the force of cohesion, which is rudely
represented by our spring. The ideas of the most wellinformed philosophers are as yet uncertain regarding the
exact nature of the motion of heat; but the great point,
at present, is to regard it as motion of some kind, leaving
its more precise character to be dealt with in future investigations.
We might extend the notion of revolving atoms to
gases also, and deduce their phenomena from a motion of
this kind. But I have just thrown out an idea regarding
gaseous particles, which is at present very ably maintained: * the idea, namely, that such particles fly in
straight lines through space. Everybody must have remarked how quickly the perfume of an odorous body fills
a room, and this fact harmonizes with the idea of the direct projection of the particles. But it may be proved,
that if the theory of rectilinear motion be true, the particles must move at the rate of several hundred feet a second. Hence it might be objected that, according to the
above hypothesis, odours ought to spread much more
quickly than they are observed to do.
The answer to this objection is, that they have to make
their way through a crowd of air particles, with which
they come into incessant collision.  On an average, the
* By Joule, KrSnig, Maxwell; and, in a series of extremely able papers,
by Clausius.




ATOMIC PROJECTILES.               77
distance through which an odorous particle can travel in
common air, without striking against a particle of air, is
infinitesimal, and hence the propagation of a perfume
through air is enormously retarded by the air itself. It is
well known that when a free communication is opened between the surface of a liquid and a vacuum, the vacuous
space is much more speedily filled to saturation with the
vapour of the liquid, than when air is present.
According to this hypothesis, then, we are to figure a
gaseous body as one whose particles are flying in straight
lines through space, impinging like little projectiles upon
each other, and striking against the boundaries of the space
which they occupy. Mr. Anderson will place this bladder,
half filled with air, under the receiver of the air-pump; he
will now work the pump, and remove the air that surrounds
the bladder. The bladder swells; the air within it appears
quite to fill it, so as to remove all its folds and creases.
How is this expansion of the bladder produced? According to our present theory, it is produced by the shooting
of atomic projectiles against its interior surface, which
drive the envelope outwards, until its tension is able to
cope with their force. When air is admitted into the receiver, the bladder shrivels up to its former size; and here
we must figure the discharge of the air particles against the
outer surface of the bladder, which drive the envelope inwards, causing, at the same time, the particles within to
concentrate their fire, until finally the force from within
equals that from without, and the envelope remains quiescent. All the impressions, then, which we derive from
heated air or vapour are, according to this hypothesis,
due to the impact of the gaseous atoms. They stir the
nerves in their own peculiar way, the nerves transmit
thne motion to the brain, and the brain declares it to be
heat. Thus the impression one receives on entering the
hot room of a Turkish bath, is caused by the atomic can



78                   LECTURE III.
nonade which is there maintained agains the surface of
the body.
If, instead of placing this bladder under the receiver of
an air-pump, and withdrawing the external air, I augment,
by heat, the projectile force of the particles within it, these
particles, though comparatively few in number, will strike
with such impetuous energy against the inner surface as to
cause the envelope to retreat: the bladder swells and becomes apparently filled with air; I hold the bladder close
to the fire, and here it is, you see, with all its creases removed. But you will retort, perhaps, by saying that this
ought not to be the case, inasmuch as the air outside the
bladder is also near the fire, and therefore animated with a
like projectile energy, which tends to drive the envelope
in. True, the bladder and the air in contact with it are
equally near the fire; but in a future lecture you will learn
that the air outside the bladder allows the rays of heat to
pass through it with very little augmentation of temperature, while the bladder intercepts the radiant heat; the envelope becomes first -warmed and then communicates its heat,
by contact, to the air within. The air, moreover, in contact with the bladder on the outside, though heated by the
bladder, has free space to dilate in, and is therefore incompetent to resist the expansion of the confined air which the
bladder contains.
This, then, is a simple illustration of the expansive force
of heat, and I have here an apparatus intended to show you
the same fact in another manner. Here is a flask, F (fig.
20), empty, except as regards air, which I intend to heat
by this little spirit-lamp underneath. From the flask a bent
tube passes to this dish, containing a coloured liquid. In
the dish, a 2-foot glass tube, t t, is inverted, closed at the
top, but with its open end downwards; you know that the
pressure of the atmosphere is competent to keep the column
of liquid in this tube, and here you have it quite filled to




EXPANSION OF GASES BY HEAT.             79
the top with the liquid. The tube passing from the flask
is caused to turn up exactly underneath the open end of
this upright tube, so that if a bubble of air should issue
from the former, it will ascend the latter. I now heat the
Fig. 20.
t
flask, and as I do so, the air expands,. for the reasons already given; bubbles are driven from the end of the bent
tube,. and they ascend in the tube t t. The air speedily depresses the liquid column, until now, in the course of a very
few seconds, the whole column of liquid has been superseded by air.
It is perfectly manifest that the air, thus expanded by
heat, is lighter than the unexpanded air. Our flask, at the
conclusion of this experiment, is lighter than it was at the
commencement, by the weight of the air transferred from
it into the upright tube. Supposing, therefore, a light bag
to be filled with such air, it is plain that the bag would,
with reference to the heavy air outside it, be like a drop
of oil in water; the oil being lighter than the water, will




80                  LECTUTE m.
ascend through the latter; so also our bag, filled with
heated air, will ascend in the atmosphere; and this is the
principle of the so-called fire-balloon. Mr. Anderson will
ignite some tow in this vessel, over it he will place this
funnel, and over the funnel I will hold the mouth of this
paper balloon. The heated air ascending from the burning
tow enters the balloon, causes it to swell; its tendency to
rise is already manifest. I let it go, and thus it sails aloft
till it strikes the ceiling of the room.
But we must not be content with regarding these phenomena in a general way; without exact quantitative determinations our discoveries would confound and bewilder
us. We must now enquire what is the amount of expansion which a given quantity of heat is able to produce in a
gas? This is an important point, and demands our special
attention. When we speak of the volume of a gas, we
should have no distinct notion of its real quantity, if its
temperature were omitted, the volume varies so largely
with the temperature. Take, then, a measure of gas at the
precise temperature of water when it begins to fireeze, or
of ice when it commences to melt, that is to say, at a temperature of 32~ Fahr. or 0~ Cent., and raise that volume of
gas one degree in temperature, the pressure on every square
inch of the envelope which holds the gas being preserved
constant. The volume of the gas will become expanded
by a quantity which we may call a; raise it another degree
in temperature, its volume will be expanded by 2a, a third
degree will cause an expansion of 3a, and so on. Thus, we
see, that for every degree which we add to the temperature
of the gas, it is expanded by the same amount. What is
this amount? No matter what the quantity of gas may be
at the freezing temperature, by raising it one degree Fcahrenheit we augment its volume by 4 -th of its own amount;
while by raising it one degree Centigrade we augment the
volume by: 7 3rd of its own amount. A cubic foot of gas,




COEFFICIENT OF EXPANSION.             81
for example, at 0~ C., becomes, on being heated to 10,
7lw1 cubic foot, or, expressed in decimals,
1 vol. at 0~ C. becomes    1 +'00367 at 1~ C.
at 2~ C. it becomes  1 +'00367 x 2
at 30 C. it becomes  1 -+'00367 x 3, and so on.
The constant number'00367, which expresses the fraction of its own volume, which a gas, at the freezing temperature, expands on being heated one degree, is
called the coefficient of exp ansion of the gas. Of  Fig. 21.
course if we use the degrees of Fahrenheit, the co-  A
efficient will be smaller in the proportion of 9 to 5.
This much made clear, we shall now approach,
by slow degrees, an interesting but difficult subject. Suppose I have a quantity of air contained
in a very tall cylinder, A B (fig. 21), the transverse
section of which is one square inch in area. Let
the top A of the cylinder be open to the air, and
let P be a piston, which, for reasons to be explained
immediately, I will suppose to weigh two pounds
one ounce, and which moves air-tight and without
friction, up or down in the cylinder. At the com- M'"'i' P
mencement of the experiment, let the piston be at
the point P of the cylinder, and let the height of
the cylinder from its bottom B to the point p be
273 inches, the air underneath the piston being at
a temperature of 0~ C. Then, on heating the air  0
from 0~ to 1~ C. the piston will rise one inch; it
will now stand at 274 inches above the bottom. If
the temperature be raised two degrees, the piston
will stand at 275, if raised three degrees it will
stand at 276, if raised ten degrees it will stand at  B
283, if 100 degrees it will stand at 373 inches
above the bottom; finally, if the temperature were raised
to 273~ C., it is quite manifest 273 inches would be added
4*




8'2                  LECTURE III.
to the height of the column, or, in other words, by heating
the air to 273~ C., its volume would be doubled.
It is evident that the gas, in this experiment, executes
work. In expanding from P upwards, it has to overcome
the downward pressure of the atmosphere, which amounts
to 15 lbs. on every square inch, and also the weight of the
piston itself, which is 2 lbs. I oz. Hence, the section of
the cylinder being one square inch in area, in expanding
from P to P' the work done by the gas is equivalent to the
raising a weight of 17 lbs. 1 oz., or 273 ounces, to a height
of 273 inches. It is just the same as what it would accomplish, if the air above P were entirely abolished, and a piston weighing 17 lbs. 1 oz. were placed at P.
Let us now alter our mode of experiment, and instead
of allowing our gas to expand when heated, let us oppose
its expansion by augmenting the pressure upon it. In
other words, let us keep its volume constant while it is
being heated. Suppose, as before, the initial temperature
of the gas to be O0 C., the pressure upon it, including the
weight of the piston P, being, as formerly, 273 ounces.
Let us warm the gas from O~ C. to 1~ C.; what weight
must we add to P in order to keep its volume constant?
Exactly one ounce. But we have supposed the gas, at the
commencement, to be under a pressure of 273 ounces, and
the pressure it sustains is the measure of its elastic force;
hence, by being heated one degree, the elastic force of the
gas has augmented by 23-rd of what it possessed at O~.
If we warm it 2~, 2 ozs. must be added to keep its volume
constant; if 3~, 3 ozs. must be added. And if we raise its
temperature 273~, we should have to add 273 ozs.; that is,
we should have to double the original pressure to keep the
volume constant.
It is simply for the sake of clearness, and to avoid fractions in our reflections, that I have supposed the gas to be
under the original pressure of 273 ozs. No matter what its




WORK )DONE BY EXPANDING GAS.          83
pressure may be, the addition of 10 C. to its temperature
produces an augmentation of ward of the elastic force
which the gas possesses at the freezing temperature; and
by raising its temperature 273~, while its volume is kept
constant, its elastic force is doubled. Let us now compare
this experiment with the last one. There we heated a certain amount of gas from 0~ to 273~, and doubled its volume by so doing, the double volume being attained while
the gas lifted a weight of 273 ozs. to a height of 273
inches. Were we heat the same amount of gas from 0~ to
273~, but we do not permit it to lift any weight. We
keep its volume constant. The quantity of matter heated
in both cases is the same; the temperature to which it is
heated is in both cases the same; but are the absolute
quantities of heat imparted in both cases the same? By
no means. Supposing that to raise the temperature of the
gas, whose volume is kept constant, 273~, 10 grains of
combustible matter are necessary; then to raise the temperature of the gas whose pressure is kept constant an
equal number of degrees, would require the consumption
of 14~ gramins of the same combustible matter. The heat
2produced by the combustion of the additional 41 grains, in
the latter case, is entirely consumed in lifting the weight.
Using the accurate numbers, the quantity of heat applied
when the volume is constant, is to the quantity applied
when the pressure is constant, in the proportion of
1 to 1'421.
This extremely important fact constitutes the basis
from which the mechanical equivalent of heat was first calculated. And here we have reached a point which is worthy of, and which will demand, your entire attention. I
will endeavour to make this calculation before you.
Let c (fig. 21a) be a cylindrical vessel with a base one
square foot in area. Let P P mark the upper surface of a




84                    LECTURE III.
cubic foot of air at a temperature of 320 Fahr. The height
A P will be then one foot. Let the air be heated till this
volume is doubled; to effect this it must,
as before explained, be raised 273~ C., or
14 —-____...490~ F. in temperature; and, when expanded, its upper surface will stand at'r
P', one foot above its initial position. But
in rising from P P to P'' it has forced
back the atmosphere, which exerts a pressure of 15 lbs. on every square inch of its
upper surface; in other words, it has lifted
a weight of 144 X 15 =  2,160 lbs. to a
A   eight of one foot.
The'capacity' for'heat of the air thus expanding is
0'24; water being unity. The weight of our cubic foot of
air is 1'29 oz., hence the quantity of heat required to raise
1'29 oz. of air 490~ Fahr. would raise a little less than onefourth of that weight of water 490~.' The exact quantity
of water equivalent to our 1'29 oz. of air is 1'29 X 0'24 =
0'31 oz.
But 0'31 oz. of water, heated to 490~, is equal to 152
ozs. or 9~ lbs. heated 1~. Thus the heat imparted to our
cubic foot of air, in order to double its volume, and enable
it to lift a weight of 2,160 lbs. one foot high, would be
competent to raise 91 lbs. of water one degree in temperature.
The air has here been heated under a constant pressure,
and we have learned, that the quantity of heat necessary to
raise the temperature of a gas under constant pressure a
certain number of degrees, is to that required to raise the
gas to the same temperature, when its volume is kept constant, in the proportion of 1'42: 1; hence we have the
statementlbs. lbs.
1'42: 1 = 9'5: 6'7




MAYER'S CALCULATION.               85
which shows that the quantity of heat necessary to augment the temperature of our cubic foot of air, at constant
volume, 490~, would heat 6'7 lbs. of water 1~.
Deducting 6'7 lbs. from 9-5 lbs., we find that the excess
of heat imparted to the air, in the case where it is permitted to expand, is competent to raise 2'8 lbs. of water 1~ in
temperature.
As explained already, this excess is employed to lift the
weight of 2,160 lbs. one foot high. Dividing 2,160 by 2'8,
we find that a quantity of heat sufficient to raise one pound
of water 1~ Fahr. in temperature, is competent to raise a
weight of 771'4 lbs. a foot high.
This method of calculating the mechanical equivalent
of heat was followed by Dr. Mayer, a physician in Heilbron, Germany, in the spring of 1842.
Mayer's first paper contains merely an indication of the
way in which he had found the equivalent; but does not
contain the calculation. The paper was evidently a kind
of preliminary note, from which date might be taken. In
it were enunciated the convertibility and indestructibility
of force, and its author referred to the mechanical equivalent of heat, merely in illustration of his principles. Had
this first paper stood alone, Mayer's relation to the dynamical theory of heat would be very different from what it
now is; but in 1845 he published an Essay on Organic
Motion, which, though exception might be taken to it here
and there, is, on the whole, a production of extraordinary
merit. This was followed in 1848 by an Essay on'Celestial Dynamics,' in which, with remarkable boldness, sagacity, and completeness, he developed the meteoric theory
of the sun. Taking him all in all, the right of Mayer to
stand, as a man of true genius, in the front rank of the
founders of the dynamical theory of heat, cannot be disputed.
On August 21, 1843, Mr. Joule communicated a paper




86                   LECTURE III.
to the British Association, then meeting at Cork, and in the
third part of this paper* he describes a series of experiments on magneto-electricity, executed with a view to determine the'mechanical value of heat.' The results of this
elaborate investigation gave the following weights raised
one foot high, as equivalent to the warming of 1 lb. of
water 1~ Fahr.
1. 896 lbs.               5. 1026 lbs.
2. 1001,,                 6. 587,,
3. 1040,,                 7. 742,,
4. 910,,                  8. 860,,
In 1844 Mr. Joule deduced from experiments on the
condensation of air, the following equivalents to 1 lb. of
water heated 1~ Fahr.
823 foot pounds
795
820
814,,
760
As the experience of the experimenter increased, we
find that the coincidence of his results becomes closer. In
1845 Mr. Joule deduced from experiments with water, agitated by a paddle-wheel, an equivalent of
890 foot pounds.
Summing up his results in 1845, and taking the mean,
he found the equivalent to be
817 foot pounds.
In 1847 he found the mean of two experiments to give
as equivalent
781'8 foot pounds.
* Phil. Mag., 1843, vol. xxiii. p. 435.




JOULE S EXPERIMENTS.               87
Finally, in 1849, applying all the precautions suggested
by seven years' experience, he obtained the following numbers for the mechanical equivalent of heat:7 72'692, from friction of water, mean of 40 experiments
774-083,,,,, mercury,,,  50,,
774'987,,,,,  cast-iron,,,  20,,
For reasons assigned in his paper, Mr. Joule fixes the
exact equivalent of heat at
772 foot pounds.
According to the method pursued by Mayer, inl 1842,
the mechanical equivalent of heat is
771'4 foot pounds.
Such a coincidence relieves the mind of every shade of
uncertainty, regarding the correctness of our present me.
chanical equivalent of heat.
Do I refer to these things in order to exalt Mayer, at
the expense of Joule? It is far from my intention to do
so. The man who through long years, without encourage.
ment, and in the face of difficulties which might well be
deemed insurmountable, could work with such unswerving
steadfastness of purpose to so triumphant an issue, is safe
from depreciation. And it is not the experiments alone,
but the spirit which they incorporate, and the applications
which their author made of them, that entitle Mr. Joule to
a place in the foremost rank of physical philosophers.
Mayer's labours have, in some measure, the stamp of a profound intuitions which rose, however, to the energy of undoubting conviction in the author's mind. Joule's labours,
on the contrary, are an experimental demonstration. True
to the speculative instincts of his country, Mayer drew
large and weighty conclusions from slender premises, while
the Englishman aimed, above all things, at the firm establishment of facts. And he did establish them. The future




88                  LECTURE III.
historian of science will not, I think, place these men in
antagonism. To each belongs a reputation which will not
quickly fade, for the share he has had, not only in establishing the dynamical theory of heat, but also in leading
the way towards a right appreciation of the general energies
of the universe.
Let us now check our conclusion regarding the influence
which the performance of work has on the quantity of heat
communicated to a gas. Is it not possible to allow a gas
to expand, without performing work? This question is answered by the following important experiment, which was
first made by Gay Lussac. I have here two copper vessels,
A, B (fig. 22), of the same size, one of which, A, is exhausted, and the other, B, filled with air. I turn the cock c;
the air rushes out of B into A, until the same pressure exists
in both vessels. Now the air in
Fig. 22.      driving its own particles out of B
performs work, and experiments
which we have already made inform us, that the residue of air
which remains in B must be chilled. The particles of air enter A
xB   with a certain velocity, to generate
which the heat of the air in B has
been sacrificed; but they immediately strike against the interior
surface of A, their motion of translation is annihilated, and the exact quantity of heat lost by
B appears in A. Mix the contents of A and B together, and
you have air of the original temperature. There is no work
performed, and there is no heat lost. MIr. Joule made this
experiment by compressing twenty-two atmospheres of air
into one of his vessels, while the other was exhausted. On
surrounding both vessels by water, kept properly agitated,
no augmentation of temperature was observed in the water,




EXPANSION WITHOUT REFRIGERATION.         89
when the gas was allowed to stream from one vessel into
the other.* In like manner, supposing the top of the cylinder (fig. 20) to be closed, and the half above the piston a
perfect vacuum; and suppose the air in the lower half to
be heated 273~, its volume being kept constant. If the
pressure were removed, the air would expand and fill the
cylinder; the lower portion of the column would thereby
be chilled, but the upper portion would be heated, and
mixing both portions together, we should have the whole
column at a temperature of 273~. In this case we raise the
temperature of the gas from 0~ to 273~, and afterwards allow it to double its volume; the state of the gas at the
commencement, and at the end, is the same as when the
gas expands against a constant pressure, or lifts a constant
weight; but the absolute quantity of heat in the latter case
is 1'421 times that employed in the former, the difference
being due to the fact that the gas, in the one case, performs mechanical work, and in the other not.
We are taught by this experiment that mere rarefaction
is not of itself sufficient to produce a lowering of the mean
temperature of a mass of air. It was, and is still, a current
notion, that the mere expansion of a gas produced refrigeration, no matter how that expansion was effected. The
coldness of the higher atmospheric regions was accounted
for by reference to the expansion of the air. It was
thought that what we have called the' capacity for heat'
was greater in the case of the rarefied than of the unrare.
fled gas. But the refrigeration which accompanies expansion is, in reality, due to the consumption of heat in the
performance of work by the expanding gas. Where no
work is performed there is no absolute refrigeration.
All this needs reflection to arrive at clearness, but every
effort of this kind which you make will render your subse* Phil. Mag. 1845, vol. xxvi. p. 378.




90                   LECTURE III.
quent efforts easier, and should you fail, at present, to gain
clearness of comprehension, I repeat my recommendation
of patience. Do not quit this portion of the subject without an effort to comprehend it-wrestle with it for a time,
but do not despair if you fail to arrive at clearness.
I have now to direct your attention to one other interesting question. We have seen the elastic force of our gas
augmented by an increase of temperature. In an inflexible
envelope we have, for every degree of temperature, a certain definite increment of elastic force, due to the augmented energy of the gaseous projectiles. Reckoning from 0~
C. upwards, we find that every degree added to the temperature produces an augmentation of elastic force, equal
to 2- rd of that which the gas possesses at 0~, and hence,
that by imparting 273~ we double the elastic force. Supposing the same law to hold good when we reckon from 0~
downwarcds-that for every degree of temperature withdrawn from the gas we diminish its elastic force, or the
motion which produces it, by  I a3rd of what it possesses at
0~, it is manifest that at a temperature of 273~ Centigrade
below 0~ we should cease to have any elastic force whatever. The motion to which the elastic force is due must
here vanish, and we reach what is called the absolute zero
of temperature.
No doubt, practically, every gas deviates from the
above law of contraction before it sinks so low, and it
would become solid before reaching-273~ C., or the absolute zero. This is considerably below any temperature
which we have as yet been able to obtain.
I will not subject your minds to any further strain in
connection with this subject to-day, but will now pass on
to illustrate experimentally the expansion of liquids by
heat.
ILlere is a Florence flask filled with alcohol, and tightly
corked; through the cork a tube, t' (fig. 23), passes water



EXPANSION OF LIQUIDS.               91
tight, and the liquid rises a foot or so in this tube. I will
heat this flask, the alcohol will expand, and it will rise in
the tube. But I wish you to see it rising, and to enable
you to do so I will place the tube t t' in front of the electric lamp E, and send a strong beam of light across it, at
Fig. 23.
the place t', where the liquid column ends; I thus illuminate the tube and column. In front of the tube I place this
lens L, and arrange its distance so that it shall cast an enlarged image i i, of the column upon the screen. You now
see clearly where the column ends; you see this quivering
of the top of the column, and if it moves, you will bVe able
to see its motion. I now fill this beaker, B, with hot wa



92                   LECTURE III.
ter, and I will raise the beaker so that the hot water shall
surround the Florence flask. It is needless to say that the
image upon the screen is inverted, and that when the liquid
expands, the top of the column will descend along the
screen. Observe the experiment from the commencement;
the flask is now in the hot water, and the head of our column ascends, as if the liquid contracted. Now it stops
and commences to descend, and it will continue to do so
permanently. But why the first ascent? It is not due to
the contraction of the liquid, but to the momentary expansion of the flask, to which the heat is first communicated.
The glass expands before the heat can fairly reach the
liquid, and hence the column falls; but soon the expansion
of the liquid exceeds that of the glass, and the column
rises. Two things are here illustrated; the expansion of
the solid glass by heat, and the fact that the observed dilatation of the liquid does not give us its true augmentation
of volume, but only the difference of dilatation between
the glass and it.
I have here another flask filled with water, exactly
similar in size to the former, and furnished with a similar
tube. I place it in the same position, and repeat with it
the experiment made with the alcohol. You see, first of all,
the transitory effect due to the expansion of the glass, and
afterwards, the permanent expansion of the liquid; but you
can observe that the latter proceeds much more slowly than
in the case of alcohol; the alcohol expands more speedily
than the water. Now we might go over a hundred liquids
in this way, and find them all expanding by heat, and we
might thus be led to conclude that expansion by heat is a
law without exception; but we should err in this conclusion. And it is really to illustrate an exception of this kind
that I have introduced this flask of water. I will cool the
flask by plunging it into a substance somewhat colder than
water, when it first freezes. This substance I obtain by




DEPORTMENT OF WATER.                93
mixing pounded ice with salt. You see the column gradually sinking, the heat is being given up to the freezing mixture, and the water contracts. This contraction is now very
slow, and now it stops altogether. A slight motion commences in the opposite direction, and now the liquid is visibly expanding. I stir the freezing mixture, so as to bring
colder portions of it into contact with the flask; the colder
the mixture the quicker is the expansion. Here then we
have Nature stopping in her ordinary course, and reversing!ter ordinary habits. The fact is, that the water goes on
contracting till it reaches a temperature of 39~ Fahr., or 4~
Cent., at which point the contraction ceases. This is the
so-called point of maximum density of the water; from
this downwards, to its freezing point, the liquid expands;
and when it is converted into ice, the expansion is large
and sudden. Ice, we know, swims upon water, being
lightened by this expansion. If I now apply heat, the
series of changes are reversed: the column descends, showing the contraction of the liquid by heat. After a time the
contraction ceases, and permanent expansion sets in.
The force with which these molecular changes are
effected is all but irresistible. The changes usually occur
under conditions which allow us no opportunity of observing
the energy involved in their accomplishment. But to give
you an example of this energy, I have confined a quantity
of water in this iron bottle. The iron is fully half an inch
thick, and the quantity of water is small, though sufficient
to fill the bottle. The bottle is closed by a screw firmly
fixed in its neck. I have here a second bottle of the same
kind, and prepared in a similar manner. Both of them I
place in this copper vessel, and surround them with a freezing mixture. They cool gradually, the water within approaches its point of maximum density; no doubt, at this
moment, the water does not quite fill the bottle, a small
vacuous space exists within.  But soon the contraction




94                     LECTURE II.
ceases, and expansion sets in; the vacuous space is slowly
filled, the water gradually changes from liquid to solid; in
doing so it requires more room, which the rigid iron refuses to grant. But its rigidity is powerless in the presence of the atomic forces. These atoms are giants in disguise; you hear that sound; the bottle is shivered by the
crystallising molecules-there goes the other; and here are
the fragments of the vessels, which show  their thickness,
and impress you with the might of that energy by which
they were thus riven.*
You have now no difficulty in understanding the effect
of frosty weather upon the water pipes of your houses. I
have here a number of pieces of such pipes, all rent. You
become first sensible of the damage when the thaw sets in,
but the mischief is really done at the time of freezing; the
pipes are then rent, and through the rents the water escapes, when the solid within is liquefied.
It is hardly necessary for me to say a word on the importance of this property of water in the economy of nature. Suppose a lake exposed to a clear wintry sky; the
superficial water is chilled, contracts, becomes thus heavier,
and sinks by its superior weight, its place being supplied
by the lighter water from below. In time this is chilled,
and sinks in turn. Thus a circulation is established, the
cold, dense water descending, and the lighter and warmer
water rising to the top. Supposing this to continue, even
after the first pellicles of ice were formed at the surface;
the ice would sink as it was formed,t and the process
* Metal cylinders, an inch in thickness, are unable to resist the decomposing force of a small galvanic battery. M. Gassoit has burst many such
cylinders by electrolytic gas.
t Prof. William Thomson has recently raised a point which deserves
the grave consideration of theoretic geologists: Supposing the constituents
of the earth's crust to contract on solidifying, as the experiments thus far
made indicate, a breaking in, and sinking of the crust would assuredly




DEPORTMENT OF BISMUTH.                95
would not cease until the entire water of the lake would be
solidified. Death to every living thing in the water would
be the consequence. But just when matters become critical,
Nature steps aside from her ordinary proceeding, causes
the water to expand by cooling, and the cold water swims
like a scum on the surface of the warmer water underneath. Solidification ensues, but the solid is much lighter
than the subjacent liquid, and the ice forms a protecting
roof over the living things below.
Such facts naturally and rightly excite the emotions;
indeed, the relations of life to the conditions of life-the
general adaptation of means to ends in Nature, excite, in
the profoundest degree, the interest of the philosopher.
But in dealing with natural phenomena, the feelings must
be carefully watched. They often lead us unconsciously
to overstep the bounds of fact. Thus, I have heard this
wonderful property of water referred to as an irresistible
proof of design, unique of its kind, and suggestive of pure
benevolence.' Why,' it is urged,' should this case of water stand out isolated, if not for the purpose of protecting
Nature against herself?' The fact, however, is, that the
case is not an isolated one. You see this iron bottle, rent
from neck to bottom; I break it with this hammer, and
you see a core of metal within. This is the metal bismuth,
which, when it was in a molten condition, I poured into
this bottle, and closed the bottle by a screw, exactly as in
the case of the water. The metal cooled, solidified, expanded, and the force of its expansion was sufficient to
burst the bottle. There are no fish here to be saved, still
the molten bismuth acts exactly as the water acts.  Once
for all, I would say that the natural philosopher, as such,
follow its formation. Under these circumstances, it is extremely difficult
to conceive that a solid shell should be formed, as is generally assumed,
round a liquid nucleus.




96                  LECTURE III.
has nothing to do with purposes and designs. His vocation is to enquire what Nature is, not wfhy she is; though
he, like others, and he, more than others, must stand at
times rapt in wonder at the mystery in which he dwells,
and towards the final solution of which his studies furnish
him with no clue.
We must now pass on to the expansion of solid bodies,
by heat, and I will illustrate it in this way: I have here
two wooden stands, A and B (fig. 24), with plates of brass,
p p', riveted against them. I hold in my hand two bars
of equal length, one of brass, the other of iron, and these,
as you observe, are not sufficiently long to stretch from
Fig. 24.
stand to stand. I will support them on two little projections of wood attached to the stand at p andp'. I connect
one of the plates of brass, p, with one pole of a small voltaic battery, D, and from the other, p', a wire proceeds to




EXPANSION OF SOLIDS.              97
the little instrument c, which you see in front of the table;
and again from that instrument a wire returns direct to the
other pole of the battery. The instrument in front consists merely of an arrangement to support a spiral c of platinum wire, which will glow with a pure white light when
the current from D passes through it. At the present moment the only break in the circuit is due to the insufficient
length of the bars of brass and iron to bridge the space from
stand to stand. Underneath the bars is a row of gas jets,
which I will now ignite; the bars are heated, the metals
expand, and I expect that in a few minutes they will
stretch quite across from plate to plate; when this occurs,
the current will pass, and the fact of the gap being bridged
will be declared by the sudden glowing of the platinum
spiral. It is still non-luminous, the bridge is not yet complete; but now it brightens up, showing that one, or both,
of these bars have expanded so as to stretch quite across
from stand to stand. Which of the bars is it? I remove
the iron, but the platinum still glows: I restore the iron,
and remove the brass; the light disappears. It was the
brass that bridged the gap. So that we have here an illustration, not only of the general fact of expansion, but also
of the fact that different bodies expand in different degrees.
The expansion of both brass and iron is very small: and
various instruments have been devised to measure the expansion. Such instruments go under the general name of
pyrometers. But I have here a means of multiplying the
effect, far more powerful than the ordinary pyrometer.
Here is a solid upright bar of iron two feet long, and on a
mirror connected with the top of the bar I throw a beam
of light from the electric lamp, which beam is reflected to
the upper part of the wall. If the bar shorten, the mirror
will turn in one direction: if it lengthen, the mirror will
turn in the opposite direction. Every movement of the
mirror, however slight, is multiplied by this long index of
5




98                    LECTURE III.
light; which, besides its length, has the advantage of moving with twice the angular velocity of the mirror. Even
the breath, projected against this massive bar of iron, produces a sensible motion of the beam; and if I warm it for
a moment with the flame of a spirit-lamp, the luminous index will travel downwards, the patch of light upon the
wall moving through a space of full thirty feet. I withdraw the lamp, and allow the bar to cool; it contracts, and
the patch of light reascends the wall: I hasten the contraction by throwing a little alcohol on the bar of iron, the
light moves more speedily upwards, and now it occupies a
place near the ceiling, as at the commencement of the experiment.*
I have stated that different bodies possess different
powers of expansion; that brass, for example, expands
more, on being heated, than iron. Here are two rulers,
one of brass and the other of iron, riveted together so as
to form, at this temperature, a straight compound ruler.
But if the temperature be changed, the ruler is no longer
straight. I heat it, it bends in one direction: I cool it, it
bends in the opposite direction. When heated, the brass
expands most, and forms the convex side of the curved
ruler. When cooled, the brass contracts most, and forms
the concave side of the ruler. Facts like these must, of
course, be taken into account, in structures where it is necessary to avoid distortion. The force with which bodies
expand when heated, is quite irresistible by any mechanical
appliances that we can make use of. All these molecular
forces, though operating in such minute spaces, are almost
infinite in energy. The contractile force of cooling has
* The piece of apparatus with which this experiment was made is intended for a totally different purpose. I therefore indicate its principle
merely.
1 The coefficients of expansion of a few well-known substances are
given in the Appendix to this Lecture.




ENERGY OF FORCE OF EXPANSION.           99
been applied by engineers to draw leaning walls into an
upright position. If a body be brittle, the heating of one
portion of it, producing expansion, may so press or stratain
another portion, as to produce fracture. Hot water poured
into a glass often cracks it, through the sudden expansion
of the interior. It may also be cracked by the contraction
produced by intense cold.
I have here some flasks of very thick glass, which, when
blown, were allowed to cool quickly. The external portions become first chilled and rigid. The internal portions
cooled more gradually, but they found themselves, on
cooling, surrounded, as it were, by a rigid shell, on which
they exerted the powerful strain of their contraction. The
consequence is, that the superficial portions of these flasks
are in such a state of tension that the slightest scratch produces rupture. I throw into this glass a grain of quartz;
the mere dropping of the little bit of hard quartz into the
flask causes the bottom to fly out of it. Here, also, I have
these so-called Rupert drops, or Dutch tears, produced
by glass being fused to drops, which are suddenly cooled.
The external rigid shell has to bear the strain of the inner
contraction; but the strain is distributed so equally all over
the surface, that no part gives way. But by simply breaking this filament of glass, which forms the tail of the drop,
the solid mass is instantly reduced to powder. I dip the
drop into a small flask filled with water, and break the tail
of the drop outside the flask; the drop is shivered with
such force that the shock, transferred through the water, is
sufficient to break the bottle in pieces.
A very curious effect of expansion was observed, and
explained, some years ago by the Reverend Canon Mosely.
The choir of Bristol Cathedral was covered with sheet lead,
the length of the covering being 60 feet, and its depth 19
feet 4 inches. It had been laid on in the year 1851, and
two years afterwards-viz., in 1853-it had moved bodily




100                 LECTURE III.
down for a distance of eighteen inches. The descent had
been continually going on from the time the lead had been
laid down, and an attempt made to stop it by driving nails
into the rafters had failed; for the force with which the
lead descended was sufficient to draw out the nails. The
roof was not a steep one, and the lead would have rested
on it for ever, without sliding down by gravity. What,
then, was the cause of the descent? Simply this. The
lead was exposed to the varying temperatures of day and
night. During the day the heat imparted to it caused it to
expand. Had it lain upon a horizontal surface, it would
have expanded equally all round, but as it lay upon an inclined surface, it expanded more freely downwards than
upwards. When, on the contrary, the lead contracted at
night, its upper edge was drawn more easily downwards
than its lower edge upwards. Its motion was therefore
exactly that of a common earthworm; it pushed its lower
edge forward during the day, and drew its upper edge
after it during the night, and thus by degrees it crawled
through a space of eighteen inches in two years. Every
local change of temperature during the day and during the
night contributed also to the result; indeed Canon Mosely
afterwards found the main effect to be due to these quicker
alternations of temperature.
Not only do different bodies expand differently by heat,
but the same body may expand differently in different directions. In crystals the atoms are laid together according
to law, and along some lines they are more closely packed
than along others. It is also likely that the atoms of many
crystalline bodies oscillate more freely and widely in some
directions than in others. The consequence of this would
be an unequal expansion by heat in different directions.
This crystal I hold in my hand (Iceland spar) has been
proved by Professor Mitscherlich to expand more along its
crystallographic axis than in any other direction. Nay,




THERMAL EFFECTS OF STRETCHING.          101
while the crystal expands as a whole-that is to say, while
its volume is augmented by heat-it actually contracts in a
direction at right angles to the crystallographic axis. Mlany
other crystals also expand differently in different directionil-;
and, I doubt not, most organic structures would, if examined, exhibit the same fact.
Nature is full of anomalies which no foresight can predict, and which experiment alone can reveal. From the
deportment of a vast number of bodies, we should be led
to conclude that heat always produces expansion, and that
cold always produces contraction. But water steps in, and
bismuth steps in to qualify this conclusion. If a metal be
compressed, heat is developed: but if a metal wire be
stretched, cold is developed. Mr. Joule and others have
worked at this subject, and found the above fact all but
general.
One striking exception to this rule (I have no doubt
there are many others) has been known for a great number
of years; and I will now illustrate this exception by an experiment.  My assistant will hand me a sheet of Indiarubber, which I have placed in the next room to keep it
quite cold. From this sheet I cut a strip three inches long,
and an inch and a half wide; I turn my thermo-electric pile
upon its back, and upon its exposed face I lay this piece of
India-rubber. From the deflection of the needle, you see
that that piece of rubber is cold. I now lay hold of the
ends of the strip, suddenly stretch it, and press it, while
stretched, on the face of the pile. See the effect! The
needle moves with energy, and showing that the stretched
rubber has heated the pile.
But one deviation from a rule always carries other deviations in its train. In the physical world, as in the moral,
acts are never isolated. Thus with regard to our Indiarubber; its deviation from the rule referred to is only part
of a series of deviations. In many of his investigations




102                 LECTURE Il.
Fig. 25,      Mr. Joule has been associated
a  with an eminent natural philosopher-Professor William Thomson-and when Mr. Thomson was
l  made aware of the deviation of
India-rubber from an almost general rule, he suggested that the
stretched  India - rubber might
shorten, on being heated.  The
test was applied by Mr. Joule,
c and the shortening was found to
take place. This singular experiment, thrown into a suitable
form, I will now perform before
you.
I fasten to this arm, a a (fig.
25), a length of common vulcanised India-rubber tubing, and
stretch it by a weight, w, of ten
pounds, to about three times its
former length. Here is an index,
i i, formed first of a piece of
light wood moving freely on a




DEPORTMENT OF INDIA RUBBER.           103
pivot, and prolonged by a stout straight straw. At the end
of the straw I place a spear-shaped piece of paper, which
can range over the graduated circle drawn on this black
board. The index is now pressed down by a projection
which I have attached to the weight; but if the weight
should be lifted by the contraction of the India-rubber, the
lever will follow it, being drawn after it by a spring, s s,
which acts upon the short arm of the index. The Indiarubber tube, you observe, passes through a sheet iron
chimney, c, through which I will now allow a current of
hot air to ascend from this lamp L. You see the effect;
the index rises, showing that the rubber contracts, and by
continuing to apply the heat for a minute or so, I cause the
end of my index to describe an arc fully three feet long.
I withdraw the "amp, and as the India-rubber returns to its
former temperature, it lengthens; the index moves downwards, and now it rests even below the position which it
first occupied.




APPENDIX TO LECTURE III.
FURTHER REMARKS ON DILATATION.
IT is not within the scope of these lectures to dwell in detail
on all the phenomena of expansion by heat; but for the sake of
my young readers, I will supplement this lectuwe by a few additional remarks.
The linear, superficial, or cubic coefficient ofexpansion, is that
fraction of a body's length, surface, or volume,which it expands
on being heated one degree.
Supposing one of the sides of a square plat3 of metal, whose
length is 1, to expand, on being heated one degee, by the quantity a; then the side of the new square is 1 + a,and its area is
1 + 2a + a2.
In the case of expansion by heat, the quantity a is so small, that
its square is almost insensible; the square ofa small fraction is,
of course, greatly less than the fraction itsef.  Hence without
sensible error, we may throw away the a2 it the above expression, and then we should have the area of thenew square
1 + 2a.
2a, then, is the superficial coefficient of exparion; hence we infer
that by multiplying the linear coefficient byl, we obtain the superficial coefficient.
Suppose, instead of a square, that we hd a cube, having a
side = 1; and that on heating the cube on degree, the side expanded to 1 + a; then the volume of the epanded cube would
be
1 + 3a + 3a2 + a3.




EXPANSION: THE THERMOMETER.                   105
In this, as in the former case, the square of a, and much more the
cube of a, may be neglected, on account of their exceeding smallness; we then have the volume of the expanded cube
=1 + 3a;
that is to say, the cubic coefficient of expansion is found by trebling the linear coefficient.
The following table contains the coefficients of expansion, for
a number of well-known substances:Copper... 0000017         0'000051     0'000051
Lead... 0000029        0'000087     0'000089
Tin.. 0'000023                   0-000069     0'000069
Iron.... 00000123    0'000037        0'000037
Zinc.. 0'0000294    0'000088                   0'000089
Glass.... 0000080       0-000024      0'000024
The second column here gives the linear coefficient of expansion
for 10 C.; the third column contains this coefficient trebled,
which is the cubic expansion of the substance; and the fourth
column gives the cubic expansion of the same substance, determined directly by Professor Kopp.*  It will be seen that Kopp's
coefficients agree almost exactly with those obtained by the trebling of the linear coefficients.
The linear coefficient of glass for 10 C. is
0'0000080.
That of platinum is
0'0000088.
Hence glass and platinum expand nearly alike. This is of the
greatest importance to chemists, who often find it necessary to
fuse platinum wires into their glass tubes. Were the coefficients
different, the fracture of the glass would be inevitable during the
contraction.
The Thermometer.
Water owes its liquidity to the motion of heat; when this
motion sinks sufficiently, crystallisation, as we have seen, sets in.
* Phil. Mag., 1852, vol. iii. p. 268.
5*




106             APPENDIX TO LECTURE II.
The temperature of crystallisation is perfectly constant if the
water be kept under the same pressure. For example, water crystallises in all climates at the sea-level, at a temperature of 32~ F.,
or of 0~ C. The temperature of condensation from the state of
steam is equally constant, as long as the pressure remains the
same. The melting of ice and the freezing of water touch each
other, if I may use the expression, at 32~ F.; the condensation of
steam and the boiling of water touch each other at 212: 32~ then
is the freezing point of water, and it is the melting point of ice;
212~ is the condensing point of steam and the boiling point of
water. Both are invariable as long as the pressure remains the
same. Here, then, we have two invaluable standard points of
temperature, and they have been used for this throughout the
world. The mercurial thermometer consists of a bulb and a stem
with capillary bore. The bore ought to be of aquable diameter
throughout.  The bulb and a portion of the stem are filled with
mercury. Both are then plunged into melting ice, the mercury
shrinking, the column descends, and finally comes to rest. Let
the point at which it becomes stationary be marked; it is the
freezing point of the thermometer. Let the instrument be now
removed and thrust into boiling water; the mercury expands, the
column rises, and finally attains a stationary height. Let this
point be marked; it is the boiling point of the thermometer. The
space between the freezing point and the boiling point has been
divided by Reaumur into 80 equal parts, by Fahrenheit into 180
equal parts, and by Celsius into 100 equal parts, called degrees. The
thermometer of Celsius is also called the Centigrade thermometer.
Both Reaumur and Celsius call the freezing point 0~, Fahrenheit calls it 32~, because he started from a zero which he incorrectly imagined was the greatest terrestrial cold. Fahrenheit's
boiling point is therefore 212~. Reaumur's boiling point is 80~,
while the boiling point of Celsius is 100~.
The length of the degrees being in the proportion of 80: 100:
180, or of 4: 5: 9; nothing can be easier than to convert one into
the other. If you want to convert Fahrenheit into Celsius, multiply by 5 and divide by 9; if Celsius into Fahrenheit, multiply
by 9 and divide by 5. Thus 200 of Celsius are equal to 360
Fahrenheit; but if we would know what temperature by Fahrenheit's thermometer corresponds to 20~ of Celsius, we must




CALORIC DOES NOT EXIST.                107
add 382 to the 36, which would make the temperature 20~, as
shown by Celsius, equal the temperature 68~, as shown by Fahrenheit.
EXTRACTS FROM SIR H. DArY'S FIRST SCIENTIFIC M1IEMOIR, BEARING THE TITLE'ON HEAT, LIGHT, AND THE COMBINATIONS OF
LIGHT.' *
THE peculiar modes of existence of bodies, solidity, fluidity,
and gazity, depend (according to the calorists) on the quantity
of the fluid of heat entering into their composition. This substance insinuating itself between their corpuscles, separating them
from each other, and preventing their actual contact, is by them
supposed to be the cause of repulsion.
Other philosophers, dissatisfied with the evidences produced in
favour of the existence of this fluid, and perceiving the generation of heat by friction and percussion, have supposed it to be the
motion. Considering the discovery of the true cause of the repulsive power as highly important to philosophy, I have endeavoured to investigate this part of chemical science by experiments;
from these experiments (of which I am now about to give a detail)
I conclude that heat or the power of repulsion is not matter.
The Phenomena of Repulsion are not dependent on a peculiar elastic
fluid for their existence, or Caloric does not exist.
Without considering the effects of the repulsive power on
bodies, or endeavouring to prove from these effects that it is motion, I shall attempt to demonstrate by experiments, that it is not
matter; and in doing this, I shall use the method called by
mathematicians, reductio ad absurdum.
First, let the increase of temperature produced by friction and
percussion be supposed to arise from a diminution of the capacities of the acting bodies. In this case it is evident some change
must be induced in the bodies by the action, which lessens their
capacities and increases their temperatures.
Experiment.-I procured two parallelopipedons of icet, of the
* Sir iumphry Davy's works, vol. ii.
t The result of this experiment is the same, if wax, tallow, resin, or




108             APPENDIX TO LECTURE III.
temperature of 29~, six inches long, two wide, and two-thirds of
an inch thick; they were fastened by wires to two bars of iron.
By a peculiar mechanism, their surfaces were placed in contact,
and kept in a continued and most violent friction for some minutes. They were almost entirely converted into water, which
water was collected, and its temperature ascertained to be 35~,
after remaining in an atmosphere of a lower temperature for some
minutes. The fusion took place only at the plane of contact of
the two pieces of ice, and no bodies were in friction but ice.
From this experiment it is evident that ice by friction is converted into water, and according to the supposition, its capacity
is diminished; but it is a well-known fact, that the capacity of
water for heat is much greater than that of ice; and ice must have
an absolute quantity of heat added to it, before it can be converted into water. Friction consequently does not diminish the capacities of bodies for heat.
From this experiment it is likewise evident, that the increase
of temperature consequent on friction cannot arise from the decomposition of the oxygen gas in contact, for ice has no attraction for oxygen. Since the increase of temperature consequent
on friction cannot arise from the diminution of capacity, or oxydation of the acting bodies, the only remaining supposition is,
that it arises from an absolute quantity of heat added to them,
which heat must be attracted from the bodies in contact. Then
friction must induce some change in bodies, enabling them to attract heat from the bodies in contact.
Experiment.-I procured a piece of clockwork, so constructed
as to be set at work in the exhausted receiver; one of the external
wheels of this machine came in contact with a thin metallic plate.
A considerable degree of sensible heat was produced by friction
between the wheel and plate when the machine worked, uninsulated from bodies capable of communicating heat. I next procured a small piece of ice; * round the superior edge of this a
any substance fusible at a low temperature, be used; even iron may be
fused by collision.
* The temperature of the ice and of the surrounding atmosphere at
the commencement of the experiment was 320, that of the machine was
likewise 330. At the end of the experiment the temperature of the coldest




FUSION OF ICE BY FRICTION.               109
small canal was made, and filled with water. The machine was
placed on the ice, but not in contact with the water. Thus disposed, the whole was placed under the receiver (which had been
previously filled with carbonic acid), a quantity of potash (i. e.
caustic vegetable alkali) being at the same time introduced.
The receiver was now exhausted. From the exhaustion and
from the attraction of the carbonic acid gas by the potash, a vacuum nearly perfect, was, I believe, made.
The machine was now set to work; the wax rapidly melted,
proving an increase of temperature.
Caloric then was collected by friction; which caloric, on the
supposition, was communicated by the bodies in contact with the
machine. In this experiment, ice was the only body in contact
with the machine. Had this ice given out caloric, the water on
the top of it must have been frozen. The water on the top of it
was not frozen, consequently the ice did not give out caloric.
The caloric could not come from the bodies in contact with the
ice, for it must have passed through the ice to penetrate the machine, and an addition of caloric to the ice would have converted
it into water.
Heat, when produced by friction, cannot be collected from the
bodies in contact, and it was proved, by the first experiment, that
the increase of temperature consequent on friction cannot arise
from diminution of capacity or oxydation. But if it be considered
as matter, it must be produced in one of these modes. Since (as
is demonstrated by these experiments) it is produced in neither
of these modes, it cannot be considered as matter. It has therefore been experimentally demonstrated that caloric, or the matter
of heat, does not exist.
Solids, by long and violent friction, become expanded, and if
of a higher temperature than our bodies, affect the sensory organs
with the peculiar sensation known by the common name of heat.
part of the machine was near 33~, that of the ice and surrounding atmosphere the same as at the commencement of the experiment; so that the
heat produced by the friction of the different parts of the machine was
sufficient to raise the temperature of near half a pound of metal at least
one degree; and to convert eighteen grains of wax (the quantity employed)
into a fluid.




110            SA-PENDIX  TO LECTURE III.
Since bodies become expanded by friction, it is evident that
their corpuscles must move or separate from each other.
Now a motion or vibration of the corpuscles of bodies must
be necessarily generated by friction and percussion. Therefore
we may reasonably conclude that this motion or vibration is heat,
or the repulsive power.
Heat, then, or that power which prevents the actual contact
of the corpuscles of bodies, and which is the cause of our peculiar
sensations of heat and cold, may be defined a peculiar motion,
probably a vibration of the corpuscles of bodies, tending to
separate them. It may with propriety be called the repulsive
motion.
Since there exists a repulsive motion, the particles of bodies
may be considered as acted on by two opposing forces; the approximating power, which may (for greater ease of expression) be
called attraction, and the repulsive motion. The first of these is
the compound effect of the attraction of cohesion, by which the
particles tend to come in contact with each other; the attraction
of gravitation, by which they tend to approximate to the great
contiguous masses of matter, and the pressure under which they
exist, dependent on the gravitation of the superincumbent bodies.
The second is the effect of a peculiar motory or vibratory impulse given to them, tending to remove them farther from each
other, and which can be generated, or rather increased, by friction
or percussion. The effects of the attraction of cohesion, the great
approximating cause, on the corpuscles of bodies, is exactly similar to that of the attraction of gravitation on the great masses of
matter composing the universe, and the repulsive force is analogous to the planetary projectile force.
In his' Chemical Philosophy,' pp. 94 and 95, Davy expresses
himself thus: —' By a moderate degree of friction, as it would
appear from Rumford's experiments, the same piece of metal may
be kept hot for any length of time; so that, if the heat be pressed
out, the quantity must be inexhaustible. When any body is
cooled, it occupies a smaller volume than before; it is evident,
therefore, that its parts must have approached each other; when
the body has expanded by heat, it is equally evident that its
parts must have separated from each other. The immediate cause
of the phenomenon of heat, then, is motion, and the laws of its




DAVY 1ON THE MOTION OF HEAT.                111
communication are precisely the same as the laws of the communication of motion.'
Since all matter may be made to fill a smaller space by cooling, it is evident that the particles of matter must have space between them; and since every body can communicate the power
of expansion to a body of a lower temperature-that is, can give
an expansive motion to its particles-it is a probable inference
that its own particles are possessed of motion; but as there is no
change in the position of its parts, as long as its temperature is
uniform, the motion, if it exist, mufst be a vibratory or undulatory
motion, or a motion of the particles round their axes, or a motion
of the particles round each other.
It seems possible to account for all the phenomena of heat, if
it be supposed that in solids the particles are in a constant state
of vibratory motion, the particles of the hottest bodies moving
with the greatest velocity, and through the greatest space; that
in fluids and elastic fluids, besides the vibratory motion, which
must be conceived greatest in the last, the particles have a motion
round their own axes with different velocity, the particles of
elastic fluids moving with the greatest quickness, and that in
ethereal substances the particles move round their own axes, and
separate from each other, penetrating in right lines through space.
Temperature may be conceived to depend upon the velocity of
the vibrations; increase of capacity in the motion being performed
in greater space; and the diminution of temperature during the
conversion of solids into fluids or gases, may be explained on the
idea of the loss of vibratory motion, in consequence of the revolution of particles round their axes, at the moment when the body
becomes fluid or a6riform, or from the loss of rapidity of vibration
in consequence of the motion of the particles through space.




LECTURE IV.
[February 13, 1862.]
THE TREVELYAN INSTRUMENT-GORE'S REVOLVING BALLS-INFLUENCE OF
PRESSURE ON FUSING POINT-LIQUEFACTION AND LAMINATION OF ICE BY
PRESSURE-DISSECTION OF ICE BY A CALORIFIC BEAM-LIQUID FLOWERS
AND THEIR CENTRAL SPOT-MECHANICAL PROPERTIES OF WATER PURGED
OF AIR-THE BOILING POINT OF LIQUIDS: INFLUENCING CIRCUMSTANCES
-THE GEYSERS OF ICELAND.
APPENDIX:-NOTE ON THE TREVELYAN INSTRUMENT-PHYSICAL PROPERTIES
OF ICE.
BEFORE finally quitting the Subject of expansion, I
wish to show you an experiment which illustrates in a
curious and agreeable way the conversion of heat into mechanical energy. The fact which I wish to reproduce was
first observed by a gentleman named Schwartz, in one of
the smelting works of Saxony. A quantity of silver which
had been fused in a ladle was allowed to solidify, and to
hasten its cooling it was turned out upon an anvil. Some
time afterwards, a strange buzzing sound was heard in the
locality, and was finally traced to the hot silver, which was
found quivering upon the anvil. Many years subsequent to
this, Mr. Arthur Trevelyan chanced to be using a hot soldering-iron, which he laid by accident against a piece of
lead. Soon afterwards, his attention was excited by a
most singular sound which, after some searching, was found
to proceed from the soldering-iron. Like the silver of




VIBRATIONS OF HEATED METALS.           113
Schwartz, the soldering-iron was found in a state of vibration. Mr. Trevelyan made his discovery the subject of a
very interesting investigation.  He determined the best
form to be given to the' rocker' as the vibrating mass is
now called, and throughout Europe at present this instrument is known as'Trevelyan's Instrument.'  Since that
time the subject has engaged. the attention of Prof. J. D.
Forbes, Dr. Seebeck, MIr. Faraday, M. Sondhaus, and myself; but to Trevelyan and Seebeck we owe most.
Here is such a rocker made of brass. Its length, A C
(fig. 26), is five inches, the width A B, /15 in., and the length
of the handle, which terminates in the knob F, is ten inches.
Fig. 26.
A groove runs at the back of the rocker, along its centre;
the cross section of the rocker and its groove is given at Mi.
I heat the rocker to a temperature somewhat higher than
that of boiling water, and lay it on this block of lead, allowing its knob                 Fin. 27.
to rest upon the
table. You hear
a quick succession of forcible  
taps. But you
cannot see the
oscillations of
the  rocker to
which the taps
are   due.   I
therefore place
on it this rod 
of brass, A B (fig. 27), with two balls of brass at its end,




114                 LECTURE IV.
the oscillations are thereby rendered much slower, and you
can easily follow with the eye the pendulous motion of the
rod and balls. This motion will continue as long as the
rocker is able to communicate sufficient heat to the carrier
on which it rests. Thus we render the vibrations slow, but
I can also render them quick by using a rocker with a
wider groove. The sides of this rocker donot overhang so
much as those of the last; it is virtually a shorter pendulum, and will vibrate more quickly. Placed upon the lead,
as before, it commences an unsteady and not altogether
pleasant music. It is still restless, sometimes seeming to
expostulate, sometimes even to objurgate, as if it disliked
the treatment to which it is subjected. Now it becomes
mellow, and fills the room with a clear full note. Its taps
have become periodic and regular, and have linked themselves together to produce music. Here is a third rocker,
with a still wider groove, and with it I can obtain a shriller
tone. You know of course that the pitch of note augments
with the number of the vibrations; this wide-grooved
rocker oscillates more quickly, and therefore emits a higher
note. By casting a beam of light upon the rocker I obtain
a better index than the rod and balls. This index is without weight, and therefore does not retard the motion of the
rocker. To the latter I have fastened, by a single screw
at its centre, a small disk of polished silver; on which the
beam of the electric lamp now falls, and is reflected against
the screen. When the rocker vibrates, the beam vibrates
also, but with twice the angular velocity, and there you see
the patch of llght quivering upon the screen'.
What is the cause of these singular vibrations and
tones? They are due simply to the sudden expansion by
heat of the body on which the rocker rests. Whenever the
hot rocker comes into contact with its lead carrier, a nipple
suddenly juts from the latter, being produced by the heat
communicated to the lead at the point of contact. The




THE TREVELYAN INSTRUMENT.             115
rocker is tilted up, and some other point of it comes into
contact with the lead, a fresh nipple is produced, and the
rocker is again tilted. Let A B (fig. 28) be the surface of
the lead, and a the cross section of the hot rocker; tilted
to the right, the nipple-is formed as at n; tilted to the left,
it is formed as at L. The consequence is that until its temFig. 28.
L                          R
perature falls sufficiently, the rocker is tossed to and fro,
and the quick succession of its taps against the lead produces a musical sound.
I have here fixed two pieces of sheet lead in a vice;
their edges are exposed, and are about half an inch asunder. I balance a long bar of heated brass across the two
lead edges. It rests first on one edge, which expands at
the point of contact and jerks it upwards; it then falls upon
the second edge which also rejects it; and thus it goes on
oscillating, and will continue to do so as long as the bar
Fig. 29.
can communicate sufficient heat to the lead. This fire-shovel
will answer quite as well as the prepared bar. I balance




116                 LECTURE IV.
the heated shovel thus upon the edges of the lead, and it
oscillates exactly as the bar did (fig. 29). I may add, that
by properly laying either the poker or the fire-shovel upon
a block of lead, supporting the handle so as to avoid friction, you may obtain notes as sweet and musical as any
which you have heard to-day. A heated hoop placed
upon a plate of lead may be caused to vibrate and sing;
and a hot penny-piece or half crown may be caused to
do the same.*
Looked at with an eye to the connection of natural
forces, this experiment is interesting. The atoms of bodies
must be regarded as all but infinitely small, but then they
must be regarded as all but infinitely numerous. The augmentation of the amplitude of any oscillating atom by the
communication of heat, is insensible, but the summation
of an almost infinite number of such augmentations become
sensible. Such a summation, effected almost in an instant, produces our nipple, and tilts the heavy mass of the
rocker. Here we have a direct conversion of heat into
common mechanical motion. But the tilted rocker falls
again by gravity, and in its collision with the block restores
almost the precise amount of heat which was consumed in
lifting it. Here we have the direct conversion of common
gravitating force into heat. Again the rocker is surrounded
by a medium capable of being set in motion. The air of
this room weighs some tons, and every particle of it is
shaken by the rocker, and every tympanic membrane, and
every auditory nerve present, is similarly shaken. Thus we
have the conversion of a portion of heat into sound. And,
finally, every sonorous vibration which speeds through the
air of this room, and wastes itself upon the walls, seats,
and cushions, is converted into the form with which the
cycle of actions commenced-namely, into heat.
* For further information see Appendix to this lecture.




ROTATION BY ELECTRICITY.                11I
Here is another curious effect, for which we are indebted to Mr. George Gore, and which admits of a similar explanation. You see this line of rails. Two strips of brass,
s s, s' si (fig. 30), are set edgeways, and about an inch
asunder. I place this hollow metal ball B upon the rails;
if I push it, it rolls along them; but if I do not push
it, it stands still. I connect these two rails, by the wires
Fig. 30.
ili~11il lllllr llllll'l li lllll t lll ll i ll  Illlllllllllll llllllltlilllllll ll' l  It[  Illlll  llit lllilll`tll~tl~ll l llillllil  ",
w w', with the two poles of a Voltaic battery. A current
now passes down one rail to the metal ball, thence along
the ball to the other rail, and finally back to the battery.
In passing from the rail to the ball, and from the ball to
the other rail, the current encounters resistance, and whereever a current encounters resistance, heat is developed.
Heat, therefore, is generated at the two points of contact
of the ball with the rails; and this heat produces an elevation of the rail at these points. Observe the effect; the
ball which a moment ago was tranquil is now very uneasy.
It vibrates a little at first without rolling; now it actually
rolls a little way, stops, and rolls back again. It gradually
augments its excursion, now it has gone further than I
intended: it has quite rolled off the rails, and injured itself
by falling on the floor.
Here is another apparatus for which I am indebted to
Mr. Gore himself, and in which the rails form a pair of
concentric hoops; when the circuit is established, the ball
F (fig. 31) rolls round the circle.*  Mr. Gore has also obtained the rotation of light balls, by placing them on cir* Phil. Mag., vol. 15, p. 521.




118                 LECTURE m.
cular rails of hot copper, the rolling force in this case
being the same as the rocking force in the Trevelyan in.
strument.
In my last lecture I made evident to you the expansion
of water when it passes from the liquid to the solid condition; with most other substances solidification is accomFig. 31.
panied by contraction. I have here a round glass dish into
which I pour some hot water. Over the water I pour from
a ladle a quantity of melted bees'-wax. The wax now
forms a liquid layer nearly half an inch thick above the
water. We will suffer both water and wax to cool, and
when they are cool you will find that the wax which now
overspreads the entire surface, and is attached all round to
the glass, will retreat, and we shall finally obtain a cake of
wax of considerably smaller area than the dish.
The wax, then, in passing from the solid to the liquid
state expands. To assume the liquid form, its particles
must be pushed more widely apart, a certain play between
the particles being necessary to the condition of liquidity.
Now supposing we resist the expansion of the wax by an
external mechanical force; suppose we have a very strong
vessel completely filled with solid wax, and which offers a
powerful resistance to the expansion of the mass within it;




INFLUENCE OF PRESSURE ON FUSING POINT.  119
what would you expect if you sought to liquefy the wax in
this vessel? When the wax is free, the heat has only to
conquer the attraction of its own particles, but in the strong
vessel it has not only to conquer the attraction of the particles, but also the resistance offered by the vessel. By a mere
process of reasoning, we should thus be led to infer that a
greater amount of heat would be required to melt the wax
under pressure, than when it is free; or, in other words,
that the point of fusion of the wax is elevated by pressure.
This reasoning is completely justified.by experiment, not
only with wax, but with other substances which contract on
solidifying, and expand on liquefying.  Messrs. Hopkins
and Fairbairn have, by pressure, raised the melting point
of some substances which contract considerably on solidifying as much as 20~ and 30~ Fahr.
These experiments bear on a very remarkable speculation. The earth is known gradually to augment in temperature as we pierce it deeper, and the depth has been calculated at which all known terrestrial bodies would be in a
state of fusion. Mr. Hopkins, however, observes that
owing to the enormous pressure of the superincumbent
layers, the deeper strata would require a far higher temperature to fuse them, than would be necessary to fuse the
strata near the earth's surface. Hence he infers that the
solid crust must have a considerably greater thickness than
that given by a calculation, which assumed the fusing
points of the superficial and the deeper strata to be the
same.
Now let us turn from wax to ice. Ice, on liquefying,
contracts; in the arrangement of its atoms to form a solid,
more room is required than they need in the neighbouring
liquid state. No doubt this is clue to crystalline arrangement; the attracting poles of the molecules are so placed
that when the crystallising force comes into play, the molecules unite so as to leave larger inter-atomic spaces in the




120                  LECTURE IV.
mass. We may suppose them to attach themselves by their
corners; and in turning corner to-corner, to cause a recession of the atomic centres. At all events their centres retreat from each other when solidification sets in. By cooling, then, this power of retreat, and of consequent enlargement of volume, is conferred. It is evident that pressure
in this case would resist the expansion which is necessary
to solidification, and hence the tendency of pressure, in the
case of water, is to keep it liquid. Thus reasoning, we
should be led to the conclusion that the fusing points of
substances which expand on solidifying are lowered by
pressure.
Professor James Thomson first drew attention to this
fact, and his theoretic reasonings have been verified by the
experiments of his brother Professor William Thomson.
Let us illustrate these principles by a striking experiment. I have here a square pillar of clear ice an inch and
a half in height and about a square inch in cross section.
At present the temperature of this ice is 0~ C. But suppose I subject this ice to pressure, I lower its point of fusion: the' ice under pressure will melt at a temperature
under 0~ C., and hence the temperature which it now possesses is in excess of that at which it will melt under pressure. I have cut this ice so that its planes of freezing are
perpendicular to the height of the pillar. The direction of
the stratified air-bubbles in the ice from which this clear
piece was taken, enabled me to fix at once upon its planes
of freezing. Well, I place the column of ice, L, upright
between two slabs of boxwood,  B' (fig. 32), and place the
whole between the plates of this small hydraulic press;
through the ice I send a beam from the electric lamp. In
front of the ice I place a lens, and by it project a magnified
image of the ice upon the screen before you. The beam
which passes through the ice has been purified beforehand,
so that, although it is still hot, its heat is not of such a




LIQUEFACTION OF ICE BY PRESSURE.         121
quality as can melt the ice; hence the light passes through
the substance without causing fusion. I work the arm of
the press; the pillar of ice is now gently squeezed between
the two slabs of boxwood. I apply the pressure cautiously,
and now you see dark streaks beginning to show themselves across the ice, at right angles to the direction of
pressure. Right in the middle of the mass they are apFig. 32.
pearing; and as I continue the pressure, the old streaks
expand and new ones appear. The entire column is now
scarred across by these strie.'What are they?  They are
simply liquid layers foreshortened, and when you examine
this column and look into it obliquely, you see these surfaces. We have liquefied the ice in planes perpendicular to
the pressure, and these liquid planes interspersed throughout the mass give it this strongly pronounced laminated
appearance.*
Whether as a solid, a liquid, or a gas, water is one of
See Appendix to this lecture for further information.




122                  LECTURE IV.
the most wonderful substances in nature. Let us consider
its wonders a little further. At all temperatures above 32~
Fahr. or 0~ C., the motion of heat is sufficient to keep the
molecules of water from rigid union. But at 0~ C. the motion becomes so reduced that the atoms then seize upon
each other and aggregate to a solid. This union, however,
is a union according to law. To many persons here present
this block of ice may seem of no more interest and beauty
than a block of glass; but in the estimation of science it
bears the same relation to glass, that an oratorio of Handel
does to the cries of a market-place. The ice is music, the
glass is noise; the ice is order, the glass is confusion. In
the glass, molecular forces constitute an inextricably entangled skein; in the ice they are woven to a symmetric web;
the miraculous texture of which I will now try to reveal.
How shall I dissect this ice? In the solar beam,-or,
failing that, in the beam of an electric lamp, we have an
anatomist competent to perform this work. I remove the
agent by which this beam was purified in the last experiment, and will send the rays direct from the lamp through
this slab of pellucid ice. It shall pull the crystal edifice to
pieces by accurately reversing the order of its architecture.
Silently and symmetrically the crystallizing force builds the
atoms up, silently and symmetrically the electric beam will
take them down. I place this slab of ice in front of the
lamp, the jlight of which now passes through the ice. Compare the beam before it enters with the beam after its passage through the substance: to the eye there is no sensible
difference; the light is scarcely diminished. Not so with
the heat. As a thermic agent, the beam, before entering,
is far more powerful than it is after its emergence. A portion of the beam has been arrested in the ice, and that portion is our working anatomist. Well, what is he doing?
I place a lens in front of the ice, and cast a magnified image




DISSECTION OF ICE.              123
of the slab upon the screen. Observe that image (fig. 33),
which, in beauty, falls far short of the actual effect. Here
we have a star and there a star; and as the action continues, the ice appears to resolve itself into stars, each one
possessing six rays, each one resembling a beautiful flower
of six petals. And as I shift my lens to and fro, I bring
new stars into view, and as the action continues, the edges
of the petals become serrated, and spread themselves out
like fern leaves upon the screen. Probably few here present were aware of the beauty latent in a block of common
ice.  And only think of lavish Nature operating thus.
throughout the world. Every atom of the solid ice which
sheets the frozen lakes of the North has been fixed according to this law. Nature'lays her beams in music,' and it
is the function of science to purify our organs, so as to
enable us to hear the strain.
And now I have to draw your attention to two points
connected with this experiment, of great minuteness, but
of great interest. You see these flowers by transmitted
light-by the light which has passed through both the
flowers and the ice. But when you examine them, by allowing a beam to fall upon them and to be reflected from
them to your eye, you find in the centre of each flower a
spot which shines with the lustre of burnished silver. You
might be disposed to think this spot a bubble of air; but
you can, by immersing it in hot water, melt away the ice
all round the spot; and the moment the spot is thus laid
bare, it collapses, and no trace of a bubble of air is to be
seen. The spot is a vacuum. Observe how truly Nature
works; observe how rigidly she carries her laws into all
her operations. We learned in the last lecture, that ice in
melting contracted, and here we find the fact turning up.
The water of these flowers cannot fill the space occupied
by the ice by whose fusion they are produced, hence the




124               LECTURE IV.
Fig. 33.;an.=si (7   F";'5       X:w~~~~~~~~~~~~~~~~~~~t~'




LIQUID FLOWERS AND CENTRAL SPOT.        125
production of a vacuum necessarily accompanies the formation of every liquid flower.
When I first observed these beautiful figures, I thought
at the moment when the central spot appeared, like a point
of light suddenly formed within the ice, that I heard a
clink, as if the ice had split asunder when the bright spot
was formed. At first I suspected that it was my imagination which associated sound with the appearance of the
spot, as it is said that people who see-meteors often imagine a rushing noise when they really hear none. The
clink, however, was a reality; and if you will allow me, I
will now make this trivial fact a starting point from which
I will conduct you through a series of interesting phenomena, to a far-distant question of practical science.
All water holds a large quantity of air within it in a
state of solution; by boiling you may liberate this imprisoned air. On heating a flask of water you see air bubbles
crowding on its sides long before it boils, and you see the
bubbles rising through the liquid without condensation,
and often floating on the top. One of the most remarkable
effects of this air in the water is, that it promotes the ebullition of the liquid. It acts as a kind of elastic spring,
pushing the atoms of the water apart, and thus helping
them to take the gaseous form.
Now suppose this air removed; having lost the cushion
which separated them, the atoms lock themselves together
in a far tighter embrace. The cohesion of the water is
vastly augmented by the removal of the air. Here is a
glass vessel, the so-called water hammer, which contains
water purged of air. One effect of the withdrawal of the
elastic buffer is, that the water here falls with the sound
of a solid body. You hear how the liquid rings against
the end of the tube when I turn it upside down. Here is
another tube, A B C (fig. 34), bent into the form of a V, and
intended to show how the cohesion of the water is affected




126                   LECTURE IV.
by long boiling. I bring this water into one arm of the
V; by tilting the tube it flows, as you, see, freely into the
other arm. I restore it to the first arm, and now tap the
end of this arm against the table. You hear, at first, a
loose and jingling sound. As long as you hear it the water is not in true contact with the surface of the tube. I
Fig. 34.
continue my tapping: you mark an alteration in the sound;
the jingling has disappeared, and the sound is now hard,
like that of solid against solid. I now raise my tube. Observe what occurs. I turn the column of water upside
down, but there it stands in A B. Its particles cling so tenaciously to the sides of the tube, and lock themselves so
firmly together, that it refuses to behave like a liquid
body; it declines to obey the law of gravity.
So much for the augmentation of cohesion; but this
very cohesion enables the liquid to resist ebullition. Water thus freed of its air can be raised to a temperature 100~




DEPORTMENT OF WATER PURGED OF AIR.    127
and more above its ordinary boiling point, without ebullition. But mark what takes place when the liquid does
boil. It has an enormous excess of heat stored up; the
locked atoms finally part company, but they do so with the
violence of a spring which suddenly breaks under strong
tension, and ebullition is converted into explosion. For
the discovery of this interesting property of water we are
indebted to N. Donny, of Ghent.
Turn we now to our ice: —Water, in freezing, completely excludes the air from its crystalline architecture. All
foreign bodies are squeezed out in the act of freezing, and
ice holds no air in solution. Supposing then that we melt
a piece of pure ice under conditions where air cannot approach it, we have water in its most highly cohesive condition; and such water ought, if heated, to show the effects to
which I have referred. That it does so has been proved
by Mr. Faraday. He melted pure ice under spirit of turpentine, and found that the liquid thus formed could be
heated far beyond its boiling point, and that the rupture
of the liquid, by the act of ebullition, took place with almost explosive violence. And now, let us apply these facts
to the six-petaled ice-flowers and their little central star.
They are formed in a place where no air can come. Imagine the flower forming and gradually augmenting in size.
The cohesion of the liquid is so great, that it will pull the
walls of its chamber together, or even expand its own volume, sooner than give way. But as its size augments, the
space which it tries to occupy becomes too large for it,
until finally the liquid snaps, a vacuum is formed, and a
clink is heard.
Let us now take our final glance at this web of relations. It is very remarkable that a great number of locomotives have exploded on quitting the shed where they
had remained for a time quiescent. The number of explosions which have occurred just as the engineer turned on




128                  LECTURE IV.
the steam is quite surprising. Now supposing that a locomotive had been boiling sufficiently long to expel the air
contained in its water; that liquid would possess, in a
greater or less degree, the high cohesive quality to which I
have drawn your attention. It is at least conceivable that
while resting previous to starting on its journey, an excess
of heat might be thus stored up in the boiler, and if stored
up, the certain result would be, that the engineer on turning on the steam would, by a mechanical act, produce the
rupture of the cohesion, and steam of explosive force would
instantly be generated. I do not say that this is the case;
but who can say that it is not the case. We have been
dealing throughout with a real agency, which is certainly
competent, if its power be invoked, to produce the most
terrible effects.
We have here touched on the subject of steam; let us
bestow a few minutes' further consideration on its formation and action. As you add heat, or in other words, motion, to water, the particles from its free surface fly off in
augmented numbers. We at length approach what is called
the boiling point of the liquid, where the conversion into
vapour is not confined to the free surface, but is most copious at the bottom of the vessel to which the heat is applied. When water boils in a glass beaker, the steam is
seen rising in spheres from the bottom to the top, where it
often swims for a time, enclosed above by a dome-shaped
liquid film. Now, to produce these bubbles, certain resistances must be overcome. First, we have the adhesion of
the water to the vessel which contains it, and this force
varies with the substance of the vessel. In the case of a
glass vessel, for example, the boiling point may be raised
two or three degrees by adhesion; while in metal vessels
this is impossible. The adhesion is overcome by fits and
starts, which may be so augmented by the introduction of
salts into the liquid, that a loud bumping sound accompa



BOILING POINTS OF LIQUIDS.          129
nies the ebullition; the detachment is in some cases so sudden and violent as to cause the liquid to jump bodily out
of the vessel.
A second antagonism to the boiling of the liquid is the
attraction of the liquid particles for each other, a force
which, as we have seen, may become very powerful when
the liquid is purged of air. This is not only true of water,
but of other liquids-of all the ethers and alcohols, for
example. If we connect a small flask containing ether or
alcohol with an air pump, a violent ebullition occurs in the
liquid when the pump is first worked; but after all the air
has been removed, we may, in many cases, continue to work
the pump, without producing any sensible ebullition; the
free surface alone of the liquid yielding vapour.
But that steam should exist in bubbles, in the interior
of a mass of liquid, it must be able to resist two other
things, the weight of the water above it, and the weight
of the atmosphere above the water. What the atmosphere
is competent to do may be thus illustrated. I have here a
tin vessel containing a little water, which is kept boiling by
this small lamp. At the present moment all the space above
the water is filled with steam, which issues from this stopcock. I shut off the cock, withdraw the lamp, and pour
cold water upon the tin vessel. The steam within it is condensed, the elastic cushion which pushed the sides outwards
in opposition to the pressure of the atmosphere is withdrawn,
and observe the consequence. The sides of the vessel are
crushed and crumpled up by the atmospheric pressure.
This pressure amounts to 15 lb. on every square inch: how
then, can a thing so frail as a bubble of steam exist on the
surface of boiling water? simply because the elastic force
of the steam within is exactly equal to that of the atmosphere without; the liquid film is pressed between two elastic cushions which exactly neutralize each other. If the
steam were predominant, the bubble would burst from
6*




130                 LECTURE IV.
within outwards; if the air were predominant, the bubble
would be crushed inwards. Here, then, we have the true
definition of the boiling point of a liquid. It is that temperature at which the tension of its vapour exactly balances
the pressure of the atmosphere.
As we ascend a mountain the pressure of the atmosphere above us diminishes, and the boiling point is correspondingly lowered. On an August morning in 1859 I
found the temperature of boiling water on the summit of
Mont Blanc to be 184'95~ Fahr.; that is, about 27~ lower
than the boiling point at the sea level. On August 3, 1858,
the temperature of boiling water on the summit of the
Finsteraarhorn was 187~ Fahr. On August 10, 1858, the
boiling point on the summit of Monte Rosa was 184'92~
Fahr. The boiling point on:Monte Rosa is shown by these
observations to be almost the same as it was found to be
on Mont Blanc, though the latter exceeds the former in
height by 500 feet. The fluctuations of the barometer are
however quite sufficient to account for this anomaly. The
lowering of the boiling point is about 1~ Fahr. for every
590 feet that we ascend; and from the temperature at
which water boils we may approximately infer the elevation. It is said that to make good tea in London, boiling
water is essential; if this be so it is evident that the beverage cannot be procured, in all its excellence, at the higher
stations in the Alps.
Let us now make an experiment to illustrate the dependence of the boiling point on external pressure. Here
is a flask, r (fig. 35), containing water; here is another and
a much larger one, G, from which I have had the air removed by an air pump. The two flasks are connected together by a system of cocks, which enables me to establish
a communication between them. The water in the small
flask has been kept boiling for some time, the steam generated escaping from the cock y. I now remove the spirit




INTFLUENCE OF PRESSURE ON BOILING POINT.  131
lamp and turn this cock so as to shut o'elt the air. The
water ceases to boil, and pure steam now fills the flask
above it. Give the water time to cool a little. At intervals you see a bubble of steam rising, because the pressure
of the vapour above is gradually becoming less through its
slow condensation. I hasten the condensation by pouring
cold water on the flask, the bubbles are more copiously
generated. By plunging the flask bodily into cold water
we might cause it to boil violently. The water is now at
Fig. 35.
rest and some degrees below its ordinary boiling point. I
turn this cock c, which opens a way for the escape of the




132                 LECTURE IV.
vapour into the exhausted vessel G; the moment the pressure is diminished ebullition sets in in F; and observe how
the condensed steam showers in a kind of rain against the
sides of the exhausted vessel. By intentionally promoting
this condensation, and thereby preventing the vapour in the
large flask from reacting upon the surface of the water, we
can keep the small flask bubbling and boiling for a considerable length of time.
By high heating, the elastic force of steam becomes
enormous. The Marquis of Worcester burst cannon with
it, and our calamitous boiler explosions are so many illustrations of its power. By the skill of man this mighty
agent has been controlled: by it Denis Papin raised a piston, which was pressed down again by the atmosphere,
when the steam was condensed; Savery and Newcomen
turned it to practical account, and James Watt completed
the grand application of the moving power of heat. Pushing the piston up by steam, while the space above the piston is in communication with a condenser or with the free
air, and again pushing down the piston, while the space
below it is in communication with a condenser or with
the air, we obtain a simple to and fro motion, which, by
mechanical arrangements, may be made to take any form
we please.
But the grand principle of the conservation of force is
illustrated here as elsewhere. For every stroke of work
done by the steam-engine, for every pound that it lifts, and
for every wheel that it sets in motion, an equivalent of heat
disappears. A ton of coal furnishes by its combustion a
certain definite amount of heat. Let this quantity of coal
be applied to work a steam-engine; and let all the heat
communicated to the machine and the condenser, and all
the heat lost by radiation and by contact with the air be
collected; it would fall short of the amount produced by
the simple combustion of the ton of coal, and it would fall




HEAT AND WORK IN THE STEAM ENGINE.    133
short of it by an amount exactly equivalent to the quantity
of work performed. Suppose that work to consist in lifting a weight of 7,720 lbs. a foot high; the heat produced
by the coal would fall short of its maximum, by a quantity
just sufficient to warm a pound of water 10~.
But my object in these lectures is to deal with nature
rather than art, and the limits of our time compel me
to pass quickly over the triumphs of man's skill in the
application of steam to the purposes of life. Those who
have walked through the workshops of Woolwich, or
through any of our great factories where machinery is extensively employed, will have been sufficiently impressed
with the aid which this great power renders to man. And
be it remembered, every wheel which revolves, every chisel, and plane, and saw, and punch, which forces its way
through solid iron as if it were so much cheese, derives its
moving energy from the clashing atoms in the furnace.
The motion of these atoms is communicated to the boiler,
thence to the water, whose particles are shaken asunder,
and fly from each other with a repellent energy commensurate with the heat communicated. The steam is simply
the apparatus through the intermediation of which the
atomic motion is converted into the mechanical. And the
motion thus generated can reproduce its parent. Look at
the planing tools; look at the boring instruments-streams
of water gush over them to keep them cool. Take up the
curled iron shavings which the planing tool has pared off:
you cannot hold them in your hand they are so hot. Here
the moving force is restored to its first form; the energy
of the machine has been consumed in reproducing the
power from which that energy was derived.
I must now direct your attention to a natural steamengine which long held a place among the wonders of the
world. I allude to the Great Geyser of Iceland. The surface of Iceland gradually slopes from the coast towards the




134                  LECTURE IV.
centre, where the general level is about 2,000 feet above
the sea. On this, as a pedestal, are planted the Jskull or
icy mountains, which extend both ways in a north-easterly
direction. Along this chain occur the active volcanoes of
the island, and the thermal springs follow the same general
direction. From the ridges and chasms which diverge from
the mountains enormous masses of steam issue at intervals
hissing and roaring; and when the escape bccurs at the
mouth of a cavern, the resonance of the cave often raises
the sound to the loudness of thunder. Lower down in the
more porous strata we have smoking mud pools, where a
repulsive blue-black aluminous paste is boiled, rising at
times in hugh bubbles, which, on bursting, scatter their
slimy spray to a height of fifteen or twenty feet. From
the bases of the hills upwards extend the glaciers, and
above these are the snow-fields which crown the summits.
From the arches and fissures of the glaciers vast masses
of water issue, falling at times in cascades over walls of
ice, and spreading for miles over the country before they
find definite outlet. Extensive morasses are thus formed,
which add their comfortless monotony to the dismal scene
already before the traveller's eye. Intercepted by the
cracks and fissures of the land, a portion of this water finds
its way to the heated rocks underneath; and here, meeting
with the volcanic gases which traverse these underground
regions, both travel together, to issue, at the first conve
nient opportunity, either as an eruption of steam or a boiling spring.
The most famous of these springs is the Great Geyser.
It consists of a tube 74 feet deep and 10 feet in diameter.
The tube is surmounted by a basin, which measures from
north to south 52 feet across, and from east to west 60
feet. The interior of the tube and basin is coated with a
beautiful smooth siliceous plaster, so hard as to resist
the blows of a hammer, and the first question is, how was




THE GREAT GEYSER OF ICELAND.         135
this wonderful tube constructed-how was this perfect
plaster laid on? Chemical analysis shows that the water
holds silica in solution, and the conjecture might therefore
arise that the water had deposited the silica against the
sides of the tube and basin. But this is not the case: the
water deposits no sediment; no matter how long it may be
kept, no solid substance is separated from it. It may be
bottled up and preserved for years as clear as crystal, without showing the slightest tendency to form a precipitate.
To answer the question in this way would moreover assume
that the shaft was formed by some foreign agency, and
that the water merely lined it. The geyser basin, however,
rests upon the summit of a mound about 40 feet high, and
it is evident, from mere inspection, that the mound has
been deposited by the geyser. But in building up this
mound the spring must have formed the tube which perforates the mound, and hence the conclusion that the geyser is the architect of its own tube.
If we place a quantity of the geyser water in an evaporating basin the following takes place: In the centre of the
basin the liquid deposits nothing, but at the sides, where
it is drawn up by capillary attraction, and thus subjected
to speedy evaporation, we find silica deposited. Round
the edge a ring of silica is laid on, and not until the evaporation has continued a considerable time do we find the
slightest turbidity in the middle of the water. This experiment is the microscopic representant of what occurs in
Iceland. Imagine the case of a simple thermal siliceous
spring, whose waters trickle down a gentle incline; the
water thus exposed evaporates speedily, and silica is deposited. This deposit gradually elevates the side over
which the water passes until finally the latter has to take
another course. The same takes place here, the ground is
elevated as before and the spring has to move forward.
Thus it is compelled to travel round and round, discharg



136                 LEOTUTRE IV.
ing its silica and deepening the shaft in which it dwells,
until finally, in the course of ages, the simple spring has
produced that wonderful apparatus which has so long puzzled and astonished both the traveller and the philosopher.
Previous to an eruption, both the tube and basin are
filled with hot water; detonations which shake the ground,
are heard at intervals, and each is succeeded by a violent
agitation of the water in the basin. The water in the pipe
is lifted up so as to form an eminence in the middle of the
basin, and an overflow is the consequence. These detonations are evidently due to the production of steam in the
ducts which feed the geyser tube, which steam escaping
into the cooler water of the tube is there suddenly condensed, and produces the explosions. Professor Bunsen
succeeded in determining the temperature of the geyser
tube, from top to bottom, a few minutes before a great
eruption; and these observations revealed the extraordinary fact, that at no part of the tube did the water reach
its boiling point. In the annexed sketch (fig. 36) I have
given, on one side, the temperatures actually observed, and
on the other side the temperatures at which water would
boil, taking into account both the pressure of the atmosphere and the pressure of the superincumbent column of
water. The nearest approach to the boiling point is at A,
a height of 30 feet from the bottom; but even here the
water is 2~ Centigrade, or more than 32~ Fahr. below the
temperature at which it could boil. How then is it possible that an eruption could occur under such circumstances?
Fix your attention upon the water at the point A;
where the temperature is within 2~ C. of the boiling point.
Call to mind the lifting of the column when the detonations are heard. Let us suppose that by the entrance of
steam from the ducts near the bottom of the tube, the
geyser column is elevated 6 feet, a height quite within the




THEORY OF THE GEYSER.                  137
limits of actual observation; the water at A is thereby
transferred to B. Its boiling point at A is 123. 8, and its
actual temperature 121'8~; but at B its boiling point is
only 120'8~, hence, when transferred from A to B the heat
which it possesses is in excess of that necessary to make it
boil. This excess of heat is instantly applied to the generation of steam: the column is thus lifted higher, and the
Fig. 86.
OESERVED     10 F'EET-    BOILING
TEMPERATURES      1     TEMPERATURES
r~r
iloo-  m   -116f
120.8
218 _   12.S
124      18
136
water below is further relieved. More steam is generated;
from the middle downwards the mass suddenly bursts into




138               LECTURE IV.
Fig 37.;      -..1\
-J   6




GEYSER OPERATIONS IN ICELAND.            139
ebullition, the water above, mixed with steam clouds, is
projected into the atmosphere, and we have the geyser
eruption in all its grandeur.
By its contact with the air the water is cooled, falls
back into the basin, partially refills the tube, in which it
gradually rises, and finally fills the basin as before. Detonations are heard at intervals, and risings of the water in
the basin. These are so many futile attempts at an eruption, for not until the water in the tube comes sufficiently
near its boiling temperature, to make the-lifting of the column effective, can we have a true eruption.
To Bunsen we owe this beautiful theory, and now let
us try to justify it by experiment. Here is a tube of galvanized iron, 6 feet long, A B (fig. 37), and surmounted by
this basin c D. It is heated by a fire
underneath; and to imitate as far as        Fig. 38.
possible the condition of the geyser,
I have encircled the tube by a second
fire F, at a height of 2 feet from the
bottom.  Doubtless the high  temperature of the water at the corresponding part of the geyser tube is
due to a local action of the heated
rocks.  I fill the tube with water,
which gradually becomes heated; and
regularly, every five minutes, the water is ejected from the tube into the
atmosphere.
But there is another famous spring
in Iceland, called the Strokkur, which
is usually forced to explode by stopping its mouth with clods. We can
imitate the action of this spring by
stopping the mouth of our tube A B with a cork. I do
so: and now the heating progresses.  The steam  below




140                  LECTURE TV.
will finally attain sufficient tension to eject the cork, and
the water, suddenly relieved from  the pressure, will
burst forth in the atmosphere.  There it goes!  The
ceiling of this room is nearly 30 feet from the floor, but
the eruption has reached the ceiling, from  which the
water now drips plentifully. In fig. 38, I have given a
section of the Strokkur.
By stopping the tube with corks, through which tubes
of various lengths and widths pass, the action of many of
the other eruptive springs may be accurately imitated.
Here, for example, I have an intermittent action; discharges of water and impetuous steam gushes follow each
other in quick succession, the water being squirted in jets
15 or 20 feet high. Thus, it is proved experimentally,
that the geyser tube itself is the sufficient cause of the
eruptions, and we are relieved from the necessity of imagining underground caverns filled with water and steam,
which were formerly regarded as necessary to the production of these wonderful phenomena.
A moment's reflection will suggest to us that there
must be a limit to the operations of the geyser. When
the tube has reached such an altitude that the water in the
depths below, owing to the increased pressure, cannot attain its boiling point, the eruptions of necessity cease. The
spring, however, continues to deposit its silica, and often
forms a Laug or cistern. Some of those in Iceland are 40
feet deep. Their beauty, according to Bunsen, is indescribable; over the surface curls a light vapour, the water
is of the purest azure, and tints with its lovely hue the fantastic incrustations on the cistern walls; while, at the bottom, is often seen the mouth of the once mighty geyser.
There are in Iceland vast, but now extinct, geyser operations. Mounds are observed whose shafts are filled with
rubbish, the water having forced a passage underneath and
retired to other scenes of action. We have in fact the




GEYSER OPERATIONS IN ICELAND.        141
geyser in its youth, manhood, old age, and death, here presented to us. In its youth, as a simple thermal spring; in
its manhood, as the eruptive column; in its old age, as the
tranquil Laug; while its death is recorded by the ruined
shaft and mound which testify the fact of its once active
existence.




APPENDIX TO LECTURE IV.
ABSTRACT OF A LECTURE ON THE VIBRATION AND TONES PRO.
DUCED BY THE CONTACT OF BODIES OF DIFFERENT TEMPERATURES.
LGiven at the Royal Institution on Friday, January 27, 1854.]
IN the year 1805, M. Schwartz, an inspector of one of the
smelting works in Saxony, placed a cup-shaped mass of hot silver
upon a cold anvil, and was surprised to find that musical tones
proceeded from the mass. In the autumn of the same year, Professor Gilbert of Berlin visited the smelting works and repeated
the experiment. He observed, that the sounds were accompanied
by a quivering of the hot silver, and that when the vibrations
ceased, the sound ceased also. Professor Gilbert merely stated the
facts, and made no attempt to explain them.
In the year 1829, Mr. Arthur Trevelyan, being engaged in
spreading pitch with a hot plastering iron, and once observing
that the iron was too hot for his purpose, he laid it slantingly
against a block of lead which chanced to be at hand; a shrill
note, which he compared to that of the chanter of the small
Northumberland pipes, proceeded from the mass, and, on nearer
inspection, he observed that the heated iron was in a state of vibration. He was induced by Dr. Reid of Edinburgh to pursue
the subject, and the results of his numerous experiments were
subsequently printed in the Transactions of the Royal Society of
Edinburgh.
On April 1, 1831, these singular sounds and vibrations formed
the subject of a Friday evening discourse by Professor Faraday at
the Royal Institution. Professor Faraday expanded and further
established the explanation of the sounds given by Mr. Trevelyan




GENERAL LAWS OF PROFESSOR FORBES.            143
and Sir John Leslie. He referred them to the tapping of the hot
mass against the cold one underneath it, the taps being in many
cases sufficiently quick to produce a high musical note. The alternate expansion and contraction of the cold mass at the points
where the hot rocker descends upon it, he regarded as the sustaining power of the vibrations. The superiority of lead he ascribed to its great expansibility, combined with its feeble power
of conduction, which latter prevented the heat from being quickly diffused through the mass.
Professor J. D. Forbes of Edinburgh was present at this lecture, and not feeling satisfied with the explanation, undertook the
farther examination of the subject; his results are described in
a highly ingenious paper communicated to the Royal Society of
Edinburgh in 1833.  He rejects the explanation supported by
Professor Faraday, and refers the vibrations to' a new species of
mechanical agency in heat'-a repulsion exercised by the heat
itself on passing from a good conductor to a bad one. This conclusion is based upon a number of general laws established by
Professor Forbes. If these laws be correct, then indeed a great
step has been taken towards a knowledge of the intimate nature
of heat itself, and this consideration was the lecturer's principal
stimulus in resuming the examination of the subject.
He had already made some experiments, ignorant that the subject had been farther treated by Seebeck, until informed of the
fact by Professor Magnus of Berlin. On reading Seebeck's interesting paper, he found that many of the results which it was his
intention to seek had been already obtained. The portion of the
subject which remained untouched was, however, of sufficient interest to induce him to prosecute his original intention.
The general laws of Professor Forbes were submitted in succession to an experimental examination. The first of these laws
affirms fhat' the vibrations never take place between substances of the
same nature.' This the lecturer found to be generally the case when
the hot rocker rested upon a block, or on the edge of a thick plate
of the same metal; but the case was quite altered when a thin
plate of metal was used. Thus a copper rocker laid upon the
edge of a penny-piece did not vibrate permanently; but when the
coin was beaten out by a hammer, so as to present a thin sharp
edge, constant vibrations were obtained. A silver rocker resting




144             APPENDIX TO LECTURE IV.
resting on the edge of a half-crown refused to vibrate permanently;
but on the edge of a sixpence continuous vibrations were obtained.
An iron rocker on the edge of a dinner knife gave continuous vibrations. A flat brass rocker placed upon the points of two common brass pins, and having its handle suitably supported, gave
distinct vibrations. In these experiments the plates and pins
were fixed in a vice, and it was found that the thinner the plate,
within its limits of rigidity, the more certain and striking was the
effect. Vibrations were thus obtained with iron on iron, copper
on copper, brass on brass, zinc on zinc, silver on silver, tin on
tin. The list might be extended, but the cases cited are sufficient
to show that the proposition above cited cannot be regarded as
expressing a' general law.'
The second general law enunciated by Professor Forbes is, that'both substances must be metallic.'  This is the law which first attracted the lecturer's attention. During the progress of a kindred
enquiry, he had discovered that certain non-metallic bodies are
endowed with powers of conduction far higher than has been
hitherto supposed, and the thought occurred to him that such
bodies might, by suitable treatment, be made to supply the place
of metals in the production of vibrations. This anticipation was
realized. Rocks of silver, copper, and brass, placed upon the
natural edge of a prism of rock crystal, gave distinct tones; on
the clean edge of a cube of fluor spar, the tones were still more
musical; on a mass of rock-salt the vibrations were very forcible.
There is scarcely a substance, metallic or non-metallic, on which
vibrations can be obtained with greater ease and certainty than
on rock-salt. In most cases a high temperature is necessary to
the production of the tones, but in the case of rock-salt the temperature need not exceed that of the blood. A new and singular
property is thus found to belong to this already remarkable substance. It is needless to enter into a full statement regarding the
various minerals submitted to experiment. Upwards of twenty
non-metallic substances had been examined by the lecturer, and
distinct vibrations obtained with every one of them.
The number of exceptions here exhibited far exceeds that of
the substances which are mentioned in the paper of Professor
Forbes, and are, it was imagined, sufficient to show that the second general law is untenable.




LAWS TESTED EXPERIMENTALLY.                 145
The third general law states, that'the vibrations take place
with an intensity proportional (within certain limits) to the difference of the conducting powers of the metals for heat, the metal having
the least conducting power, being necessarily the coldest.' The evidence adduced against the first law appears to destroy this one
also; for if the intensity of the vibrations be proportional to the
difference of the conducting powers, then, where there is no such
difference, there ought to be no vibrations. But it has been
proved in half a dozen cases, that vibrations occur between different pieces of the same metal. The condition stated by Professor
Forbes was, however, reversed. Silver stands at the head of conductors; a strip of the metal was fixed in a vice, and hot rockers
of brass, copper, and iron, were successively laid upon its edge:
distinct vibrations were obtained with all of them. Vibrations
were also obtained with a brass rocker which rested on the edge
of a half-sovereign. These and other experiments show that it is
not necessary that the worst conductor should be the cold metal,
as affirmed in the third general law above quoted. Among the
metals, antimony and bismuth were found perfectly inert by
Professor Forbes; the lecturer however had obtained musical
tones from both of these substances.
The superiority of lead as a cold block, Professor Faraday, as
already stated, referred to its high expansibility, combined with
its deficient conducting power. Against this notion, which he
considers to be'an obvious oversight,' Professor Forbes contends
in an ingenious and apparently unanswerable manner. The vibrations, he urges, depend upon the difference of temperature
existing between the rocker and the block; if the latter be a bad
conductor and retain the heat at its surface, the tendency is to
bring both the surfaces in contact to the same temperature, and
thus to stop the vibration instead of exalting it. Farther: the
greater the quantity of heat transmitted from the rocker to the
block during contact, the greater must be the expansion; and
hence, if the vibrations be due to this cause, the effect must be a
maximum when the block is the best conductor possible.- But
Professor Forbes, in this argument, seems to have used the term
expansion in two different senses. The expansion which produces
the vibration is the sudden upheaval of the point where the
hot rocker comes in contact with the cold mass underneath: but
7




146             APPENDIX TO LECTURE IV.
the expansion due to good conduction would be an expansion of
the general mass. Imagine the conductive power of the block to
be infinite-that is to say, that the heat imparted by the rocker is
instantly diffused equally throughout the block; then, though
the general expansion might be very great, the local expansion at
the point of contact would be wanting, and no vibrations would
be possible. The inevitable consequence of good conduction is,
to cause a sudden abstraction of the heat from the point of contact of the rocker with the substance underneath, and this the
lecturer conceived to be the precise reason why Professor Forbes
had failed to obtain vibrations when the cold metal was a good
conductor. He made use of blocks, and the abstraction of heat
from the place of contact by the circumjacent mass of metal, was
so sudden as to extinguish the local elevation on which the vibrations depend. In the experiments described by the lecturer, this
abstraction was to a great extent avoided, by reducing the metallic masses to thin laminae; and thus the very experiments adduced
by Professor Forbes against the theory supported by Professor
Faraday, appear, when duly considered, to be converted into
strong corroborative proofs of the correctness of the views of the
philosopher last mentioned.
EXTRACT FROM A PAPER ON SOME PHYSICAL PROPERTIES OF
ICE.*
In a very interesting paper communicated to the British Association during its last meeting, Mr. James Thomson has explained
the freezing together of two pieces of ice at 32~ Fahr., in the following manner:-' The two pieces of ice, on being pressed together at their point of contact, will at that place, in virtue of the
pressure, be in part liquefied and reduced in temperature, and the
cold evolved in their liquefaction will cause some of the liquid
film intervening between the two masses to freeze.'
I am far from denying the operation under proper circumstances of the vera causa to which Mr. Thomson refers, but I do
X Phil. Trans., 1858, p. 225.




LAMIl'ATION OF ICE BY PRESSURE.             147
not think it explains the facts. For freezing takes place without
the intervention of any pressure by which Mr. Thomson's effect
could sensibly come into play.
It is not necessary to squeeze the pieces of ice together; one
bit may be simply laid upon the other, and they will still freeze.
Other substances besides ice are also capable of being frozen to
the ice. If a towel be folded round a piece of ice at 32~ the towel
and ice will freeze together. Flannel is still better; a piece of
flannel wrapped round a piece of ice, freezes to it sometimes so
firmly that a strong tearing force is necessary to separate both.
Cotton, wool, and hair may also be frozen to ice, without the intervention of any pressure which would render Mr. Thomson's
cause sensibly active.
But there is a class of effects to the explanation of which the
lowering of the freezing point of water, by pressure, may, I think,
be properly applied. The following statement is true of fifty experiments, or more, made with ice from various quarters. A cylinder of ice, two inches high and an inch in diameter, was placed
between two slabs of box-wood, and submitted to a gradually
increasing pressure. Looked at perpendicularly to the axis,
cloudy lines were seen drawing themselves across the cylinder;
and when the latter was looked at obliquely, these lines were
found to be sections of dim hazy surfaces which traversed the
cylinder, and gave it an appearance closely resembling that of a
crystal of gypsum whose planes of cleavage had been forced out
of optical contact by some external force.
Fig. 39 represents the cylinder looked at perpendicularly to
its axis, and fig. 40 the same cylinder when looked at obliquely.
Fig. 89.                    Fig. 40.
lima




148             APPENDIX TO LECTURE IV.
To ascertain whether the rupture of optical contact which
these experiments disclosed was due to the intrusion of air between two separated surfaces of ice, a cylinder of ice, two inches
long and one inch wide, was placed in a copper vessel containing
ice-cold water. The ice cylinder projected half an inch above the
surface of the water. Placing the copper vessel on a slab of
wood, and a second slab of wood upon the cylinder of ice, the
whole was subjected to pressure. When the hazy surfaces were
well developed in the portion of ice above the water, the cylinder
was removed and examined. The planes of rupture extended
throughout the entire length of the cylinder, just the same as it
had been squeezed in free air.
Still the removal of the cylinder from its vessel might be attended with the intrusion of air into the fissures. I therefore
placed a cylinder of ice, two inches long and one inch wide, in a
stout vessel of glass, which was filled with ice-cold water. Squeezing the whole, as in the last experiment, the surfaces of discontinuity were seen under the liquid quite as distinctly as in air.
The surfaces are due to compression, and not to any tearing
asunder of the mass by tension, and they are best developed
where the pressure, within the limits of fracture, is a maximum.
A cylindrical piece of ice, one of whose ends was not parallel to
the other, was placed between slabs of wood and subjected to
pressure. Fig. 41 shows the disposition of the experiment. The
Fig. 41.                  Fig. 42.
effect upon the ice-cylinder was that shown in fig. 42, the surfaces being developed along that side which had suffered the
pressure.




LIQUEFACTION OF ICE BY PRESSURE.             149
Sometimes the surfaces commence at the centre of the cylinder. A dim small spot is first observed, which, as the pressure
continues, expands until it sometimes embraces the entire transverse section of the cylinder.
On examining these surfaces with a pocket lens, they appeared
to me to be composed of very minute water-parcels, like what is
produced upon a smooth cold surface by the act of breathing.
Were they either vacuous plates, or plates filled with air, their
aspect would, on optical grounds, be far more vivid than it really
was.
A concave mirror was so disposed, that the diffused light of
day was thrown full upon the cylinder while under pressure.
Observing the expanding surfaces through a lens, they appeared
in a state of intense commotion; this was probably due to the
molecular tensions of the little water-parcels. This motion followed closely on the edge of the surface as it advanced through
the solid ice. Once or twice I observed the hazy surfaces pioneered through the mass by dim offshoots apparently liquid.
They constituted a kind of negative crystallization, having the
exact form of the crystalline spines and spurs produced by the
congelation of water upon a surface of glass. I have no doubt,
then, that these surfaces are produced by the liquefaction of the
solid in planes perpendicular to the direction of pressure.
The surfaces are developed with great facility when they correspond to the surfaces of freezing. By care I succeeded in some
cases in producing similar effects in surfaces at right angles to the
planes of freezing, but this was difficult and uncertain. Wherever
the liquid disks before described were observed, the surfaces were
always easily developed in the planes of the disks.




LECTURE V.
[February 20, 1862.]
APPLICATION OF THE DYNAMICAL THEORY TO THE PHENOMENA OF SPECIFIC
AND LATENT HEAT-DEFINITION OF ENERGY: POTENTIAL AND DYNAMIC
ENERGY-ENERGY OF MOLECULAR FORCES-EXPERIMENTAL ILLUSTRATIONS OF SPECIFIC AND LATENT HEAT-MECHANICAL VALUES OF TIHE
ACTS OF COMBINATION, CONDENSATION, AND CONGELATION IN THE CASE
OF WATER-SOLID CARBONIC ACID-THE SPHEROIDAL STATE OF LIQUIDSFLOATING OF SPHEROID ON ITS OWN VYAPOUR-FREEZING OF WATER AND
MERCURY IN A RED-HOT CRUCIBLE.'W F% HENEVER  a difficult expedition is undertaken in
the Alps, the experienced mountaineer commences
the day at a slow pace, so that when the real hour of trial
arrives, he may find himself hardened instead of exhausted
by his previous work. We, to-day, are about to enter on
a difficult ascent, and I propose that we commence it in the
same spirit; not with a flush of enthusiasm which the
necessity of labour extinguishes, but with patient and
determined hearts which will not recoil should a difficulty
arise.
I have here a lead weight attached to a string which
passes over a pulley at the top of the room. We know
that the earth and the weight are mutually attractive; the
weight now rests upon the earth and exerts a certain pressure upon its surface.  The earth and the weight here
touch each other; their mutual attractions are as far as
possible satisfied, and motion by their mutual approach is
no longer possible.  As far as the attraction of gravity is




POTENTIAL AND DYNAMIC ENERGY.            151
cuncerned, the possibility of producing motion ceases as
soon as the two attracting bodies are actually in contact.
I draw up this weight. It is now suspended at a height
of sixteen feet above the floor; it is just as motionless as
when it rested on the floor; but by introducing a space
between the floor and it, I entirely change the condition
of the weight. By raising it I have conferred upon it a
motion-producing power. There is now an action possible
to it, which was not possible when it rested upon the earth;
it can fall, and in its descent can turn a machine or perform other work. It has no energy as it hangs there dead
and motionless; but energy is possible to it, and we might
fairly use the term possible energy, to express this power
of motion which the weight possesses, but which has not
yet been exercised by falling; or we might call it' potential energy,' as some eminent men have already done. This
potential energy is derived, in the case before us, from the
pull of gravity, which pull, however, has not yet eventuated
in motion. But I now let the string go; the weight falls,
and reaches the earth's surface with a velocity of thirty-two
feet a second. At every moment of descent it was pulled
down by gravity, and its final moving force is the summation of the pulls. While in the act of falling, the energy
of the weight is active. It may be called actual energy, in
antithesis to possible; or it may be called dynamic energy,
in antithesis to potential, or we might call the energy with
which the weight descends moving force. Do not be inattentive to these points; we must be able promptly to distinguish between energy in store and energy in action.
Once for all then, let us take the terms of Mr. Rankine,
and call the energy in store'potential,' and the energy in
action'actual.'* If, after this, I should use the terms
* Helmholtz, in his admirable memoir on'Die Erhaltung der Kraft,'
(1847), divided all energy into Tension and vis viva. (Spannkrifte und
Lebendige Krifte.)




152                  LECTI4RE V' possible energy,' or' dynamic energy,' or'moving force,'
you will have no difficulty in affixing the exact idea to these
terms. And remember exactness is here essential. We
must not now tolerate vagueness in our conceptions.
Our weight started from a height of sixteen feet; let
us fix our attention upon it after it has accomplished the
first foot of its fall. The total pull, if I may use the term,
to be expended on it has been then diminished by the
amount expended in its passing through the first foot. At
the height of fifteen feet it has one foot less of potential
energy than it possessed at the height of sixteen feet, but
at the height of fifteen feet it has got an equivalent amount
of dynamic or actual energy which, if reversed in direction,
would raise it again to its primitive height. Hence as potential energy disappears, dynamic energy comes into play.
Throughout the universe the sum of these two energies is
constant.
It is as yet too early to refer to organic processes, but
could we observe the molecular condition of my arm as I
drew up that weight, it would be seen that in accomplishing this mechanical act, an equivalent amount of some
other form of motion was consumed. If the weight were
raised by common heat, a portion of heat would disappear
exactly equivalent to the work done. The weight is about
one pound, and to raise it sixteen feet would consume as
much heat as would raise the temperature of a cubic foot
of air about 1~ F. Conversely, this quantity of heat would
be generated by the falling of the weight from a height of
sixteen feet. It is easy to see that, if the force of gravity
were immensely greater than it is, an immensely greater
amount of heat would have to be expended to raise the
weight. The greater the attraction, the greater would be
the amount of heat necessary to overcome it; but conversely,
the greater would be the amount of heat which a falling
body would then develope by its collision with the earth.




ENERGY OF MOLECULAR FORCES.                153
Having made our minds clear that heat is consumed
when a weight is forcibly separated from the earth by this
agent, and that the amount of heat consumed depends on
the energy of the attracting force overcome, we must turn
these conceptions, regarding sensible masses, to account,
in forming conceptions regarding insensible masses. As
an intellectual act it is quite as easy to conceive of the separation of two mutually attracting atoms, as to conceive of
the separation of the earth and weight. I have already
had occasion to refer more than once to the energy of
molecular forces, and here I have to return to the subject.
Closely locked together as they are, the atoms of bodies,
though we cannot suppose them to be in contact, exert
enormous attractions. It would require an almost incredible amount of ordinary mechanical force to widen the distances intervening between the atoms of any solid or liquid,
so as to increase the volume of the solid or liquid in any
considerable degree.  It would also require a force of great
magnitude to squeeze the particles of a liquid or solid together, so as to make the body less in size.  I have vainly
tried to augment the density of a soft metal by pressure
Water, for example, which yields so freely to the hand
plunged in it, was for a long time regarded as absolutely
incompressible. Great force was brought to bear upon it;
but sooner than shrink sensibly, it oozed through the pores
of the metal vessel which contained it, and spread like a
dew on the surface.* By refined and powerful means we
* I have to thank my friend, Mr. Spedding, for the following extract
in reference to this experiment:-'Now it is certain that rarer bodies (such as air) allow a considerable
degree of contraction, as has been stated; not that tangible bodies (such as
water) suffer compression with much greater difficulty and to a less extent.
How far they do suffer it, I have investigated in the following experiment:
I had a hollow globe of lead made capable of holding about two pints, and
sufficiently thick to bear considerable force; having made a hole in it, I
7*:




154                        LECTURIE V.
can now  compress water, but the force necessary to accomplish this is very great.
When we wish to overcome molecular forces we must
attack them by their peers. Heat accomplishes what mechanical energy, as generally wielded, is incompetent to
perform. Bodies when heated expand, and to effect this
expansion their molecular attractions must be overcome.
In masses equally large this is a work, in comparison with
which the erection of the Egyptian pyramids dwindles to
the labour of mites; and where the attractions to be overcome are so vast, we may infer that the quantity of heat
necessary to overcome them will be commensurate.
And now I must ask your entire attention. I hold in
filled it with water, and then stopped up the hole with melted lead, so that
the globe became quite solid. I then flattened the two opposite sides of the
globe with a heavy hammer, by which the water was necessarily contracted
into less space, a sphere being the figure of largest capacity; and when the
hammering had no more effect in making the water shrink, I made use of
a mill or press; till the water, impatient of further pressure, exuded
through the solid lead like a fine dew. I then computed the space lost by
the compression, and concluded that this was the extent of compression
which the water had suffered, but only when constrained by great violence.'
(Bacon's Novum Organum published in 1620: vol. iv. 209 of the translation.) Note by R. Leslie Ellis, vol. i. p. 324.-This is perhaps the most
remarkable of Bacon's experiments, and it is singular that it was so little
spoken of by subsequent writers. Nearly fifty years after the production
of the " Novum Organum," an account of a similar experiment was published by Megalotti, who was secretary of the Academia del Cimento at Florence; and it has since been familiarly known as the Florentine experiment.
I quote his account of it, "Facemmo lavorar,"' &c.
The writer goes on to remark that the absolute incompressibility of
water is not proved by this experiment, but merely that it is not to be compressed in the manner described; but the experiment is on other grounds
inconclusive.
It is to be remembered that Leibnitz (' Nouveaux Essais') in mentioning
the Florentine experiment, says that the globe was of gold (p. 229 Erdmann), whereas the Florentine academicians expressly say why they preferred silver to either gold or lead.




INTERIOR WORK.                155
my hand a lump of lead; suppose I communicate a certain
amount of heat to the lead, how is that heat disposed of
within the substance? It is applied to two distinct purposes-it performs two different kinds of work. One portion of it imparts that species of motion which raises the
temperature of the lead, and which is sensible to the thermolleter; but another portion of it goes to force the atoms
of the lead into new positions, and this portion is lost as
heat. The pushing asunder of the atoms of the lead in
this case, in opposition to their mutual attractions, is exactly analogous to the raising of our weight in opposition to
the force of gravity. Let me try to make the comparison
between the two actions still more strict; suppose that I
have a definite amount of force, to be expended on our
weight, and that I divide this force into two portions, one
of which I devote to the actual raising of the weight, while
I employ the other to cause the weight, as it ascends, to
oscillate, or revolve, like a pendulum or governor, and to
oscillate, moreover, with gradually:augmented energy; we
have, then, the analogue of that which occurs when heat is
imparted to the lead. The atoms are pushed apart, but
during their recession they vibrate, or revolve, with gradually augmented intensity. Thus the heat communicated
to the lead resolves itself, in part, into atomic potential
energy, and in part into a kind of atomic music, the musical part alone being competent to act upon our thermometers or to affect our nerves.
In this case, then, the heat accomplishes what we may
call interior work; * it performs work within the body
heated, by forcing its particles to take up new positions.
When the body cools, the forces which were overcome in
the process of heating come into play, and the heat which
was consumed by the forcing asunder of the atoms is now
restored by the drawing together of the atoms.
* See the excellent memoirs of Clausius in the Philosophical Magazine.




156                 LECTURE V.
Chemists have determined the relative weights of the
atoms of different substances. Calling the weight of a hydrogen atom 1, the weight of an oxygen atom, you know,
is 16. Hence to make up a pound weight of hydrogen,
sixteen times the number of atoms contained in a pound
of oxygen would be necessary. The number of atoms required to make up a pound is evidently inversely proportional to the atomic weight. We here approach a very
delicate and important point. The experiments of Dulong
and Petit, and of MM. Regnault and Neumann, render it
extremely probable that all elementary atoms, great and
small, light and heavy, when at the same temperature,
possess the same amount of the energy which we call heat,
the lighter atoms making good by velocity what they want
in mass. Thus, each of the atoms of hydrogen has the
same moving energy as an atom of oxygen at the same
temperature. But, inasmuch as a pound weight of hydrogen contains sixteen times the number of atoms, it must
also contain sixteen times the amount of heat possessed by
a pound of oxygen, at the same temperature.
From this it follows that to raise a pound of hydrogen,
a certain number of degrees in temperature-say from 500
to 600-would require sixteen times the amount of heat
needed by a pound of oxygen under the same circumstances.  Conversely, a pound of hydrogen, in falling
through 10~, would yield sixteen times the amount of heat
yielded by a pound of oxygen, in falling through the same
number of degrees.
In oxygen and hydrogen we have no sensible amount
of' interior work,' to be performed; there are no molecular attractions of sensible magnitude to be overcome. But
in solid and liquid bodies, besides the differences due to
the number of atoms present in the unit of weight, we have
also differences due to the consumption of heat in interior
work. Hence it is clear that the amount of heat which




RELATIONS OF ATOMIC NUMBERS TO HEAT.   157
different bodies contain is not at all declared by their temperature. To raise a pound of water, for example, 1~,
would require thirty times the amount of heat necessary to
raise a pound of mercury 1~. Conversely, the pound of
water, in falling through 1~, would yield up thirty times
the amount of heat yielded up by the pound of mercury.
Let me illustrate, by a simple experiment, the differences which exist between bodies, as to the quantity of
heat which they contain. I have here a cake of beeswax
six inches in diameter and half an inch thick. Here I have
a vessel containing oil, which is now at a temperature of
180~ C. In the hot oil I have immersed a number of balls
of different metals-of iron, lead, bismuth, tin and copper.
At present they all possess the same temperature, namely,
that of the oil. Well, I lift them out of the oil, and place
them upon this cake of wax c D
(fig. 43), which is supported by       Fi. 43.
the ring of a retort-stand; they
melt the wax underneath and
sink in it. But I see that they     __
are sinking with different velocities. The iron and the copper  are working  themselves
much more vigorously into the
fusible mass than the others; the
tin comes next, while the lead
and the bismuth lag entirely behind. There goes the iron clean
through, the copper follows; I
can see the bottom of the tin
ball just peeping through the lower surface of the cake,
but it cannot go farther; while the lead and bismuth have
made but little way, being unable to sink to much more
than half the depth of the cake.
Supposing, then, I take equal weights of different sub



158                    LECTURE V.
stances, heat them all (say to 1000) and then determine the
exact amount of heat which each of them gives out in
cooling from 100~ to 0~, I should find very different
amounts of heat for the different substances. How could
this problem be solved? It has been solved by eminent
men by observing the time which a body requires to cool.
Of course the greater the amount of heat possessed and
generated by its atoms, the longer would the body take to
cool. The relative quantities of heat yielded up by different bodies have also been determined by plunging them,
when heated, into cold water, and observing the gain on
the one hand and the loss on the other. The problem has
also been solved by observing the quantities of ice which
different bodies can liquefy, in falling from  2120 Fahr. to
32~, or from 100~ C. to 0~. These different methods have
given concordant results. Calling the amount of heat given
out by a pound of water, in sinking through one degree of
temperature, unity, the following numbers express the
amount of heat given out by a pound weight of each of the
substances whose names are annexed.
Water. 1'0000
Sulphur. 0'2026
Arsenic. 0'0814
Antimony.. 0'0508
Bismuth. 0-0308
Zinc.. 0'0955
Cadmium. 0'0567
Tin...             00562
Lead.. 0-0314
Iron.. 0'1138
Cobalt. 0-1070
Nickel. 0'1086
Copper. 0'0951
Mercury.       0'0333
Silver.. 00570
Gold.. 0-0324
Platinum.... 00324




SPECIFIC HEAT.                159
A moment's inspection of this table explains why it is
that, in the case of iron and copper, our balls melted
through the wax, while the lead and bismuth balls were incompetent to do so; it will also be seen that tin here occupies the position which we should assign to it from the experiment with the cake of wax; water, we see, stands at
the head of all.
Each of these numbers denotes what has been hitherto
called the' specific heat' or the' capacity for heat' of the
substance to which it is attached. As I stated in a former
lecture, those who hold that heat is a fluid, explained these
differences by saying that some substances had a greater
store of this fluid than others. We may, without harm,
continue to use the term'specific heat' or'capacity for
heat;' now that we know the true nature of the actions
covered by the term.
The energy of the forces engaged in this atomic motion
and interior work, as measured by any ordinary mechanical
standard, is enormous. I have here a pound of iron, which
on being heated from 32~ to 212~ F. expands by about W-rth
of the volume which it possesses at 320. Its augmentation
of volume would certainly escape the most acute eye; still
to give its atoms the motion corresponding to this augmentation of temperature, and to shift them through the
small space indicated, an amount of heat is requisite which
would raise about eight tons one foot high. Gravity almost vanishes in comparison with these molecular forces;
the pull of the earth upon the pound weight, as a mass, is
as nothing compared with the mutual pull of its own molecules. Water furnishes a still subtler example. WVater
expands on both sides of 4~ C. or 39~ F.; at 40 C. it has its
maximum density. Suppose a pound of water heated from
32~ C. to 42~ C. —that is, 10 —its volume at both temperatures is the same; there has been no forcing asunder whatever of the atomic centres, and still, though the volume is




160                  LECTURE V.
unchanged, an amount of heat has been imparted to the
water, sufficient, if mechanically applied, to raise a weight
of 1390 lbs. a foot high. The interior work done here by
the heat can be nothing more than the turning round of
the atoms of water. It separates the attracting poles of
the atoms by a tangential movement, but leaves their centres at the same distance asunder first and last. The conceptions with which I here deal, may not be easy to those
unaccustomed to such studies, but they are capable of perfect clearness of realization to all who have the patience to
dwell upon them sufficiently long.
This is the place to note further, that there are descriptions of interior work different from that of pushing the
atoms more widely apart. Enormous interior work may
be accomplished while the atoms, instead of being pushed
apart, as a whole, approach each other.  Polar forcesforces emanating from distinct points, and acting in distinct directions, give to crystals their symmetry, and the
overcoming of these forces, while it necessitates a consumption of heat, may also be accompanied by a diminution of volume. This is illustrated by the deportment of
both ice and bismuth on liquefying. I could readily
sketch a system of atoms illustrative of this position, but
every instructed mind will be able to imagine such combinations for itself.
The high specific heat of water has one important bearing which I do not wish to pass over here. Comparing
equal weights, the specific heat of water being 1~, that of
air is about 0'25. Hence a pound of water in losing 1~ of
temperature, would warm 4 lbs. of air 1~. But water is
770 times heavier than air; hence, comparing equal volumes, a cubic foot of water in losing 1~ of temperature
would raise 770 X 4 = 3080 cubic feet of air 1~.
The vast influence which the ocean must exert as a
moderator of climate here suggests itself. The heat of




THE OCEAN A MODERATOR OF CLIMATE.        161
summer is stored up in the ocean, and slowly given out
during the winter. Hence one cause of the absence of
extremes in an island climate. The summers of the island
can never attain the fervid heat of the continental summer,
nor can the winter of the island be so severe as the continental winter. In various parts of the continent fruits
grow which ouv summers cannot ripen; but in these same
parts our evergreens are unknown; for they cannot live
through the winters. The winter of Iceland is, as a general rule, milder than that of Lombardy.
We have hitherto confined our attention to the heat
consumed in the molecular changes of solid and liquid
bodies while these bodies continue solid and liquid. We
shall now direct our attention to the phenomena which accompany changes of the state of aggregation. When sufficiently heated, a solid melts, and when sufficiently heated,
a liquid assumes the form of gas. Let us take the case of
ice, and trace it through the entire cycle. This block of
ice has now a temperature of 20~ F. I warm it; a thermometer fixed in it rises to 32~, and at this point the ice
begins to melt; the thermometric column, which rose previously, is now arrested in its march, and becomes perfectly
stationary. I continue to apply warmth, but there is no
augmentation of temperature; and not till all the solid has
been reduced to liquid does the thermometer resume its
motion. It is now again ascending; it reaches 100~, 200~,
212~: here steam-bubbles show themselves in the liquid;
it boils, and from this point onwards the thermometer remains stationary at 212~.
But during the melting of the ice and during the evaporation of the water, heat is incessantly communicated: to
simply liquefy the ice, as much heat has been imparted to
it as would raise the same weight of water 143~ Fahr., or
as would raise 143 *imes the weight 1~ F. in temperature;
and to convert a pound of water at 212~ into a pound of




162                  LECTURE V.
steam at the same temperature, 967 times as much heat is
required as would raise a pound of water 1~ in temperature.
The former number, 143~, represents what has been hitherto called the latent heat of water; and the latter number,
967~, represents the latent heat of steam. It was manifest
to those who first used these terms, that, throughout the
entire time of melting, and throughout the entire time of
boiling, heat was communicated; but inasmuch as this heat
was not revealed by the thermometer, the fiction was invented that it was rendered latent. The fluid of heat hid
itself in some unknown way in the interstitial spaces of the
water and of the steam. According to our present theory,
the heat expended in melting is consumed in conferring
potential energy upon the atoms. It is virtually the lifting
of a weight. So likewise as regards the steam, the heat is
consumed in pulling the liquid molecules asunder, conferring upon them a still greater amount of potential energy;
and when the heat is withdrawn, the vapour condenses and
the molecules again clash with a dynamic energy equal to
that which was employed to separate them, and the precise
quantity of heat then consumed now reappears.
The act of liquefaction consists of interior work expended in moving the atoms into new positions. The act
of vaporisation is also, for the most part, interior work; to
which however must be added the external work performed
in the expansion of the vapour, which makes place for itself by forcing back the atmosphere.
We are indebted to the eminent man to whom I have
referred so often, for the first accurate determinations of
the calorific power of fuel.'Rumford estimated the calorific power of a body by the number of parts, by weight,
of water, which one part, by weight, of the body would,
on perfect combustion, raise 1~ in temperature. Thus one
part, by weight, of charcoal, in combining with 22 parts
of oxygen to form carbonic acid, will evolve heat sufficient




LATENT HEAT OF LIQUIDS.              163
to raise the temperature of about 8,000 parts by weight of
water 1~ C. Similarly, one pound of hydrogen, in combining with eight pounds of oxygen to form water, will
raise 34,000 lbs. of water 1~ C. The relative calorific powers, therefore, of carbon and hydrogen are as 8: 34.' 
The recent refined researches f Favre and Silbermann entirely confirm the determinations of Rumford.
Let us, then, fix our attention upon this wonderful substance, water, and trace it through the various stages of
its existence. First we have its constituents as free atoms,
which attract each other, fall, and clash together. The
mechanical value of this atomic act is easily determined;
knowing the number of foot-pounds corresponding to the
heating of 1 lb. of water 1~ C., we can readily calculate the
number of foot-pounds equivalent to the heating of 34,000
lbs. of water 1~ C. Multiplying the latter number by
1,390,t we find that the concussion of our 1 lb. of hydrogen
wth 8 lbs. of oxygen is equal, in mechanical value, to the
raising of forty-seven million pounds one foot high! I
think I did not overrate matters when I said that the force
of gravity, as exerted near the earth, was almost a vanishing quantity, in comparison with these molecular forces;
and bear in mind the distances which separate the atoms
before combination-distances so small as to be utterly
immeasurable; still it is in passing over these distances
that the atoms acquire a velocity sufficient to cause them
to clash with the tremendous energy indicated by the above
numbers.
After combination the substance is in a state of vapour,
which sinks to 212~, and afterwards condenses to water.
In the first instance -the atoms fell together to form the
compound; in the next instance the molecules of the com* Percy's Metallurgy, p. 53.
7 772 foot-pounds being the mechanical equivalent for 1~ F., 1,390
foot-pounds is the equivalent for 1~ C.




164                    LECTURE V.
pound fall together to form a liquid. The mechanical value of this act is also easily calculated: 9 lbs. of steam in
falling to water, generate an amount of heat sufficient to
raise 967 X 9 = 8,703 lbs. of water 1~ F.  Multiplying
this number by 772, we have a product of 6,718,716 footpounds as the mechanical value of the mere act of condensation.* The next great fall of our 9 lbs. of water is from
the state of liquid to that of ice, and the mechanical value
of this act is equal to 993,564 foot-pounds.  Thus our 9
lbs. of water, in its origin and progress, falls down three
great precipices: the first fall is equivalent to the descent
of a ton weight urged by gravity down a precipice 22,320
feet high; the second fall is equal to that of a ton down
a precipice 2,900 feet high; and the third is equal to the
descent of a ton down a precipice 433 feet high. I
have seen the wild stone-avalanches of the Alps, which
smoke and thunder down the declivities with a vehemence
almost sufficient to stun the observer. I have also seen
snow-flakes descending so softly as not to hurt the fragile
spangles of which they were composed; yet to produce,
from aqueous vapour, a quantity of that tender material
which a child could carry, demands an exertion of energy
competent to gather up the shattered blocks of the largest
stone-avalanche I have ever seen, and pitch them to twice
the height from which they fell.
I will now relieve the strain which I have hitherto put
upon your attention, by introducing a few experimental
illustrations of the calorific effects which accompany the
change of. aggregation. I place my thermo-electric pile
thus upon its back on the table, and on its naked face I
* In Rumford's experiments the heat of condensation was included in
his estimate of calorific power; deducting the above number from that
found for the chemical union of the hydrogen and oxygen, forty millions of
foot-pounds would still remain as the mechanical value of the act of combination.




EXPERIMENTAL ILLUSTRATIONS.           165
place this thin silver basin, B (fig. 44), into which I pour a
quantity of water slightly warmed, the needle of the gal,
vanometer moves to 900, and remains permanently deflected
to 70~. I now place a little powdered nitre, not more than
can fit upon a three-penny piece, in the basin, and allow it
to dissolve. I had placed the nitre previously before the
fire, so that not only was the liquid warm, but the solid
powder was also warm. Observe the effect of their mixFig. 44.
ture! The nitre dissolves in the water; and to produce
this change, all the heat which both the water and the nitre
possess, in excess of the temperature of this room, is consumed, and, indeed, a great deal more. The needle, you
see, sinks not only to zero, but goes strongly up at the
other side, showing that now the face of the pile is powerfully chilled.
I remove the basin, pour the liquid out, and resupply it
with warm water, into which I introduce a pinch of common salt. The needle was at 70~ when the salt was introduced: it is now sinking, reaches zero, and goes up on the
side which indicates cold. But the action is not at all so
strong as in the case of saltpetre. The reason is that the
amount of interior work required by the salt, and which
necessitates the consumption of heat, is much less than that
demanded by the nitre. As regards latent heat, then, we
have differences similar to those which we have already
illustrated as regards specific heat. Again, I cleanse the
basin, put fresh water in it, and put a little sugar in the
water; the amount of heat absorbed in the solution of the




166                  LECTURE V.
sugar is sensible, the liquid is chilled, but the amount of
chilling is much less than in either of the former cases.
Thus, when you sweeten your hot tea, you cool it in the
most philosophical manner; when you put salt in your
soup, you do the same; and if you were concerned with the
act of cooling alone, and careless of the flavour of your
soup, you might hasten its refrigeration by adding saltpetre.
In a former lecture I made use of a mixture of pounded
ice and salt to obtain great cold. Both the salt and the ice
when they are thus mixed together, change their state of
aggregation; the amount of interior work is here so great,
that during its performance the temperature of the mixture sinks 30~ Fahr., and more, below the freezing point
of water. Here is a nest of watch-glasses which I have
wrapped in tinfoil, and immersed in a mixture of ice and salt.
Into each watch-glass I had poured a little water, in which
the next glass rested. They are now all frozen together to
a solid cylinder, by the cold of this mixture of ice and salt.
I will now reverse the process, and endeavour to show
Fig. 45,      you the heat developed in passing from the liquid to the solid
state. But first let me show you
that heat is rendered latent when
sulphate of soda is dissolved. I
experiment with the substance
exactly as I experimented with
the nitre, and you see, that as the
crystals melt in the water the pile
is chilled. And now for the complementary experiment.   This
with this long neck, is now filled
with a solution of sulphate of so-. I^  eLda.  Yesterday Mr. Anderson
dissolved the substance in a pan




HEAT ACCOMPANYING SOLIDIFICATION.       167
over our laboratory fire, and filled this bolt-head with the
solution. He then covered the top carefully with a piece
of bladder, and placed the bottle behind this table, where
it has remained undisturbed throughout the night.
The liquid is, at the present moment, supersaturated
with sulphate of soda. When the water was hot, it melted
more than it could melt when cold. But now the temperature has sunk much lower than that which corresponds to
the point of saturation. This state of things is secured by
keeping the solution perfectly still, and permitting nothing
to fall into it. Water, kept thus still, may be cooled many
degrees below its freezing point. Some of you may have
noticed the water in your jugs, after a cold winter night,
suddenly freeze on being poured out in the morning. In
cold climates this is not uncommon. Well, the particles of
sulphate of soda in this solution are on the brink of a precipice, and I can push them over it, by simply dropping a
small crystal of the substance, not larger than a grain of
sand, into the solution. Observe what takes place; the
bottle now contains a clear liquid; I drop the bit of crystal in, it does not sink; the molecules have closed round it
to form a solid in which it is now embedded. The passage
of the atoms from a state of freedom to a state of bondage
goes on quite gradually; you see the solidification extending down the neck of the bottle. Observe where I have
placed my thermo-electric pile P. Its naked face rests
against the convex surface of the bottle, and the needle of
the galvanometer points to zero. The process of crystallisation has not yet reached the liquid in front of the pile,
but you see it approaching. It is now solidified opposite
the pile, and mark the effect. The atoms, in falling to the
solid form, develope heat; this heat communicates itself to
the glass envelope, the glass envelope warms the pile, and
the needle, as you see, flies to 90~. The quantity of heat




168                 LECTURE V.
thus rendered sensible by solidification is exactly equal to
that which was rendered latent by liquefaction.
We have, in these experiments, dealt with the latent
heat of liquids; let me now direct your attention to a few
experiments illustrative of what has been called the latent
heat of vapours-in other words, the heat consumed in
conferring potential energy, when a body passes from the
liquid to the gaseous state. As before, I turn my pile upon
its back with its naked face upwards, and on this face I
place the silver basin already used, into which I have
poured a small quantity of a volatile liquid, which I have
purposely warmed. The needle now moves, indicating
heat. But scarcely has it attained 90~ when it turns
promptly, descends to 0~, and flies with violence up on the
side of cold. The liquid here used is sulphuric ether; it is
very volatile, and the speed of its evaporation is such that
it consumes, rapidly, the heat at first communicated to it,
and then abstracts heat from the face of the pile. I remove the ether, and supply its place by alcohol, slightly
warm; the needle, as before, goes up on the side of heat.
But wait a moment; I will use these small bellows to promote the evaporation of the alcohol; now you see the needle descending, and now it is up at 90~ on the side of cold.
Water is not nearly so volatile as alcohol, still I can show
the absorption of heat by the evaporation of water also.
We use a kind of pottery for holding water, which admits
of a slight percolation of the liquid, so as to cause a kind
of dewiness on the external surface. Evaporation goes on
from that surface, and the heat necessary to this work,
being drawn in great part from the water within, keeps it
cool. Butter-coolers are made on the same principle.
To show you the extent to which refrigeration may be
carried by the evaporation of water, I have here an instrument (fig. 46), by which water is frozen, through the simple abstraction of its heat by its own vapour. The instru



LATENT HEAT OF VAPORS.            169
ment is called the cryophorus, or ice-carrier, and it was
invented by Dr. Wollaston. It is made in this way-a little water is put into one of these bulbs; the other bulb, B,
when softened by heat, had a tube drawn out from it with
a minute aperture at the end. Well, the water was boiled
in A, and steam was produced, until it had chased all the
air away through the small aperture in the distant bulb.
When the bulbs and connecting tube were filled with pure
steam, the small orifice was sealed with a blow-pipe. Here,
Fig. 46.
then, we have water and its vapour, with scarcely a trace
of air. You hear how the liquid rings, exactly as it does in
the case of the water-hammer.
I turn all the liquid into one bulb, A, whieI dip into
an empty glass to protect it from currents Tof air. The
empty bulb, B, I plunge into a freezing mixture6:' thus, the
vapour which escapes from the liquid in the builb, x, is condensed by the cold, to water, ih;,Thisi condensation
permits of the formation of new quaa s of vapour. As
the evaporation continues, the water which supplies the
vapour becomes more and more chilled. In a quarter of
an hour, or twenty minutes, it will be converted into a
cake of ice. Here is the opalescent solid formed in a second instrument, which you saw me arranging before the
commencement of the lecture. The whole process consists
in the uncompensated transfer or motion from the one bulb
to the other.




170                  LECTURE V.
But the most striking example of the consumption of
heat in changing the state of aggregation is furnished by
the substance which I have imprisoned in this strong iron
bottle. This bottle contains carbonic acid, liquefied by
enormous pressure. The substance you know is a gas under
ordinary circumstances; here is a jar full of it, which,
though it manifests its nature by extinguishing a taper, is
not to be distinguished, by the eye, from common air.
When the cock attached to the iron bottle is turned, the
pressure which acts upon the gas is relieved, the liquid
boils-flashes, as it were, suddenly into gas, which rushes
from the orifice with impetuous force. But you can see
this current of gas; mixed up with it you see a white substance, which is now blown against me, to a distance of
eight or ten feet, through the air. What is this white
substance? It is carbonic acid snow. The cold produced
in passing from the liquid to the gaseous state is so intense
that a portion of the carbonic acid is actually frozen to form
this snow, and mingles in small flakes with the issuing
stream of gas. I can collect this snow in a suitable vessel.
Here is a cylindrical box with two hollow handles, through
which I will allow the gas to pass. Right and left you see
the streams, but a large portion of the frozen mass is retained in the box. I open it, and you see it filled with this
perfectly white carbonic acid snow.
The solid very gradually disappears; its conversion into
vapour is slow, because it can only slowly collect from surrounding substances the heat necessary to vaporise it. You
can handle it freely, but not press it too much, lest it should
burn you. It is cold enough to burn the hand. I plunge
a piece of it into water, and hold it there: you see bubbles
rising through the water-these are pure carbonic acid gas.
I collect this gas, and show you that it possesses all the
properties of the gas as commonly prepared. The solid
acid does not melt in the water; when I release it, it rises




SOLID CARBONIC ACID.             171
to the surface, and floats upon it. I put a bit of the acid
into my mouth, taking care not to inhale while it is there.
I breathe against this candle; my breath extinguishes the
flame. Before the conclusion of the lecture, I will show
you how it is possible to preserve so cold a body in the
mouth without injury. A piece of iron of equal coldness
-ould do serious damage.
Here, then, we have a solid body intensely cold, which,
however, does not chill bodies in contact with it, as it
might be expected to do. In fact, no real contact has been
established with the acid. Water, we see, will not dissolve
it, but sulphuric ether will; and by pouring a quantity of
this ether on the snow, I obtain a pasty mass, which has
an enormous power of refrigeration. Here I have some
thick and irregular masses of glass-the feet, in fact, of
drinking-glasses.  I place a portion of the solid acid on
them, and wet it with ether; you hear the glass crack; it
has been shattered by the contraction produced by the intense cold.
In this basin I spread a little paper, and over the paper
I pour a pound or two of mercury; on the mercury I place
some solid carbonic acid, and over the acid I pour a little
ether. Mercury, you know, requires a very low temperature to freeze it. Well, here it is frozen; I turn it out before you, a solid mass; I can hammer the solid; I can also
cut it with a knife. To enable me to lift the mercury out
of the basin, I have dipped this wire into it; by this I
raise it,' and plunge it into a glass jar containing water. It
liquefies, and showers downwards through the water; but
every fillet of mercury freezes the water with which it
comes into contact, and thus round each fillet is formed a
tube of ice, through which you can see the liquid metal
descending. These experiments might be multiplied almost indefinitely; but enough, I trust, has been shown to
illustrate our present subject.




172                   LECTURE V.
I have now to direct your attention to another and very
singular class of phenomena, connected with the production
of vapour. Here is a broad porcelain basin, B (fig. 47),
filled with hot water. Here is a silver basin, s, which I
now heat to redness. If I place the silver basin in the hot
water, what will occur?  You might naturally reply, that
the basin will impart its
Fig. 4T.
excess of heat instantly to
_____________ IIII the water, and be cooled
down to the temperature
of the latter. But nothing
(-~..~  Z:-~   of this kind occurs. The
____'-______g        basin for a time developes
Y — ~     ~q ~ —--— ~~~-a sufficient amount of
vapour underneath it, to
lift it entirely out of contact with the water; or, in the language of the hypothesis, developed in our third lecture, it
is lifted by the discharge of molecular projectiles against its
under surface. This will go on until the temperature of
the basin sinks, and it is no longer able to produce vapour
of sufficient tension to support it. Then it comes into contact with the water, and the ordinary hissing of a hot
metal, together with the cloud which forms overhead, declares the fact.
I now reverse the experiment, and instead of placing
the basin in the water, I place the water in the basin —first
of all, however, heating the latter to redness by a lamp.
You hear no noise of ebullition, no hissing of the water as
I pour it into the hot basin; the drop rolls about on its
own vapour-that is to say, it, is sustained by the recoil of
the molecular projectiles discharged from its under surface.
I withdraw the lamp, and allow the basin to cool, until it is
no longer able to produce vapour strong enough to support
the drop. The liquid then touches the metal; the instant




SPHEROIDAL STATE.                173
it does so, violent ebullition sets in, and the cloud which
you now observe forms above the basin.
You cannot, from your present position, see this flattened spheroid rolling about in the hot basin, but I can
show it to you, and, if I am fortunate, I shall show you
something very beautiful. You will bear in mind that
there is an incessant developement of vapour underneath
the drop, which, as incessantly, escapes from it laterally.
If the drop rest upon a flattish surface, so that the lateral
Fig. 48.
escape is very difficult, the vapour will burst up through the
middle of the drop. But I have here arranged matters, so
that the vapour shall issue laterally; and it sometimes happens that the escape of the vapour is rythmic; it issues in
regular pulses, and then we have our drop of water moulded to a most beautiful rosette. I have it now,-a round
mass of liquid, two inches in diameter, with a beautifully
crimped border. I will throw the beam of the electric
lamp upon this drop so as to illuminate it, and holding this
lens over it, I hope to cast its image on the ceiling, or on
the screen. There it is (fig. 48), a figure eighteen inches in




1~74                 LECTURE V.
diameter, and the vapour breaking, as if in music, from its
edge. If I add a little ink, so as to darken the liquid, the
definition of its outline is augmented, but the pearly lustre
of its surface is lost. I withdraw the heat; the undulation
continues for some time: the border finally becomes unindented. The drop is now perfectly motionless-a liquid
spheroid-and now it suddenly spreads upon the surface,
Fig. 49.
contact has been established, and the spheroidal condition
ends.
I dry the silver basin and place it, with its bottom upwards, in front of the electric lamp, and with a lens in
front I bring the rounded outline of the basin to a focus
on the screen; I dip this bit of sponge in alcohol and
squeeze it over the cold basin, so that the drops fall upon
the surface of the metal: you see their magnified images
upon the screen, and you observe that when they strike the
surface they spread out and trickle down along it. Now I
will heat this basin by placing a lamp underneath. Ob



THE DROP IS SUPPORTED ON A VAPOUR SPRING. 175
serve what occurs: when I squeeze the sponge the drops
descend as before, but when they come in contact with the
basin they no longer spread but roll over the surface as
liquid spheres (fig. 49). See how they bound and dance
as if they had fallen upon elastic springs; and so in fact
they have. Every drop, as it strikes the hot surface, and
as it rolls along the surface, developes vapour which lifts
it out of contact, thus destroying all1 cohesion between the
surface and the drop, and enabling the latter to preserve its
spherical or spheroidal form.
I have here an arrangement suggested by Professor
Poggendorf, which shows, in a very beautiful manner, the
interruption of contact between the spheroidal drop and its
supporting surface. From this silver basin, B (fig. 50), inFig. 50.
_
tended to hold the drop, I carry a wire, w, round yonder
magnetic needle; the other end of the galvanometer wire
I attach to one end of this battery, A. From the opposite
pole of the little battery I carry a wire, w', and so attach
it to the arm, a b, of this retort-stand, k, that I can readily
lower it. I heat the basin, pour in the water, and lower
my wire till the end of it dips into the spheroidal mass:
you see no motion of the galvanometer needle; the only




176                 LECTURE v.
gap in the entire circuit is that which now exists underneath the drop. If the drop were in contact the current
would pass. I prove this thus: I withdraw the lamp; the
spheroidal state will soon end; the liquid will touch the
bottom. It now does so, and the needle instantly flies
aside.
You can actually see the interval between the drop and
the hot surface upon which it rests. A private experiment
may be made in this way: Let a flattish basin, B (fig. 51),
be turned upside down, and let the bottom of it be slightly
indented so as to be able to bear a drop; heat the basin by
a spirit lamp, and place upon it a drop of ink, d, with which
a little alcohol has been mixed. Stretch a platinum wire,
Fig. 51.
a b, vertically behind the drop, and render the wire incandescent by sending a current of electricity through it. Bring
your eye to a level with the bottomn of the drop, and you
will be able to see the red-hot wire through the interval
between the drop and the surface which supports it. Let
me show you this interval. I place my basin, B (fig. 52),
as before, with its bottom upward in front of the lamp; I
heat the basin and bring carefully down upon it a drop, d,
dependent from a pipette. When it rests upon the prop



INTERVAL BETWEEN DROP AND HOT SURFACE. 17T
er part of the surface, and the lens in front is brought to
its proper position, you see a line of bright light between
the drop and the silver, indicating that the beam of the
lamp has passed underneath the drop to the screen.
The spheroidal condition was first observed by Leidenfrost, and I might give you -fifty other illustrations of it.
Fig. 52.
Liquids can be made to roll on liquids. If, moreover, I
take this red-hot copper ball and plunge it into a vessel of
hot water, a loud sputtering is produced, due to the escape
of the vapour generated; still the contact of the liquid and
solid is only very partial: let'the ball cool, the liquid >M
length touches it, and then the ebullition is so violent as
to project the water from the vessel on all sides.
M. Boutigny has of late lent new interest to this subject by expanding the field of illustration, and applying it
to the explanation of many extraordinary effects. If the
hand be wet, it may be passed though a stream of molten
metal without injury. I have seen M. Boutigny myself pass
his wet hand through a stream of molten iron, and toss with
his fingers the fused metal from a crucible: a blacksmith
will lick a white hot iron without fear of burning his
8*




178                  LECTURE V.
tongue. The tongue is effectually preserved from contact
with the iron, by the vapour developed; and it was to the
vapour of the carbonic acid, which shielded me from itscontact, that I owed my safety when I put the substance
into my mouth. To the same protective influence many
escapes from the fiery ordeal of ancient times have been
attributed by M. Boutigny. I may add, that the explanation of the spheroidal condition given by Mi. Boutigny has
not been accepted by scientific men.
Boiler explosions have also been ascribed to the water
in the boiler assuming the spheroidal state; the sudden
developement of steam, by subsequent contact with the
heated metal, causing the explosion. We are more ignorant of these things than we ought to be. Experimental
Fig. 53.
science has brought a series of true causes to light, which
may produce these terrible catastrophes, but practical science has not yet determined the extent to which they actually come into operation. The effect of a sudden generation of steam has been illustrated by an experiment which
I will now make in your presence. Here is a copper vessel, v (fig. 53), with a neck which I can stop with this
cork, through which half an inch of fine glass tubing passes.




FIERY ORDEAL: BODLER EXPLOSIONS.        179
I heat the copper vessel, and pour into it a little water.
The liquid is now in the spheroidal state. I cork the vessel,
and the small quantity of steam developed, while the water
remains spheroidal, escapes through the glass tube. I now
remove the vessel from the lamp, and wait for a minute or
two: very soon the water will come into contact with the
copper; it now does so, and you observe the result: the
cork is driven, as if by the explosion of gunpowder, to a
considerable height in the atmosphere.
I have reserved what you will probably think the most
interesting experiment in connection with this subject, for
the conclusion of to-day's lecture. 3A. Boutigny, by means
of sulphurous acid, first froze water in a red-hot crucible;
and Mr. Faraday subsequently froze mercury, by means of
solid carbonic acid. I will try and reproduce this latter resuit; but first let me operate with water. I have here a
hollow sphere of brass about two inches in diameter, now
accurately filled with water; into the sphere I have had
this wire screwed, which is to serve as a handle. I heat
this platinum crucible to glowing redness, and place within
it some lumps of solid carbonic acid. I pour some ether on
the acid-neither of them comes into contact with the hot
crucible-they are protected from contact by the elastic
cushion of vapour which surrounds them; I lower my
sphere of water down upon the mass, and carefully pile
fragments of carbonic acid over it, adding also a little
ether. The pasty mass within the red-hot crucible remains
intensely cold; and now you hear a crack! I am thereby
assured that the experiment will succeed. The freezing
water has burst the brass sphere, as it burst the iron bottles
in a former experiment. Round the sphere I have wound
a bit of wire to prevent the ice from falling out. I now
raise the sphere, peel off the shattered brass shell, and there
you have a solid sphere of ice, extracted from the red-hot
crucible.




180                 LECTURE V.
I place a quantity of mercury in a conical copper spoon,
and dip it into the crucible. The ether in the crucible has
taken fire, which I did not intend it to do. The experiment
ought to be so made, that the carbonic acid gas-the chokedamp of mines-ought to keep the ether from ignition.
But the mercury will freeze notwithstanding. Out of the
fire, and through the flame, I draw the spoon, and there is
the frozen mass turned out before you on the table.




LECTURE VI
[February 27, 1862.]
CONVECTION OF HEATED AIR-WINDS-THE UPPER AND LOWER'TRADES'
-EFFECT OF THE EARTH'S ROTATION ON THE DIRECTION OF WIND-INFLUENCE OF AQUEOUS VAPOUR UPON CLIMATE-EUROPE THE CONDENSER
OF THE WESTERN ATLANTIC-RAINFALL IN IRELAND-THE GULF STREAM
-FORMATION OF SNOW-FORMATION OF ICE FROM SNOW-GLACIERSPHENOMENA OF GLACIER MOTION-REGELATION-MOULDING OF ICE BY
PRESSURE-ANCIENT GLACIERS.
APPENDIX:-mDATA CONCERNING GLACIER MOTION.
PROPOSE devoting an hour to-day to the consideration of some of the physical phenomena which exhibit
themselves on a large scale in Nature.  And first, with regard to winds. You see those sunburners now almost
wholly turned down, which are intended to illuminate this
room when the daylight is intercepted or gone. Not to
give light alone were they placed there; they were set up,
in part, to promote ventilation. The air, heated by the
gas flames, expands, and issues in a strong vertical current
into the atmosphere.  The air of the roomi is thereby incessantly drawn upon, and a fresh supply must be introduced
to make good the loss. Our chimney draughts are so many
vertical winds due to the heating of the air by our fires.
I ignite this piece of brown paper, the flamre ascends; I
blow out the flame, leaving the edges of the paper smoking; the heated edges warm the air, and produce currents
which carry the smoke upward. I dip the smoking paper




182                  LECTURE VI.
into a large glass vessel, and stop the neck of the vessel to
prevent the escape of the smoke; the smoke ascends with
the light air in the middle, spreads out laterally above, is
cooled, and falls like a cascade of cloud along the sides of
the vessel. I have here a heavy iron spatula, heated to
dull redness; as I hold it thus, you cannot see the currents
of heated air ascending from it. But I can show them to
you by their action on strong light. I place the spatula in
the beam of the electric lamp; here is the shadow of the
spatula on the screen, and those waving lines of light and
shade mark the streaming upwards of the heated air. Here
also is an iron spoon containing a fragment of sulphur,
which I heat until it ignites; I plunge the sulphur into this
jar of oxygen: the combustion becomes more brilliant and
energetic, and the air of the jar is thrown into intense commotion. The fumes of the sulphur enable you to track the
storms which the heating of the air produces within the
jar. I use the word' storms' advisedly, for the hurricanes
which desolate the earth are nothing more than large illustrations of the effect which we have produced in this glass
jar.
From the heat of the sun our winds are all derived.
We live at the bottom of an aerial ocean, which is to a remarkable degree permeable to the sun's rays, and is but
little disturbed by their direct action. But those rays,
when they fall upon the earth, heat its surface; the air in
contact with the surface shares its heat, is expanded, and
ascends into the upper regions of the atmosphere. Where
the rays fall vertically on the earth, the heating of the surface is greatest, that is to say, between the tropics. Here
aerial currents ascend and flow laterally north and south
towards the poles, the heavier air of the polar regions
streaming in to supply the place vacated by the light and
warm air. Thus we have an incessant circulation. Yesterday I made the following experiment in the hot room




CONVECTION OF HEATED AIR.           183
of a Turkish bath. I opened wide the door, and held a
lighted taper in the doorway, midway between top and
bottom. The flame rose straight from the taper. I placed
the taper at the bottom, it was blown violently inwards; I
placed it at the top, it was blown violently outwards. Here
we had two currents, or winds, sliding over each other,
and moving in opposite directions. Thus, also, as regards
our hemisphere, we have a current from the equator setting
in towards the north and flowing in the higher regions of
the atmosphere, and another flowing towards the equator in
the lower regions of the atmosphere. These are the upper
and the lower Trade Winds.
Were the earth motionless, these two currents would
run directly north and south, but the earth rotates from
west to east round its axis once in twenty-four hours. In
virtue of this rotation, an individual at the equator is carried round with a velocity of 1,000 miles an hour. You
have observed what takes place when a person incautiously
steps out of a carriage in motion. He is animated by the
motion of the carriage, and when his feet touch the earth
he is thrown forward in the direction of the motion. This
is what renders leaping from a railway carriage, when the
train is at full speed, almost always fatal. As we withdraw from the equator, the velocity due to the earth's rotation diminishes, and becomes nothing at the poles. It is
proportional to the radius of the parallel of latitude, and
diminishes as these circles diminish in size. Imagine, then,
an individual suddenly transferred from the equator to a
place where the velocity, due to rotation, is only 900 miles
an hour; on touching the earth here he would be thrown
forward in an easterly direction, with a velocity of 100
miles an hour, this being the difference between the equatorial velocity with which he started, and the velocity of
the earth's surface in his new locality.
Similar considerations apply to the transfer of air from




184                 LECTURE VI.
the equatorial to the northern regions, and vice versa. At
the equator the air possesses the velocity of the earth's surface there, and on quitting this position, it not only has its
tendency northwards to obey, but also a tendency to the
east, and it must take a resultant direction. The farther it
goes north the more is it deflected. from its original course;
the more it turns towards the east, the more it becomes
what we should call a westerly wind. The opposite holds
good for the current proceedingfrom the north; this passes
from places of slow motion to places of quick motion:' it is
met by the earth; hence the wind which started as a north
wind becomes a north-east wind, and as it approaches the
equator it becomes more and more easterly.
It is not by reasoning alone that we arrive at a knowledge of the existence of the upper atmospheric current,
though reasoning is sufficient to show that compensation
must take place somehow, —that a wind cannot blow in any
direction without an equal displacement of air taking place
in the opposite direction.  But clouds are sometimes seen in
the tropics high in the atmosphere, and moving in a direction
opposed to that of the constant wind below. Could we discharge a light body with sufficient force to cause it to penetrate the lower' current, and reach the higher, the direction
of that body's motion would give us the direction of the
wind above. Human strength cannot perform this experiment, but it has nevertheless been made. Ashes have been
shot through the lower current by volcanoes, and, from the
places where they have subsequently fallen, the direction
of the wind which carried them has been inferred. Professor Dove in his' Wittertmgs Verhaltnisse von Berlin'
cited the following instance:'On the night of April 30th,
explosions like those of heavy artillery were heard at Barbadoes, so that the garrison at Fort St. Anne remained all night
under arms. On May 1, at daybreak, the eastern portion
of the horizon appeared clear, while the rest of the firma



CURRENTS OF THE ATMOSPHERE.          185
ment was covered by a black cloud, which soon extended
to the east, quenched the light there, and at length produced a darkness so dense that the windows in the rooms
could not be discerned. A shower of ashes descended,
under which the tree branches bent and broke. Whence
came these ashes? From the direction of the wind, we
should infer that they came from the Peak of the Azores:
they came, however, from the volcano Morne Garou in St.
Vincent, which lies about 100 miles west of Barbadoes. The
ashes had been cast into the current of the upper trade. A
second example of the same kind occurred on January 20,
1835. On the 24th and 25th the sun was darkened in Jamaica by a shower of fine ashes, which had been discharged
from the mountain Coseguina, distant 800 miles. The people learned in this way that the explosions previously heard
were not those of artillery. These ashes could only have
been carried by the upper current, as Jamaica lies northeast from the mountain. The same eruption gives also a
beautiful proof that the ascending air-current divides itself
above, for ashes fell upon the ship Conway in the Pacific,
at a distance of 700 miles south-west of Coseguina.' Even on the highest summits of the Andes no traveller
has as yet reached the upper trade. From this some notion
may be formed of the force of the explosions; they were
indeed tremendous in both instances. The roaring of Coseguina was heard at San Salvador, a distance of 1,000 miles.
Union, a seaport on the west coast of Conchagua, was in
absolute darkness for forty-three hours; as light began to
dawn it was observed that the sea-shore had advanced 800
feet upon the ocean, through the mass of ashes which had
fallen. The eruption of Morne Garou forms the last link
of a chain of vast volcanic actions. In June and July
1811, near St. Miguel, one of the Azores, the island Sabrina rose, accompanied by smoke and flame, from the bottom
of a sea 150 feet deep, attained a height of 300 feet and a




186                  LECTURE VI.
circumference of a mile. The small Antilles were afterwards shaken, and subsequently the valleys of the Mississippi, Arkansas, and Ohio. But the elastic forces found no
vent; they sought one, then, on the north coast of Columbia. March 26 began as a day of extraordinary heat in
Caraccas; the air was clear and the firmament cloudless.
It was Green Thursday, and a regiment of troops of the
line stood under arms in the barracks of the quarter San
Carlos ready to join in the procession. The people
streamed- to the churches. A loud subterranean thunder
was heard, and immediately afterwards followed an earthquake shock so violent, that the church of Alta Gracia, 150
feet in height, borne by pillars fifteen feet thick, formed a
heap of crushed rubbish not more than six feet high. In
the evening the almost full moon looked down with mild
lustre upon the ruins of the town, under which lay the
crushed bodies of upwards of 10,000 of its inhabitants.
But even here there was no exit granted to the elastic
forces underneath. Finally, on April 27, they succeeded in
opening once more the crater of Morne Garou, which had
been closed for a century; and the earth, for a distance
equal to that from Vesuvius to Paris, rung with the thunder-shout of the liberated prisoner.'
I have here a terrestrial globe, on which I now trace
with my hand two meridians; they start from the equator
of the globe a foot apart, which would correspond to about
1,000 miles on the earth's surface. But these meridians,
as they proceed northward, gradually approach each other,
and meet at the north pole. It is manifest that the air
which rises between these meridians in the equatorial regions must, if it went direct to the pole, squeeze itself into
an ever-narrowing bed. Were the earth a cylinder instead
of a sphere, we might have a circulation from the middle
of the cylinder quite to each end, and a return current from
each end to the middle. But this, in the case of the earth,




THE UPPER MAD LOWER TRADES.           187
is impossible, simply because the space around the poles is
unable to embrace the air frorm the equator. The cooled
equatorial air sinks, and the return current sets in before
the poles are attained, and this occurs more or less irregularly. The two currents, moreover, instead of flowing one
over the other, often flow beside each other. They constitute rivers of air, with incessantly shifting beds.
These are the great winds of our atmosphere which,
however, are materially modified by the irregular distribution of land and water. Winds of minor importance also
occur, through the local action of heat, cold, and evaporation. There are winds produced by the heating of the air
in Alpine valleys, and which sometimes rush with sudden
and destructive violence down the gulleys of the mountains: gentler down-flows of air are produced by the presence of glaciers upon the heights. There are the land
breeze and the sea breeze, due to the varying temperature
of the sea-board soil, by day and night. The morning sun
heating the land, produces vertical displacement, and the
air from the sea moves landward. In the evening the land
is more chilled by radiation than the sea, and the conditions
are reversed; the heavy air of the land now flows seaward.
Thus, then, a portion of the heat of the tropics is sent
by an aerial messenger towards the poles, a more equable
distribution of terrestrial warmth being thus secured. But
in its flight northward the air is accompanied by another
substance-by the vapour of water, which, you know, is
perfectly transparent.  Imagine the ocean of the tropics,
giving forth its vapour, which promotes by its lightness the
ascent of the associated air. They expand as they ascend:
at a height of 16,000 feet the air andtvapour occupy twice
the volume which they embraced at the sea level. To secure this space they must, by their elastic force, push away
the air in all directions round them; they must perform
work; and this work cannot be performed, save at the ex



188                  LECTURE VI.
pense of the warmth with which they were in the first instance charged.
The vapour thus chilled is no longer competent to retain
the gaseous form. It is precipitated as cloud: the cloud
descends as rain; and in the region of calms, or directly
undcler the sun, where the air is first drained of its aqueous
load, the descent of rain is enormous. The sun does not
remain always vertically over the same parallel of latitude
-he is sometimes north of the equator, sometimes south
of the equator, the two tropics limiting his excursion.
When he is south of the equator, the earth's surface north
of it is no longer in the region of calms, but in a region
across which the aerial current from  the north flows
towards the region of calms. -The moving air is but
slightly charged with vapour, and as it travels from north
to south it becomes ever warmer; it constitutes a dry
wind, and its capacity to retain vapour is continually augmenting. It is plain, from these considerations, that each
place between the tropics must have its dry season and
rainy season; dry when the sun is at the opposite side of
the equator, and wet when the sun is overhead.
Gradually, however, as the upper stream, which rises
from the equator, and flows towards the poles, becomes
chilled and dense, it sinks towards the earth; at the Peak
of Teneriffe it has already sunk below the summit of the
mountain. With the contrary wind blowing at the base,
the traveller finds the stream from the equator blowing
strong over the top. Farther north the equatorial wind
sinks lower still, and finally quite reaches the surface of
the earth. Europe, for the most part, is overflowed by
this equatorial current. Here in London, for eight or nine
months in the year, southwesterly winds prevail.  But
mark what an influence this must have upon our climate.
The moisture of the equatorial ocean comes to us endowed
with potential energy; with its molecules separate, and




ATMOSPHERIC CONDENSATION.           189
therefore competent to clash and develope heat by their
collision; it comes, if you will, charged with latent heat.
In our northern atmosphere the collision takes place, and
the heat generated is a main source of warmth to our climate. Were it not for the rotation of the earth, we should
have over us the hot dry blasts of Africa; but owing to
this rotation, the wind which starts northward from the
Gulf of Mexico is deflected to Europe. Europe is, therefore, the recipient of those stores of latent heat which were
amassed in the western Atlantic. The British Isles come
in for the greatest share of this moisture and heat, and this
circumstance adds itself to that already dwelt upon-the
high specific heat of water-to preserve our climate from
extremes. It is this condition of things which makes our
fields so green, and which gives the blossom to our maidens' cheeks. A German writer, Moritz, expresses himself
on these points in the following ardent words: —' Ye
blooming youthful faces,'ye green meadows and streams
of this happy land, how have ye enchanted me! 0 Richmond, Richmond! never can I forget the evening when,
full of delight, I wandered near you up and down along
the flowery banks of the Thames. This, however, must
not detain me fiom that dry and sand-strewn soil on which
fate has appointed me my sphere of action.' All this poetry and enchantment are derived directly from aqueous
vapour.*
As we travel eastward in Europe, the amount of aqueous precipitation grows less and less; the air becomes more
and more drained of its moisture. Even between the east
and west coasts of our own islands the difference is sensible, and local circumstances also have a powerful influence
on the amount of precipitation. Dr. Lloyd finds the mean
yearly temperature of the western coast of Ireland about
* Its relation to Radiant Heat is developed in Lecture XI.




190                     LECTURE VI.
two degrees higher than that of the eastern coast, at the
same height, and in the same parallel of latitude. The
total amount of rain which fell in the year 1851, at various
stations in the island, is given in the following tableStation               Rain in inches
Portarlington.. 21'2
Killough.... 23'2
Dublin.... 26'4
Athy.. 26'7
Donaghadee. 27'9
Courtown.. 2996
Kilrush..32'6
Armagh..33'1
Killybegs..... 33'2
Dunmore.                    33.5
Portrush.                   37.2
Burincrana. 39'3
]Markree....     40'3
Castletownsend.. 42-5
Westport.. 45'9
Cahirciveen.... 594'With reference to this table, Dr. Lloyd remarks-' 1. That there is great diversity in the yearly amount
of rain at the different stations, all of which (excepting
four) are but a few feet above the sea level; the greatest
rain (at Cahirciveen) being nearly three times as great as
the least (at Portarlington).' 2. That the stations of least rain are either inland or
on the eastern coast, while those of the greatest rains are
at or near the western coast.'3. That the amount of rain is greatly dependent on
the proximity of a mountain chain or group, being always
considerable in such neighbourhood, unless the station lie
to the north-east of the same.'Thus, Portarlington lies to the north-east of Slievebloom; Killough to the north-east of the Mourne range;
Dublin, north-east of the Wicklow range, and so on. On




RAIN-FALL IN IRELAND.                   191
the other hand, the stations of greatest rain, Cahirciveen,
Castletownsend, Westport, &c., are in the vicinity of high
mountains, but on a different side.' *
This distribution of heat by the transfer of masses of
heated air from place to place, has been called' convection,'
in contradistinction to the process of conduction, which
will be treated in our next lecture.  Heat is distributed in
a similar manner through liquids. I have here a glass cell,
c (fig. 54), containing warm water; I place it in front of
Fig. 54.
the electric lamp, and by means of a converging lens,
throw a magnified image of the cell upon the screen. I
now introduce the end of this pipette into the water of the
cell, and allow a little cold water to gently enter the hot.
The difference of refraction between both enables you to
see the heavy cold water falling through the lighter warm
water.  The experiment succeeds still better when I allow
a fragment of ice to float upon the surface of the water.
As the ice melts, it sends long heavy strive downwards to
the bottom  of the cell.  You observe, as I cause the ice to
* The greatest rainfall recorded by Sir John Herschel in his table
(Meteorology, 110, &c.) occurs at Cherra Pungee, where the annual fall is
592 inches. It is not my object to enter far into the subject of meteorology; for the fullest and most accurate information the reader will refer to
the excellent works of Sir John Herschel and Professor Dove.




192                  LECTURE VI.
move along the top, how these streams of cold water
descend through the hot. I now reverse the experiment,
placing cold water in the cell, and hot water in the pipette.
Care is here necessary to allow the warm water to enter
without any momentum, which would carry it mechanically
down. You notice the effect. The point of the pipette is
in the middle of the cell, and you see, as the warm water
Fig. 55.
enters, it speedily turns upwards (fig. 55) and overflows the
top, almost as oil would do under the same circumstances.
When a vessel containing water is heated at the bottom, the warmth communicated is thus diffused. You may
see the direction of the ascending warm currents by means
of the electric lamp, and also that of the currents which
descend to occupy the place of the lighter water. Here is
a vessel containing cochineal, the fragments of which,
being not much heavier than the water, freely follow the
direction of its currents. You see the pieces of cochineal
breaking loose from the heated bottom; ascending along
the middle of the jar, and descending again by the sides.
In the Geyser of Iceland this convection occurs on a grand
scale. A fragment of paper thrown upon the centre of the
water which fills the pipe is instantly drawn towards the
side, and there sucked down by the descending current.
Partly to this cause, but mainly, perhaps, to the action
of winds, currents establish themselves in the ocean, and




CONVECTION IN LIQUIDS.             193
powerfully influence climate by the heat which they distribute. The most remarkable of these currents, and by
far the most important for us, is the so-called Gulf-stream,
which sweeps across the Atlantic from the equatorial regions through the Gulf of Mexico, whence it derives its
name. As it quits the straits of Florida it has a temperature of 83~ Fahr., thence it follows the coast of America as
far as Cape Fear, whence it starts across the Atlantic, taking a north-easterly course, and finally washing the coast
of Ireland, and the north-western shores of Europe generally. As might be expected, the influence of this body
of warm water makes itself most evident in our winter.
It then entirely abolishes the difference of temperature
due to the difference of latitude of north and south Britain; if we walk from the Channel to the Shetland Isles, in
January, we encounter everywhere the same temperature.
The Isothermal line runs north and south. The presence
of the water renders the climate of western Europe totally
different from that of the opposite coast of America. The
river Hudson, for example, in the latitude of Rome, is
frozen over for three months in the year. Starting from
Boston in January, and proceeding round St. John's, and
thence to Iceland, we meet everywhere the same temperature. The harbour of Hammerfest derives great value from
the fact that it is clear of ice all the year round. This is
due to the Gulf-stream which sweeps round the North
Cape, and so modifies the climate there, that at some
places, by proceeding northward, you enter a warmer region. The contrast between northern Europe and the east
coast of America caused Halley to surmise that the north
pole of the earth had shifted; that it was formerly situate
somewhere near Behring's Straits, and that the intense
cold observed in these regions is really the cold of the ancient pole, which had not been entirely subdued since the
axis changed its direction. But now we know that the
9




194:                  LECTURE VI.
Gulf-stream and the diffusion of heat by winds and vapours
are the real causes of European mildness. On the western
coast of America, between the Rocky mountains and the
ocean, we find a European climate.
Europe, then, is the condenser of the Atlantic; and the
mountains are the chief condensers in Europe. On them,
moreover, when they are sufficiently high, the condensed
vapour descends, not in a liquid, but a solid form. Let us
look to this water in. its birthplace, and follow it through
its subsequent course. Clouds float in the air, and hence
the surmise that they are composed of vesicles or bladders
of water, thus forming shells instead of spheres. Eminent
travellers say that they have seen these bubbles, and their
statements are entitled to all respect. It is certain, however, that the water-particles at high elevations possess, on
or after precipitation, the powers of building themselves
into crystalline forms; they thus bring forces into play
which we have hitherto been accustomed to regard as
molecular, and which could not be ascribed to the aggregates necessary to form vesicles.
Snow, perfectly formed, is not an irregular aggregate
of ice-particles; in a calm atmosphere, the aqueous atoms
arrange themselves so as to form the most exquisite figures.
You have seen those six-petalled flowers which form themselves within a block of ice when a beam of heat is sent
through it. The snow-crystals, formed in a calm atmosphere, are built upon the same type: the molecules arrange
themselves to form hexagonal stars. From a central nucleus shoot six spiculve, every two of which are separated
by an angle of 60~. From these central ribs smaller spiculke shoot right and left with unerring fidelity to the
angle 60~, and from these again other smaller ones diverge
at the same angle. The six-leaved blossoms assume the
most wonderful variety of form; their tracery is of the
finest frozen gauze; and round about their corners other




CAUSE OF EUROPEAN MILDNESS.           195
rosettes of smaller dimensions often cling. Beauty is superposed upon beauty, as if Nature, once committed to her
task, took delight in showing, even within the narrowest
limits, the wealth of her resources.*
These frozen blossoms constitute our mountain snows;
they load the Alpine heights, where their frail architecture
is soon destroyed by the accidents of the weather. Every
winter they fall, and every summer they disappear, but this
rythmic action does not perfectly compensate itself. Below a certain line warmth is predominant, and the quantity
which falls every winter is entirely swept away; above
this line cold is predominant, the quantity which falls is in
excess of the quantity melted, and an annual residue remains. In winter the snows reach to the plains; in summer they retreat to the snow-line,-to that particular line
where the snow-fall of every year is exactly balanced by
the consumption, and above which is the region of eternal
snows. But if a residue remains annually above the snow
line, the mountains must be loaded with a burden which
increases every year.  Supposing at a particular point
above the line referred to, a layer of three feet a year is
added to the mass; this deposit, accumulating even through
the brief period of the Christian era, would produce an
elevation of 5,580 feet. And did such accumulations continue throughout geologic instead of historic ages, there is
no knowing the height to which the snows would pile
themselves. It is manifest no accumulation of this kind
takes place; the quantity of snow on the mountains is not
augmenting in this way; for some reason or other the sun
is not permitted to lift the ocean out of its basins and pile
its waters permanently upon the hills.
But how is this annually augmenting load taken off the
* See fig. 56, in which are copied some of the beautiful drawings of
Mr. Glaisher.




,A'0
WST., f,.j'     a
~IA  flwnLo:             961




SNOW CRYSTALS.                 197
shoulders of the mountains? The snows sometimes detach
themselves and rush down the slopes in avalanches, melting
to water in the warmer air below. But the violent rush
of the avalanche is not their only motion; they also creep
by almost insensible degrees down the slopes. As layer,
moreover, heaps itself upon layer, the deeper portions of
the mass become squeezed and consolidated; the air first
entrapped in the meshes of the snow is squeezed out, and
the compressed mass approximates more and more to the
character of ice. You know how the granules of a snowball will adhere; you know how hard you can make it if
mischievously inclined: the snow-ball is incipient ice; augment your pressure, and you actually convert it into ice.
But even after it has attained a compactness which would
entitle it to be called ice, it is still capable of yielding
more or less, as the snow yields, to pressure. When, therefore, a sufficient depth of the substance collects upon the
earth's surface, the lower portions are squeezed out by the
pressure of the upper ones, and if the snow rests upon a
slope, it will yield principally in the direction of the slope,
and move downwards.
This motion is incessantly going on along the slopes of
every snow-laden mountain; in the Himalayas, in the
Andes, in the Alps; but in addition to this motion, which
depends upon the power of the substance itself to yield to
pressure, there is also a sliding motion over the inclined
bed. The consolidated snow moves bodily over the mountain slope, grinding off the asperities of the rocks, and polishing their hard surfaces.  The under surface of the
mighty polisher is also scarred and furrowed by the rocks
over which it has passed; but as the compacted snow descends, it enters a warmer region, is more copiously melted
and sometimes, before the base of its slope is reached, it is
wholly cut off by fusion. Sometimes, however, large and
deep valleys receive the gelid masses thus sent down; in




198                  LECTURE VI.
these valleys it is further consolidated, and through them
it moves, at a slow but measurable pace, imitating in all its
motions those of a river. The ice is thus carried far beyond the limits of perpetual snow, until, at length, the consumption below equals the supply above, and at this point
the glacier ceases. From the snow-line downwards in summer, we have ice; above the snow-line, both summer and
winter, we have, on the surface, snow. The portion below
the snow-line is called a glacier, that above the snow-line is
called the nrve'.  The n6ve, then, is the feeder of the
glacier.
Several valleys thus filled may unite in a single valley,
the tributary glaciers welding themselves together to form
a trunk glacier. Both the main valley and its tributaries
are often sinuous, and the tributaries must change their direction to form the trunk. The width of the valley, also,
often changes; the glacier is forced through narrow
gorges, widening after it has passed them; the centre of
the glacier moves more quickly than the sides, and the
surface more quickly than the bottom. The point of
swiftest motion follows the same law as that observed in
the flow of rivers, changing from one side of the centre to
the other, as the flexure of the valley changes.*  Most of
the great glaciers in the Alps have, in summer, a central
velocity of two feet a day. There are points on the Merde-Glace, opposite the Montenvert, which have a daily motion of thirty inches in summer, and in winter have been
found to move at half this rate.
The power of accommodating itself to the channel
through which it moves has led eminent men to assume
that ice is viscous; and the phenomena at first sight seem
to enforce this assumption. The glacier widens, bends, and
narrows, and its centre moves more quickly than its sides;; For the data on which this law is founded see Appendix to this Lecture.




GLACIERS.                   199
a viscous mass would undoubtedly do the same. But the
most delicate experiments on the capacity of ice to yield
to strain, to stretch out like treacle, honey or tar, have
failed to detect this stretching power. Is there, then, any
other physical quality to which the power of accommodation possessed by glacier ice, may be referred?
Let us approach this subject gradually. We know that
vapour is continually escaping from the free surface of a
liquid; that the particles at the surface attain their gaseous
liberty sooner than the particles within the liquid; it is
natural to expect a similar state of things with regard to
ice; that when the temperature of a mass of ice is uniformly augmented, the first particles to attain liquid liberty
are those at the surface; for here they are entirely free,
on one side, from the controlling action of the surrounding
particles. Supposing, then, two pieces of ice raised
throughout to 320, and melting at this temperature at their
surfaces; what may be expected to take place if we place
the liquefying surfaces close together?  We thereby virtually transfer these surfaces to the centre of the ice, where
the motion of each molecule is controlled all round by its
neighbours. As might reasonably be expected, the liberty
of liquidity at each point where the surfaces touch each
other, is arrested, and the two pieces freeze together at
these points. Let us make the experiment: Here are two
masses which I have just cut asunder by a saw; I place
their fiat surfaces together; half a minute's contact will
suffice; they are now frozen together, and by taking hold
of one of them I thus lift them both.
This is the effect to which attention was first directed
by Mr. Faraday in June 1850, and which is now known
under the name of Regelation. On a hot summer's day, I
have gone into a shop in the Strand where fragments of ice
were exposed in a basin in the window; and with the
shopman's permission have laid hold of the topmost piece




200                  LECTURE VI.
of ice, and by means of it have lifted the whole of the
pieces bodily out of the dish. Though the thermometer at
the time stood at 80~, the pieces of ice had frozen together
at their points of junction. Even under hot water this
effect takes place; I have here a basin of water as hot as
my hand can bear; I plunge into it these two pieces of
ice, and hold them together for a moment: they are now
frozen together, notwithstanding the presence of the heated liquid. A pretty experiment of Mr. Faraday's is to
place a number of small fragments of ice in a dish of water
deep enough to float them. When one piece touches the
other, if only at a single point, regelation instantly sets in..
Thus a train of pieces may be caused to touch each other,
and, after they have once so touched, you may take the
terminal piece of the train, and, by means of it, draw all
the others after it. When we seek to bend two pieces
thus united at their point of junction, the frozen points
suddenly separate by fracture, but at the same moment
other points come into contact, and regelation sets in between them. Thus a wheel of ice might be caused to roll
on an icy surface, the contacts being incessantly ruptured,
with a crackling noise, and others as quickly established by
regelation. In virtue of this property of regelation, ice is
able to reproduce many of the phenomena which are usually ascribed to viscous bodies.*
Here, for example, is a straight bar of ice: I can by
passing it successively through a series of moulds, each
more curved than the last, finally turn it out as a semi-ring.
The straight bar in being squeezed into the curved mould
breaks, but by continuing the pressure new surfaces come
into contact, and the continuity of the mass is restored. I
take a handful of those small ice fragments and squeeze
* See note on the Regelation of Snow Granules in the Appendix to this
Lecture.




REGELATION OF ICE.              201
them together, they freeze at their points of contact and
now the mass is one aggregate. The making of a snowball, as remarked by Mr. Faraday, illustrates the same principle. In order that this freezing shall take place, the snow
ought to be at 320 and moist. When below 32~ and dry,
on being squeezed it behaves like salt. The crossing of
snow-bridges in the upper regions of the Swiss glaciers is
often rendered possible solely by the regelation of the
snow granules. The climber treads the mass carefully,
and causes its granules to regelate: he thus obtains an
amount of rigidity which, without the act of regelation,
would be quite unattainable. To those unaccustomed to
such work, the crossing of snow bridges, spanning, as they
often do, fissures 100 feet and more in depth, must appear
quite appalling.
If I still further squeeze this mass of ice fragments, I
bring them into still closer proximity. My hand, however,
is incompetent to squeeze them very closely together. I
place them in this boxwood mould, which is a shallow cylinder, and placing a flat piece of boxwood overhead, I introduce both between the plates of a small hydraulic press,
and squeeze the mass forcibly into the mould. I now
relieve the pressure and turn the substance out before you:
it is converted into a coherent cake of ice. I place it in
this lenticular cavity and again squeeze it. It is crushed
by the pressure, of course, but new contacts establish
themselves, and there you have the mass a lens of ice. I
now transfer my lens to this hemispherical cavity, i (fig.
57), and bring down upon it a hemispherical protuberance,
P, which is not quite able to fill the cavity. I squeeze the
mass: the ice, which a moment ago was a lens, is now
squeezed into the space between the two spherical surfaces: I remove the protuberance, and here I have the interior surface of a cup of glassy ice. By care I release it
from the mould, and there it is, a hemispherical cup, which
9*




202                  LECTURE VI.
I can fill with cold sherry, without the escape of a drop.
I scrape with a chisel a quantity of ice from this block, and
Fig. 57.
placing the spongy mass within this spherical cavity, c (fig.
58), I squeeze it and add to it, till finally I can bring down
another spherical cavity, D, upon it, enclosing it as a sphere
between both. As I work the press the mass becomes
more and more compacted. I add more material, and
again squeeze; by every such act the mass is made harder,
and there you have a snow-ball before you such as you
never saw before. It is a sphere of hard translucent ice,
B. Thus, you see, broken ice can be compacted together
Fig. 58.
by pressure, and in virtue of the property of regelation,
which cements its touching surfaces, the substance may be
made to take any shape we please. Were the experiment
worth the trouble, I feel satisfied that I could form a rope
of ice from this block, and afterwards coil the rope into a
knot. Nothing of course can be easier than to produce
statuettes of the substance from suitable moulds.
It is easy to understand how a substance so endowed




MOULDING -OF ICE BY PRESSUXEE         203
can be squeezed through the gorges of the Alps-can bend
so as to accommodate itself to the flexures of -the Alpine
valleys, and can permit of a differential motion of its parts,
without at the same time possessing a sensible trace of viscosity. The hypothesis of viscosity, first started by Rendu, and worked out with such ability by Prof. Forbes, accounts, certainly, for half the facts. Where pressure
comes into play, the deportment of ice is apparently that
of a viscous body; where tension comes into play, the analogy with a viscous body ceases.
I have thus briefly sketched the phenomena of existing
glaciers, as far as they are related to our present subject;
but the scientific explorer of mountain r~egions soon meets
with appearances which carry his mind back to a state of
things very different from that which now obtains. The
unmistakable traces which they have left behind them show
that vast glaciers once existed in places, from which they
have for ages disappeared. Go, for example, to the glacier of the Aar in the Bernese Alps and observe its present
performances; look to the rocks upon its flanks as they are
at this moment, rounded, polished, and scarred by the
moving ice. And having by patient and varied exercise
educated your eye and judgment in these matters, walk
down the glacier towards its end, keeping always in view
the evidences of the glacier's action. After quitting the
ice, continue your walk down the valley towards the Grimsel: you see everywhere the same unmistakable record.
The rocks which rise from the bed of the valley are rounded like hogs' backs; these are the'roches moutonn6s' of
Charpentier and Agassiz; you observe upon them the larger
flutings of the ice, and also the smaller scars scratched by
pebbles, which the glacier held as emery on its under surface. All the rocks of the Grimsel have been thus planed
down. Walk down the valley of Hasli and examine the
moumtain sides right and left; without the key which I




204                 LECTURE VI.
now suppose you to possess, you would be in a land of
enigmas; but with this key all is plain, you see everywhere
the well-known scars and flutings and furrowings. In the
bottom of the valley you have the rocks filed down in some
places to dome-shaped masses, and, in others, polished so
smooth that to pass over them, even when the inclination is
moderate, steps must be hewn. All the way down to
Meyringen and beyond it, if you wish to pursue the enquiry, these evidences abound. For a preliminary lesson in
the recognition of the traces of ancient glaciers no better
ground can be chosen than this.
Similar evidences are found in the valley of the Rhone;
you may track them through the valley for eighty miles,
and lose them at length in the lake of Geneva. But on the
flanks of the Jura, at the opposite side of the Canton de
Vaud, the evidences reappear. All along these limestone
slopes you have strewn the granite boulders of Miont Blanc.
Right and left also from the great Rhone valley the lateral
valleys show that they were once held by ice. On the
Italian side of the Alps the remains are, if possible, more
stupendous than on the northern side. Grand as are the
present glaciers to those who explore them in all their
lengths, they are mere pigmies in comparison with their
predecessors.
Not in Switzerland alone-not alone in proximity with
existing glaciers-are these well-known vestiges of the ancient ice discernible; in the hills of Cumberland they are
almost as clear as in the Alps. Where the bare rock has
been exposed for ages to the action of the weather, the
finer marks have in most cases disappeared; and the mammillated forms of the rocks are the only evidences. But
the removal of the soil which has protected them, often
discloses rock surfaces which are scarred as sharply, and
polished as cleanly as those which are now being scratched
and polished by the glaciers of the Alps. Round about




ANCIENT GLACIERS.              205
Scawfell the traces of the ancient ice appear, both in
roches moutonnds and blocs perches; and there are ample
facts to show that Borrodale was once occupied by glacier
ice. In North Wales, also, the ancient glaciers have placed
their stamp so firmly upon the rocks, that the ages which
have since elapsed have failed to obliterate even their
superficial marks.  All round Snowdon these evidences
abound. On the south-west coast of Ireland also rise the
Reeks of Magillicuddy, which tilt upwards, and catch upon
their cold crests the moist winds of the Atlantic; precipitation is copious, and rain at Killarney seems the rule of
Nature. In this moist region every crag is covered with
rich vegetation; but the vapours which now descend as
mild and fertilising rain, once fell as snow, which formed
the material for noble glaciers. The Black Valley was
once filled by ice, which planed down the sides of the Purple Mountain, as it moved towards the Upper Lake. The
ground occupied by this lake was entirely held by the ancient ice, and every island that now emerges from its sur*face is a glacier-dome. The fantastic names which many
of the rocks have received are suggested by the shapes into
which they have been sculptured by the mighty moulding
plane which once passed over them. North America is also
thus glaciated. But the most notable observation in connection with this subject is one recently made by Dr. Hooker
during a visit to Syria: he has found that the celebrated
cedars of Lebanon grow upon ancient glacier moraines.
To determine the condition which permitted of the formation of those vast masses of ice has long been a problem
with philosophers, and a consideration of the solutions
which have been offered fromn time to time will not be uninstructive. I have no new hypothesis, but it seems possible to give a truer direction and more definite aim to our
enquiries. The aim of all the writers on this subject, with
whom I am acquainted, has been directed to the attain



206                 LECTURE VI.
ment of cold. Some eminent men have thought, and some
still think, that the reduction of temperature during the
glacier epoch was due to a temporary diminution of solar
radiation, others have thought that, in its motion through
space, our system may have traversed regions of low tem.
perature, and that during its passage through these regions,
the ancient glaciers were produced. Others, with greater
correctness, have sought to lower the temperature by a redistribution of land and water. If I understand the writings of the eminent men who have propounded and advocated the above hypotheses, many of them seem to have
overlooked the fact, that the enormous extension of glaciers in bygone ages, demonstrates, just as rigidly, the
operation of heat as the action of cold.
Cold will not produce glaciers. You may have the bitterest north-east winds here in London throughout the
winter without a single flake of snow. Cold must have the
fitting object to operate upon, and this object-the aqueous
vapour of the air-is the direct product of heat. Let us
put this glacier question in another form: the latent heat
of aqueous vapour, at the temperature of its production in
the tropics, is about 1,000~ lFahr., for the latent heat grows
larger as the temperature of evaporation descends.  A
pound of water then vaporised at the equator, has absorbed
1,000 times the quantity of heat which would raise a pound
of the liquid one degree in temperature. But the quantity
of heat which would raise a pound of water one degree
would raise a pound of cast-iron ten degrees: hence, simply
to convert a pound of the water of the equatorial ocean
into vapour, would require a quantity of heat sufficient to impart to a pound of cast-iron 10,000 degrees of temperature.
But the fusing point of cast-iron is 2,000 Fahr.; therefore,
for every pound of vapour produced, a quantity of heat has
been expended by the sun sufficient to raise 5 lbs. of castiron to its melting point. Imagine, then, every one of




HYP'OTHIESES TO ACCOUNT FOR ANCIENT GLACIERS. 207
those ancient glaciers with its mass of ice quintupled; and
let the place of the mass, so augmented, be taken by an
equal mass of cast-iron raised to the white heat of fusion,
and we have the exact expression of the solar action involved in the production of the ancient glaciers. Substitute the hot iron for the cold ice-our speculations would
instantly be directed to account for the high temperature
of the glacial epoch, and a complete reversal of some of
the hypotheses above quoted would probably ensue.
It is perfectly manifest that by weakening the sun's action, either through a defect of emission, or by the steeping of the entire solar system in space of a low temperature, we should be cutting off the glaciers at their source.
Vast masses of mountain ice indicate, infallibly, commensurate masses of atmospheric vapour, and a proportionately
vast action on the part of the sun. In a distilling apparatus, if you required to augment the quantity distilled, you
would not surely attempt to obtain the low temperature
necessary to distillation, by taking the fire from under your
boiler; but this, if I understand them aright, is what has
been done by those philosophers who have sought to produce the ancient glaciers by diminishing the sun's heat. It
is quite manifest that the thing most needed to produce the
glaciers is an improved condenser; we cannot afford to lose
an iota of solar action; we need, if anything, more vapour,
but we need a condenser so powerful that this vapour, instead of falling in liquid showers to the earth, shall be so
far reduced in temperature as to descend in snow. The
problem, I think, is thus narrowed to the precise issue on
which its solution depends.




APPENDIX TO LECTURE VI.
ABSTRACT OF A DISCOURSE ON THE MER-DE-GLACE.*
A PORTION of a series of observations made upon the Mer-deGlace of Chamouni during the months of July and August last
year, formed the basis of this discourse.
The law first established by [M. Agassiz and] Prof. J. D.
Forbes, that the central portions of a glacier moved faster than
the sides, was amply illustrated by the deportment of lines of
stakes placed across the Mer-de-Glace at several places, and across
the tributaries of the glacier. The portions of the Mer-de-Glace
derived from these tributaries were easily traceable throughout
the glacier by means of the moraines. Thus, for example, that
portion of the trunk stream derived from the Glacier du Geant,
might be distinguished, in a moment, from the portion derived
from the other tributaries, by the absence of the debris of the
moraines upon the surface of the former. The commencement of
the dirt formed a distinct junction between both portions. Attention has been drawn by Prof. Forbes to the fact, that the eastern
side of the glacier in particular is' excessively crevassed;' and he
accounts for this crevassing by supposing that the Glacier du
G6ant moves most swiftly, and in its efforts to drag its more sluggish companions along with it, tears them asunder, and thus produces*-the fissures and dislocations for which the eastern side of
the glacier is remarkable. The speaker said that too much weight
must not be attached to this explanation. It was one of those
suggestions which are perpetually thrown out by men of science
* Given at the Royal Institution of Great Britain, on Friday, June 4,
1858. By John Tyndall, F.R.S.




MOTION OF MER-DE-GLACE.                    209
during the course of an investigation, and the fulfillment or nonfulfillment of which cannot materially affect the merits of the investigator. Indeed, the merits of Forbes must be judged on far
broader grounds; and the more his labours are compared with
those of other observers, the more prominently does his comparative intellectual magnitude come forward. The speaker would
not content himself with saying that the book of Prof. Forbes was
the best book which had been written upon the subject. The
qualities of mind, and the physical culture invested in that excellent work, were such as to make it, in the estimation of the physical investigator at least, outweigh all other books upon the subject taken together.* While thus acknowledging its merits, let a
free and frank comparison of its statements with facts be instituted.  To test whether the Glacier du Geant moved quicker than
its fellows, five different lines were set out across the Mer-deGlace, in the vicinity of the Montenvert, and in each of these it
was found that the point of swiftest motion did not lie upon the
Glacier du Geant at all; but was displaced so as to bring it comparatively close to the eastern side of the glacier. These measurements prove that the statement referred to is untenable; but the
deviation of the point of swiftest motion from the centre of the
glacier will doubtless be regarded by Prof. Forbes as of far greater importance to his theory. At the place where these measurements were made, the glacier turns its convex curvature to the
eastern side of the valley, being concave towards the Montenvert.
Let us take a bolder analogy than even that suggested in the explanation of Forbes, where he compares the Glacier du Geant to a
strong and swiftly-flowing river. Let us enquire how a river
would behave in sweeping round a curve similar to that here
existing.  The point of swiftest motion would undoubtedly lie on
* Since the above was written, my'Glaciers of the Alps' has been
published, and, soon after its appearance, a' Reply' to those portions of the
book which referred to the labours of M. Rendu was extensively circulated
by Principal Forbes. For more than two years I have abstained from
answering my distinguished censor; not from inability to do so, but because
I thought, and think, that, within the limits of the case, it is better to submit to misconception, than to make science the arena of a purely personal
controversy.




210              APPENDIX TO LECTURE VI.
that side of the centre of the stream towards which it turns its
convex curvature. Can this be the case with the ice? If so, then
we ought to have a shifting of the point of maximum motion
towards the western side of the valley, when the curvature of the
glacier so changes as to turn its convexity to the western side.
Such a change of flexure occurs opposite the passages called Les
Fonts, and at this place the view just enunciated was tested. It
was soon ascertained that the point of swiftest motion here lay at
a different side of the axis from that observed lower down. But
to confer strict numerical accuracy upon the result, stakes were
fixed at certain distances from the western side of the glacier, and
others at equal distances from the eastern side. The velocities of
these stakes were compared with each other, two by two; a stake
on the western side being always compared with a second one,
which stood at the same distance from the eastern side. The results of this measurement are given in the following table, the
numbers denoting inches:1st pair    2nd pair   3rd pair   4th pair   5th pair
West 15   West 1h7  West 221  West 231  West 23g
East 12~   East 15~  East 15~   East 181  East 19~
It is here seen that in each case the western stake moved more
rapidly than its eastern fellow stake; thus proving, beyond a
doubt, that opposite the Ponts the western side of the Mer-deGlace moves quickest-a result precisely the reverse of that observed where the curvature of the valley was different.
But another test of the explanation is possible. Between the
Ponts and the promontory of Trelaporte, the glacier passes a
point of contrary flexure, its convex curvature opposite to Trelaporte being turned towards the base of the Aiguille du Moine,
which stands on the eastern side of the valley. A series of stakes
was placed across the glacier here; and the velocities of those
placed at certain distances from  the western side were compared,
as before, with those of stakes placed at the same distances from
the eastern side. The following table shows the result of these
measurements; the numbers, as before, denote inches:1st pair         2nd pair        3rd pair
West.. 121    West.. 15         West 17:
East.. 14      East.. 17~     East 19




MOTION QF MER-DE-GLACE.                 211
Here we find that in each case the eastern stake moved faster
than its fellow. The point of maximum motion has therefore
once more crossed the axis of the glacier, being now upon its eastern side.
Determining the points of maximum motion for a great number of transverse sections of the Mer-de-Glace, and uniting these
points, we have the locus of the curve described by the point referred to. Fig. 59 represents a sketch of the Mer-de-Glace. The
dotted line is drawn along the centre of the glacier; the defined line, which crosses the axis of  Fig.?9.
the glacier at t&he points A A, is then the locus of
the point of swiftest motion. It is a curve more
deeply sinuous than the valley itself, and crosses
the central line of the valley at each point of
contrary flexure. The speaker drew attention to       A
the fact that the position of towns upon the
banks of rivers is usually on the convex side of
the stream, where the rush of the water renders
silting-up impossible: the Thames was a case in
point; and the same law which regulated its
flow and determined the position of the adjacent
towns, is at this moment operating, with silent
energy, among the Alpine glaciers.
Another peculiarity of glacier motion is now
to be noticed.
Before any observations had been made upon
the subject, it was surmised by Prof. Forbes that
the portions of a glacier near its bed were retarded by friction
against the latter.  This view was afterwards confirmed by his
own observations, and by those of M. Martins. Nevertheless the
state of our knowledge upon the subject, rendered further confirmation of the fact highly desirable. A rare opportunity for
testing the question was furnished by an almost vertical precipice
of ice, constituting the side of the Glacier de Geant, which was
exposed near the Tacul. The precipice was about 140 feet in
height. At the top and near the bottom stakes were fixed, and
by hewing steps in the ice, the speaker succeeded in fixing a
stake in the face of the precipice, at a point about 40 feet above
the base. After the lapse of a sufficient number of days, the prog



212             APPENDIX TO LECTURE VI.
ress of the three stakes was measured; reduced to the diurnal
rate, the motion was as follows:Top stake.. 600 inches
Mfiddle stake.. 4'59
Bottom stake.. 256 
We thus see that the top stake moved with more than twice
the velocity of the bottom one; while the velocity of the middle
stake lies between the two. But it also appears that the augmentation of velocity upwards is not proportional to the distance
from the bottom, but increases in a quicker ratio. At a height
of 100 feet from the bottom, the velocity would undoubtedly be
practically the same as at the surface. Measurements made upon
an adjacent ice-cliff proved this. We thus see the perfect validity
of the reason assigned by Forbes for the continued verticality of
the walls of transverse crevasses. Indeed a comparison of the result with his anticipations and reasonings will prove alike their
sagacity and their truth.
The most commanding view of the Mer-de-Glace and its tributaries is obtained from a point above the remarkable cleft in the
mountain range underneath the Aiguille de Charmoz, which is
sure to attract the attention of an observer standing at the Montenvert. This point, which is marked G on the map of Forbes,
the speaker succeeded in attaining. A Tiibingen professor once visited the glaciers of Switzerland, and seeing these apparently rigid
masses enclosed in sinuous valleys, went home and wrote a book,
flatly denying the possibility of their motion. An inspection firom
the point now referred to would have doubtless confirmed him in
his opinion; and indeed nothing can be more calculated to impress the mind with the magnitude of the forces brought into
play than the squeezing of the three tributaries of the Mer-deGlace through the neck of the valley at Trelaporte. But let us
state numerical results. Previous to its junction with its fellows,
the Glacier du G6ant measures 1,134 yards across. Before it is
influenced by the thrust of the Talefre, the Glacier de Lechaud
had a width of 825 yards; while the width of the TaIefre branch
across the base of the cascade, before it joins the Lechaud, is approximately 638 yards. The sum of these widths is 2,597 yards.
At Trelaporte,those three branches are forced through a gorge




MOTION OF MER-DE-GLtACE.                 213
893.yards wide, with a central velocity of 20 inches a day! The
result is still more astonishing, if we confine our attention to one
of the tributaries-that of the Lechaud. Before its junction with
the Talefre, the glacier has a width of 37~ English chains. At
Trelaporte this broad ice river is squeezed to a driblet of less
than 4 chains in width-that is to say, to about one-tenth of its
previous horizontal transverse dimension.
Whence is the force derived which drives the glacier through
the gorge? The speaker believed that it must be a pressure from
behind. Other facts also suggest that that the Glacier du Geant is
throughout its length in a state of forcible longitudinal compression. Taking a series of points along the axis of this glacier-if
these points, during the descent of the glacier, preserved their
distances asunder perfectly constant-there could be no longitudinal compression. The mechanical meaning of this term, as applied to a substance capable of yielding like ice, must be that the
hinder points are incessantly advancing upon the forward ones.
The speaker was particularly anxious to test this view, which first
occurred to him from' priori considerations. Three points, A 3 C,
were therefore fixed upon the axis of the Glacier du Geant, A being the highest up the glacier. The distance between A and s
was 545 yards, and that between B and c was 487 yards. The
daily velocities of these three points, determined by the theodolite, were as follows:A. 20-55 inches
B. 15'43
C. 1275,,
The result completely corroborates the foregoing anticipation.
The hinder points are incessantly advancing upon those in front,
and that to an extent sufficient to shorten a segment of this glacier, measuring 1,000 yards'in length, at the rate of 8- inches a
day. Were this rate uniform at all seasons, the shortening would
amount to 240 feet in a year. When we consider the compactness
of this glacier, and the uniformity in the width of the valley
which it fills, this result cannot fail to excite surprise; and the
exhibition of force thus rendered manifest must, in the speaker's
opinion, be mainly instrumental in driving the glacier through the
jaws of the granite vice at Trelaporte.




214             APPENDIX TO LECTURE VI.
In virtue of what quality, then, can ice be bent and squeezed,
and change its form in the manner indicated in the foregoing observations? The only theory worthy of serious consideration at
the present day is that- of Prof. Forbes, which attributes these
effects to the viscosity of the ice. The speaker did not agree with
this theory; as the term viscosity appeared to him to be wholly
inapplicable as expressive of the physical constitution of the glacier ice. He had already moulded ice into cups, bent it into
rings, changed its form in a variety of ways by artificial pressure,
and he had no doubt of his ability to mould a compact mass of
Norway ice which stood upon the table into a statuette; but
would viscosity be the proper term to apply to the process of
bruising and regelation by which this result could be attained?
He thought not. A mass of ice at 32~ is very easily crushed, but
it has as sharp and definite a fracture as a mass of glass. There
is no sensible evidence of viscosity.
The very essence of viscosity is the ability of yielding to a
force of tension, the texture of the substance, after yielding, being
in a state of equilibrium, so that it has no strain to recover from;
and the substances chosen by Prof. Forbes, as illustrative of the
physical condition of a glacier, possess this power of being drawn
out in a very eminent degree. But it has been urged, and justly
urged, that we ought not to conclude that viscosity is absent because hand specimens do not show it, any more than we ought to
conclude that ice is not blue because small fragments of the substance do not exhibit this colour. To test the question of viscosity, then, we must appeal to the glacier itself. Let us do so.
First, an analogy between the motion of a glacier through a sinuous valley, and of a river in a sinuous channel, has been already
pointed out. But the analogy fails in one important particular:
the river, and much more so a mass of flowing treacle, honey, tar,
or melted caoutchouc, sweeps round its curves without rupture of
continuity. The viscous mass stretches, but the icy mass breaks, and
the' excessive crevassing' pointed out by Prof. Forbes himself is
the consequence. Secondly, the inclinations of the Mer-de-Glace
and its three tributaries were taken, and the association of transverse crevasses with the changes of inclination was accurately
noted. Every Alpine traveller knows the utter dislocation and
confusion produced by the descent of the Mer-de-Glace friom the




FRAGILITY OF ICE.                   215
Chapeau downwards. A similar state of things exists in the icecascade of the Talefre. Descending from the Jardin, as the ice
approaches the fall, great transverse chasms are formed, which at
length follow each other so speedily as to reduce the ice masses
between them to mere plates and wedges, along which the explorer has to creep cautiously. These plates and wedges are in
some cases bent and crumpled by the lateral pressure, and on
some masses vortical forces appeared to have acted, turning large
pyramids 90~ round, so as to set their structure at right angles to
its normal position. The ice afterwards descends the fall, the
portions exposed to view being a fantastic assemblage of frozen
boulders, pinnacles, and towers, some erect, some leaning, falling
at intervals with a sound like thunder, and crushing the ice crags
on which they fall to powder. The descent of the ice through
this outlet has been referred to as a proof of its viscosity; but
the description just given does not, it was believed, harmonise
with our ideas of a viscous substance.
But the proof of the non-viscosity of the substance must be
sought at places where the change of inclination is very small.
Nearly opposite l'Angle there is a change from 4 to 9 degrees, and
the consequence is a system of transverse fissures, which renders
the glacier here perfectly impassable. Further up the glacier,
transverse crevasses are produced by a change of inclination from
3 to 5 degrees. This change of inclination is accurately protracted
in fig.- 60; the bend occurs at the point B; it is scarcely percepFig. 60.
tible, and still the glacier is unable to pass over it without breaking across. Thirdly, the crevasses are due to a state of strain,
from which the ice relieves itself by breaking: the rate at which
they widen may be taken as a measure of the amount of relief
demanded by the ice. Both the suddenness of their formation,
and the slowness with which they widen, are demonstrative of
the noif-viscosity of the ice. For were the substance capable of
stretching even at the small rate at which they widen, there would
be no necessity for their formation.
Further, the marginal crevasses of a glacier are known to be a




216             APPENDIX TO LECTURE VI.
consequence of the swifter flow of its central portions, which
tltrows the sides into a state of strain, from which they relieve
themselves by breaking. Now it is easy to calculate the amount
of stretching demanded of the ice in order to accommodate itself
to the speedier central flow. Take the case of a glacier, half a
mile wide. A straight transverse element, or slice, of such a glacier, is bent in twenty-four hours to a curve. The ends of the
slice move a little, but the centre moves more: let us suppose the
versed side of the curve formed by the slice in twenty-four hours
to be a foot, which is a fair average. Having the chord of this
arch, and its versed side, we can calculate its length. In the case
of the Mer-de-Glace, which is about half-a-mile wide, the amount
of stretching demanded would be about the eightieth of an inch
in twenty-four hours. Surely, if the glacier possessed a property
which could with any propriety be called viscosity, it ought to
be able to respond to this moderate demand; but it is not able
to do so: instead of stretching as a viscous body, in obedience
to this slow strain, it breaks as an eminently fragile one, and marginal crevasses are the consequence. It may be urged that it is
not fair to distribute the strain over the entire length of the
curve: but reduce the distance as we may, a residue must remain
which is demonstrative of the non-viscosity of the ice.
To sum up, then, two classes of facts present themselves to the
glacier investigator-one class in harmony with the idea of viscosity, and another as distinctly opposed to it. Where pressure
comes into play we have the former, where tension comes into play
we have the latter. Both classes of facts are reconciled by the
assumption, or rather the experimental verity, that the fragility
of ice and its power of regelation render it possible for it to
change its form without prejudice to its continuity.
NOTE ON THE REGELATION OF SNOW-GRANULES.*
I this morning (March 21, 1862) noticed an extremely interesting case of regelation. A layer of snow, between one and two
* Phil. Nag. 1862, vol. xxiii. p. 312.




REGELATION OF SNOW-GRANULES.              217
inches thick, had fallen on the glass roof of a small green-house
into which a door opened from the mansion to which the greenhouse was attached. Air, slightly warmed, acting on the glass
surface underneath, melted the snow in immediate contact with
the glass, and the layer in consequence slid slowly down the glass
roof. The inclination of the roof was very gentle, and the motion
correspondingly gradual. When the layer overshot the edge of
the roof, it did not drop off, but bent like a flexible body and
hung down over the edge for several inches. The continuity of
the layer was broken into rectangular spaces by the inclined longitudinal sashes of the rooft and from local circumstances one side
of the roof was warmed a little more than the other: hence the
subdivisions of the layer moved with different velocities, and overhung the edge to different depths. The bent and down-hanging
layer of snow in some cases actually curved up inwards.
Faraday has shown that when small fragments of ice float on
water, if two of them touch each other, they instantly cement
themselves at the point of contact; and on causing a row of fragments to touch, by laying hold of the terminal piece of the row,
you can draw all the others after it. A similar cementing must
have taken place among the particles of snow now in question,
which were immersed in the water of liquefaction near the surface
of the glass. But Faraday has also shown that when two fragments
of ice are thus united, a hinge-like motion sets in when you try to
separate the one from the other by a lateral push: one fragment
might, in fact, be caused to roll round another, like a wheel, by
the incessant rupture, and re-establishment of regelation.
The power of motion thus experimentally demonstrated, rendered it an easy possibility for the snow in question to bend itself in the manner observed. The lowermost granules, when the
support of the roof had been withdrawn, rolled over each other
without a destructionof continuity, and thus enabled the snow-layer
to bend as if it were viscous. The curling up was evidently due
to a contraction of the inner surface of the layer, produced, no
doubt, by the accommodation of the granules to each other, as
they slowly diminished in size.
J.T.
10




LECTURE VII.
[March 6, 1862.]
CONDUCTION A TRANSMISSION OF MOTION-GOOD CONDUCTORS AND BAD
CONDUCTORS-CONDUCTIVITY OF THE METALS FOR HEAT: RELATION BETWEEN THE CONDUCTIVITY OF HEAT AND THAT OF ELECTRICITY-INFLUENCE OF TEMPERATURE ON THE CONDUCTION'OF ELECTRICITY-INFLUENCE OF MOLECULAR CONSTITUTION ON THE CONDUCTION OF HEAT-RELATION OF SPECIFIC HEAT TO CONDUCTION-PHILOSOPHY OF CLOTHES:
RUMFORD'S EXPERIMENTS-INFLUENCE OF MECHANICAL TEXTURE ON
CONDUCTION-INCRUSTATIONS  OF BOILERS-THE SAFETY LAMP-CONDUCTIVITY OF LIQUIDS AND GASES: EXPERIMENTS OF RUMFORD AND
DESPRETZ —COOLING EFFECT OF HYDROGEN GAS-EXPERIMENTS OF MAGNUS ON THE CONDUCTIVITY OF GASES.
J   THINK  we are now  sufficiently conversant with our
subject to distinguish between the sensible motions produced by heat, and heat itself.  Heat is not the clash of
winds; it is not the quiver of a flame, nor the ebullition
of water, nor the rising of a thermometric column, nor the
motion which animates steam as it rushes from a boiler in
which it has been compressed. All these are mechanical
motions into which the motion of heat may be converted;
but heat itself is molecular motion-it is an oscillation
of ultimate particles. But such particles, when closely
grouped, cannot oscillate without communication of motion
from one to the other. To this propagation of the motion
of heat, through ordinary matter, we must this day devote
our attention.
Here is a poker, the temperature of which I am scarce



CONDUCTION OF HEAT.             219
ly conscious of: I feel it as a hard and heavy body, but it
neither warms me nor chills me; it has been before the
fire, and the motion of its particles at the present moment
chances to be the same as that which actuates my nerves;
there is neither communication nor withdrawal, and hence
the temperature of the poker on the one hand, and my
sensations on the other, remain unchanged. But I thrust
the end of the poker into the fire; it is heated; the particles in contact with the fire are thrown into a state of more
intense oscillation; the swinging atoms strike their neighbours, these again theirs, and thus the molecular music
rings along the bar. The motion, in this instance, is communicated from particle to particle of the poker, and finally
appears at its most disant end. If I now lay hold of the
poker, its motion is communicated to my nerves, and produces pain; the bar is what we call hot, and my hand, in
popular language, is burned. Convection we have already
defined to be the transfer of heat, by sensible masses, from
place to place; but this molecular transfer, which consists
in each atom taking up the motion of its neighbours, and
sending it on to others, is called the conduction of heat.
Let me exemplify this property of conduction in a
homely way. I have here a basin filled with warm water,
and in the water I place this cylinder of iron, an inch in
Fig. 61.
diameter, and two inches in height; this cylinder is to be
my source of heat. I lay my thermo-electric pile, o (fig.
61), thus flat, with its naked face turned upwards and on




220                 LECTURE VII.
that face I place a cylinder of copper, c, which now possesses the temperature'of this room. We observe no deflection of the galvanometer. I now place my warm cylinder,
i, having first dried it, upon the cool cylinder, which is
supported by the pile. The upper cylinder is not at more
than a blood heat; but you see that I have scarcely time
to make this remark before the needle flies aside, indicating
that the heat has reached the face of the pile. Thus the
molecular motion imparted to the iron cylinder by the
warm water has been communicated to the copper one,
through which it has been transmitted, in a few seconds,
to the face of the pile.
Different bodies possess different powers of transmitting molecular motion; in other words, of conducting
heat. Copper, which we have just used, possesses this
power in a very eminent degree. I will now remove the
copper, allow the needle to return to 00, and then lay upon
the face of the pile this cylinder of glass. On the cylinder
of glass I place my iron cylinder, which has been re-heated
in the warm water. There is, as yet, no motion of the
needle, and you would have to wait a long time to see it
move. We have already waited thrice the time which the
copper required to transmit the heat, and you see the needle
continues motionless. I place cylinders of wood, chalk,
stone, and fireclay, in succession on the pile, and heat their
upper ends in the same manner; but in the time which we
can devote to an experiment, not one of these substances is
competent to transmit the heat to the pile. The molecules
of these substances are so hampered or entangled, that
they are incompetent to pass the motion freely from one to
another. The bodies are all bad conductors of heat. On
the other hand, I place cylinders of zinc, iron, lead, bismuth, &c., in succession on the pile; each of them, you
see, has the power of transmitting the motion of heat
swiftly through its mass. In comparison with the wood,




CONDUCTION OF HEAT.                221
stone, chalk, glass, and clay, they are all good conductors
of heat.
As a general rule, though it is not without its exceptions, the metals are the best conductors of heat. But the
metals differ notably among themselves as regards their
powers of conduction. In illustration of this I will compare copper and iron. Here, behind me, are two bars, A B,
Fig. 62.
A 
[ llll iiiililllllll i  l 1[.l leilIl lllll l i ii i {1 lllis]nliiiil ll llllillllllllllllllllll
A C (fig. 62), placed end to end, with balls of wood attached by wax at equal distances from the place of junction.
Under the junction I place a spirit-lamp, which heats the
ends of the bars; the heat will be propagated right and
left through both. This bar is iron, this one is copper;
the heat will travel to the greatest distance along the best
conductor, liberating a greater number of its balls.
But for my present purpose I want a quicker experiment. Here, then, are two plates of metal, the one of copper, the other of iron, which are united together, so as to
form a long continuous plate c I (fig. 63). To it a handle
is attached, which gives the whole instrument the shape of
a T. From c to the middle, the plate is copper, from I to
the middle it is iron. At c I have soldered a small bar of
bismuth to the plate; at I a similar bar; and from both
bars wires, g g, lead to the galvanometer.  I warm  the
junction I by placing my finger on it; an electric current is there generated, and you observe the deflection.
The red end of the needle moves towards you. I withdraw my finger, and the needle sinks to 0~. I now warm,
in the same manner, the junction c; the needle is deflected,




222                 LECTURE VII.
but in the opposite direction. If I place a finger on each
end, at the same time, these currents neutralise each other,
and we have no deflection. I now place a spirit-lamp, with
Fig. 63.
a very small flame, directly under the middle of the compound plate; the heat will propagate itself from the centre towards the two ends, passing on one side through
copper, and on the other through iron. If the heat reach
both ends at the same instant, the one end will neutralize
the other, and the needle will rest quiescent. But if one
end be reached sooner than the other, we shall obtain a
deflection, and the direction in which the needle moves will
declare which end is heated. Now for the experiment: I
place the lamp underneath, and in three seconds the needle
flies aside. The red end moves towards me, which proves
that the end c is heated; the molecular motion has propagated itself most swiftly through the copper. I allow the
lamp to remain until each metal has taken up as much heat
as it can appropriate, until the ends of the plates become
stationary in temperature; that is to say, until the quantity of heat which they receive from the lamp is exactly
equal to the quantity dissipated in the space around them.
The copper still asserts its predominance; the needle still
indicates that the end c is most heated: and thus we prove
copper to be a better conductor of heat than iron. This
little experiment illustrates how in natural philosophy
we turn one agent to account in the investigation of an



EXPERIMENTAL ILLUSTRATIONS OF CONDUCTION. 223
other. Every new discovery is a new instrument: it was
once an end, but it is soon a means; and thus the growth
of science is secured.
One of the first attempts to determine with accuracy
the conductivity of different bodies for heat, was that suggested by Franklin, and carried out by Ingenhausz. He
coated a number of bars of various substances with wax,
and immersing the ends of the bars in hot oil, he observed
the distance to which the wax was melted on each of the
bars. The good conductors melted the wax to the greatest distance; and the melting distance furnished a measure
of the conductivity of the bar.
The second method was that pointed out by Fourier,
and followed out experimentally by M. De'spretz. A B (fig.
64) represents a bar of metal with holes drilled in it, intended to contain small thermometers. At the end of the
Fig. 6t.
bar was placed a lamp as a source of heat; the heat propagated itself through the bar, reaching the thermometer a
first, b next, c next, and so on. For a certain time the
thermometers continued to rise, but afterwards the state
of the bar became stationary, each thermometer marking a
constant temperature.  The better the conduction, the
smaller is the difference between any two successive thermometers. The decrement, or fall of heat, if I may use
the term, from the hot end towards the cold, is greater in
the bad conductors than in the good ones, and from the




224                   LECTURE VII.
decrement of temperature shown by the thermometers we
can deduce, and express by a number, the conductivity of
the bar.  This same method was followed by MM. Wiedemann and Franz, in a very important investigation, but instead of using thermometers they employed a suitable
modification of the thermo-electric pile. Of the numerous
and highly interesting results of these experiments the following is a r6sum6:Conductivity
Name of Substance    For Electricity  For Heat
Silver.... 100        100
Copper...  73           74
Gold..          59          53
Brass....  22          24
Tin....  23           15
Iron....  13            12
Lead....  11            9
Platinum...  10          8
German Silver.    6           6
Bismuth...   2           2
This table shows, that, as regards their conductive
powers, the metals differ very widely from  each other..
Calling, for example, the conductive power of silver 100,
that of German silver is only 6. You may illustrate this
difference in a very simple way by plunging two spoons,
one of German silver and the other of pure silver, into the
same vessel of hot water. After a little time you find the
free end of the silver spoon much hotter than that of its
neighbour; and if bits of phosphorus be placed on the ends
of the spoons, that on the silver will fuse and ignite in a
very short time, while the heat transmitted through the
other spoon will never reach an intensity sufficient to ignite,he phosphorus.
Nothing is more interesting to the natural philosopher
than the tracing out of connections and relationships between the various agencies of nature. We know that they




TABLE OF CONDUCTrVITIES.            225
are a common brotherhood, we know that they are mutually convertible, but as yet we know very little as to the precise form of the conversion. We have every reason to
conclude that heat and electricity are both modes of motion; we know experimentally that from electricity we can
get heat, and from heat, as in the case of our thermo-electric pile, we can get electricity. But although we have, or
think we have, tolerably clear ideas of the character of the
motion of heat, our ideas are very unclear as to the precise
nature of the change which this motion must undergo, in
order to appear as electricity-in fact, we know as yet
nothing about it.
Our table, however, exhibits one important connection
between heat and electricity. Beside the numbers expressing conductivity for heat, MM. Wiedemann and Franz have
placed the numbers expressing the conductivity of the same
metals for electricity. They run side by side: the good
conductor of heat is the good conductor of electricity, ind
the bad conductor of heat is the bad conductor of electricity.*  Thus we may infer, that the physical quality which
interferes with the transmission of heat, interferes, in a
proportionate degree, with the transmission of electricity.
This common susceptibility of both forces indicates a relationship which future investigations will no doubt clear up.
Let me point out another evidence of communion between heat and electricity. I have here a length of wire
made up of pieces of two different kinds of wire; there
are three pieces of platinum, each four or five inches long,
and three pieces of silver of the same length and thickness.
It is a proved fact that the amount of heat developed in a
wire by a current of electricity of a certain strength, is directly proportional to the resistance of the wire.t We
* Professor Forbes had previously noticed this.
f Joule, Phil. Mag. 1841, vol. xix. p. 263,
10'




226                  LECTURE Vet.
may figure the atoms as throwing themselves as barriers
across the track of the electric current-the current knocking against them, and imparting its motion to them, and
rendering the wire hot. In the case of the good conductor, on the contrary, the current may be figured as gliding
freely round the atoms without disturbing them in any
great degree. I will now send the self-same current from
a battery of twenty of Grove's cells through this compound wire. You see three spaces white-hot, and three
dark spaces between them. The white-hot portions of the
wire are platinum, and the dark portions are silver. The
electric current breaks impetuously upon the molecules of
the platinum, while it glides with little resistance among
the atoms of silver thus producing, in the metals, different
calorific effects.*
Now I wish to show you that the motion of heat interferes with the motion of electricity. You are acquainted
with the little platinum lamp which stands in front of the
table. It consists simply of a little coil of platinum wire
suitably attached to a brass stand. I can send a current
through that coil and cause it to glow. But you see I have
introduced into the circuit two feet additional of thin platinum wire, and on establishing the connection, the same
current passes through this wire and the coil. Both, you
see, are raised to redness-both are in a state of intense
molecular motion. What I wish now to prove is, that this
motion of heat, which the electricity has generated in these
two feet of wire, and in virtue of which the wire glows,
offers a hindrance to the passage of the current. The electricity has raised up a foe in its own path. I will cool this
wire, and thereby cause the heat to subside. I shall thus
open a wider door for the passage of the electricity. But
* May not the condensed ether which surrounds the atoms be the
vehicle of electric currents?




CONDUITTION OF HEAT AND ELECTRICITY.    227
if more electricity passes, it will announce itself at the platinum lamp; it will raise that red heat to whiteness, and
the change in the intensity of the light will be visible to
you all.
Fig. 05.
Thus, then, I plunge my red-hot wire into a beaker of
water w (fig. 65): observe the lamp, it becomes almost too
bright to look at. I raise the wire out of the water and
allow the motion of heat once more to develope itself; the
motion of electricity is instantly impeded, and the lamp
sinks in brightness. I again dip the wire into the cold
water, deeper and deeper: observe how the lightbecomes
intensified-deeper still, so as to quench the entire two feet
of wire; the augmented current raises the lamp to its maximum brightness, and now it suddenly goes out. The circuit is broken, for the coil has actually been fused by the
additional flow of electricity.
Let us now devote a moment's time to the conduction of
cold. To all appearance cold may be conducted like heat.
Here is a copper cylinder, which I warm a little by holding




228                 LECTuRE VII.
it for a moment in my hand. I place it on the pile, and the
needle goes up to 90~, declaring heat. On this cylinder I
place a second one, which, as you observe, I have chilled
by sinking it for some time in this mass of ice. We wait
a moment, the needle moves: it is now descending to zero,
passes it, and goes on to 90~ on the side of cold. Analogy
might well lead you to suppose that the cold is conducted
downwards from the top cylinder to the bottom one, as
the heat was conducted in our former experiments. I have
no objection to the term' conduction of cold,' if it be used
with a clear knowledge of the real physical process involved. The real process is, that the warm intermediate
cylinder first delivers up its motion, or heat, to the old cylinder overhead, and, having thus lost its own possession
of heat, it draws upon that of the pile. In our former experiments we had conduction of motion to the pile; in our
present one we have conduction of motion from the pile.
In the former case the pile is heated, in the latter chilled;
the heating produces a positive current, the chilling produces a negative current; but it is in both cases the propagation of motion with which we have to do, the heating
and the chilling depending solely upon the direction of
propagation. I place one of these metal cylinders, which I
have purposely cooled, on the face of our pile; a violent
deflection follows, declaring the chilling of the instrument.
Are we to suppose the cold to be an entity communicated
to the pile? No. The pile here is the warm body; its
molecular motion is in excess of that possessed by the cylinder; and when both come into contact the pile seeks to
make good the defect. It imparts a quantity of its own
motion to the cylinder, and by its bounty becomes impoverished: it chills itself, and generates the current due to
cold.
I remove the cold metal cylinder, and place upon the
pile a cylinder of wood, having the same temperature as




CONDUCTION OF COLD.              229
the metal one. The chill is very feeble, and the consequent
deflection very small. Why does not the cold wood produce an action equal to that of the cold metal? Simply because the heat communicated to it by the pile is accumulated at its under surface; it cannot escape through the bad
conducting wood as it escapes through the metal, and thus
the quantity of heat withdrawn from the pile, by the wood,
is less than that withdrawn by the copper. A similar effect
is produced when the human nerves are substituted for the
pile. Suppose you come into a cold room and lay your
hand upon the fire-irons, the chimney-piece, the chairs, the
carpet, in succession; they appear to you of different temperatures: the iron chills you more than the marble, the
marble more than the wood, and so on. Your hand is affected
exactly as the pile was affected in the last experiment. It
is needless to say that the reverse takes place when you
enter a hot room; that is to say, a room hotter than your
own bodies. I should certainly suffer if I were to lie down
upon a plate of metal in a Turkish bath; but I do not suffer
when I lie down on a bench of wood. By preserving the
body fiom contact with good conductors, very high temr
peratures may be endured. Eggs may be boiled and beefsteaks cooked, by the heat of an apartment in which the
living bodies of men sustain no injury.
The exact philosophy of this last experiment is worthy
of a moment's consideration. With it the names of Blagden and Chantrey are associated, those eminent men having exposed themselves, in ovens, to temperatures considerably higher than that of boiling water. Let us compare
the condition of the two living human beings, with that
of two marble statues placed in the same oven. The statues become gradually hotter, until finally they assume the
temperature of the air of the oven; the two sculptors,
under the same circumstances, do not similarly rise in temperature. If they did, the tissues of the body would be




230                 LECTURE VII.
infallibly destroyed, the temperature which they endured
being more than sufficient to stew the muscles in their own
liquids. But the fact is, that the heat of the blood is
scarcely affected by an augmentation of the external heat.
This heat, instead of being applied to increase the temperature of the body, is applied to the performance of
work, in altering the aggregation of the body; it prepares
the perspiration, forces it through the pores, and in part
vaporises it. Heat is here converted into potential energy; it is consumed in work. This is the waste-pipe, if I
may use the term, through which the excess of heat overflows; and hence it is, that under the most varying conditions of climate the temperature of the human blood is
practically constant. The blood of the Laplander is sensibly as warm as that of the Hindoo; while an Englishman,
in sailing from the north pole to the south, finds his bloodtemperature hardly heightened by his approach to the
equator, and hardly diminished by his approach to the antarctic pole.
When the communication of heat is gradual-as it always is when the body is surrounded by an imperfect
conductor-the heat is consumed in the manner indicated
as fast as it is supplied; but if the supply of heat be so
quick (as it would be in the case of contact with a good
conductor) that the conversion into this harmless potential
energy cannot be executed with sufficient rapidity, the injury of the tissues is the result. Some people have professed to see in this power of the living body to resist a
high temperature, a conservative action peculiar to the
vital force. No doubt all the actions of the animal organism are connected with what we call its vitality; but the
action here referred to is the same in kind as the melting
of ice, or the vaporisation of water. It consists simply in
the diversion of heat from the purposes of temperature to
the performance of work.




HEAT OF ItUMAN BODY CONSTANT.          231
Thus far we have compared the conducting power of
different bodies together; but the same substance may
possess different powers of conduction in different directions. Mlany crystals are so built that the motion of heat
runs with greater facility along certain lines of atoms than
along others. Here, for instance, is a large rock-crystala crystal of quartz forming an hexagonal pillar, which, if
complete would be terminated by two six-sided pyramids.
Heat travels with greater facility along the axis of this
crystal than across it. This has been proved in a very
simple manner by M. de Senarmont. I have here two
plates of quartz, one of which is cut parallel to the axis
of the crystal, and the other perpendicular to it. I coat
the plates with a layer of white wax, laid on by a camel's
hair pencil. The plates are pierced at the centre, and into
the hole I insert' o wire, which I warm by an electric curFig.                       Fig. 67.
Fig. 67a.
rent. B (fig. 66) is the battery whence the current proceeds; c is a capsule of wood, through the bottom of which
a sewing-needle passes; d is a second capsule, into which
dips the point of the needle, and Q is the perforated plate




232                  LECTURE VII.
of quartz.  Each capsule contains a drop of mercury.
When the current passes from c to d, the needle is heated,
and the heat is propagated in all directions. The wax
melts around the place where the heat is applied; and on
this plate, which is cut perpendicular to the axis of the
quartz, I find the figure of the melted wax to be a perfect
circle (fig. 67). The heat has travelled with the same rapidity all round, and melted the wax to the same distance
in all directions. I make a similar experiment with the
other plate: the wax is now melting; but I notice that its
figure is no longer a circle. The heat travels more speedily
along the axis than across it, and hence the wax figure is an
ellipse instead of a circle (fig. 67a). When the wax dries,
I will project magnified images of these two plates upon
the screen, and you will then see the circular figure of the
melted wax on the one, and the oval figure of the wax on
the other. Iceland spar conducts better along the crystallographic axis than at right angles to it, while a crystal of
tourmaline conducts best at right angles to its axis. The
metal bismuth, with which you are already acquainted,
cleaves with great facility in one direction, and, as has been
well shown by MM. Svanberg and MIatteucci, it conducts
both heat and electricity better along the planes of cleavage than across them.
In wood we have an eminent example of this difference
of conductivity. Upwards of twenty years ago MM. De
la Rive and De Candolle instituted an inquiry into the conductive power of wood,* and, in the case of five specimens
examined, established the fact that the velocity of transmission was greater along the fibre than across it. The manner
of experiment was that usually adopted in inquiries of this
nature, and which was applied to metals by M. Despretz.t
* M6m. de la Soc. de Geneve, vol. iv. p. 70.
f Annales de Chim. et de Phys. December 1827.




EFFECT OF MOLECIULAR STRUCTURE.        233
A bar of the substance was taken, one end of which was
brought into contact with a source of heat, and allowed to
remain so until a stationary temperature was assumed.
The temperatures attained by the bar, at various distances
from its heated end, were ascertained by means of thermometers fitting into cavities made to receive them; from
there data, with the aid of a well-known formula, the conductivity of the wood was determined.
To determine the velocity of calorific transmission in
different directions through wood, the instrument shown in
fig. 68 was devised some years ago by myself. Q Q' R i' is
an oblong piece of mahogany, A is a bar of antimony, B is
a bar of bismuth. The united ends of the two bars are
kept in close contact by the ivory jaws I i', and the other
ends are let into a second piece of ivory, in which they are
firmly fixed. Soldered to these ends are two pieces of
platinum wire, which proceed to the little ivory cups M M,
enter through the sides of the cups, and communicate with
a drop of mercury placed in the interior. The mahogany
is cut away, so that the bars A and B are sunk to a depth
which places their upper surfaces a little below the general
level of the slab of mahogany. The ivory jaws I I' are sunk
similarly. Two small projections are observed in the figure
jutting from I I'; across, from one projection to the other,
a fine membrane is stretched, thus enclosing a little chamber m, in front of the wedge-like end of the bismuth and
antimony junction; the chamber has an ivory bottom. s is
a wooden slider, which can be moved smoothly back and
forward along a bevelled groove, by means of the lever L.
This lever turns on a pivot near Q, and fits into a horizontal
slit in the slider, to which it is attached by the pin p' passing through both; in the lever an oblong aperture is cut,
through which p' passes, and in which it has a certain
amount of lateral play, so as to enable it to push the slider
forward in a straight line. Two projections are seen at




c9'
t-41i'3TJ;D  a                                 t1




CONDUCTION OF CRYSTALS AND OF WOOD.   235
the end of the slider, and across, from projection to projection, a thin membrane is stretched; a chamber m' is thus
formed, bounded on three sides and the bottom by wood,
and in front by the membrane. A thin platinum wire, bent
up and down several times, so as to form a kind of grating,
is laid against the back of this chamber, and imbedded in
the end of the slider by the stroke of a hammer; the end
in which the wire is imbedded is then filed down, until
about half the wire is removed, and the whole is reduced
to a uniform flat surface. Against the common surface of
the slider and wire, an extremely thin plate of mica is
glued, sufficient, simply, to interrupt all contact between
the bent wire and a quantity of mercury which the chamber m' is destined to contain; the ends w w' of the bent
wire proceed to two small cisterns c c', hollowed out in a
slab of ivory; the wires enter through the substance into
the cisterns, and come thus into contact with mercury, which
fills the latter. The end of the slider and its bent wire are
shown in fig. 68a. The rectangular space e fg h (fig. 68)
is cut quite through the slab of mahogany, and a brass
plate is screwed to the latter underneath; from this plate
(which, for reasons to be explained presently, is cut away,
as shown by the dotted lines in the figure) four conical
ivory pillars a b c d project upwards; though appearing to
be upon the same plane as the upper surfaces of the bismuth and antimony bars, the points are in reality 0'3 of an
inch below the said surfaces.
The body to be examined is reduced to the shape of a
cube, and is placed, by means of a pair of pliers, upon the
four supports a b c d; the slider s is then drawn up against
the cube, and the latter becomes firmly clasped between
the projections of the piece of ivory I I' on the one side,
and those of the slider s on the other. The chambers rn m'
being filled with mercury, the membrane in front of each
is pressed gently against the cube by the interior fluid




236                 LECTURE VII.
mass, and in this way perfect contact, which is absolutely
essential, is secured.
The problem which requires solution is the following:
-It is required to apply a source of heat of a strictly
measurable character, and always readily attainable, to
that face of the cube which is in contact with the membrane at the end of the slider, and to determine the quantity of this heat which crosses the cube to the opposite
face, in a minute of time. For the solution of this problem, two things are required-first, the source of heat to
be applied to the left hand of the face of the cube, and secondly, a means of measuring the amount which has made
its appearance at the opposite face at the expiration of a
minute.
To obtain a source of heat of the nature described, the
following method was adopted:-B is a small galvanic battery, from which a current proceeds to the tangent galvanometer T; passes round the ring of the instrument, deflecting in its passage the magnetic needle, which hangs in the
centre of the ring. From T the current proceeds to the
rheostat R; this instrument consists of a cylinder of serpentine stone, round which a German silver wire is coiled
spirally; by turning the handle of the instrument, any
required quantity of this powerfully resisting wire is
thrown into the circuit, the current being thus regulated at
pleasure. The sole use of these two last instruments, in the
present series of experiments, is to keep the current perfectly constant from day to day. From the rheostat the
current proceeds to the cistern c, thence through the bent
wire, and back to the cistern c', from which it proceeds to
the other pole of the battery.
The bent wire, during the passage of the current, becomes gently heated; this heat is transmitted through the
mercury in the chamber m' to the membrane in front of
the chamber; this membrane becomes the proximate source




INSTRUMENTS.                 237
of heat which is applied to the left-hand face of the cube.
The quantity of heat transmitted from this source, through
the mass of the cube, to the opposite face, in any given
time, is estimated from the deflection which it is able to
produce upon the needle of a galvanometer, connected with
the bismuth and antimony pair. G is a galvanometer used
for this purpose; from it proceed wires to the mercury
cups M M, which, as before remarked, are connected by platinum wires with A and B.
The action of mercury upon bismuth, as a solvent, is
well known; an amalgam is speedily formed when the two
metals come into contact. To preserve the thermo-electric
couple from this action, their ends are protected by a
sheathing of the same membrane as that used in front of
the chambers n m'.
Previous to the cube's being placed between the two
membranes, the latter, by virtue of the fluid masses behind
them, bulge out a little, thus forming a pair of soft and
slightly convex cushions. When the cube is placed on its
supports, and the slider is brought up against it, both cushions are pressed flat, and thus make the contact perfect.
The surface of the cube is larger than the surface of the
membrane; * and thus the former is always firmly caught
between the opposed rigid projections, the slider being held
fast in this position by means of the spring r, which is then
attached to the pin p. The exact manner of experiment is
as follows:-lHaving first seen that the needle of the galvanometer points to zero, when the thermo-circuit is complete, the latter is interrupted by means of the break-circuit key k'. At a certain moment, marked by the secondhand of a watch, the voltaic circuit is closed by the key k,
and the current is permitted to circulate for sixty seconds;
at the sixtieth second the voltaic circuit is broken by the
* The edge of each cube measured 0'3 inch.




238                 LECTURE VII.
left hand at k, while, at the same instant, the thermo-circuit is closed by the right hand at k'. The needle of the
galvanometer is instantly deflected, and the limit of the
first impulsion is noted; the amount of this impulsion depends, of course, upon the quantity of heat which has
reached the bismuth and antimony junction through the
mass of the cube, during the time of action. The limit of
the first impulsion being noted, the cube is removed and
the instrument is allowed to cool, until the needle of the
galvanometer returns to zero. Another cube being introduced, the voltaic circuit is once more closed, the current
permitted to circulate sixty seconds, then interrupted by
the left hand, the thermo-circuit being closed at the same
moment with the right, and the limit of the first swing is
noted as before.
Judging from the description, the mode of experiment
may appear complicated, but in reality it is not so. A single experimenter has the most complete command over the
entire arrangement. The wires from the small galvanic
battery (a single cell) remain undisturbed from day to day;
all that is to be done is to connect the battery with them,
and everything is ready for experiment.
There are in wood three lines, at right angles with each
other, which the mere inspection of the substance enables
us to fix upon as the necessary resultants of molecular action: the first line is parallel to the fibre; the second is
perpendicular to the fibre, and to the ligneous layers which
indicate the annual growth of the tree; while the third is
perpendicular to the fibre, and parallel, or rather tangential,
to the layers. From each of a number of trees a cube was
cut, two of whose faces were parallel to the ligneous layers, two perpendicular to them, while the remaining two
were perpendicular to the fibre. It was proposed to examine the velocity of calorific transmission through the wood
in these three directions. It may be remarked that the




INSTRUMENTS.                  239
cubes were fair average specimens of the woods, and were
in all cases well-seasoned and dry.
The cube was first placed upon its four supports a 6 c d,
so that the line of flux from m' to m was parallel to the
fibre, and the deflection produced by the heat transmitted
in sixty seconds was observed. The position of the cube.
was then changed, so that its fibre stood vertical, the line
of flux from m' to m being perpendicular to the fibre, and
parallel to the ligneous layers; the deflection produced by
a minute's action in this case was also determined. Finally, the cube was turned 90~ round, its fibre being still vertical, so that the line of flux was perpendicular to both lfibre
and layers, and the consequent deflection was observed.
In the comparison of these two latter directions the chief
delicacy of manipulation is necessary. It requires but a
rough experiment to demonstrate the superior velocity of
propagation along the fibre, but the velocities in all directions perpendicular to the fibre are so nearly equal that
it is only by great care, and, in the majority of cases, by
numerous experiments, that a difference of action can be
securely established.
The following table contains some of the results of the
enquiry; it will explain itself:



240                         LECTURE VII.
DEFLECTIONS.
Description of Wood.                      II.           III.
Perpendicular Perpendicular
Parallel to   to fibre and    to fibre and
fibre.     parallel to       to
ligneous layers. ligneous layers.
1 American Birch............   35            9-0          110
2 Oak.........................  34           95            11'0
3 Beech......................  33            8'8          10'8
4 Coromandel-wood...........   33          9-8           12'3
5 Bird's-eye Maple............    31       11'0           12'0
6 Lance-wood................   31          10'6           12-1
7 Box-wood....................  31           9'9           12-0
8 Teak-wood.................   31           9'9           12-4
9 Rose-wood...................    31       10'4           12'6
10 Peruvian-wood............    30          10'7           11'7
11 Green-heart................  29          11'4           12-6
12 Walnut.....................    28        11'0           13'0
13 Drooping Ash...............  28          11'0           12'0
14 Cocoa-wood.................  28          11'9           13-6
15 Sandal-wood...............   28          10'0           11-7
16 Tulip-wood..................  28         110           12-1
17 Camphor-wood...........    28             8'6           10'0
18 Olive-tree..................  28         10'5           13'2
19 Ash...........................  27        9'5           11'5
20 Black Oak..................  27            8'0           9'4
21 Apple-tree.................  26          10'0           12'5
22 Iron-wood..................  26          10'2           12'4
23 Chestnut....................  26         10'1           11'5
24 Sycamore....................  26          10'6           12'2
25 Honduras Mahogany......       25            9'0          10'0
26 Brazil-wood................    25         11'9          13'9
27 Yew....................      24           11'0           12'0
28 Elm.........................    24        10'0           11'5
29 Plane-tree..................  24          10'0           12'0
30 Portugal Laurel............   24           10.0           11-5
31 Spanish Mahogany.........    23           11'5           12'5
32 Scotch Fir..................  22          10'0           12'0
The above table  furnishes us with  a corroboration  of
the result arrived  at by De la Rive and De Candolle, regarding the superior conductivity of the wood in the direction of the fibre.  Evidence is also afforded as to how  little




AXES OF CONDUCTION IN WOOD.            241
mere density affects the velocity of transmission. There
appears to be neither law nor general rule here. American
Birch, a comparatively light wood, possesses undoubtedly
a higher transmissive power than any other in the list.
Iron-wood, on the contrary, with a specific gravity of 1'426,
stands low. Again, Oak and Coromandel-wood-the latter
so hard and dense that it is used for sharp war-instruments
by savage tribes-stand near the head of the list, while
Scotch Fir and other light woods stand low.
If we cast our eyes along the second and third columns
of the table, we shall find that in every instance the velocity
of propagation is greatest in a direction perpendicular to
the ligneous layers. The law of molecular action, as regards the transmission of heat through wood, may therefore be expressed as follows:At all the points not situate in the centre of the tree,
qwood possesses three unequal axes of calorific conduction,
which are at right angles to each other.  Tlhe first, and
principal axis, is parallel to the fibre of the wood; the
second, and intermediate axis, is perpendicular to the fibre
and to the ligneous layers; while the third and least axis
is perpenzdicular to the fibre and parallel to the layers.
MM. De la Rive and DIe Candolle have remarked upon
the influence which its feeble conducting power in a lateral
direction must exert in preserving within a tree the warmth
which it acquires fiom the soil. In virtue of this property
a tree is able to resist sudden changes of temperature which
would probably be prejudicial to it: it resists alike the sudden abstraction of heat from within and the sudden accession of it from without. But Nature has gone further, and
clothes the tree with a sheathing of worse-conducting material than the wood itself, even in its worst' direction.
The following are the deflections obtained by submitting
11'




242                    LECTURE VII.
a number of cubes. of bark, of the same size as the cubes
of wood, to the same conditions of experiment:Deflection  Corresponding deflection
produced by the wood
Beech-tree Bark...7                      10'8~
Oak-tree Bark.... 7            11'0
Elm-tree Barkl...'7             115
Pine-tree Bark.... 7            12'0
The direction of transmission, in these cases, was from
the interior surface of the bark outwards.
The average deflection produced by a cube of wood,
when the flux is lateral, may be taken at
120;
a cube of rock crystal (pure silica), of the same size, produces the deflection of
90~.
Two bodies so diverse, where they cover any considerable portion of the earth's surface, must affect the climate
very differently.  There are the strongest experimental
grounds for believing that rock-crystal possesses a higher
conductive power than some of the metals.
The following numbers express the transmissive power
of a few other organic structures: cubes of the substances
were examined in the usual manner:Tooth of Walrus... 16
Tusk of East-Indian Elephant.. 17
Whalebone....                    9
Rhinoceros'-horn....  9
Cow's-horn......               9
Sudden changes of temperature are prejudicial to animal and vegetable health; the substances used in the construction of organic tissues are exactly such as are best calculated to resist those changes.
The following results further illustrate this point. Each




LOW CONDUCTIVITY OF ORGANIC TISSUES.    243
of the substances mentioned was reduced to the cubical
form, and submitted to an examination similar in every
respect to that of wood and quartz. While, however, a
cube of the latter substance produces a deflection of 90~, a
cube of
Sealing-wax produces a deflection of.    00
Sole leather...0
Bees'-wax...0
Glue......                       0 
Gutta-percha..                    0
India-rubber.0.. 
Filbert-kernel.                0
Almond-kernel... 0
Boiled ham-muscle.                0
Raw veal-muscle.... 0
The substances here named are animal and vegetable
productions; and the experiments demonstrate the extreme
imperviousness of every one of them. Starting from the
principle that sudden accessions or deprivations of heat are
prejudicial to animal and vegetable health, we see that the
materials chosen are precisely those which are best calculated to avert such changes.
I wish now to direct your attention to what may, at
first sight, appear to you a paradoxical experiment. Here
is a short prism of bismuth, and here another of iron, of
the same size. I coat the ends of both prisms with white
wax, and then place them, with their coated surfaces upwards, on the lid of this vessel, which contains hot water.
The motion of heat will propagate itself through the
prisms, and you are to observe the melting of the wax. It
is already beginning to yield, but on which? On the bismuth. And now the white has entirely disappeared from
the bismuth, the wax overspreads it in a transparent liquid
layer, while the wax on the iron is not yet melted. How
is this result to be reconciled with the fact stated in our




244                  LECTURE VII.
table (page 224), that, the conduction of iron being 12,
conduction of bismuth is only 2?  In this experiment the
bismuth seems to be the best conductor. We solve this
enigma by turning to our table of specific heat (Lecture
V.); we there find that, the specific heat of iron being
1138, that of bismuth is only 308; to raise it, therefore, a
certain number of degrees in temperature, iron requires
more than three times the absolute quantity of heat required by bismuth. Thus, though the iron is really a much
better conductor than the bismuth, and is at this moment
accepting, in every unit of time, a much greater amount
of heat than the bismuth, still, in consequence of the number of its atoms, or the magnitude of its interior work, the
augmentation of temperature, in the case of iron, is slow.
Bismuth, on the contrary, can immediately devote a large
proportion of the heat imparted to it to the augmentation
of temperature; and thus it apparently outstrips the iron in
the transmission of that motion to which temperature is
due.
You see here very plainly the incorrectness of the
statements sometimes made in books, and certainly made
very frequently by candidates in our science examinations,
regarding the experiment of Ingenhausz, to which I have
already referred. It is usually stated, that the greater the
quickness with which the wax melts, the better is the conductor. If the bad conductor and the good conductor have
the same specific heat, this is true, but in other cases, as
proved by our last experiment, it may be entirely incorrect.
The proper way of proceeding, as already indicated, is to
wait until both the iron and the bismuth have attained a
constant temperature-till each of them, in fact, has accepted, and is transmitting, all the motion which it can accept, or transmit, from the source of heat; when this is
done, it is found that the quantity transmitted by the iron
is six times greater than that transmitted by the bismuth.




INFLUEKCFE OF SPECIFIC HEAT.         245
You remember our experiments with the Trevelyan instrument, and know the utility of having a highly expansible
body as the bearer of the rocker. Lead is good, because
it is thus expansible. But the coefficient of expansion of
zinc is slightly higher than that of lead; still zinc does not
answer well as a block. The reason is, the specific heat of
zinc is more than three times that of lead, so that the heat
communicated to the zinc by the contact of the rocker,
produces only about one-third the augmentation of temperature, and a correspondingly small amount of local expansion.
These considerations also show that in our experiments
on wood the quantity of heat transmitted by our cube in
one minute's time, cannot, in strictness, be regarded as the
expression of the conductivity of the wood, unless the
specific heat of the various woods be the same. On this
point no experiments have been made. But as regards the
influence of molecular structure, the experiments hold
good, for here we compare one direction with another, in
thoe same cube. With respect to organic structures, I may
add that, even allowing them time to accept all the motion
which they are capable of accepting, from a source of heat,
their power of transmitting that motion is exceedingly
low. They are really bad conductors.
It is the imperfect conductibility of woollen textures
which renders them so eminently fit for clothing. They
preserve the body from sudden accessions or losses of heat.
The same quality of non-conductibility manifests itself when
we wrap flannel round a block of ice. The ice thus preserved is not easily melted. In the case of a human body
on a cold day, the woollen clothing prevents the transmission of motion from within outwards; in the case of the
ice on a warm day, the self-same fabric prevents the transmission of motion from without inwards. Animals which
inhabit cold climates are furnished by Nature with their




246                    LECTURE VII.
necessary clothing. Birds especially need this protection,
for they are still more warm-blooded than the mammalia.
They are furnished with feathers, and between the feathers
the interstices are filled with down, the molecular constitution and mechanical texture of which render it, perhaps,
the worst of all conductors.  Here we have another example of that harmonious relation of life to the conditions of
life, which is incessantly presented to the student of natural science.
The indefatigable Rumford made an elaborate series
of experiments on the conductivity of the substances used in
clothing.*  His method was this: —A mercurial thermometer was suspended in the axis of a cylindrical glass tube
ending with a globe, in such a manner that the centre of
the bulb of the thermometer occupied the centre of the
globe; the space between the internal surface of the globe
and the bulb was filled with the substance whose conductive power was to be determined; the instrument was then
heated in boiling water, and afterwards, being plunged into
a freezing mixture of pounded ice and salt, the times of
cooling down 135~ Fahr. were noted.  They are recorded
in the following table:Surrounded with            Seconds
Twisted silk. 917
Fine lint....1032
Cotton wool.. 1046
Sheep's wool. 1118
Taffety.1169
Raw silk....1264
Beavers' fur. 1296
Eider down... 1305
Hares' fur. 1312
Wood ashes.        927
Charcoal.          937
Lamp-black..1117
* Phil. Trans. 1792, p. 48.




ACTION OF CLOTHING.              247
Among the substances here examined, hares' fur offered
the greatest impediment to the transmission of the heat.
The transmission of heat is powerfully influenced by
the mechanical state of the body through which it passes.
The raw and twisted silk of Rumford's table illustrate
this. Pure silica, in the state of hard rock-crystal, is a
better conductor than bismuth or lead; but if the crystal
be reduced to powder, the propagation of heat through
that powder is exceedingly slow.  Through transparent
rock-salt heat is copiously conducted, through common
table-salt very feebly. I have here some asbestos, which
is composed of certain silicates in a fibrous condition; I
place it on my hand, and on it I place a red-hot iron ball:
you see I can support the ball without inconvenience. The
asbestos intercepts the heat. That this division of the substance should interfere with the transmission might reasonably be inferred; for, heat being motion, anything which
disturbs the continuity of the molecular chain, along which
the motion is conveyed, must affect the transmission. In
the case of the asbestos the fibres of the silicates are separated from each other by spaces of air; to propagate itself,
therefore, the motion has to pass from the silicate to the
air, a very light body, and again from the air to the silicate, a comparatively heavy body; and it is easy to see
that the transmission of motion through this composite texture must be very imperfect. In the case of an animal's
fur, this is more especially the case; for here not only do
spaces of air intervene between the hairs, but the hairs
themselves, unlike the fibres of the asbestos, are very bad
conductors. Lava has been known to flow over a layer of
ashes underneath which was a bed of ice, and the non-conductivity of the ashes has saved the ice from fusion. Redhot cannon-balls may be wheeled to the gun's mouth in
wooden barrows partially filled with sand. Ice is packed
in sawdust to prevent it from melting; powdered charcoal




248                  LECTIJRE VII.
is also an eminently bad conductor. But there are cases
where sawdust, chaff, or charcoal could not be used with
safety, on account of their combustible nature. In such
cases, powdered gypsum may be used with advantage; in
the solid crystalline state it is incomparably a worse conductor than silica, and it may be safely inferred, that in
the powdered state its imperviousness far transcends that
of sand, each grain of which is a good conductor. A
jacket of gypsum powder round a steam boiler would materially lessen its loss of heat.
Water usually holds certain minerals in solution. In
percolating through the earth, it dissolves more or less of
the substances with which it comes into contact.  For
example, in chalk districts the water always contains a
quantity of carbonate of lime; such water is called hard
water. Sulphate of lime is also a common ingredient of
water. In evaporating, the water is only driven off, the mineral is left behind, and often in quantities too great to be
held in solution by the water. Many springs are strongly
impregnated by carbonate of lime, and the consequence is,
that when the waters of such springs reach the surface and
are exposed to the air, where they can partially evaporate,
the mineral is precipitated, and forms incrustations on the
surfaces of plants and stones over which the water trickles.
In the boiling of water the same occurs; the minerals are
precipitated, and there is scarcely a kettle in London which
is not internally coated with a mineral incrustation. This
is an extremely serious difficulty as regards steam boilers;
the crust is a bad conductor, and it may become so thick as
materially to intercept the passage of heat to the water. I
have here an example of this mischief. This is a portion
of a boiler belonging to a steamer, which was all but lost
through the exhaustion of her coals: to bring this vessel
into port her spars and every piece of available wood were
burnt. On examination this formidable incrustation was




WITHDRAWAL OF HEAT 3BY GOOD CONDUCTORS. 249
found within the boiler: it is mainly carbonate of lime,
which by its non-conducting power rendered a prodigal expenditure of fuel necessary to generate the required quantity of steam. Doubtless the slowness of many kettles in
boiling would be found due to a similar cause.
I wish now to bring before you one or two instances of
the action of good conductors in preventing the local accumulation of heat. I have here two spheres of the same
size, both covered closely with white paper. One of them
is copper, the other is wood. I place a spirit lamp underneath each of them, and after a time we will observe the
effect. The motion of heat is, of course, communicating itself to each ball, but in one it is quickly conducted away
from the place of contact with the flame, through the entire
mass of the ball; in the other this quick conduction does
not take place, the motion therefore accumulates at the
point where the flame plays upon the ball; and here you
have the result. I turn up the wooden ball, the white paper is quite charred; I turn up the other ball,-so far from
being charred, it is wet at its under surface by the condensation of the aqueous vapour generated by the lamp. Here
is a cylinder covered closely with paper; I hold its centre
thus over the lamp, turning it so that the flame shall play
all round the cylinder: you see a well-defined black mark,
on one side of which the paper is charred, on the other
side not. The cylinder is half brass and half wood, and
this black mark shows their line of junction: where the
paper covers the wood, it is charred; where it covers the
brass, it is not sensibly affected.
If the entire moving force of a common rifle bullet
were communicated to a heavy cannon-ball, it would produce in the latter a very small amount of motion. Supposing the rifle bullet to weigh two ounces, and to have a
velocity of 1,600 feet a second, the moving force of this
bullet communicated to a 100 lb. cannon-ball would impart
11'




250                 LECTURE Vu.
to the latter a velocity of only 32 feet a second. Thus
with regard to a flame; its molecular motion is very intense, but its weight is extremely small, and if communicated to a heavy body, the intensity of the motion must
fall. For example, I have here a sheet of wire gauze, with
meshes wide enough to allow air to pass through them
with the utmost freedom; and here is a jet of gas burning
brilliantly. I bring down the wire gauze upon the flame;
you would imagine that the flame could readily pass
through the meshes of the gauze; but no, not a flicker gets
through (fig. 69). The combustion is entirely confined to
Fig. 69.                   Fig. T0.
the space under the gauze. I extinguish the flame, and allow the unignited gas to stream from the burner. I place
the wire gauze thus above the burner: the gas, I know, is
now freely passing through the meshes. I ignite the gas
above; there you have the flame, but it does not propagate
itself downwards to the burner (fig. 70). You see a dark
space of four inches between the burner and the gauze, a
space filled with gas in a condition eminently favourable to
ignition, but still it does not ignite. Thus, you see, this
metallic gauze, which allows the gas to pass freely through,
intercepts the flame. And why? A certain heat is necessary to cause the gas to ignite; but by placing the wire
gauze over the flame, or the flame over the wire gauze, you
transfer the motion of that light and quivering thing to




THE SAFETY-LAMP.               251
the comparatively heavy gauze. The intensity of the molecular motion is greatly lowered by being communicated
to so great a mass of matter-so much lowered, indeed,
that it is incompetent to propagate the combustion to the
opposite side of the gauze.
We are all, unhappily, too well acquainted with the terrible accidents that occur through explosions in coal mines.
You know that the cause of these explosions is the presence
of a certain gas-a compound of carbon and hydrogengenerated in the coal strata. When this gas is mixed with
a sufficient quantity of air, it explodes on ignition, the carbon of the gas uniting with the oxygen of the air, to produce carbonic acid; the hydrogen of the gas uniting with
the oxygen of the air to produce water. By the flame of
the explosion the miners are burnt; but even should this
not destroy life, they are often suffocated afterwards by
the carbonic acid produced. The original gas is the miner's'fire-damp,' the carbonic acid is his'choke-damp.' Sir
Humphry Davy, after having assured himself of the action
of wire gauze, which I have just exhibited before you, applied it to the construction of a lamp which should enable
the miner to carry his light into an explosive atmosphere.
Previous to the introduction of the safety-lamp, the miner
had to content himself with the light from sparks produced by the collision of flint and steel, for it was found
that these sparks were incompetent to ignite the firedamp.
Davy surrounded a common oil lamp by a cylinder of
wire gauze (fig. 71). As long as this lamp is fed by pure
air, the flame burns with the ordinary brightness of an oilflame; but when the miner comes into an atmosphere which
contains'fire-damp,' his flame enlarges, and becomes less
luminous; instead of being fed by the pure oxygen of the
air, it is now in part surrounded by inflammable gas. This
he ought to take as a warning to retire. Still, though a




252                 LECTURE VII.
continuous explosive atmosphere may extend from the air
outside, through the meshes of the gauze, to the flame
within, the ignition is not propagated
across the gauze. The lamp may be filled
with an almost lightless flame, and still
explosion does not occur. A defect in
lt 0   l~    the gauze, the destruction of the wire at
/.i l.....any point by oxidation, hastened by the
flame playing against it, would cause an
explosion.  The motion of' the lamp
through the air might also force, mechanically, the flame through the meshes. In
short, a certain amount of intelligence
and caution is necessary in using the
lamp. The intelligence, unhappily, is not
l  always possessed, nor the caution always
exercised, by the miner; and the consequence is, that even with the safety-lamp,
explosions still occur. Before permitting
a man or a boy to enter a mine, would it
not be well to place these results, by experiment, visibly before him? Mere advice will not enforce caution; but let the miner have the
physical image of what he is to expect, clearly and vividly
before his mind, and he will find it a restraining and a
monitory influence, long after the effect of cautioning
words has passed away.
A word or two now on the conductivity of liquids and
gases. Rumford made numerous experiments on this subject, showing at once clearness of conception and skill of
execution. He supposed liquids to be non-conductors,
clearly distinguishing the'transport' of heat by convection from true conduction; and in order to prevent convection in his liquids, he heated them at the top. In this way
he found the heat of a warm iron cylinder incompetent to




CHILLING BY HYDROGEN AND AIR.            253
pass downwards through 02 of an inch of olive oil; he also
boiled water in a glass tube, over ice, without melting the
substance.  The later experiments of M. Despretz show,
however, that liquids possess true, though extremely feeble,
powers of conduction. Rumford also denied the conductivity of gases, though he was well acquainted with their convection.*  The subject of gaseous conduction has been recently taken up by Professor Magnus, of Berlin, who considers that his experiments prove that hydrogen gas conducts heat like a metal.
The cooling action of air may be thus prettily illustrated-here is a platinum wire, formed into a coil; I send a
voltaic current through the coil, till it glows bright red. I
now stretch out the coil so as to form a straight wire; the
glow instantly sinks-you can now hardly see it. This effect
is due entirely to the freer access of the cold air to the
stretched wire. Here, again, is a receiver R (fig. 72) which
can be exhausted at pleasure; attached to the bottom is a
vertical metal rod, m n, and through the top another rod,
a b, passes, which can be moved up and down through an
air-tight collar, so as to bring the ends of the two rods
within any required distance of each other. At present
the rods are united by two inches of platinum wire, b m,
which I can heat to any required degree of intensity by a
voltaic current. I have here a small battery, and now I
make my connections; the wire is barely luminous enough
to be seen; in fact, the current from a single cell only is
now sent'through it. It is surrounded by air, which, no
doubt, is carrying off a portion of its heat.' I exhaust
the receiver-the wire glows more brightly than before.
I allow  air to -enter —the wire, for a time, is quite
quenched, rendered perfectly black; but after the air has
ceased to enter, its first feeble glow is restored. The cur* Phil. Trans. 1792: Essays,'vol. ii. p. 56.




254                  LECTURE VII.
rent of air here passing over the wire, and destroying its
glow, acts like the current which the wire itself establishes
by heating the air in contact with it.
Fi, 72.    The cooling of the wire in both cases is
la
due to convection and not to true conduction.
The same effect is obtained in a greatly increased degree, if hydrogen be used
instead of air. We owe this interesting
Kl b 1  observation to Mr. Grove, and it formed
the starting-point of M. Magnus's investigation. The receiver is now exhausted,
and the wire is almost white-hot.  Air
1,~ m l~l/~  cannot do more than reduce that whiteness to bright redness; but observe what
hydrogen can do. On the entrance of
this gas the wire is totally quenched, and
even after the receiver has been filled
with the gas, and the inward current has
ceased, the glow of the wire is not restored. The electric current now passing
through the wire is from two cells; I try
three cells, the wire glows feebly; five cause it to glow
more brightly, but even with five it is but a bright red.
WVere the hydrogen not there, the current now passing
through the wire would infallibly fuse it.  Let us see
whether this is not the case. I commence exhaustion,-the
first few strokes of the pump produce a scarcely sensible
effect; but I continue to work the pump, and now the
effect begins to be visible. The wire whitens and appears
to thicken. To those at a distance it is now as thick as a
goose-quill; and now it glows upon the point of fusion; I
continue to work the pump, the light suddenly vanishes,
the wire is fused.
This extraordinary cooling power of hydrogen has been




EXPERIMENTS OF MA GNUS.            255
usually ascribed to the mobility of its particles, which enables currents to establish themselves in this gas with greater facility than in any other. But Prof. Magnus conceives
the chilling of the wire to be an effect of conduction. To
impede, if' not prevent, the formation of currents, he passes
his platinum wire along the axis of a narrow glass tube,
which he fills with hydrogen. Although in this case the
wire is surrounded by a mere film of the gas, and currents,
in the ordinary sense, are scarcely to be assumed, the film
shows itself just as competent to quench the wire, as when
the latter is caused to pass through a large vessel containing the gas. He also heated the closed top of a vessel, and
found that the heat was conveyed more quickly from it to
a thermometer, placed at some distance below the source
of heat, when the vessel was filled with hydrogen, than
when it was filled with air. He found this to be the case,
even when the vessel was loosely filled with cotton wool or
eider down. Here, he contends, currents could not be
formed; the heat must be conveyed to the thermometer by
the true process of conduction, and not by convection.
Beautiful and ingenious as these experiments are, I do
not think they conclusively establish the conductivity of
hydrogen. Let us suppose the wire in Prof. Magnus's first
experiment to be stretched along the axis of a wide cylinder containing hydrogen, we should have convection, in the
ordinary sense, on heating the wire. Where does the heat
thus dispersed ultimately go? It is manifestly given up to
the sides of the cylinder, and if we narrow our cylinder we
simply hasten the transfer. The process of narrowing may
continue till a narrow tube is the result,-the convection
between centre and sides, will continue and produce the
same cooling effect as before. The heat of the gas being
instantly lowered by communication to the heavy tube, it
is prepared to re-abstract the heat from the wire. With
regard also to the vessel heated at the top, it would require




256                 LECTURE VI.
a surface mathematically horizontal, and a perfectly uniform
application of heat to that surface-it would, moreover, be
necessary to cut the heat sharply off from the sides of the
vessel-to prevent convection. Even in the interstices of
the eider down and of the cotton wool the convective mobility of hydrogen will make itself felt, and taking everything into account, I think the experimental question of
gaseous conduction is still an open one.




LECTURE VIII.
[March 13, 1862.]
COOLING A LOSS OF MOTION: TO WHAT IS THIS MOTION IMPARTED?-EXPERIMENTS ON SOUND BEARING ON THIS QUESTION-EXPERIMENTS ON
LIGHT BEARING ON THIS QUESTION-THE THEORIES OF EMISSION AND
UNDULATION-LENGTH OF WAVES AND NUMBER OF IMPULSES OF LIGHT
— PHYSICAL CAUSE'OF COLOUR-INVISIBLE RAYS OF THE SPECTRUMTHE CALORIFIC RAYS BEYOND THE RED-THE CHEKMICAL RAYS BEYOND
THE BLUE-DEFINITION OF RADIANT HEAT-REFLECTION OF RADIANT
HEAT FROMI PLANE AND CURVED SURFACES: LAWS THE SAME AS THOSE
OF LIGHT-CONJUGATE MIRRORS.
APPENDIX: —-ON SINGING FLAMES.
E have this day reached the boundary of one of the
two great divisions of our subject; hitherto we
have dealt with heat while associated with solid, liquid, or
gaseous bodies.  We have found it competent to produce
changes of volume in all these bodies.  We have also observed it reducing solids to liquids, and liquids to vapours;
we have seen it transmitted through solids by the process
of conduction, and distributing itself through liquids and
gases by the process of convection.  We have now to follow it into conditions of existence, different from any
which we have examined hitherto.
I hang this heated copper ball in the air; you see it
glow, the glow sinks, the ball becomes obscure; in popular
language the ball cools. Bearing in mind what has been
said on the nature of heat, we must regard this cooling as




258                 LECTURE VIII.
a loss of motion on the part of the ball. But motion cannot be lost without being imparted to something; to what
then is the molecular motion of this ball transferred?
You would, perhaps, answer to the air, and this is partly
true: over the ball air is passing, and rising in a heated
column, which is quite visible against the screen, when we
allow the electric beam to pass through the warmed air.
But not the whole, nor even the chief part, of the molecular motion of the ball is lost in this way. If the ball were
placed in vacuo it would still cool. Rumford, of whom we
have heard so much, contrived to hang a small thermometer, by a singlefibre of silk, in the middle of a glass globe
exhausted by means of mercury, and he found that the calorific rays passed to and fro across the vacuum; thus proving that the transmission of the heat was independent of
the air. Davy, with an apparatus which I have here before me, showed that the heat rays from the electric light
passed freely through an air-pump vacuum; and we can
repeat his experiment substantially for ourselves. I simply
take the receiver made use of in our last lecture (fig. 72),
and removing the remains of the platinum wire, then destroyed, I attach to each end of the two rods, m n and a b,
a bit of retort carbon. I now exhaust the receiver, bring
the coal points together, and send a current from point to
point. The moment I draw the points a little apart, the
electric light blazes forth: and here I have the thermoelectric pile ready to receive a portion of the rays. The
galvanometer needle at once flies aside, and this has been
accomplished by rays which have crossed the vacuum.
But if not to the air, to what is the motion of our cooling ball communicated? We must ascend by easy stages
to the answer to this question. It was a very considerable
step in science when men first obtained a clear conception
of the way in which sound is transmitted through air, and
it was a very important experiment which Hauksbee made




ANALOGIES OF LIGHT,  HEAT; AND SOUND.    259
before the Royal Society in 1705, by which he showed that
sound could not propagate itself through a vacuum. NTow
I wish to make manifest to you this conveyance of the vi.
brations of sound through the air. I have here a bell
turned up-side-down, and supported by a stand. I draw a
fiddle-bow across the edge of the bell, you hear its tone;
the bell is now vibrating, and if I throw sand upon its fiattish bottom, it would arrange itself there so as to form a
definite figure, or if I filled it with water I should see the
surface fretted with the most beautiful crispations. These
crispations would show that the bell, when it emits this
note, divides itself into four swinging parts, which are separated from each other by lines of no swinging. Here is
a sheet of tracing paper, drawn tightly over this hoop, so
as to form a kind of fragile drum. I hold it over the vibrating bell, but not so as to touch the latter; you hear the
shivering of the membrane. - It is a -little too slack, so I
will tighten it by warming it before the fire, and repeat
the experiment. You no longer hear a shivering, but a loud
musical tone superadded to that of the bell. I raise the
membrane and lower it; I move it to and fro, and you hear
the rising and the sinking of the tone. Here is a smaller
drum, which I pass round the bell, holding the membrane
vertical; it actually bursts into a roar when I bring it within
half an inch of the bell. The motion of the bell, communicated to the air, has been transmitted by it to the membrane, and the latter is thus converted into a sonorous
body.
I have here two plates of brass, A B (fig. 73), united together by this metal rod. I have darkened the plates by
bronzing them, and on both of them I strew a quantity of
white sand. I now take the connecting brass rod by its
centre, between the finger and thumb of my left hand, and
holding it upright I draw, with my right, a piece of flannel, over which I have shaken a little powdered resin, along




260                 LECTrRE VIII.
the rod. You hear the sound; but observe the behaviour
of the sand: a single
Fig. 78.          stroke of my finger, you
see, has caused it to
jump into a series of
concentric rings, Which
must be quite visible to
you all.  I repeat the
experiment   operating
more gently; you hear
-the clear, weak, musical
sound, you see the sand
shivering, and creeping,
by degrees, to the lines
-which it formerly occupied; and there are the
curves as sharply drawn
upon the surface of the
lower disk as if they had
been arranged  with a
camel's hair pencil. On
the upper disk you see a
series of concentric circles of the same kind.
_-__ —     b           In fact, the vibrations
which I have imparted
to the rod have communicated themselves to
both the disks, and divided each of them into
a series of vibrating segments, which are separated from  each other
by lines of no vibration, on which the sand finds peace.
Now let me show you the transmission of these vibra



COMMUNICATION OF VIBRATIONS THROUGH AIR. 261
tions from the lower disk through the air. On the floor I
place this paper drum, D, strewing dark-coloured sand uniformly over it; I might stand on the table-I might stand
as high as the ceiling, and produce the effect which I'am
now going to show you. Pointing the rod which unites
my plates in the direction of the paper drum, I draw my
resined rubber vigorously over the rod: observe the effect,
-a single stroke has caused that sand to spring into a reticulated pattern. A precisely similar effect is produced by
sound on the drum of the ear; the tympanic membrane is
caused to shudder in the same manner as that drum-head
of paper, and its motion, conveyed to the auditory nerves
and transmitted thence to the brain, awakes in us the sensation of-sound.
HIere is a still more striking example of the conveyance
of the motion of sound through the air. By permitting a
jet of gas to issue through the small orifice of this tube, I
obtain a slender flame, and by turning the cock I reduce
the flame to a height of about half an inch. I introduce
the flame into this glass tube, A B (fig. 74), which is twelve
inches long. Now I must ask your permission to address
that flame, and if I am skilful enough to pitch my voice to
the precise note, I am sure the flame will respond; it will
start suddenly into a melodious song, and continue singing
as long as the gas continues to burn. The burner is now
arranged within the tube, which covers it to a depth of a
couple of inches. If I were to lower it more, the flame
would start into singing on its own account, as in the wellknown case of the hydrogen harmonica; but, with the
present arrangement, it cannot sing till I tell it to do so.
Now I emit a sound, which you will pardon if it is not
musical. The flame does not respond; I have not spoken
to it in the proper language. Let me try again; I pitch
my voice a little higher; there, the flame stretches its little
throat, and every individual in this large audience hears




262                   LECTURE VIII.
the sound of it. I stop the song, and stand at a greater
distance from the flame, and now that I have ascertained
the proper pitch, the experiment is sure  to sucA        ceed; from  a distance of
twenty or thirty feet I can
cause that flame to sing. I
now stop it, turn my back
upon it, and strike the note
as before; you see how
obedient it is to my voice;
when I call, it answers, and
with a little practice I have
been able to command the
flame to sing and to stop,
and it has strictly obeyed
the injunction. Here, then,
we have a striking example
of the conveyance of the
vibrations of the organ of
voice through the air, and
of their communication to
a body which is eminently
Add -A5 M Isensitive to their action.*
Why do I make these
experiments  on   sound?
Simply to give you clear
conceptions regarding what takes place in the case of heat;
to lead you up from the tangible to the intangible; from
the region of sense into that of physical theory.
After philosophers had become aware of the manner in
* Though not belonging to our present subject, so many persons have
evinced an interest in this experiment that I have been induced to reprint
two short papers in the Appendix to this Lecture, in which the experiment
is more fully described.




THEORIES OF EBMISSION AND UNDULATION.   263
which sound was produced and transmitted, analogy led
some of them to suppose that light might be produced and
transmitted in a somewhat similar manner. And perhaps
in the whole history of science there was never a question
more hotly contested than this one. Sir Isaac Newton supposed light to consist of minute particles darted out from
luminous bodies: this was the celebrated Emission Theory.
Huyghens, the contemporary of Newton, found great difficulty in conceiving of this cannonade of particles; that
they should shoot with inconceivable velocity through space
and not disturb each other. This celebrated man entertained the view that light was produced by vibrations similar to those of sound. Euler supported Huyghens, and
one of his arguments, though not quite physical, is so
quaint and curious that I will repeat it here. He looks at
our various senses, and at the manner in which they are
affected by external objects.' With regard to smell,' he
says,'we know that it is produced by material particles
which issue from a volatile body. In the case of hearing,
nothing is detached from the sounding body, and in the
case of feeling we must touch the body itself. The distance at which our senses perceive bodies is, in the case of
touch, no distance, in the case of smell a small distance, in
the case of hearing, a considerable distance, but in the case
of sight greatest of all. It is therefore more probable that
the same mode of propagation subsists for sound and light,
than that odours and light should be propagated in the
same manner;-that luminous bodies should behave, not as
volatile substances, but as sounding ones.'
The authority of Newton bore these men down, and
not until a man of genius within these walls took up the
subject, had the Theory of Undulation any chance of coping with the rival Theory of Emission. To. Dr. Thomas
Young, who was formerly Professor of Natural Philosophy in this Institution, belongs the immortal honour of




264                  LECTURE Vi.
stemming this tide of authority, and of establishing on a
safe basis, the theory of undulation. There have been great
things done in this edifice, but hardly a greater than this.
And Young was led to his conclusion regarding light, by a
series of investigations on sound. He, like ourselves, at
the present moment, rose from the known to the unknown,
from the tangible to the intangible. This subject has been
illustrated and enriched by the labours of genius ever since
the time of Young; but one name only will I here associate with his, —a name which, in connection with this subject, can never be forgotten: that is, the name of Augustin
Fresnel.
According to the notion now universally received, light
consists, first, of a vibratory motion of the particles of the
luminous body; but how is this motion transmitted to our
organs of sight? Sound has the air as its medium, and
long pondering on the phenomena of light, and refined
and conclusive experiments, devised with the express intention of testing the idea, have led philosophers to the conclusion, that space is occupied by a substance almost infinitely elastic, through which the pulses of light make
their way. Here your conceptions must be perfectly clear.
The intellect knows no difference between great and small:
it is, just as easy, as an intellectual act, to conceive of a vibrating atom as to conceive of a vibrating.cannon-ball; and
there is no more difficulty in conceiving of this Ether, as it
is called, which fills space, that in imagining all' space to be
filled with jelly. You must imagine the atoms vibrating,
and their vibrations you must figure as communicated to
the ether in which they swing, being propagated through it
in waves; these waves enter the pupil, cross the ball of the
eye, and break upon the retina at the back of the eye. The
act, remember, is as real, and as truly mechanical as the
breaking of the sea waves upon the shore. Their motions
are communicated to the retina, transmitted thence along




INTERSTELLAR MEDIUM.             265
the optic nerve to the brain, and there announce themselves to consciousness as light.
I have here an electric lamp, known well to all of you,
and on the screen in front of you I project an image of the
incandescent coal points which produce the electric light.
I will first bring the points together and then separate them.
Observe the effect. You have first the place of contact rendered luminous, then you see the glow conducted downwards
to a certain distance along the stem of coal.  This, as you
know, is in reality the conduction of motion. I interrupt
the circuit. The points continue to glow for a short time;
the light is now subsiding. The coal points are now quite
dark, but have they ceased to radiate? By no means. At
the present moment there is a copious radiation from these
points, which, though incompetent to affect sensibly the
nerves of vision, are quite competent to affect other nerves
of the human system. To the eye of the philosopher who
looks at such matters without reference to sensation, these
obscure radiations are precisely the same in kind as those
which produce the impression of light. You must therefore figure the particles of the heated body as being in a
state of motion; you must figure the motion communicated
to the surrounding ether, and transmitted through the ether
with a velocity, which we have the strongest reason for believing is the same as that of light. Thus when you turn
towards a fire on a cold day, and expose your chilled hands
to its influence, the warmth that you feel is due to the impact of these ethereal billows upon your skin; they throw
the nerves into motion, and the consciousness corresponding to this motion is what we popularly call warmth. Our
task during the lectures which remain to us is to examine
heat under this rcadiant form.
To investigate this subject we possess our valuable thermo-electric pile, the face of which is now coated with lampblack, a powerful absorber of radiant heat. I hold the in12




266                  LECTURE VIII.
strument in front of the cheek of Mr. Anderson; he is a radiant body, and observe the effect produced by his rays;
the pile drinks them in, they generate electricity, and the
needle of the galvanometer moves up to 90~. I withdraw
the pile from the source of heat, and allow the needle to
come to rest, and now I place this slab of ice in front of
the pile. You have a deflection in the opposite direction,
as if rays of cold were striking on the pile. But you know
that in this case the pile is the hot body; it radiates its
heat against the ice; the face of the pile is thus chilled,
and the needle, as you see, moves up to 90~ on the side of
cold. Our pile is therefore not only available for the
examination of heat communicated to it by direct contact,
but also for the examination of radiant heat. Let us apply it at once to a most important investigation, and examine, by means of it, the distribution of thermal power in
the electric spectrum.
Let me in the first place show you this spectrum. I do
so by sending a slice of pure white light'from the orifice
o (fig. 75), through this prism, a b c, which is built up of
Fig. 75.
plane glass sides, but is filled with the liquid bisulphide of
carbon, It gives a richer display of colour than glass does,
and this is one reason why I use it in preference to glass.
Here then you have the white beam disentangled, and re



HEAT OF SPECTRUM.                 267
duced to the colours which compose it; you have this burning red, this vivid orange, this dazzling yellow, this brilliant green, and these various shades of blue; the blue space
being usually subdivided into blue, indigo, and violet. I
will now cause a thermo-electric pile of particular construction to pass gradually through all these colours in succes..
sion, so as to test their heating powers, and I will ask you
to observe the needle of the galvanometer which is to declare the magnitude of that power.
For this purpose I have here (fig. 76) a beautiful piece
of apparatus, designed by Melloni, and executed, with his
accustomed skill, by M. Ruhmkorff.*  You observe here a pol-          Fig.TO.
ished brass plate, A B, attached
to  a stem, and  this stem  is
mounted on a horizontal bar,
which, by means of a screw, has
motion imparted to it. By turning this ivory handle in one direction I cause the plate of brass
to approach; by turning it in the
other, I cause it to recede, and
the motion is so fine and gradual,
that I could, with ease and ce- 
tainty, push the screen through a
space less than   2 owith of an inch.
You observe a narrow  vertical -— A
slit in the middle of this plate,
and something dark behind it.
That dark space is the blackened face of a thermo-electric
pile, P, the elements of which are ranged in a single row,
and not in a square, as in our other instrument. I will allow distinct slices of the spectrum to fall on that slit; each
will impart whatever heat it possesses to the pile, and the
* Kindly lent to me by M. Gassiot.




268                  LECTURE VIII.
quantity of the heat will be marked by the needle of our
galvanometer.
At present a small but brilliant spectrum falls upon the
plate, A B, but the slit is quite out of the spectrum. I turn
the handle, and the slit gradually approaches the violet
end of the spectrum; the violet light now falls upon the
slit, but the needle does not move sensibly. I pass on to
the indigo, the needle is still quiescent; the blue also
shows no action. I pass oh to the green, the needle barely stirs: now the yellow falls upon the slit; the motion of
the needle is now perhaps for the first time visible to you;
but the deflection is small, though I now expose the pile
to the most luminous part of the spectrum.*  I will now
pass ion to the orange, which is less luminous than the yellow, but you observe, though the light diminishes the heat
increases; the needle moves still'farther. I pass on to the
red, which is still less luminous than the orange, and you
see that I here obtain the greatest thermal power exhibited
by any of the visible portions of the spectrum.
The appearance, however, of this burning red might lead
you to suppose it natural for such a colour to be hotter
than any of the others. But now pay attention. I will
cause my slit to pass entirely out of the spectrum, quite
beyond the extreme red. Look to the galvanometer! The
needle goes promptly up to the stops. So that we have
here a heat-spectrum which we cannot see, and whose thermal power is far greater than that of any visible part of
the spectrum. In fact, the electric light with which we
deal, emits an infinity of rays which are converged by our
lens, refracted by our prism, which form the prolongation
of our spectrum, but which are utterly incompetent to excite the optic nerve to vision. It is the same with the sun.
Our orb is rich in these obscure rays; and though they are
* I am here dealing with a large lecture-room galvanometer.




.EXTRA RED AND EXTRA VIOLET RAYS.       269
for the most part cut off by our atmosphere, multitudes of
them still reach us. To the great William Herschel we are
indebted for the discovery of them.
Thus we prove that the spectrum extends on the red
side much beyond its visible limits; and were I, instead of
being compelled to make use of lenses and prisms of glass,
fortunate enough to possess lenses and prisms of rock salt, I
could show you, as Melloni has done, that those rays extend
a great way farther than it is now in my power to prove.
In fact, glass, though sensibly transparent to light, is, in a
great measure, opaque to these obscure rays; instead of
reaching the screen, they are for the most part lodged in
the glass.
The visible spectrum, then, simply marks an interval of
radiant action, in which the radiations are so related to our
organisation that they excite the impression of light; beyond this interval, in both directions, radiant power is
exerted-obscure rays fall-those falling beyond the red
being powerful to produce heat, while those falling beyond
the violet are powerful to promote chemical action. These
latter rays can actually be rendered visible; or more strictly expressed, the undulations or waves which are now
striking here beyond the violet against the screen, and
which are scattered from it so as to strike the eyes of every
person present, though they are incompetent to excite
vision in those eyes; those waves, I say, may be caused to
impinge upon another body, and to impart their motion to
it, and actually to convert the dark space beyond the violet
into a brilliantly illuminated one. I have here the proper
substance. The lower half of this sheet of paper has been
washed with a solution of sulphate of quinine, while I have
left the upper half in its natural state. I will hold the
sheet, so that the straight line dividing its prepared from
its unprepared half, shall be horizontal and shall cut the
spectrum into two equal parts; the upper half will remain




270                 LECTURE VIII.
unaltered, and you will be able to compare with it the
under half, on which I hope to find the spectrum elongated.
You see this effect; we have here a splendid fluorescent
band, several inches in width, where a moment ago there
was nothing but darkness. I remove the prepared paper,
and the light disappears. I re-introduce it, and the light
flashes out again, showing you, in the most emphatic manner, that the visible limits of the ordinary spectrum by no
means mark the limits of radiant action. I dip my brush
in this solution of sulphate of' quinine, and dab it against
the paper; wherever the solution falls, light flashes forth.
The existence of these extra violet rays has been long
known; it was known to Thomas Young, who actually experimented on them; but to Prof. Stokes we are indebted
for the complete investigation of this subject. He rendered
the rays thus visible.
How then are we to conceive of the rays, visible and
invisible, which fill this large space upon the screen? Why
are some of them visible and others not? Why are the
visible ones distinguished by various colours?  Is there
anything that we can lay hold of in the undulations which
produce these colours, to which, as a physical cause, we
must assign the colour? Observe first, that the entire
beam of white light is drawn aside, or refracted by the
prism, but the violet is pulled aside more than the indigo,
the indigo more than the blue, the blue more than the
green, the green more than the yellow, the yellow more
than the orange, and the orange more than the red. These
colours are differently refrangible, and upon this depends
the possibility of their separation. To every particular degree of refraction belongs a definite colour and no other.
But why should light of one degree of refrangibility produce the sensation of red, and of another degree the sensation of green? This leads us to consider more closely the
cause of these sensations.




PHYSICAL CAUSE OF COLOUR.           271
A reference to the phenomena of sound will materially
help our conceptions here. Figure clearly to your minds a
harp-string vibrating to and fro; it advances and causes the
particles of air in front of it to crowd together; it thus
produces a conzdensation of the air. It retreats, and the
air particles behind it separate more widely; in other
words, a rarefaction of the air occurs behind the retreating
wire. The string again advances and produces the condensation as before, it again retreats and produces a rarefaction. Thus the condition of the air through which the
sound of the string is propagated consists of a regular
sequence of condensations and rarefactions, which travel
with a velocity of about 1,100 feet a second.
The condensation and rarefaction constitute what is
called a sonorous pulse or wave, and the length of the wave
is the distance from the middle of the condensation to the
middle of the rarefaction. Of course these blend gradually
into each other. The length of the wave is also measured
by the distance from the centre of one condensation to the
centre of the next one. Now the quicker a string vibrates
the more quickly will these pulses follow each other, and
the shorter, at the same time, will be the length of each individual wave. Upon these differences the pitch of a note
in music depends. If a violin player wishes to produce a
higher note, he shortens his string by pressing his finger
on it; he thereby augments the rapidity of vibration. If
his point of pressure exactly halves the length of his string,
he obtains the octave of the note which the string emits
when vibrating as a whole.' Boys are chosen as choristers
to produce the shrill notes, men to produce the bass notes;
the reason being, that the boy's organ vibrates more speedily than the man's;' and the hum of a gnat is shriller than
that of a beetle, because the smaller insect can send a
greater number of impulses per second to the ear.
We have now cleared our way towards the clear com



272                 LECTURE VIII.
prehension of the physical cause of colour. This spectrum
is to the eye what the gamut is to the ear; its different
colours represent notes of different pitch. The vibrations
which produce the impression of red are slower, and the
ethereal waves which they generate are longer, than those
which produce the impression of violet, while the other
colours are excited by waves of some intermediate length.
The length of the waves both of sound and light, and the
number of shocks which they respectively impart to the ear
and eye, have been strictly determined.. Let us here go
through a simple calculation. Light travels through space
at a velocity of 192,000 miles a second. Reducing this to
inches, we find the number to be 12,165,120,000. Now it
is found that 39,000 waves of red light placed end to end
would make up an inch; multiply the number of inches in
192,000 miles by 39,000, we obtain the number of waves
of red light in 192,000 mniles: this number is 474,439,680,000,000. All these waves enter the eye in a single second.
To produce the impression of red in the brain, the retina
must be hit at this almost incredible rate. To produce the
impression of violet, a still greater number of impulses is
necessary; it would take 57,500 waves of violet to fill an
inch, and the number of shocks required to produce the
impression of this colour, amounts to six hundred and ninety-nine millions of millions per second. The other colours
of the spectrum, as already stated, rise gradually in pitch
from the red to the violet.
But beyond the violet we have rays of too high a pitch
to be visible, and beyond the red we have rays of too low
a pitch to be visible. The phenomena of light are in this
case also paralleled by those of sound. If it did not -involve a contradiction, we might say that there are musical
sounds of too high a pitch to be heard, and also sounds of
too low a pitch to be heard. Speaking strictly, there are
waves transmitted through the air from vibrating bodies,




THEORY OF EXCHANGES.               273
which, though they strike upon the air in regular recurrence, are incompetent to excite the sensation of a musical
note. Probably sounds are heard by insects which entirely
escape our perceptions; and, indeed, as regards human beings, the selfsame note may be of piercing shrillness to one
person, while it is absolutely unheard by another. Both
as regards light and sound, our organs of sight and hearing
embrace a certain practical range, beyond which, on both
sides, though the objective cause exists, our nerves cease to
be influenced by it.
When therefore I place this red-hot copper ball before
you, and watch the waning of its light, you will have a
perfectly clear conception of what is occurring here. The
atoms of the ball oscillate, but they oscillate in a resisting
medium on which their moving force is expended, and
which transmits it on all sides with inconceivable velocity.
The oscillations competent to produce light are now exhausted; the ball is quite dark, still its atoms oscillate, and
still their oscillations are taken up and transmitted on all
sides by the ether.  The ball cools as it thus loses its
molecular motion, but no cooling to which it can be practically subjected can entirely deprive it of its motion. That
is to say, all bodies, whatever may be their temperature,
are radiating heat. From the body of every individual
here present, waves are speeding away, some of which
strike upon this cooling ball and restore a portion of its
lost motion. But the motion thus received by the ball is
far less than what it communicates, and the difference between them expresses the ball's loss of motion. As long
as this state of things continues the ball will continue to
show an ever-lowering temperature: its temperature will
sink until the quantity it emits is equal to the quantity
which it receives, and at this point its temperature becomes
constant. Thus, though you are conscious of no reception
of heat, when you stand before a body of your own termn12*




2174                 LECTIORE VIII.
perature, an interchange of rays is passing between you.
Every superficial atom of each mass is sending forth its
waves, which cross those that move in the opposite direction, every wave asserting its own individuality amid the
entanglement of its fellows. When the sum of motion received is greater than that given out, warming is the consequence; when the sum of motion given out is greater
than that received, chilling takes place. This is Prevost's
Theory of Exchanges, expressed in the language of the
Wave Theory.
Let us occupy the remainder of this lecture by illustrating experimentally the analogy between light and radiant
heat, as regards reflection. You observed when I placed
my thermo-electric pile in front of Mr. Anderson's face, that
I had attached to it an open cone which I did not use in
my former experiments. This cone is silvered inside, and
it is intended to augment the action of feeble radiations,
by converging them upon-the face of the thermo-electric
pile. It does this by reflection; instead of shooting wide
of the pile, as they would do if the reflector were removed,
they meet the silvered surface and glance from it against the
pile. The augmentation of the effect is thus shown. I place
the pile at this end of the table with its reflector off, and at
a distance of four or five feet I place this copper ball, hot
-but not red-hot; you observe scarcely any motion of the
needle of the galvanometer. Disturbing nothing, I now attach the reflector to the pile; the needle instantly goes up
to 90~, declaring the augmented action.
The law of this reflection is precisely the same as that
of light. Observe this apparently solid luminous cylinder,
issuing from our electric lamp, and marking its track thus
vividly upon the dust of our darkened room. I take a mirror in my hand, and permit the beam to fall upon it; the
beam rebounds from the mirror; it now strikes the ceiling.
This horizontal beam is the incident beam, this vertical




REFLECTION FROM PLANE SURFACES.         215
one is the reflected beam, and the law of light, as many of
you know, is, that the angle of incidence is equal to the
angle of reflection. The incident and reflected beams now
enclose a right angle, and when this is the case I may be
Fig. 77.
sure that both lbeams form, with a perpendicular to the
surface of the mirror, an angle of 45~.
I place the lamp at this corner, E, of the table (fig. 77);
behind the table I place a looking-glass, L, and on the table
you observe I have drawn a large arec, a b. Attached to
the mirror is this long straight lath, m n, and the lookingglass, resting upon rollers, can be turned by the lath, which
is to serve as an index. I have here drawn a dark central
line, and when the mirror exactly faces the middle of the
audience, our lath and this line coincide. Those in front
may see that the lath itself and its reflection in the mirror
form a straight line, which proves that the central dark
line is now perpendicular to the mirror. Right and left of
this central line I have divided the are into ten equal
parts; commencing at the end E with 00, I have graduated
the are up to 20~. I first turn the index so that it shall be
in the line of the beam emitted by the lamp. The beam
now falls upon the mirror, striking it as a perpendicular,
and you see it is reflected back along the line of incidence.
I now move my index to I; the reflected beam, as you ob



276                 LECTURfE vm.
serve, draws itself along the table, cutting the figure 2. I
move the index to 2, the beam is now at 4; I move the index to 3, the beam is now at 6; I move it to 5, the beam
is now at 10; I move it to 10, the beam is now at 20. If I
stand midway between the incident and reflected beams,
and stretch out my arms, my finger tips touch each of them.
One lies as much to the left of the perpendicular as the
other does to the right. The angle of incidence is equal to
the angle of reflection. But we have also demonstrated
that the beam moves twice as fast as the index; and this
is usually expressed in the statement, that the angular velocity of a reflected ray is twice that of the mirror which
reflects it.
I have already shown you that these incandescent coalpoints emit an abundance of obscure rays-of rays of pure
heat, which have no illuminating power; my object now is
to show you that those rays of heat emitted by the lamp,
have obeyed precisely the same laws as the rays of light.
I have here a piece of black glass; so black that when I
look through it at the electric light, or even at the noonday
sun, I see nothing. You observe the disappearance of the
beam when I place this glass in front of the lamp. It cuts
off every ray of light; but, strange as it may appear to
you, it is, in a considerable degree, transparent to the obscure rays of the lamp. I now extinguish the light by interrupting the current, and I lay my thermo-electric pile on
the table at the number 20, where the luminous beam fell a
moment ago. The pile is connected with the galvanometer,
and the needle of the instrument is now at zero. I ignite
the lamp, no light makes its appearance, but observe the
galvanometer; the needle has already swung to 90~,
through the action of the non-luminous rays upon the pile.
If I move the instrument right or left from its present position the needle immediately sinks; the calorific rays have
pursued the precise track of the luminous rays; and for




RAD)IANT HEAT AND LIGHT OBEY THE SAME LAW. 277
them, also, the angle of incidence is equal to the angle of
reflection.  Repeating the experiments that I have already
executed with light, bringing the index in succession to 1,
2, 3, 5, &c., I prove that in the case of radiant heat also,
the angular velocity of the reflected ray is twice that of the
mirror.
The heat of the fire obeys the same law. I have here a
sheet of tin-a homely reflector, but it will answer my purpose. At this end of the table I place the thermo-electric
pile, and at the other end my tin screen. The needle of
the galvanometer is now at zero. Well, I turn the reflector
so as to cause the heat striking it to rebound towards the
pile; it now meets the instrument, and the needle at once
declares its arrival. Observe the positions of the fire, of
the reflector, and of the pile; you see that they are just in
the positions which make the angle of incidence equal to
that of reflection.
But in these experiments the heat is, or has been, associated with light. Let me now show that the law holds
good for rays emanating from a truly obscure body. Here
is a copper ball, c (fig. 78), heated to dull redness; I plunge
it in water until its light totally disappears, but I leave it
warm. It is still giving out radiant heat of a slightly
greater intensity than that emitted by the human body.  I
place it on this candlestick as a support, and here I place
my pile, P, turning its conical reflector away from the ball, so
that no direct ray from  the latter can reach the pile. You
see the needle remains at zero. I place here my tin reflector, AM N, SO that a line drawn to it from the ball, shall make
the same angle with a perpendicular to the polished tin reflector, as a line drawn from the pile. The axis of the conical reflector lies in this latter line. True to the law, the
heat-rays emanating from  the ball rebound from  it and
strike the pile, and you observe the consequent prompt motion of the needle.




278                   LECTURE VIII.
Like the rays of light, the rays of heat emanating from
our ball proceed in straight lines through space, diminishFig. 78.
ing in intensity exactly as light diminishes. Thus, this ball,
which when close to the pile causes the needle of the galvanometer to fly up to 90~, at a distance of 4 feet 6 inches,
shows scarcely a sensible action. Its rays are squandered on
all sides, and comparatively few of them reach the pile. But
I now introduce between the pile and the ball this tin tube,
A B (fig. 79), 4 feet long. It is polished within, and thereFig. 79.
fore capable of reflection. The calorific rays which strike
the interior surface obliquely, are reflected from side to side




REFLECTION FROM CURVED SURFACES.        2T9
of the tube, and thus those rays which, when the tube is
absent, are squandered in space, are caused, by internal reflection, to reach the pile. You see the result: the needle,
which a moment ago showed no sensible action, moves
promptly to its stops.
We have now dwelt sufficiently long on the reflection of
radiant heat by plane surfaces; let us turn for a moment to
reflection from curved surfaces. I have here a concave
mirror, m N (fig. 80) formed of copper, but coated with silFig. 80.
ver. I place this warm  copper ball, B, at a distance of
eighteen inches from the pile, which, has now its conical
reflector removed; you observe scarcely any motion of the
needle. If I placed the reflector, Ar N, properly behind a
candle, I should collect its rays, and send them back in a cylinder of light. I shall do the same with the calorific rays
emitted by the ball B; you cannot, of course, see the track
of these obscure rays, as you can that of the luminous
ones; but you observe that while I speak, the galvanometer has revealed the action; the needle of the instrument
has gone up to 90~.




280                 LECTURE VPII.
I have here a pair of much larger mirrors, one of which
is placed flat upon the table: now, the curvature of this
mirror is so regulated that if I place a light at this point,
which is called the focus of the mirror, the rays which fall
divergent upon the mirror are reflected upward from it parallel. Let us make the experiment: In the focus I place
our coal-points, bring them into contact, and then draw
them a little apart; there is the electric light, and there is
a splendid vertical cylinder, cast upwards by the reflector,
and marked by the action of the light on the dust of the
room. If we reversed the experiment, and allowed a parallel beam of light to fall upon the mirror, the rays of that
beam, after reflection, would be collected in the focus of
the mirror. We can actually make this experiment by introducing a second mirror; here it is suspended from the
ceiling. I will now draw it up to a height of 20 or 25 feet
above the table; the vertical beam, which before fell upon
the ceiling, is now received by the upper mirror; I have
hung in the focus of the upper mirror a bit of oiled paper,
to enable you to see the collection of the rays of the focus.
You observe how intensely that piece of paper is now illuminated, not by the direct light from below, but by the reflected light converged upon it from above.
Many of you know the extraordinary action of light
upon a mixture of hydrogen and chlorine. I have here a
transparent collodion balloon filled with the mixed gases;
I lower my upper reflector, and suspend the balloon from
a hook attached to it, so that the little globe shall swing
in the focus; we will now draw the mirror quite up to the
ceiling (fig. 81); and as before I place my coal-points in
the focus of the lower mirror; the moment I draw them
apart, the light gushes from them, and the gases explode.
And remember this is the action of the light; you know
collodion to be an inflammable substance, and hence might
suppose that it was the heat of the coal-points that ignited




CONrUGATE  MIRRORS.              281
it, and that it commu-            Fig. 81.
nicated its combustion....'.......
to the gases; but look
here! you see, as I
speal, the flakes of the
balloon descending on
the table; the luminous rays went harmlessly through it, caused the gases to explode, and the hydrochloric acid, formed
by their combustion,,',,
has actually preserved
the inflammable  envel-                             i
ope from sharing in
the combustion.
i lower the upper
mirror and hang in its
focus a second balloon,
containing a mixture
of oxygen and hydrog~en, on which light
has no sensible effect;
I raise the mirror, and
in the focus of' the
lower one place this
red-hot copper ball.
The  calorifie rays are            _____
now reflected and con-_     _
verged above, as the
luminous ones were
reflected and converged in the last experiment; but they act




282                 LECTURE VIII.
upon the envelope, which I have purposely blackened a little, so as to enable it to intercept the heat-rays; the action
is not so' sudden as in the last case, but there is the explosion, and you now see no trace of the balloon; the inflammable substance is entirely dissipated.
But here, you may object, light is associated with the
heat; very well, I lower the upper mirror once more and
suspend in its focus a flask of hot water. I bring my thermo-electric pile to the focus of the lower mirror, and first
turn the face of the pile upwards, so as to expose it to the
direct radiation of the warm flask-there is no sensible action produced by the direct rays. But I now turn my pile
with its face downwards. If light and heat behave alike,
the rays from the flask which strike the reflector will be
collected at its focus. You see that this is the case; the
needle, which was not sensibly affected by the direct rays,
goes up to its stops. I would ask you to observe the direction of that deflection; the red end of the needle moves
towards you.
I again lower the mirror, and, in the place of the flask
of hot water, suspend a second one containing a freezing
mixture. I raise the mirror and, as in the former case,
bring the pile into the focus of the lower one. Turned directly towards the upper flask there is no action; turned
downwards, the needle moves: observe the direction of the
motion-the red end comes towards me.
Does it not appear as if this body in the upper focus
were now emitting rays of cold which are converged by
the lower mirror exactly as the rays of heat in our former
experiment. The facts are exactly complementary, and it
would seem that we have precisely the same right to infer
from the experiments, the existence and convergence of
these cold rays, as we have to infer the existence and convergence of the heat rays. But many of you, no doubt,
have already perceived the real state of the case. The pile




RADIATION OF COLD.              283
is a warm body, but in the last experiment the quantity
which it lost by radiation was more than made good by
the quantity received from the hot flask above. Now the
case is reversed, the quantity which the pile radiates is in
excess of the quantity which it receives, and hence the pile
is chilled;-the exchanges are against it, its loss of heat
is only partially compensated-and the deflection due to
cold is the necessary consequence.




APPENDIX TO LECTURE VIII.
ON THE SOUNDS PRODUCED BY THE COMBUSTION OF GASES
IN TUBES.*
IN the first volume of Nicholson's Journal, published in 1802, the
sounds produced by the combustion of hydrogen in tubes are
referred to as having been'made in Italy:' Dr. Higgins, in the
same place, shows that he had discovered them in the year 1777,
while observing the water formed in a glass vessel by the slow
combustion of a slender stream of hydrogen. Chladni, in his'Akustik,' published in 1802, page 74, speaks of their being
mentioned, and incorrectly explained, by De Luc in his'New
Ideas on Meteorology:' I do not know the date of the volume.
Chladni himself showed that the tones produced were the same
as those of an open pipe of the same length as the tube which
encompassed the flame. He also succeeded in obtaining a tone
and its octave from the same tube, and in one case obtained the
fifth of the octave. In a paper published in the' Journal de Physique' in 1802, G. De la Rive endeavoured to account for the
sounds by referring them to the alternate contraction and expansion of aqueous vapour; basing his opinion upon a series of experiments of great beauty and ingenuity made with the bulbs of
thermometers. In 1818 Mr. Faraday took up the subject,t and
showed that the tones were produced when the glass tube was
enveloped by an atmosphere higher in temperature than 212~
Fahr. That they were not due to aqueous vapour was further
shown by the fact that they could be produced by the combustion
* From the Philosophical Magazine for July, 1857. By John Tyndall,
F.R.S.
f Journal of Science and the Arts, vol. v. p. 274.




SINGIG FLAMES.                     285
of carbonic oxide. He referred the sounds to successive explosions produced by the periodic combination of the atmospheric
oxygen with the issuing jet of hydrogen gas.
I am not aware that the dependence of the pitch of the note
on the size of the flame has as yet been noticed. To this point I
will, in the first place, briefly direct attention.
A tube 25 inches long was placed over an ignited jet of hydrogen: the sound produced was the fundamental note of the tube.
A tube 121 inches long was brought over the same flame, but
no sound was obtained.
The flame was lowered, so as to make it as small as possible,
and the tube last mentioned was again brought over it; it gave
a clear melodious note, which was the octave of that obtained
with the 25-inch tube.
The 25-inch tube was now brought over the same flame; it no
longer gave its fundamental note, but exactly the same note as
that obtained from the tube of half its length.
Thus we see, that although the speed with which the explosions succeed each other depends upon the length of the tube,
the flame has also a voice in the matter: that to produce a musical sound, its size must be such as to enable it to explode in
unison either with the fundamental pulses of the tube, or with
the pulses of its harmonic divisions.
With a tube 6 feet 9 inches long, by varying the size of the
flame, and adjusting the depth to which it reached within the
tube, I have obtained a series of notes in the ratio of the numbers
1, 2, 3, 4, 5.
These experiments explain the capricious nature of the sounds
sometimes obtained by lecturers upon this subject. It is, however, always possible to render the sounds clear and sweet, bysuitably adjusting the size of the flame to the length of the tube.*
Since the experiments of Mr. Faraday, nothing, that I am
aware of, has been added to this subject, until quite recently.
In a recent number of Poggendorff's' Annalen' an interesting
* With a tube 14, inches in length and an exceedingly minute jet of
gas, I obtained, without altering the quantity of gas, a note and its octave:
the flame possessed the power of changing its own dimensions to suit both
notes.




286            APPENDIX TO LECTURE VIII.
experiment is described by M. von Schafigotsch, and made the
subject of some remarks by Prof. Poggendorff himself. A musical
note was obtained with a jet of ordinary coal-gas, and it was
found that when the voice was pitched to the same note, the
flame assumed a lively motion, which could be augmented until
the flame was actually extinguished. M. von Schafigotsch does
not describe the conditions necessary to the success of his experiment; and it was while endeavouring to find out these conditions that I alighted upon the facts which form the principal
subject of this brief notice. I may remark that M. von Schaffgotsch's result may be produced, with certainty, if the gas be
caused to issue under sufficient pressure through a very small
orifice.
In the first experiments I made use of a tapering brass burner,
10~ inches long, and having a superior orifice about -oth of an
inch in diameter. The shaking of the singing flame within the
glass tube, when the voice was properly pitched, was so manifest
as to be seen by several hundred people at once.
I placed a syrene within a few feet of the singing-flame, and
gradually heightened the note produced by the instrument. As
the sounds of the flame and syrene approached perfect unison, the
flame shook, jumping up and down within the tube. The interval between the jumps became greater until the unison was perfect, when the motion ceased for an instant; the syrene still increasing in pitch, the motion of the flame again appeared, the
jumping became quicker and quicker, until finally it escaped
cognisance by the eye.
This experiment showed that the jumping of the flame, observed by L1. von Schaffgotsch, is the optical expression of the
beats which occur at each side of the perfect unison: the beats
could be heard in exact accordance with the shortening and
lengthening of the flame. Beyond the region of these beats, in
both directions, the sound of the syrene produced no visible
motion of the flame. What is true of the syrene is true of the
voice.
While repeating and varying these experiments, I once had a
silent flame within a tube, and on pitching my voice to the note
of the tube, the flame, to my great surprise, instantly started into
song. Placing the finger on the end of the tube, and silencing




SINGING FLAMES.                   287
the melody, on repeating the experiment the same result was
obtained.
I placed the syrene near the flame, as before. The latter was
burning tranquilly within its tube. Ascending gradually from
the lowest notes of the instrument, at the moment when the sound
of the syrene reached the pitch of the tube which surrounded the
gas flame, the latter suddenly stretched itself and commenced its
song, which continued indefinitely after the syrene had ceased to
sound.
With the burner which I have described, and a glass tube 12
inches long, and from - to' of an inch internal diameter, this result can be obtained with ease and certainty. If the voice be
thrown a little higher or lower than the note due to the tube, no
visible effect is produced upon the flame: the pitch of the voice
must lie within the region of the audible beats.
By varying the length of the tube we vary the note produced,
and the voice must be mnodified accordingly.
That the shaking of the flame, to which I have already referred, proceeds in exact accordance with the beats, is beautifully
shown by a tuning-fork, which gives the same note as the flame.
Loading the fork so as to throw it slightly out of unison with the
flame, when the former is sounded and brought near the flame,
the jumpings are seen at exactly the same intervals as those in
which the beats are heard. When the tuning-fork is brought
over a resonant jar or bottle, the beats may be heard and the
jumpings seen by a thousand people at once. By changing the
load upon the tuning-fork, or by slightly altering the size of the
flame, the quickness with which the beats succeed each other
may be changed, but in all cases the juumpings address the eye at
the same moment that the beats address the ear.
With the tuning-fork I have obtained the same results as with
the voice and syrene. Holding a fork over a tube which responds
to it, and which contains within it a silent flame of gas, the latter
immediately starts into song. I have obtained this result with a
series of tubes varying from 101 to 29 inches in length. The following experiment could be made:-A series of tubes, capable of
producing the notes of the gamut, might be placed over suitable
jets of gas; all being silent, let the gamut be run over by a
musician with an instrument sufficiently powerful, placed at a




288            APPENDIX TO LECTURE VIII.
distance of twenty or thirty yards. At the sound of each particular note, the gas-jet contained in the corresponding tube would
instantly start into song.
I must remark, however, that with the jet which I have used,
the experiment is most easily made with a tube about 11 or 12
inches long: with longer tubes it is more difficult to prevent the
flame from singing spontaneously, that is, without external excitation.
The principal point to be attended to is this. With a tube,
say of 12 inches in length, the flame requires to occupy a certain
position in the tube in order that it shall sing with a maximum
intensity. Let the tube be raised so that the flame may penetrate
it to a less extent; the energy of the sound will be thereby
diminished, and a point (A) will at length be attained, where it
will cease altogether. Above this point. for a certain distance,
the flame may be caused to burn tranquilly and silently for any
length of time, but when excited by the voice it will sing.
When the flame is too near the point (A), on being excited by
the voice or by a tuning-fork, it will respond for a short time,
and then cease. A little above the point where this cessation
occurs, the flame burns tranquilly, if unexcited, but if once caused
to sing it will contniue to sing. With such a flame, which is not
too sensitive to external impressions, I have been able to reverse
the effect hitherto described, and to stop the song at pleasure by the
sound of my voice, or by a tuning-fork, without quenching the
flame itself. Such a. flame, I find, may be made to obey the word
of command, and to sing or cease to sing, as the experimenter
pleases.
The mere clapping of the hands, producing an explosion,
shouting at an incorrect pitch, shaking of the tube surrounding
the flame, are, when the arrangements are properly made, ineffectual. Each of these modes of disturbance doubtless affects the
flame, but the impulses do not accumulate, as in the case where
the note of the tube itself is struck. It appears as if the flame
were decf to a single impulse, as the tympanum would probably
be, and, like the latter, needs the accumulation of impulses to
give it sufficient motion. A difference of half a tone between
two tuning-forks is sufficient to cause one of these to set the flame
singing, while the other is powerless to produce this effect.




SINGING FLAMES.                   289
I have said that the voice must be pitched to the note of the
tube which surrounds the flame; it would be more correct to say
the note produced by the flame when singing. In all cases this
note is sensibly higher than that due to the open tube which surroundsi the flame; this ought to be the case, because of the high
temperature of the vibrating column. An open tube, for example, which, when a tuning-fork is held over its end, gives a maximum reinforcement, produces, when surrounding a singing flame,
a note higher than that of the fork. To obtain the latter note
the tube must be sensibly longer.
What is the constitution of the flame of gas while it produces
these musical sounds? This is the next question to which I will
briefly call attention.'Looked at with the naked eye, the sounding flame appears constant, but is the constancy real? Supposing
each pulse to be accompanied by a physical change of the flame,
such a change would not be perceptible to the naked eye, on
account of the velocity with which the pulses succeed each other.
The light of flame would appear continuous, on the same principle that the troubled portion of a descending liquid yet appears
continuous, although by proper means this portion of a jet can be
shown to be composed of isolated drops. If we cause the image
of the flame to pass speedily over different portions of the retina,
the changes accompany the periodic impulses will manifest themselves in the character of the image thus traced.
I took a glass tube 3 feet 2 inches long, and about an inch and
a half in internal diameter, and placing it over a very small flame
of olefiant gas (common gas will also answer), obtained the fundamental note of the tube: on moving the head to and fro, the
image of the sounding flame was separated into a series of distinct images; the distance between the images depended upon
the velocity with which the head was moved. This experiment
is suited to a darkened lecture-room. It was still easier to obtain
the separation of the images in this way, when a tube 6 feet 9
inches in length, and a large flame, were made use of.
The same result is obtained when an opera glass is moved to
and fro before the eye.
But the most convenient mode of observing the flame is with
a mirror; and it can be seen either directly in the mirror, or by
projection upon a screen.
13




290            APPENDIX TO LECTURIE VIII.
A lens of 33 centimetres focus was placed in front of a flafhe
of common gas, upwards of an inch long, and a paper screen was
hung at about 6 or 8 feet distance behind the flame. In front of
the lens a small looking-glass was held, which received the light
that had passed through the lens, and reflected it back upon the
screen placed behind the latter. By adjusting the position of the
lens, a well-defined inverted image of the flame was obtained
upon the screen. On moving the mirror the image was displaced,
and owing to the retention of the impression by the retina, when
the movement was sufficiently speedy the image described a continuous luminous track. Holding the mirror motionless, the 6foot 9-inch tube was placed over the flame: the latter changed
its shape the moment it commenced to sound, remaining however
well defined upon the screen. On now moving the mirror, a
totally different effect was produced: instead of a continuous
track of light, a series of distinct images of the sounding flame
was observed. The distance of these images apart varied with
the motion of the mirror; and, of course, could be made, by suitably turning the reflector, to form a ring of images. The experiment is beautiful, and in a dark room may be made visible to a
large audience.
The experiment was also varied in the following manner:A triangular prism of wood had its sides coated with rectangular
pieces of looking glass: it was suspended by a thread with its
axis vertical; torsion was imparted to the thread, and the prism,
acted upon by this torsion, caused to rotate. It was so placed
that its three faces received, in succession, the beam of light sent
from the flame through the lens in front of it, and threw the
images upon the screen. On commencing its motion the images
were but slightly separated, but became -more and more so as the
motion approached its maximum. This once past, the images
drew closer together again,'until they ended in a kind of luminous
ripple. Allowing the acquired torsion to react, the same series of
effects could be produced, the motion being in an opposite direction. In these experiments, that half of the tube which was
turned towards the screen was coated with lamp-black, so as to
cut off the direct light of the jet from the screen.*
X Since these experiments were made, Mr. Wheatstone has drawn my




SINGING FLAMES.                       291
But what is the state of the flame in the interval between two
images?  The -flame of common gas, or of olefiant gas, owes its
luminousness to the solid particles of carbon discharged into it.
If we blow against a luminous gas-flame, a sound is heard,' a
small explosion in fact, and by such a puff the light may be
caused to disappear. During a windy night the exposed gas-jets
in the shops are often deprived of their light, and burn blue. In
like manner the common blowpipe-jet deprives burning coal-gas
of its brilliant light. I hence concluded, that the explosions, the
repetition of which produces the musical sound, rendered, at the
moment they occurred, the combustion so perfect as to extinguish the solid carbon particles; but I imagined that the images
on the screen would, on closer examination, be found united by
spaces of blue, which, owing to their dimness, were not seen by
the method of projection.  This in many instances was found to
be the case.
I was not, however, prepared for the following result:-A
flame of olefiant gas, rendered almost as small as it could be, was
procured. The 3-foot 2-inch tube was placed over it; the flame,
on singing, became elongated, and lost some of its light, still it
was bright at its top; looked at in the moving mirror, a beaded
line of great beauty was observed; in front of each bead was a
little luminous star, after it, and continuous with it, a spot of
rich blue light, which terminated, and left, as far as I could
judge, a perfectly dark space between it and the next following
luminous star.  I shall examine this further when time permits
me, but as far as I can at present judge, the flame was actually
extinguished and relighted in accordance with the sonorous pulsations.
When a silent flame, capable, however, of being excited by the
voice in the manner already described, is placed within a tube,
attention to the following passage, which proves that he had already made
use of the rotating mirror in examining a singing flame:'A flame of hydrogen gas burning in the open air presents a continuous circle in the
mirror; but while producing a sound within a glass tube, regular intermissions of intensity are observed, which present a chain-like appearance, and
indicate alternate contractions and dilatations of the flame corresponding
with the sonorous vibrations of the column of air.'-Phil. Trans., 1834, p.
586.




292            APPENDIX TO LECTURE VIII.
and the continuous line of light produced by it in the moving
mirror is observed, I know no experiment more pretty than the
resolution of this line into a string of richly luminous pearls at
the instant the voice is pitched to the proper note. This may be
done at a considerable distance from the jet, and with the back
turned towards it.
The change produced in the line of beads when a tuning-fork,
capable of giving beats with the flame, is brought over the tube,
or over a resonant jar near it, is also extremely interesting to
observe. I will not at present enter into a more minute description of these results. Sufficient, I trust, has been said to induce
experimenters to reproduce the effects for themselves; the sight
of them will give more pleasure than any description of mine
could possibly do.
TRANSLATION OF A PAPER ON ACOUSTIC EXPERIMENTS. *
A glass tube open at both ends, when simply blown upon by
the mouth, gives its fundamental tone, i. e. the deepest tone belonginig to it, as an open organ-pipe, feebly but distinctly. On placing
the open hand upon one of th e openings and rapidly withdrawing it, the tube yields two notes, one after the other; first the
fundamental note of the closed pipe, and then the note of the
open pipe, already mentioned, which is an octave higher. By the
application of heat these fundamental tones, of which only the
higher one will be taken into consideration here, are raised, as is
well known; this is observed immediately on blowing upon a
tube heated externally, or by a gas-flame burning in its interior.
For example, a tube 242 millims. in length, and 20 millims. in
diameter, heated throughout its whole length, when blown upon
even before it reaches a red heat, gives a tone raised a major
third, namely, the second G sharp in the treble clef, instead of the
corresponding E. If a gas-flame 14 millims. in length, and 1
millir. in breadth at the bottom, is burning in the tube, the tone
rises to the second treble F sharp. The same gas-flame raises
* By Count Schaffgotsch: Phil. Mag., December 1857.




SINGING FLAMES.                   293
the tone of a tube 273 millims. in length, and 21 millims. in
width, from the second treble D to the corresponding E. These
two tubes, which for brevity will hereafter be referred to as the
E tube and the D tube, served for all the following experiments,
the object of which was to show a well-known and by no means
surprising fact, in a striking manner, namely, that the column
of air in a tube is set in vibration when its fundamental tone, or
one nearly allied, for example, an octave, is sounded outside the
tube. The existence of the aerial vibrations was rendered perceptible by a column of smoke, a current of gas, and a gas flame.
1. A glimmering smoky taper was placed close under the E
tube held perpendicularly, and the smoke passed through the
tube in the form of a uniform thread. At a distance of 1'5 metre
from the tube, the first treble E was sung. The smoke curled,
and it appeared as if a part of it would be forced out at the upper,
and the other part at the lower opening of the tube.
2. Two gas-burners, 1 millim. in the aperture, were applied
near each other to the same conducting tube. Common gas
flowed from both of them; one projected from below into the D
tube for about one-fifth of its length; the gas flame of the other
was 3 millims. in height. At a distance of 1'5 metre therefrom
the first treble D was sung; the flame increased several times in
breadth and height, and consequently in size generally; a larger
quantity of gas therefore flowed out of the outer burner, which can
only be explained by a diminution of the stream of gas in the
inner burner, that is, in the one surrounded by the glass tube.
3. A burner, with an aperture of 1 millim. projecting from
below into the D tube, about 80 millims., yielded a gas flame
14 millims. in length. At 5'6 metres therefrom the first treble E
was sung: the flame was instantaneously extinguished. The same
thing took place at 7 metres, when the flame is only 10 millims.
in height, and the first treble D sharp is sung.
4. The last-mentioned flame is also extinguished by the note G
sharp sounded close to it. Noises, such as the clapping of hands,
pushing a chair, or shutting a book, do not produce this effect.
5. A burner with an aperture of 0'5 millim., projecting from
below 60 millims. into the D tube, yielded a globular gas flame
3 to 3'5 millims. in diameter. By gradually closing a stopcock
the passage of gas was more and more limited. The flame sud



294             APPENDIX TO LEOTURE VIII.
denly became much longer, but at the same time narrower, and
nearly cylindrical, acquiring a bluish color throughout, and from
the tube a piercing second treble D was sounded; this is the
phenomenon of the so-called chemical harmonica, which has been
known for eighty years. When the stopcock is still further
closed, the tone becomes stronger, the flame longer, narrower, and
nearly spindle-shaped; at last it disappears.
An effect exactly similar to that caused by-cutting off the
gas is produced upon the small gas flame by a D, or the first
treble D, sung or sounded from instruments; and in this case it
is to be observed that the flame generally becomes the more sensitive the smaller it is, and the further the burner projects into the
glass tube.
6. The flame in the D tube was 2 or 3 millims. in length; at
a distance of 16-3 metres (more than 51 feet) from  it, the first
treble D was sounded. The flame immediately acquired the unusual form, and the second treble D sounded and continued to
sound from the tube.
7. While the second treble D of the preceding experiment was
sounding, the first treble D was sounded loudly close to the tube,
when the flame became excessively elongated, and then disappeared.
8. The flame being only 1'5 millim. in length, the first treble
D was sounded. The flame gave out the second treble D  (and
perhaps sometimes also a higher D) only for a moment, and disappeared.  The flame is also affected by various D's of an adjustible labial pipe, by the contra D, D, D, the first treble D, and
the second treble D of a harmonium, but by no single C sharp or
D sharp of this powerful instrument. It is also affected by the
third treble D of a clarionet, although only when quite close.
The sung note also acts when it is produced by inspiration (in this
case the second treble), or when the mouth is turned from the
flame.
9. In immediate proximity the note G sung is effective.
Some influence is exerted by noises, but not by all, and often
not by the strongest and nearest, evidently because the exciting
tone is not contained in them.
10. The flame burning quietly in the interior of the D tube
was about 2'5 millims. in length. In the next room, the door of




SINGING FLAMES.                   295
which was open, the four legs of a chair were stamped simultaneously upon the wooden floor. The phenomenon of the chemical harmonica immediately occurred. A very small flame is of
course extinguished, after sounding for an instant, by the noise
of a chair. A tambourine, when struck, acts sometimes, but in
general not.
11. The flame burning in the excited singing condition in the
interior of the D tube, the latter was slowly raised as high as
possible without causing the return of the flame to the ordinary
condition. The note, the first treble D, was sung strongly and
broken, off suddenly at a distance of 1'5 metre. The harmonic
tone ceased, and the flame fell into a state of repose without being
extinguished.
12. The same result was produced by acting upon the draught
of air in the tube by a fanning motion of the open hand close
above the upper aperture of the tube.
13. In the I)D tube there were two burners close together; one
of them, 0'5 millim. in aperture, opened 5 millims. below the
other, the diameter of which was 1 millim. or more. Currents
of gas, independent of each other, flowed out of both; that
flowing from the narrower burner being very feeble, and burning
when ignited, with a flame about 1'5 millim. in length, nearly
invisible in the day; the first treble D was sung at a distance of
three metres. The strong current of gas was immediately inflamed, because the little flame situated below it, becoming
elongated, flared up into it. By a stronger action of the tone,
the small flame itself is extinguished, so that an actual transfer
of the flame from one burner to the other takes place. Soon afterwards the feeble current of gas is usually again inflamed by the
large flame, and if the latter be again extinguished alone, everything is ready for a repetition of the experiment.
14. The same result is furnished by stamping with the chair,
&c. It is evident that in this way gas-flames of any desired
size and any mechanical action may be produced by musical
tones and noises, if a wire stretched by a weight be passed through
the glass tube in such a way that the flaring gas-flame must burn
upon it.
15. If the flame of the chemical harmonica be looked at steadfastly, and at the same time the head be moved rapidly to the




296              APPENDIX TO LECTUlRE VIII.
right and left alternately, an uninterrupted streak of light is not
seen, such as is given by every other luminous body, but a series
of closely approximated flames, and often dentated and undulated
figures, especially when tubes of a metre and flames of a centimetre in length are employed.
This experiment also succeeds very easily without moving
the eyes, when the flame is looked at through an opera-glass, the
object-glass of which is moved rapidly to and fro, or in a circle;
and also when the picture of the flame is observed in a handmirror shaken about. It is, however, only a variation of the
experiment long since described and explained by Wheatstone,
for which a mirror turned by watchwork was employed.
[It is perhaps but right that I should draw attention to the relation of
the foregoing paper to one that I have published on the same subject. On
May 6, and the days immediately following, the principal facts described in
my paper were discovered; but on April 30, the foregoing results were
communicated by Prof. Poggendorff to the Academy of Sciences in Berlin.
Through the kindness of Mr. Schaffgotsch himself, I received his paper at
Chamouni, many weeks after the publication of my own, and until then I
was not aware of his having continued his experiments upon the subject.
We thus worked independently of each other, but as far as the described phenomena are common to both, all the merit of priority rests with
Count Schaffgotsch.-J. T.]




LECTURE IX.
[March 20, 1862.]
LAW OF DIMINUTION WITH THE DISTANCE-THE WAVES OF SOUND LONGITUDINAL; THOSE OF LIGHT TRANSVERSAL-WHEN THEY OSCILLATE THE
MOLECULES OF DIFFERENT BODIES COMMUNICATE DIFFERENT AMOUNTS
OF MOTION TO THE ETHER-RADIATION THE COMMUNICATION OF MOTION TO THE ETHER; ABSORPTION THE ACCEPTANCE OF MOTION FROM
THE ETHER-THOSE SURFACES WHICH RADIATE WELL ABSORB WELL-A
CLOSE WOOLLEN COVERING FACILITATES COOLING-PRESERVATIVE INFLUENCE OF GOLD-LEAF-THE ATOMS OF BODIES SELECT CERTAIN WAVES
FOR DESTRUCTION AND ALLOW OTHERS TO PASS-TRANSPARENCY AND
DIATHERMANCY-DIATHERMIC BODIES BAD RADIATORS-THE TERM QUALITY AS APPLIED TO RADIANT HEAT-THE RAYS WHICH PASS WITHOUT
ABSORPTION DO NOT HEAT THE MEDIUM: THE MOST POWERFUL SOLAR
RAYS MAY PASS THROUGH AIR WHILE THE AIR REMAINS BELOW A
FREEZING TEMPERATURE-PROPORTION OF LUMINOUS AND OBSCURE RAYS
IN VARIOUS FLAMES.
T HAVE said that the intensity of radiant heat diminishes with the distance, as light diminishes.  What is
the law  of diminution for light?  I have here a square
sheet of paper, each side of the square measuring two feet;
I fold it thus to form a smaller square, each side of which
is a foot in length. The electric lamp now stands at a distance of sixteen feet from  the screen; at a distance of
eight feet, that is exactly midway between the screen and
the lamp, I hold this square of paper; the lamp is naked,
unsurrounded by its camera, and the rays, uninfluenced by
any lens, are emitted on all sides.  You see the shadow of
the square of paper on the screen. My assistant shall measure the boundary of that shadow, and now I unfold my
sheet of paper so as to obtain the original large square;
13'




298                  LECTURE IX.
you see by the creases, that it is exactly four times the area
of the smaller one. I place this large sheet against the
screen, and find that it exactly covers the space formerly
occupied by the shadow of the small square.
On the small square, therefore, when it stood midway
between the lamp and screen, a quantity of light fell which,
when the small square is removed, is diffused over four
times the area upon the screen. But if the same quantity
of light is diffused over four times the area, it must be diluted to one-fourth of its original intensity. Hence, by
doubling the distance from the source of light, we diminish
the intensity to one-fourth. By a precisely similar mode
of experiment we could prove, that by trebling the distance we should diminish the intensity to one-ninth; and
by quadrupling the distance we should reduce the intensity
to one-sixteenth: in short, we thus demonstrate the law
that the intensity of light diminishes as the square of the
distance increases. This is the celebrated law of Inverse
Squares as applied to light.
But I have said that heat diminishes according to the
same law. Observe the experiment which I am now about
to perform before you. I have here a tin vessel; narrow,
but presenting a side a square yard in area, BIN (fig. 82).
This side, you observe, I have coated with lampblack. I
fill the vessel with hot water, intending to make this large
surface my source of radiant heat. I now place the conical
reflector on the thermo-electric pile, P, but instead of permitting it to remain a reflector, I push into the hollow cone
this lining of black paper, which fits exactly, and which,
instead of reflecting any heat that may fall obliquely on it,
completely cuts off the oblique radiation. The pile is now
connected with the galvanometer, and I place its reflector
close to this large radiating surface, the face of the pile
being about six inches distant from the surface.
The needle of the galvanometer moves: let it move




DIMINUTION WITH DISTANCE.             299
until it takes up its final position. It now points steadily
to 60~, and there it will remain as long as the temperature
Fig. 82.
of the radiating surface remains sensibly constant. I will
now gradually withdraw the pile fiom the surface, and will
ask you to observe the effect upon the galvanometer. Of
course you will expect that as I retreat from the source of
heat, the intensity of the heat will diminish, and that the
deflection of the galvanometer will diminish in a corresponding degree. I am now at double the distance, but the
needle does not move; I treble the distance, the needle is
still stationary; I successively quadruple, quintuple-go to
ten times the distance, but the needle is rigid in its adherence to the deflection of 60~. There is, to all appearance,
no diminution at all of intensity with the increase of distance.
From this experiment, which might at first sight appear
fatal to the law of inverse squares, as applied to heat, Melloni, in the most ingenious manner, proved the law. Mark
his reasoning. I again place the pile close to the radiating
surface. Imagine the hollow cone in front of the pile prolonged; it would cut the radiating surface in a circle, and




300                  LECT'UEE - x.
this circle is the only portion of the surface whose rays can
reach the pile. All the other rays are cut off by the non
reflecting lining of the cone. I move the pile to double the
distance; the section of the cone prolonged now encloses
a circle of the radiating surface, exactly four times the area
of the former circle; at treble the distance the radiating
surface is augmented nine times; at ten times the distance
the radiating surface is augmented 100 times. But the
constancy of the deflection proves that the augmentation
of the radiating surface must be exactly neutralised by the
diminution of intensity; the radiating surface augments as
the square of the distance, hence the intensity of the heat
must diminish as the square of the distance; and thus the
experiment, which might at first sight appear fatal to the
law, demonstrates the law in the most simple and conclusive manner.
Let us now revert for a moment to our fundamental
conceptions regarding radiant heat. Its origin is an oscillatory motion of the ultimate particles of matter-a motion
taken up by the ether, and propagated through it in waves.
The particles of ether in these waves do not oscillate in the
same manner as the particles of air in the case of sound.
The air-particles move to and fro, in the direction in which
the sound travels, the ether particles move to and fro,
across the line in which the light travels. The undulations
of the air are longitudinal, the undulations of the ether are
transversal. The ether waves resemble more the ripples
of water than they do the aerial pulses which produce
sound; that this is the case has been inferred from optical
phenomena. But it is manifest that the disturbance produced in the ether must depend upon the character of the
oscillating mass; one atom may be more unwieldy than another, and a single atom could not be expected to produce
so great a disturbance as a group of atoms oscillating as a
system. Thus, when different bodies are heated, we may




LAW OF INVERSE SQUARES.               301
fairly expect that they will not all create the same amount
of disturbance in the ether. It is probable that some will
communicate a greater amount of motion than others: in
other words, that some will radiate more copiously than
others; for radiation, strictly defined, is the communication
of motion from the particles of a heated body, to the ether
in which theseparticles are immersed.
Let us now test this idea by experiment. I have here
a cubical vessel, c (fig. 83)-a' Leslie's cube'-so called
from its having been used by Sir John Leslie in his beautiful researches on radiant heat.  The mass of the cube is
pewter, but one of its sides is coated with a layer of gold,
another with a layer of silver, a third with a layer of copper, while the fourth I have coated with a varnish of isinglass. I fill the cube with hot water, and keeping it at a
constant distance from the thermo-electric pile, r, I allow
Fig. 83.
its four faces to radiate, in succession, against the pile.
The hot gold surface, you see, produces scarcely any deflec
tion; the hot silver is equally inoperative, the same is the
case with the copper; but when I turn this varnished sur



302                  LECTURiE IX.
face towards the pile, the gush of heat becomes suddenly
augmented; and the needle, as you see, moves up to its
stops. Hence we infer, that through some physical cause
or other, the molecules of the varnish, when set in motion
by the hot water within the cube, communicate more motion to the ether than the atoms of the metals; in other
words, the varnish is a better radiator than the metals are.
I obtain a similar result when I compare this silver teapot
with this earthenware one; filling them both with boiling
water, the silver, you see, produces but little effect, while
the radiation from the earthenware is so copious as to drive
the needle up to 90~. Thus, also, if I compare this pewter
pot with this glass beaker, when both are filled with hot
water, the radiation from the glass is much more powerful
than that from the pewter.
You have often heard of the effect of colours on radiation, and heard a good deal, no doubt, which is unwarranted by experiment. I have here a cube, one of whose sides
is coated with whiting, another with carmine, a third with
lampblack, while the fourth is left uncoated. I present the
black surface first to the pile, the cube being filled with
boiling water; the needle moves up, and now points steadily to 65~. The cube rests upon a little turn-table, and by
turning the support I present the white face to the pile;
the needle remains stationary, proving that the radiation
from the white surface is just as copious as that from the
black. I turn the red surface towards the pile, there is no
change in the position of the needle. I turn the uncoated
side, the needle instantly falls, proving the inferiority of
the metallic surface as a radiator. I repeat precisely the
same experiments with this cube, the sides of which are
covered with velvet; one face with black velvet, another
with white, and a third with red. The results are precisely the same as in the former instances; the three velvet
surfaces radiate alike, while the naked surface radiates less




INFLUENCE OF COLOURS ON' RADIATION.    303
than any of them. These experiments show that the radiation from the clothes which cover the human body, is independent of the colour of these clothes; the colour of an
animal's fur is equally incompetent to influence the radiation. These are. the conclusions arrived at by Melloni obr
obscure heat.*
But if the coated surface communicates more motion to
the ether than the uncoated one, it necessarily follows that
the coated vessel will cool more quickly than the uncoated
one. I have here two cubes, one of which is quite coated
with lampblack, while the other is bright. At the commencement of the lecture I poured boiling water into these
vessels, and placed in each a thermometer. A short time
ago both thermometers showed the same temperature, but
now one of them is two degrees below the other. The velocity of cooling in one vessel is greater than in the other,
and the vessel which cools quickest is the coated one. Here
are two vessels, one of which is bright and the other closely coated with flannel. Half an hour ago two thermometers
plunged in these vessels showed the same temperature, but
they show it no longer; the covered vessel has now a temperature two or three degrees lower than the naked one.
It is usual to preserve the heat of teapots by a woollen
covering, but the cover must fit very loosely. In this case,
though the covering may be a good radiator, its goodness
is more than counterbalanced by the difficulty encountered
by the heat in reaching the outer surface of the covering.
A closely fitting cover would, as we have seen, promote the
loss which it is intended to diminish, and thus do more
harm than good.
One of the most interesting points connected with our
subject is the reciprocity which exists between the power
* By the application of a more powerful and delicate test than that
employed by Melloni, I find that his conclusions will require modification.




304                   LEaCTURLE IX.
of a body to communicate motion to the ether, or to radi.
ate; and its capacity to accept motion from the ether, or
to absorb. As regards radiation we have already compared
lampblack and chalk with metallic surfaces; we will now
compare the same substances with reference to their powers
of absorption. I have here two sheets of tin, X N, o Pr (fig.
84), one of them coated with whiting and the other left
uncoated. I place them thus parallel to each other, and
Fig. 84.
at a distance of about two feet asunder.  To the edge of
each sheet I have soldered a screw, and from one screw to
she other I stretch a copper wire, cc 6, which now connects
the two sheets. At the back of the sheet I have soldered
one end of a little bar of bismuth, to the other end, e, of
which a wire is soldered, and terminated by a binding
tcrew. To these two binding screws I attach the two ends




RECIPROCITY OF lRADIATION AiND ABSORPTION.  305
of the wire coming from my galvanometer at G, and you
observe I have now an unbroken circuit, in which the galvanometer is included. You know already what the bismuth bars are intended for. I place my warm finger on
this left-hand one, a current is immediately developed,
which passes from the bismuth to the tin, thence through
the wire connecting the two sheets, thence round the galvanometer, to the point from which it started. You observe the effect. The needle of the galvanometer moves
through a large arc; the red end going towards you. The
junction of tin and bismuth is now cooling, the needle returns to 0~, and now I will place my finger upon the bismuth at the back of the other plate-you see the effect-a
large deflection in the opposite direction; the red end of
the needle now comes towards me. I withdraw my finger,
the junction cools, and once more the needle sinks to
zero.
I set this stand exactly midway between the two sheets
of tin, and on the stand I intend to place a heated copper
ball; the ball will radiate its heat against both sheets; on
the right, however, the rays will strike upon a coated surface, while on the left they will strike upon a naked metallic surface. If both surfaces drink in the radiant heat-if
both accept with equal freedom the motion of the ethereal
waves-the bismuth junctions at the backs will be equally
warmed, and one of them will neutralise the other. But
if one surface be a more powerful absorber than the other,
that, which absorbs most will heat its bismuth indicator
most; a deflection of the galvanometer needle will be the
consequence, and the direction of the deflection will tell us
which is the best absorber. The ball is now upon the
stand, and you see we have not long to wait for a decision
of the question. The prompt and energetic deflection of
the needle informs us that the coated surface is the most
powerful absorber. In the same way I compare lampblack




306                   LECTURE IX.
and varnish with tin, and find the two former by far the
best absorbers.*
The thinnest metallic coating furnishes a powerful defence against the absorption of radiant heat. I have here
a sheet of' gold paper,' the gold being merely copper reduced to great tenuity. Here is a red powder, the iodide
of mercury, with which I coat the under surface of the gold
paper. This iodide, as many of you know, has its red colour discharged by heat, the powder becoming a pale yellow. I lay the paper fiat on this board with the coloured
surface downwards, and on this upper metallic surface I
paste pieces of paper-common letter paper will answer my
purpose. A figure of any desired shape is thus formed on
the surface-of the copper. I now take a red-hot spatula in
my hand and pass it several times over the sheet; the
spatula radiates strongly against the sheet, but I apprehend
this its rays are absorbed in very different degrees. The
metallic surface will absorb but little; the paper surfaces
will absorb greedily; and, on turning up the sheet, you see
the effect: the iodide underneath the metallic portion is
perfectly unchanged, while under every bit of paper the
colour is discharged, thus forming below an exact copy of
the figure pasted on the opposite surface of the sheet.
Here is another example of the same kind, for which I am
indebted to Mr. Hill, of the establishiment of Mr. Jacob
Bell in Oxford Street. A hot fire sent its rays against this
painted piece of wood (fig. 85),-on which the number 338
was printed in gold leaf letters; the paint is blistered and
charred all round the letters, but underneath the latter the
wood and paint are quite unaffected. This thin film of
gold has been quite sufficient to prevent the absorption, to
which the destruction of the surrounding surface is due.
* Colour, according to Melloni, has no influence on the absorption of
obscure heat: on luminous heat, such as that of the sun, it has great influence.




METALS BAD RADIATORS AND ABSORBERS.    307
The luminiferous ether fills stellar space; it makes the
universe a whole, and renders the intercommunication of
light and energy between star and star possible. But the
subtle substance penetrates further; it surrounds the very.
Fig. 85.
atoms of solid and liquid substances. Transparent bodies
are such, because the ether and their atoms are so related
to each other, that the waves which excite light can pass
through them, without transferring their motion to the
atoms. In coloured bodies certain waves are broken or absorbed; but those which give the body its colotlr pass
without loss. Through this solution of sulphate of copper,
for example, the blue waves speed unimpeded, but the red
waves are destroyed. I form a spectrum upon the screen;
sent through this solution you see the red end of the spectrum is cut away. This piece of red glass, on the contrary,
owes its redness to the fact that its substance can be traversed freely by the longer undulation of red, while the
shorter waves are absorbed. Interposing it in the path of
this light you see it cuts the blue end of the spectrum quite
away, leaving merely a vivid red band upon the screen.
This blue liquid then cuts off the rays which are transmitted by the red glass; and the red glass cuts off the rays
which are transmitted by the liquid; by the union of both
we ought to have perfect opacity, and so we have. When




308                 LECTURE IX.
both are placed in the path of the beam, the entire spectrum disappears; the union of these two transparent bodies
produce an opacity equal to that of pitch or coal.
I have here another liquid-a solution of the permanganate of potash-which I introduce into the path of the
beam. See the effect upon the spectrum; the two ends
pass freely through, you have the red and the blue, but between both a space of intense blackness. The yellow of
the spectrum is pitilessly destroyed by this liquid; through
the entanglement of its atoms these yellow rays cannot
pass, while the red and the blue glide round them and get
through the inter-atomic spaces without sensible hindrance.
And hence the gorgeous colour of this liquid. I will turn
the lamp round and project a disk of light two feet in
diameter upon the screen. I now introduce this liquid;
can anything be more splendid than the colour of that disk?
I again turn the lamp obliquely and introduce a prism;
here you have the components of that beautiful colour; the
violet component has slidden away from the red. You see
two definite disks of these two colours upon the screen,
which overlap in the centre, and exhibit there the colour of
the composite light which passes through the liquid.
Thus, as regards the waves of light, bodies exercise as
it were an elective power, singling out certain waves for
destruction, and permitting others to pass. Transparency
to one wave does not at all imply transparency to others,
and from this we might reasonably infer, that transparency
to light does not imply transparency to radiant heat. This
conclusion is entirely verified by experiment. I have here
a tin screen, Mr N (fig. 86), pierced by an aperture, behind
which is soldered a small stand s. I place this copper ball,
B, heated to dull redness, on a candlestick, which will serve
as a support for the ball. At the other side of the screen I
place my thermo-electric pile, P; the rays from the ball
now pass through the aperture in the screen and fall upon




ELECTIVE ABSORPTION.              309
the pile-the needle goes up, and finally comes to rest with
a steady deflection of 80~. I have here a glass cell, a
quarter of an inch wide, which I now fill with distilled water. I place the cell on the stand, so that all rays reaching
the pile must pass through it;'what takes place? The
Fig. 8G.
needle steadily sinks almost to zero; scarcely a ray from
the ball can cross this water; —to the undulations issuing
from the ball the water is practically opaque, though so
extremely transparent to the rays of light. Before removing the cell of water I place behind it a similar cell, containing transparent bisulphide of carbon; so that now,
when I remove the water cell, the aperture is still barred
by the new liquid. What occurs? The needle promptly
moves upwards and describes a large arc; so that the selfsame rays that found the water impenetrable, find easy access through the bisulphide of carbon. In the same way I
compare this alcohol with this chloride of phosphorus, and
find the former almost opaque to the rays emitted by our
warm ball, while the latter permits them to pass freely.
So also as regards solid bodies; I have here a plate of




310                  LECTURE IX.
very pure glass, which I place on the stand, and, using a
cube of hot water instead of the ball B, I permit the rays
from the heated cube to pass through it, if they can. No
movement of the needle is perceptible. I now displace the
plate of glass by a plate of rocksalt of ten times the thickness; you see how promptly the needle moves, until it is
arrested by its stops. To these rays, then, the rocksalt is
eminently transparent, while the glass is practically opaque
to them.
For these, and numberless similar results, we are indebted to Melloni, who may be almost regarded as the
creator of this branch of our subject. To express this
power of instantaneous transmission of radiant heat, he
proposes the word diathermcancy. Diathermancy bears the
same relation to radiant heat that transparency does to
light. Instead of giving you determinations of my own
of the diathermancy of various bodies, I will make a selection from the tables of the eminent Italian philosopher just
referred to. In these determinations Melloni uses four different sources of heat, the flame of a Locatelli lamp; a
spiral of platinum wire, kept incandescent by the flame of
an alcohol lamp; a plate of copper heated to 400~ Cent.,
and a plate of copper heated to 100~ Cent., the last mentioned source being the surface of a copper cube containing boiling water. The experiments were made in the following manner: —First, the radiation of the source, that is
to say the galvanometeric deflection produced by it, was
determined when nothing but air intervened between the
source and the pile; then the substance whose diathermancy was to be examined was introduced, and the consequent
deflection noted. Calling the quantity of heat represented
by the former deflection 100, the proportionate quantities
transmitted by twenty-five different substances are given in
the following table:



DIATHERMANCY.                              311
Transmissions: per centage of the total
radiation.
Names of substances reduced to a
conlmmn thickness of #eth of an inch
(26 millir.)                                   Incan(2  mocatell   Inceant  Copper at Copper at
Lamp   Platinu      4000 C.  10~ C.
1 Rocksalt..   92'3       92'3      92'3     92'3
2 Sicilian sulphur.            74        77       60        54
3 Fluor spar...    72        69       42        33
4 Beryl. ~          54        23        13         0
5 Iceland spar..    39        28         6         0
6 Glass..   39         24         6         0
7 Roclk crystal (clear)..    38        28         6          3
8 Smoky quartz..   37        28         6          3
9 Chromate of Potash.              34       28        15          0
10 White Topaz..   33         24         4         0
11 Carbonate of Lead..   32         23         4         O
12 Sulphate of Baryta.             24        18         3         0
13 Felspar...   23         19         6         0
14 Amethyst (violet)..   21          9         2         o
15 Artificial amber..   21          5         0         0
16 Borate of Soda..    18        12         8         0
17 Tourmaline (deep green).   18           16         3         0
18 Common gum..   18          3         0         O
19 Selenite...            14  5        O         O
20 Citric acid...    11         2         0         O
21 Tartrate of Potash                11         3         0         O
22 Natural amber..    11         5         0 
23 Alum...     9         2         0         0
24 Sugar-candy...     8         1         0         0
25 Ice...             6         0           
This table shows, in the first place, what very different
transmissive  powers  different  solid  bodies  possess.   It
shows us also that, with a single exception, the transparency of the bodies mentioned for radiant heat varies with the
qucality of the heat.  Rocksalt alone is equally transparent
to heat from  the four sources experimented with.  It must
be borne in mind here that the luminous rays are also calorific rays;  that the  selfsame ray, falling  upon  the nerve
of vision, produces the impression of light;,while, impinging upon other nerves of the body, it produces the impres



312                   LECTURE IX.
sion of heat. The luminous calorific rays have, however, a
shorter length than the obscure rays, and knowing, as we
do, how differently waves of different lengths are absorbed
by bodies, we are in a measure prepared for the results of
the foregoing table. Thus, while glass, of the thickness
specified, permits 39 per cent. of the rays of Locatelli's
lamp, and 24 per cent. of the rays from the incandescent
platinum to pass, it gives passage to only 6 per cent. of the
rays from copper, at a temperature of 4000 C., while it is
absolutely opaque to all rays emitted from a source of 100~
C. We also see that limpid ice, which is so highly transparent to light, allows to pass only 6 per cent. of the rays
of the lamp, and 0'5 per cent. of the rays emitted by the
incandescent platinum, while it utterly cuts off all rays issuing from the other two sources. We have here an intimation, that by far the greater portion of the rays emitted by
the lamp of Locatelli must be obscure. Luminous rays
pass through ice, of the thickness here given, without sensible absorption, and the fact that 94 per cent. of the rays
issuing from  Locatelli's flame are destroyed by the ice,
proves that this proportion of these rays must be obscure.
As regards the influence of transparency, clear and smoky
quartz are very instructive. Here are the two substances,
one perfectly pellucid, the other a dark brown; still, for
the luminous rays only, do these two specimens show a
difference of transmission. The clear quartz transmits 38
per cent., and the smoky quartz 37 per cent. of the rays
from the lamp, while, for the other three sources, the transmissions of both substances are identical.
In the following table, which I also borrow from Melloni, the calorific transmissions of different liquids are
given. The source of heat was an Argand lamp furnished
with a glass chimney, and the liquids were enclosed in a
cell with glass sides, the thickness of the liquid layer being
9'21 millemetres.




RADIATION THROUGH  SOLIDS AND LIQUIDS.    313
Calorifle
Names of Liquids                transmissions;
per centage of
the total radiation
Bisulphide of Carbon...   63
Bichloride of Sulphur...   63
Protochloride of Phosphorus..   62
Essence of Turpentine..   31
Olive Oil....   30
Naphtha...           28
Essence of Lavender...   26
Sulphuric Ether...   21
Sulphuric Acid...   17
Hydrate of Ammonia...   15
Nitric Acid...   15
Absolute Alcohol...   15
Hydrate of Potash...   13
Acetic Acid...          12
Pyroligneous Acid...   12
Concentrated Solution of Sugar.   12
Solution of Rocksalt..   12
White of Egg...   11
Distilled Water..       11
Liquids are here shown to be as diverse in their powers
of transmission as solids; and it is also worthy of remark,
that water maintains its opacity, notwithstanding the
change in its state of aggregation.
The reciprocity which we have already demonstrated
between radiation and absorption in the case of metals,
varnishes, &c., may now be extended to the bodies contained in Melloni's tables. I will content myself with one or
two illustrations, borrowed from Mr. Balfour Stewart. Here
is a copper vessel in which water is kept in a state of gentle ebullition. On the flat copper lid of this vessel I place
plates of glass and of rocksalt, till they have assumed the
temperature of the lid. I place the plate of rocksalt upon
this stand, in front of the thermo-electric pile. You observe the deflection; it is so small as to be scarcely sensible.  I now  remove the rocksalt, and put in its place a
14




314                  LECTURE IX.
plate of heated glass; the needle moves upwards through
a large are, thus conclusively showing that the glass, which
is the more powerful absorber of obscure heat, is also the
more powerful radiator. Alum, unfortunately, melts at a
temperature lower than that here made use of; but though
its temperature is not so high as that of the glass, you can
see that it transcends the glass as a radiator; the action on
the galvanometer is still more energetic than in the case of
the last experiment.
Absorption takes place within the absorbing body; and
it requires a certain thickness of the body to accomplish
the absorption. This is true of both light and radiant
heat. A very thin stratum of pale beer is almost as colourless as a stratum of water, the absorption being too inconsiderable to produce the decided colour which larger masses
of the beer exhibit. I pour distilled water into a drinking
glass; in this quantity it exhibits no trace of colour, but I
have arranged here an experiment which will show you that
this pellucid liquid, in sufficient thickness, exhibits a very
decided colour. Here is a tube fifteen feet long, A B (fig.
87), placed horizontal, the ends of which are stopped by
Fig. 87.
pieces of plate glass; at one end of the tube stands an electric lamp, L, from which I intend to send a cylinder of
light through the tube. The tube is now half filled with
water, the upper surface of which cuts the tube in two




INFLUENCE OF THICKNESS.                315
equal parts horizontally. Thus I send half of my beam
through air and half through water, and with this lens, c,
I intend to project a magnified image of the adjacent end
of the tube, upon the screen. Here it is; you see the
image, o P, composed of two semicircles, one of which is
due to the light which has passed through the water, the
other to the light which has passed through the air. Side
by side, thus, you can compare them, and you notice that
while the air semicircle is a pure white, the water semicircle is a bright and delicate blue green. Thus, by augmenting the thickness through which the light has to pass, you
deepen the colour; this proves that the destruction of the
light rays takes place within the absorbing body, and is not
an effect of its surface merely.
Melloni shows the same to be true of radiant heat.  In
our table, at page 311, the thickness of the plates used was
2'6 millimetres, but by rendering the plate thinner we enable a- greater quantity of heat to get through, and by rendering it sufficiently thin, we may, with a very opaque substance, almost reach the transmission of rocksalt. The following table shows the influence of thickness on the transmissive power of a plate of glass.
Transmission by Glass of different thicknesses; per centage of
the total Radiation
Thickness of
Plates in Millimetres
metrs  Incandescent  Copper at   Copper at
Locatelli Lamp  Platinum  400~ C.   100~ C.
2-6         39          24          6           0
0'5    1    54          37         12           1
0'07        77          57         34          12
Thus, we see, that by diminishing the thickness of the
plate from 2'6 to 1'07 mllimetres, the quantity of heat
transmitted rises, in the case of the lamp of Locatelli, from




316                    LECTURE IX.
39 to 77 per cent.; in the case of the incandescent platinum, from 24 to 57 per cent.; in the case of copper at 400-o
C. from 6 to 34 per cent.; and in the case of copper at
100~ C., from absolute opacity to a transmission of 12 per
cent.
The influence of the thickness of a plate of selenite on
the quantity of heat which it transmits is exhibited in the
following table.
Transmissions by Selenite of different thicknesses; per centage
of total radiation.
Thickness of
Plates in Millimetres..         Incandescent   Copper at    Copper at
Locatelli Lamp   Platinum  400~ C.    106~ C.
2.6         14           5           0           0
0.4         38          18           7           0
0.01        64          51          32          31
The decomposition of the solar beam gives us the solar
spectrum; luminous in the centre, calorific at one end, and
chemical at the other. The sun is therefore a source of
heterogenous rays, and there can scarcely be a doubt that
all other sources of heat, luminous and obscure, partake of
this heterogeniety. In general, when such mixed rays enter
a diathermic substance, some are struck down and others
permitted to pass.  Supposing, then, that we take a sheaf
of calorific rays which have already passed through a diathermic plate, and permit them to fall upon a second plate
of the same material, the transparency of this second plate
to the heat incident upon it will be greater than the transparency of the first plate to the heat incident on it. In
fact the first plate, if sufficiently thick, has already extinguished, in great part, the rays which the substance is
capable of absorbing; and the residual rays, as a matter of
course penetrate a second plate of the same substance with




SIFTING OF CALORIFIC BEAMS.           317
comparative freedom. The original beam is sifted by the
first plate, and the purified beam possesses, for the same
substance, a higher penetrative power than the original
beam.
This power of penetration has usually been taken as a
test of the quality of heat; the heat of the purified beam is
said to be different in quality from that of the unpurified
beam. It is not, however, that any'individual ray has
changed its quality, but that from the beam, as a whole,
certain rays have been withdrawn, and that their withdrawal has altered the proportion of the incident heat
transmitted by a second substance. This, I think, is the
true meaning of the term' quality' as appled to radiant
heat. In the path of the rays from a lamp let plates of
rocksalt, alum, bichromate of potash, and selenite be successively placed, each plate 2'6 millimetres in thickness; let
the heat emergent from these plates fall upon a second series of the same thickness; out of every 100 rays of this
latter heat, the following proportions are transmitted.
Rocksalt.        92 3
Alum..   90
Chromate of Potash.   71
Selenite..   91
Referring to the table, p. 311, we find that of the
whole of the rays emitted by the Locatelli lamp, only 34
per cent. are transmitted by the chromate of potash; here
we find the percentage 71. Of the entire radiation, selenite transmits only 14 per cent., but of the beam which has
been purified by a plate of its own substance it transmits
91 per cent. The same remark applies to the alum, which
transmits only 9 per cent. of the unpurified beam, and 90
per cent. of the purified beam. In rocksalt, on the contrary, the transmissions of the sifted and unsifted beam
are the same, because the substance is equally transparent




318                   LECTURIE IX.
to rays of all kinds.* In these cases I have supposed the
rays emergent from rocksalt to pass through rocksalt; the
rays emergent from alum to pass through alum, and so of
the others; but, as might be expected, the sifting of the
beam, by any substance, will alter the proportion in which
it will be transmitted by almost any other second substance.
I will conclude these observations with an experiment
which will show you the influence of sifting in a very
striking manner. I have here a sensitive differential airthermometer with a clean glass bulb.  You see the slightest touch of my hand causes a depression of the thermometric column. HIere is our electric lamp, and from it I
will converge a powerful beam on the bulb of that thermometer. The focus now falls directly on the bulb, and
the air within it is traversed by a beam of intense power;
but not the slightest depression of the thermometric column is discernible. When I first showed this experiment
to an individual here present, he almost doubted the evidence of his senses; but the explanation is simple. The
beam, before it reaches the bulb, is already sifted by the
glass lens used to concentrate it, and having passed
through 12 or 14 feet of air, the beam contains no constituent that can be sensibly absorbed by the air within the
bulb. Hence the hot beam passes through both air and
glass without warming either. It is competent, however,
to warm the thermo-electric pile; exposure of the pile to
it, for a single instant, suffices to drive the needle violently
aside; or let me coat with lampblack the portion of the
glass bulb struck by the beam; you see the effect: the
heat is now absorbed, the air expands, and the thermometric column is forcibly depressed.
* This was Melloni's conclusion; but the experiments of MM. Provostaye
and Desains, and of Mr. Balfour Stewart, prove that the conclusion is not
strictly correct.




ACTION OF GLASS FIRE-SCREENS.           319
We use glass fire-screens, which allow  the pleasant
light of the fire to pass, while they cut off the heat; the
reason is, that by far the greater part of the heat emitted
by a fire consists of obscure rays, to which the glass is
opaque. But in no case is there any loss. The rays absorbed by the glass go to warm the glass; the motion of
the ethereal waves is transferred to the molecules of the
solid. But you may be inclined to urge, that under these
circumstances the glass screen itself ought to become a
source of heat, and that therefore we ought to derive no
benefit from its absorption. The fact is so, but the conclusion is unwarranted. The philosophy of the screen is this:
Fig. 88.
-Let F (fig. 88) be a fire from which the rays proceed in
straight lines towards a person at P. Before the screen is
introduced, each ray pursues its course direct to P; but
now let a screen be placed at s. The screen intercepts the
rays of heat and becomes warmed; but instead of sending
on the rays in their original direction only, it emits them,
as a warm body, in all directions. Hence, it cannot restore to the person at P all the heat intercepted. A portion of the heat is restored, but by far the greater part is
diverted from P, and distributed in other directions.
Where the waves pursue their way unabsorbed, no motion of heat is imparted, as we have seen in the case of the




320                     LECTURE IX.
air thermometer.  A joint of meat might be roasted before
a fire, with the air around the joint as cold as ice. The air
on high mountains may be intensely cold, while a burning
sun is overhead; the solar rays which, striking on the human skin, are almost intolerable, are incompetent to heat
the air sensibly, and we have only to withdraw into perfect
shade to feel the chill of the atmosphere. I never, on any
occasion, suffered so much from solarheat as in descending
from the' Corridor' to the Grand Plateau of Mont Blanc,
on August 13, 185 7; though hip deep in snow at the time,
the sun blazed against me with unendurable power.  Immersion in the shadow  of the Dome du Goute at once
changed my feelings; for here the air was at a freezing
temperature. It was not, however, sensibly colder than
the air through which the sunbeams passed; and I suffered,
not from the contact of hot air, but from the impact of
calorific rays which had reached me through a medium icy
cold.
The beams of the sun also penetrate glass without sensibly heating it, and the reason is, that having passed
through our atmosphere, the beams have been in a great
measure deprived of those rays which can be absorbed by
glass.* I made an experiment in a former lecture which
you will now completely understand. I sent a beam from
the electric lamp through a mass of ice without melting
the substance. I had previously sifted the beam by sending it through a vessel of water, in which the rays capable
of being absorbed by the ice were lodged-and so copiously lodged-that the water was raised almost to the boiling
* On a priori grounds I should conclude that the obscure solar rays
which have succeeded in getting through our atmosphere, must be able to
penetrate the humours of the eye and reach the retina: the recent experiments of M. Franz prove this. Their not producing vision is, therefore,
not due to their absorption by the humours of the eye, but to their own
intrinsic incompetence to excite the retina.




READIATIOiN THROUGH OPAQUE BODIES.         321
point during the experiment. It is here worthy of remark
that the liquid water and the solid ice appear to be pervious and impervious to the same rays; the one may be used
as a sieve for the other; a result which indicates that the
quality of the absorption is not influenced by the difference
of aggregation between solid and liquid. It is easy to
prove that the beam which has traversed the ice without
melting it, is really a calorific beam, by allowing it to fall
upon our thermo-electric pile. Here is a beam which has
passed through a layer of water; I permit it to fall upon
the pile, and you instantly see its effect upon the galvanometer, causing the needle to move with energy to its
stops. Here is a beam which has passed through ice, but
you see that it is equally competent to affect the pile; here,
finally, is a beam which has passed through both water and
ice; you see it still possesses heating power.*
When the calorific rays are intercepted, they, as a general rule, raise the temperature of the body by which they
are absorbed; but when the absorbing body is ice at a
temperature of 32~ Fahr., it is impossible to raise its temperature. How then does the heat absorbed by the ice
employ itself?  It produces internal liquefaction, it takes
down the crystalline atoms, and thus forms those lovely
liquid flowers which I showed you in a former lecture.f
We have seen that transparency to light is not at all a
test of diathermancy; that -a body highly transparent to
the luminous undulations may be highly opaque to the nonluminous ones. I have also given you an example of the
opposite kind, and showed you that a body may be absolutely opaque to light and still, in a considerable degree,
transparent to heat. I set the electric lamp in action, and
* Mr. Faraday has fired gunpowder by converging the solar rays upon it
by a lens of ice.
f For the bearing of these results on air and water bubbles of ice, see
Appendix to Lecture IX.
14'




322                  LECTURE IX.
you see this convergent beam tracking itself through the
dust of the room: you see the point of convergence of the
rays here, at a distance of fifteen feet from the lamp; I
will mark that point accurately by the end of this rod.
Here is a plate of rocksalt, coated so thickly with soot that
the light, not only of every gas lamp in this room, but the
electric light itself, is cut off by it. I interpose this plate
of smoked salt in the path of the beam; the light is intercepted, but the rod enables me to find with my pile the
place where the focus fell. I place the pile at this focus:
you see no beam falling on the pile, but the violent action
of the needle instantly reveals to the mind's eye a focus of
heat at the point from which the light has been withdrawn.
You might, perhaps, be disposed to think that the heat
falling on the pile has been absorbed by the soot, and then
radiated from it as an independent source. Melloni has removed every objection of this kind; but none of his experiments, I think, are more conclusive, as a refutation of the
objection, than that now performed before you. For if
the smoked salt were the source, the rays could not converge here to a focus,. for the salt is at this side of the converging lens, and you see when I displace my pile a little
laterally, still keeping it turned towards the smoked salt,
the needle sinks to zero.
The heat, moreover, falling on the pile is, as shown by
Melloni, practically independent of the position of the plate
of rocksalt; you may cut off the beam at a distance of fifteen feet from the pile, or at a distance of one foot; the
result is sensibly the same, which could not be the case if
the smoked salt itself were the source of heat.
I make a similar experiment with this black glass, and
the result, as you see, is the same. Now the glass reflects
a considerable portion of the light and heat from the lamp;
if I hold it a little oblique to the beam you can see the re



PROPORTION OF VISIBLE TO INVISIBLE RAYS.   323
flected portion. While the glass is in this position I will
coat it with an opaque layer of lampblack so as to cause it
to absorb, not only all the rays which are now entering it,
but also the portion which it reflects. What is the result?
Though the glass plate has become the seat of augmented
absorption, it has ceased to affect the pile, the needle descends to zero, thus furnishing additional proof that the
rays which, in the first place, acted upon the pile, came direct from the lamp, and traversed the black glass, as light
traverses a transparent substance.
Rocksalt transmits all rays, luminous and obscure;
alum, of the thickness already given, transmits only the luminous rays; hence the difference between alum and rocksalt will give the value of the obscure radiation. Tested
in this way, Melloni finds the following proportions of lumninous to obscure rays for the three sources mentioned:Source           Luminous      Obscure
Flame of Oil         10           90
Incandescent Platinum  2          98
Flame of Alcohol      1           99
Thus, of the heat radiated from the flame of oil, 90 per
cent. is due to the obscure rays; of the heat radiated from
incandescent platinum, 98 per cent. is due to obscure rays,
while of the heat radiated from the flame of alcohol, fully
99 per cent. is due to the obscure radiations.




APPENDIX TO LECTURE IX.
EXTRACT FROM A MEMOIR ON SOME PHYSICAL PROPERTIES OF
ICE.*
~ I.
I AVAILED myself of the fine sunny weather with which we
were favoured last September and October, to examine the effects
of solar heat upon ice. The experiments were made with Wenham Lake.and Norway ice. Slabs were formed of the substance,
varying from one to several inches in thickness, and these
were placed in the path of a beam rendered convergent by a
double convex lens, 4 inches in diameter, possessing a focal distance of 10~ inches. The slabs were usually so placed, that the
focus of parallel rays fell within the ice. Having first found the
position of the focus in the air, the 1uns was screened; the ice was
then placed in position, the screen was removed, and the effect
was watched through an ordinary pocket lens.
A plate of ice an inch thick, with parallel sides, was first
examined: on removing the screen the transparent mass was
crossed by the sunbeams, and the path of the rays through it was
instantly studded by a great number of little luminous spots, produced at the moment, and resembling shining air-bubbles. When
the beam was sent through the edge of the plate, so that it traversed a considerable thickness of the ice, the path of the beam
could be traced by those brilliant spots, as it is by the floating
motes in a dark room.
In lake ice the planes of freezing are easily recognized by the
stratified appearance which the distribution of the air bubbles
gives to the substance. A cube was cut from a perfectly trans* Phil. Trans. December 1857.




LIQUID FLOWERS IN ICE.                   325
parent portion of the ice, and the solar beam was sent through
the cube in three rectangular directions successively. One was
perpendicular to the plane of freezing, and the other two parallel
to it. The bright bubbles were formed in the ice in all three
cases.
When the surfaces perpendicular to the planes of freezing were
examined by a lens, after exposure to the light, they were found
to be cut up by innumerable small parallel fissures, with here and
there minute spurs shooting from them, which gave the fissures,
in some cases, a feathery appearance. When the portions of the
ice traversed by the beam were examined parallel to the surface
of freezing, a very beautiful appearance revealed itself. Allowing the light from the window to fall upon the ice at a suitable
incidence, the interior of the mass was found filled with little
flower-shaped figures.  Each flower had six petals, and at its centre was a bright spot, which shone with more than metallic brilliancy.  The petals were mnacnifestly composed of water, and were
consequently dim, their visibility depending on the small difference of refrangibility between ice at 32~ Fahr. and water at the
same temperature.
For a long time I found the relation between the planes of
these flowers and the planes of freezing perfectly constant. They
were always parallel to each other. The developement of the
flowers was independent of the direction in Which the beam traversed the ice. Hence, when an irregularly shaped mass of transparent ice was presented to me, by sending a sunbeam through it
I could tell in an instant the direction in which it had been
frozen.
Allowing the beam to enter the edge of a plate of ice, and
causing the latter to move at right angles to the beam, so that the
radiant heat traversed different portions of the ice in succession,
when the track of the beam was observed through an eye-glass,
the ice, which a moment ago was optically continuous, was instantly starred by those lustrous little spots, and around each of
them the formation and growth of its associated flower could be
distinctly observed.
The maximum effect was confined to a space of about an inch
from the place at which the beam first struck the ice. In this
space the absorption, which resolved the ice into liquid flowers,




326              APPENDIX TO LECTU RE IX.
for the most part took place, but I have traced the effect to a
depth of several inches in large blocks of ice.
At a distance, however, from the point of incidence, the spaces
between the flowers became greater; and it was no uncommon
thing to see flowers developed in planes a quarter of an inch
apart, while no change whatever was observed in the ice between
these planes.
The pieces of ice experimented on appeared to be quite homogenous, and their transparency was very perfect. Why, then, did
the substance yield at particular points? Were they weak points
of crystalline structure, or did the yielding depend upon the manner in which the calorific waves impinged upon the molecules of
the body at these points? However these and other questions
may be answered, the experiments have an important bearing
upon the question of absorption. In ice the absorption which
produces the flower is fitful, and not continuous; and there is no
reason to suppose that in other solids the case is not the same,
though their constitution may not be such as to reveal it.*
I have applied the term' bubbles' to the little bright disks in
the middle of the flowers, simply because they resembled the little air-globules entrapped in the ice; but whether they contained
air or not could only be decided by experiment.
Pieces of ice were therefore prepared, through which the sunbeams were sent, so as to develope the flowers in considerable
quantity and magnitude. These pieces were then dipped into
warm water contained in a glass vessel, and the effect, when the
melting reached the bright spots, was carefully observed through
a lens.  The moment a liquid connection was established between them
and the atmosphere, the bubbles suddenly collapsed, and no trace of
air rose to the surface of the warm water.
This is the result which ought to be expected. The volume
of water at 32~ being less than that of ice at the same temperature, the formation of each flower ought to be attended with the
formation of a vacuum, which disappears in the manner described
when the ice surrounding it is melted.
* Notwithstanding the incomparable diathermancy of the substance, M.
Knoblauch finds that when plates of rocksalt are thick enough, they always
exhibit an elective absorption. Effects like those above described may
possibly be the cause of this.




LIQUID DISKS IN ICE.                327
Similar experiments were made with ice, in which true airbubbles were enclosed. When the melting liberated the air, the
bubbles rose slowly through the liquid, and floated for a time
upon its surface.
Exposure for a second, or even less, to the action of the sun
was sufficient to develope the flowers in the ice. The first appearance of the central star of light was often accompanied by an audible clink, as if the substance had been suddenly ruptured. The
edges of the petals were at the commencement definitely curved;
but when the action was permitted to continue, and sometimes even
without this, when the sun was strong, the edges of the petals became serrated, the beauty of the figure being thereby augmented.
Sometimes a number of elementary flowers grouped together
to form a thickly-leaved cluster resembing a rose. Here and there
also amid the flowers a liquid hexagon might be observed, but
such were of rare occurrence.
The act of crystalline dissection, if I may use the term, thus
performed by the solar beams, is manifestly determined by the
manner in which the crystalline forces have arranged the molecules. By the abstraction of heat the molecules are enabled to
build themselves together, by the introduction of heat this architecture is taken down. The perfect symmetry of the flowers, from
which there is no deviation, argues a similar symmetry in the
molecular architecture; and hence, as optical phenomena depend
upon the molecular arrangement, we might pronounce with perfect certainty from the foregoing experiments, that ice is, what
Sir David Brewster long ago proved it to be, optically speaking,
uniaxal, the axis being perpendicular to the surface of freezing.
~ II.
On September 25, while examining a perfectly transparent
piece of Norway ice, which had not been traversed by the condensed sunbeams, I found the interior of the mass crowded with
parallel liquid disks, varying in diameter from the tenth to the
hundredth of an inch. These disks were so thin, that when looked
at in section they were reduced to the finest lines. They had the
exact appearance of the circular spots of oily scum which float on
the surface of mutton broth, and in the pieces of ice first examined they always lay in the planes of freezing.




328             APPENDIX TO LECTURE IX.
As time progressed, this internal disintegration of the ice appeared to become more pronounced, so that some pieces of Norway ice examined in the middle of November appeared to be reduced to a congeries of water-cells entangled in a skeleton of ice.
The effect of this was rendered manifest to the hand on sawing a
block of this ice, by the facility with which the saw went
through it.
There seems to be no such thing as absolute homogeneity in
nature. Change commences at distinct centres, instead of being
uniformly and continuously distributed, and in the most apparently homogeneous substance we should discover defects, if our
means of observation were fine enough. The above observations
show that some portions of a mass of ice melt more readily than
others. The melting temperature of the substance is set down at
32~ Fahr., but the absence of perfect homogeneity, whether from
difference of crystalline texture or some other cause, makes the
melting temperature oscillate to a slight extent on both sides of
the ordinary standard. Let this limit, expressed in parts of a degree, be t. Some parts of a block of ice will melt at a temperature of 32-t, while others require a temperature of 32+t: the
consequence is, that such a block raised to the temperature of 32~,
will have some of its parts liquid, and others solid.
When a mass exhibiting the water-disks was examined by a
concentrated sunbeam, the six-leaved flowers before referred to
were always formed in the planes of the disks.
*          *       *       *       *       *
~ IIJ.
What has been already said will prepare us for the consideration of an associated class of phenomena of great physical interest. The larger masses of ice which I examined exhibited layers,
in which bubbles of air were collected in unuusal quantity, marking, no doubt, the limits of successive acts of freezing. These
bubbles were usually elongated. Between two such beds of bubbles a clear stratum of ice intervened; and a clear surface layer,
which, from its appearance, seemed to have suffered more from
external influences than the rest of the ice, was associated with
each block. In this superficial portion I observed detached airbubbles irregularly distributed, and associated with each vesicle




INTERNAL LIQUEFACTION.                      329
of air, a bleb of water which had the appearance of a drop of clear
oil within the solid.  The adjacent figure will give a notion of
these composite cavities: the unshaded circle represents the air-bubble, and -the shaded   /'/////
space adjacent, the water.                        
When the quantity of water was suffi-   
ciently large, which was usually the case, on  
turning the ice round, the bubble shifted its
position, rising always at the top of the bleb      // ///
of water.  Sometimes, however, the cell was
very flat, and the air was then quite surrounded by the liquid.
These composite cells often occurred in pellucid ice, which showed
inwardly no other sign of disintegration.
This is manifestly the same phenomenon as that which struck
M. Agassiz so forcibly during his ealier investigations on the glacier of the Aar. The same appearances have been described by
the Messrs. Schlagintweit, and finally attention has been forcibly
drawn to the subject in a recent paper by Mr. Huxley, published
in the' Philosophical Magazine.' *
The only explanation of this phenomenon hitherto given, and
adopted apparently without hesitation, is that of M. Agassiz and
the Messrs. Schlagintweit.  These observers attribute the phenomenon to the diathermancy of the ice, which permits the radiant heat to pass through the substance, to heat the bubbles of air,
and cause them to melt the surrounding ice.t
The apparent simplicity of this explanation contributed to
ensure its general acceptance; and yet I think a little reflection
will show that the hypothesis, simple as it may appear, is attended with grave difficulties.
For the sake of distinctness I will here refer to a most interesting fact, observed first by M. Agassiz, and afterwards by the Messrs.
Schlagintweit.  In the' Systeme Glaciaire' it is described in these
* October, 1857., I1 est 6vident pour quiconque a suivi le progres de la physique moderne, que ce phenomene est du uniquement a la diatherman6it6 de la
glace (Agassiz, Systeme, p. 157).
Das Wasser ist dadurch enstanden dass die Luft WVrmestrahlen absorbirte, welche das Eis als diathermaner K6rper durchliess (Schlagintweit,
Untersuchungen, S. 17).




330             APPENDIX TO LECTURE IX.
words: II ought also to mention a singular property of those
air-bubbles, which at first struck us forcibly, but which has since
recevied a very satisfactory explanation. When a fragment containing air-bubbles is exposed to the action of the sun, the bubbles augment insensibly. Soon, in proportion as they enlarge, a
transparent drop shows itself at some point of the bubble. This
drop, in enlarging, contributes on its part to the enlargement of
the cavity, and following its progress a little, it finishes by predominating over the bubble of air. The latter then swims in the
midst of a zone of water, and tends incessantly to reach the most
elevated point, at least if the flatness of the cavity does not hinder it.'
The satisfactory explanation here spoken of is that already
mentioned: let us now endeavour to follow the hypothesis to its
consequences.
Comparing equal weights of both substances, the specific heat
of water being 1, that of air is 0'25. Hence to raise a pound of
water one degree of temperature, a pound of air would have to
lose four degrees.
Let us next compare equal volumes of the substances. The
specific gravity of water being 1, that of the air is 71; hence a
pound of air is 770 times the volume of a pound of water; and
hence, for a quantity of air to raise its own volume of water one
degree, it must part with 770x 4, or 3,080 degrees of temperature.
Now the latent heat of water is 1426~  Fahr., hence the quantity of heat required to melt a certain weight of ice is 142'6 times
the quantity required to raise the same weight of water one degree in temperature; hence, a measure of air, in order to reduce
its own volume of ice to the liquid condition, must lose 3,080x
142'6, or 439,208 degrees of temperature.
This, then, gives us an idea of the amount of heat which, according to the above hypothesis, is absorbed by the bubble and
communicated to the ice during the time occupied in melting a
quantity of the latter equal in volume to the bubble, which time
is stated to be brief; that is to say the quantity of heat supposed
to be absorbed by the air would, if it had not been communicated
to the ice, have been sufficient to raise the bubble itself to a temperature 160 times that of fused cast iron. Had air this power
of absorption, it might be attended with inconvenient conse



ASSOCIATED BUBBLES OF AIR AND WATER.    331
quences to the denizens of the earth; for we should dwell at the
bottom of an atmospheric ocean, the upper strata of which would
effectually arrest all calorific radiation.
It is established by the experiments of Delaroche and Melloni,* that-a calorific beam, emerging from any medium which it
has traversed for any distance, possesses, in an exalted degree,
the power of passing through an additional length of the same
substance. Absorption takes place, for the most part, in the portion of the medium first traversed by the rays. In the case of a
plate of glass, for example, 172 per cent. of the heat proceeding
from a lamp, is absorbed in the first fifth of a millimetre; whereas, after the rays have passed through 6 millimetres of the substance, an additional distance of 2 millimetres absorbs less than 2
per cent. of the rays thus transmitted. Supposing the rays to
have passed through a plate 25 millimetres, or an inch, in thickness, there is no doubt that the heat emerging from such a plate
would pass through a second layer of glass, 1 millimetre thick,
without suffering any measurable absorption. For an incomparably stronger reason, the quantity of solar heat absorbed by a
bubble of air at the earth's surface, after the rays have traversed
the whole thickness of our atmosphere, and been sifted in their
passage through it, mut be wholly inappreciable.
Such, if I mistake not, are the properties of radiant heat which
modern physics have revealed; and I think they render it evident
that the hypothesis of M. Agassiz and the Messrs. Schlagintweit
was accepted without due regard to its consequences.
*       *       *       *        *       *       *
~ IV.
But the question still remains, how are the water-chambers
produced within the ice?... One simple test will, I think, decide the question whether the liquid is, or is not, the product of
melted ice. If it be, its volume must be less than that of the ice
which produced it, and the bubble associated with the water must
be composed of rarefied air. Hence, if on establishing a liquid connection between this bubble and the atmosphere a diminution of
* La Thermochrose, p. 202.




332              APPENDIX TO LECTURE IX.
volume be observed, this will indicate that the water has been
produced by the melting of the ice.
From a block of Norway ice, containing such compound bubbles, I cut a prism, and immersing it in warm water, contained in
a glass vessel, I carefully watched through the side of the vessel
the effect of the melting upon the bubbles. They invariably shrunkl
in volume at the moment the surrounding ice was melted, and the
diminished globules of air rose to the surface of the water. I
then arranged matters so that the wall of the cavity might be
melted away underneath, without permitting the bubble of air at
the top to escape. At the moment the melting reached the cavity
the air-bubbles instantly collapsed to a sphere possessing, in some
cases, far less than the hundredth part of its original volume.
The experiments were repeated with several distinct masses of ice,
and always with the same result. I think, therefore, it may be
regarded as certain that the liquid cells are the product of melted
ice.*
Considering the manner in which ice imported into this country is protected from the solar rays, I think we must infer that in
the specimens examined by me, the ice in contact'with the bubble
has been melted by heat, which has been conducted through the substance without visible prejudice to its solidity.
Paradoxical as this may appear, I think it is no more than
might reasonably be expected from a p2riori considerations. The
heat of a body is referred, at the present day, to a motion of its
particles. When this motion reaches an intensity sufficient to
liberate the particles of a solid from their mutual attractions, the
body passes into the liquid condition. Now, as regards the
amount of motion necessary to produce this liberty of liquidity,
the particles at the surface of a mass of ice must be very differently circumstanced from those in the interior, which are influenced
and controlled on every side by other particles. But if we suppose a cavity to exist within the mass, the particles bounding that
cavity will be in a state resembling that of the particles at the
surface; and by the removal of all opposing action on one side,
the molecules may be liberated by a force which the surrounding
mass has transmitted without prejudice to its solidity. Suppos* This of course refers only to the lake ice examined as described.




LIQUEFACTION BY CONDUCTION THROUGH ICE.  333
ing, for example, that solidity is limited by molecular vibrations
of a certain amplitude, those at the surface of the internal cavity
may exceed this limit, while those between the cavity and the external surface of the ice may, by their reciprocal actions, be preserved within it, just as the terminal member of a series of elastic
balls is detached by a force which has been transmitted by the
other members of the series without visible separation.*
Where, however, experiment is within reach we ought not to
trust to speculation; and I was particularly anxious to obtain an
unequivocal reply to the question whether an interior portion of
a mass of ice could be melted by heat which had passed through
the substance by the process of conduction. A piece of Norway
ice, containing a great number of the liquid disks already described, and several cells of air and water, was enveloped in tinfoil and placed in a mixture of pounded ice and salt. A few
minutes sufficed to freeze the disks to thin dusky circles, which
appeared, in some cases, to be formed of concentric rings, and
reminded me of the sections of certain agates. Looked at sideways, these disks were no thicker than a fine line. The watercells were also frozen, and the associated air-bubbles were greatly
diminished in size. I placed the mass of ice between me and a
gas-light, and observed it through a lens: after some time the
disks and water-cells showed signs of breaking up. The rings of
the disks disappeared; the contents seemed to aggregate so as to
form larger liquid spots, and finally, some of them were reduced
to clear transparent disks as before.
But an objection to this experiment is, that the ice may have
been liquefied by the radiation from the lamp, and I have experiments to describe which will show the justice of this objection.
A rectangular slab, 1 inch thick, 3 inches long, and 2 wide, was
therefore taken from a mass of Norway ice, in which the associated air and water-cells were very distinct. I enveloped it in tinfoil, and placed it in a freezing mixture. In about ten minutes
the water-blebs were completely frozen within the mass. It was
immediately placed in a dark room, where no radiant heat could
possibly affect it, and examined every quarter of an hour. The
dim frozen spots gradually broke up into little water parcels, and
* Of course I intend this to help the conception merely.




334               APPENDIX  TO LECTURE IX.
in two hours the water-blebs were perfectly restored in the centre
of the slab of ice.  When last examined, this plate was half an
inch thick, and the drops of liquid were seen right at its centre.
A second piece, similarly frozen and wrapped up in flannel,
showed the same deportment.  In an hour and a half the frozen
water surrounding the air-bubbles was restored to its liquid condition. Hence no doubt can remain as to the possibility of effecting liquefaction in the interior of a mass of ice, by heat which
has passed by conduction through the substance without melting it.
I have already referred to the formation of the liquid cavities
observed by M. Agassiz, when glacier ice was exposed to the sun.
The same effect may be produced by exposure to a glowing coal
fire.  On the 21st and 22nd of November, I thus exposed plates
of clear Wenham  Lake ice, which contained some scattered airbubbles. At first the bubbles were sharply rounded, and without
any trace of water.  Soon, however, those near the surface, on
which the radiant heat fell, appeared encircled by a liquid ring,
( j)~   which expanded and finally became crimped at its border,
as shown in the adjacent figure.  The crimping became
more pronounced as the action was permitted to continue.*
A second plate, crowded with bubbles, was held as near to the
fire as the hand could bear.  On withdrawing it, and examining
it through a pocket lens, the appearance was perfectly beautiful.
In many cases the bubbles appeared to be surrounded by a series
of concentric rings, the outer ring surrounding all the others like
a crimped frill.
I could not obtain these effects by placing the ice in contact
with a plate of metal obscurely heated,t nor by the radiation from
an obscure source.  Indeed ice, as before remarked, is impervious
to radiant heat from such a source.1 The rays from a common
* The blebs observed in glacier ice also exhibit this form: see fig. 8,
plate 6, of the Atlas to the' Systeme Glaciaire.' In fig. 13 we have also a
close resemblance of the flower-shaped figures produced by radiant heat in
lake ice.
f To develope water-cavities within ice a considerable time is necessary;
more time, indeed, than was sufficient to melt the entirepieces of ice made
use of in these contact experiments.
i Hence the soundness of the ice under the moraines; the sun's rays
are converted into obscure heat by the overlying debris; this only affects a




REGELATION.                          335
fire also are wholly absorbed near the surface upon which they
strike, and hence the described internal liquefaction was confined
to a thin layer close to this surface.
But not only does liquefaction occur in connection with the
bubbles, but the' flowers,' already described as produced by the
solar beams, start by hundreds into existence, when a slab of
transparent ice is placed before a glowing coal fire.  They, however, are also confined to a thin stratum of the substance close to
the surface of incidence.  In the experiments made in this way,
the central stars of the flowers were often bounded by sinuous
lines of great beauty.
The foregoing considerations show that liquefaction takes
place at the surface of a mass of ice at a lower temperature than
that required to liquefy the interior of the solid. At the surface
the temperature 32~ produces a vibration, to produce which, within the ice, would necessitate a temperature of 32~+x; the increment x being the additional temperature necessary to overcome
the resistance to liquefaction, arising from the action of the molecules upon each other.
Now let us suppose two pieces of ice at 32~, with moistened
surfaces, to be brought into contact with each other, we thereby.virtually transfer the touching portions of these pieces from the surface to the interior, where 32+ x is the melting temperature.
Liquefaction will therefore be arrested at those surfaces. Before
being brought together, the surfaces had the motion of liquidity,
but the interior of the ice has not this motion; and as equilibrium
will soon set in between the masses on each side of the liquid film
and the film  itself, the film will be reduced to a state of motion
inconsistent with liquidity.  In other words, it will be frozen, and
will cement the two surfaces of ice between which it is enclosed.*
If I am right here, the importance of the physical principles
layer of infinitesimal depth, and cannot produce the disintegration of the
deeper ice, as the direct sunbeams can.
* It is here implied that the contact of the moist surfaces must be so
perfect, or, in other words, the liquid film between them must be so thin,
as to enable the molecules to act upon each other across it. The extreme
tenuity of the film may be inferred from this. A — thick plate of water
within the ice would facilitate rather than retard liquefaction.




336             APPENDIX TO LECTURE IX.
involved are sufficiently manifest: if I am wrong, I hope I have
so expressed myself as to render the detection of my error easy.
Right or wrong, my aim has been to give as explicit utterance to
my meaning as the subject will admit of.
~ V.
Mr. Faraday's experiments on the freezing together of pieces of
ice at 32~ Fahr., and all of those recounted in the paper published
by Mr. Huxley and myself, find their explanation in the principles
here laid down. The conversion of snow into neve, and of n6ve
into glacier, is perhaps the grandest illustration of the same principle. It has been, however, suggested to me that the sticking
together of two pieces of ice may be an act of cohesion,. similar to
that which enables pieces of wetted glass, and other similar
bodies, to stick together. This is not the case. There is no sliding motion possible to the ice. When contact is broken, it breaks
with the snap due to the rupture of a solid. Glass and ice cannot
be made to stick thus together, neither can glass and glass, nor
alum and alum, nor nitre and nitre, at common temperatures. I
have, moreover, placed pieces of ice together over night and found
them in the morning so rigidly frozen together that when I sought
to separate them, the surface of fracture passed through one of
them  in preference to taking the surface of regelation. Many
sagacious persons have also suggested to me that the ice transported to this country from Norway and Wenham Lake may possibly retain a residue of its cold, sufficient to freeze a thin film
enclosed between two pieces of the substance. But the facts already adverted to are a sufficient reply to this surmise. The ice
experimented on cannot be regarded as a magazine of cold, because
parcels of liquid water exist within it.




LECTURE X.
[March 27, 1862.]
ABSORPTION OF HEAT BY GASEOUS MATTER-APPARATUS EMPLOYED —EARLY
DIFFICULTIES-DIATHERMANCY OF AIR AND OF THE TRANSPARENT ELEMENTARY GASES-ATHERMANCY (OPACITY) OF OLEFIANT GAS AND OF THE
COMPOUND GASES-ABSORPTION OF RADIANT HEAT BY VAPOURS-RADIATION OF HEAT BY" GASES-RECIPROCITY OF RADIATION AND ABSORPTION
-INFLUENCE OF MOLECULAR CONSTITUTION ON THE PASSAGE OF RADIANT HEAT.
IN our last lecture we examined the diathermancy, or
transparency to heat, of solid and liquid bodies; and we
then learned, that closely as the atoms of such bodies are
packed together, the interstitial spaces between the atoms
afford, in many cases, free play and passage to the ethereal
undulations, which were transmitted without sensible hindrance among the atoms. In other cases, however, we
found that the molecules stopped the waves of heat which
impinged upon them; but that in so doing, they themselves became centres of oscillation. Thus we learned that
while perfectly diathermic bodies allowed the waves of
heat to pass through them without suffering any change of
temperature, those bodies which stopped the calorific flux
became heated by the absorption. Through ice, itself, we
sent a powerful calorific beam; but as the beam was of
such a quality as not to be intercepted by the ice, it passed
through this highly sensitive substance without melting it.
We have now to deal with gaseous bodies; and here the
interatomic spaces are so vastly augmented, the molecules
15




338                 LECTIURE X.
are so completely released from all mutual entanglement,
that we should be almost justified in concluding that gases
and vapours furnish a perfectly open door for the passage
of the calorific waves. This, indeed, until quite recently,
was the universal belief, and the conclusion was verified by
such experiments as had been made on atmospheric air,
which was found to give no evidence of absorption.
But each succeeding year augments our experimental
powers; our predecessors were often obliged to fight with
flints, where we may use swords, and hence the conflict
with Nature is not decided by their discomfiture. Let us,
then, test once more the diathermancy of atmospheric air.
We may make a preliminary essay in the following way:
I have here a hollow tin cylinder A B (fig. 89), 4 feet long,
and nearly 3 inches in diameter, through which we may
send our calorific rays. We must, however, be able to conpare the passage of the rays through the air, with their
passage through a vacuum, and hence we must have some
means of stopping the ends of our cylinder, so as to be
able to exhaust it. Here we encounter our first experimental difficulty. As a general rule obscure heat is more
greedily absorbed than luminous heat, and as our object is
to make the absorption of a highly diathermic body sensible, we are most likely to effect this object by employing
obscure heat.
Our tube, therefore, must be stopped by a substance
which permits of the free passage of such heat. Shall we
use glass for the purpose? An inspection of the table at
page 311 shows us, that for such rays pllates of glass would
be perfectly opaque; we might as well stop our tube with
plates of metal. Observe here how an investigator's results
are turned to account by his successors. From one experiment buds another, and science grows by the continual degradation of ends to means. Had not Melloni discovered
the diathermic properties of rocksalt, we should now be ut



FIRST EXPERIMENTS WITH  GASES.                   339
terly at a loss. For a time, however, I was extremely hampered by the difficulty of obtaining plates of salt sufficiently
large and pure to stop the ends of my tube. But a scientific
worker does not long lack help, and, thanks to such friendly aid, I have here plates of this precious substance which,
by means of these caps, I can screw air-tight on to the ends
Fig. 89.
of my cylinder.*  You observe two stopcocks attached to
the cylinder; this one, c, is connected with an air-pump, by
* At a time when I was greatly in need of a supply of rocksalt, I stated
my wants in the' Philosophical Magazine,' and met with an immediate response from Sir John Herschel.  He sent me a block of salt, accompanied
by a note, from which, as it refers to the purpose for which the salt was
originally designed, I will make an extract. I have not yet been able to
examine the extremely remarkable point to which the eminent writer directs my attention. I am also greatly indebted to Dr. Szabo, the Hunga.
rian Commissioner to the International Exhibition, by whom I have been
lately raised to- comparative opulence, as regards the possession of rocksalt.
To the Messrs. Fletcher, of Northwich, and to Mr. Corbett, of Bromsgrove, my best thanks are also due for their obliging kindness.
Here follows the extract from Sir J. Herschel's note:-' After the publication of my paper in the Phil. Trans., 1840, I was very desirous to disengage myself from the influence of glass prisms and lenses, and ascertain, if
possible, whether in reality my insulated heat spots 3 y?  e in the spectrum




340                       LECTURE X.
which the tube can be exhausted; while through this other
one, c', I can allow air or any other gas to enter the tube.
At one end of the cylinder I place this Leslie's cube c,
containing boiling water; and which is coated with lampblack, to augment its power of radiation. At the other end
of the cylinder stands our thermo-electric pile, from which
wires lead to the galvanometer. Between the end of the
cylinder and the source of heat I have introduced a tin
screen, T, which, when withdrawn, will allow  the calorific
rays to pass through the tube to the pile. We first exhaust
the cylinder, then draw the screen a little aside, and now
the rays are traversing a vacuum and falling upon the pile.
The tin screen, you observe, is only partially withdrawn,
and the steady deflection produced by the heat at present
transmitted is 30 degrees.
Let us now admit dry air: I can do so by means of the
cock c', from which a piece of flexible tubing leads to the
bent tubes u, u', the use of which I will now  explain; u is
filled with fragments of pumice stone moistened with a solution of caustic potash; it is destined to withdraw whatwere of solar or terrestial origin. Rocksalt was the obvious resource, and
after many and fruitless endeavours to obtain sufficiently large and pure
specimens, the late Dr. Somerville was so good as to send me (as I understood from a friend in Cheshire) the very fine block which I now forward.
It is, however, much cracked, but I have no doubt pieces large enough for
lenses and prisms (especially if cemented together) might be got from it.'But I was not prepared for the working of it-evidently a very delicate
and difficult process, (I proposed to dissolve off the corners, &c., and, as it
were, lick it into shape) and though I have never quite lost sight of the
matter, I have not yet been able to do anything with it: meanwhile, I put
it by. On looking at it a year or two after, I was dismayed to find it had
lost much by deliquescence. Accordingly, I potted it up in salt in an
earthen dish, with iron rim, and placed it on an upper shelf in a room with
an Arnott stove, where it has remained ever since.'If you should find it of any use I would ask you, if possible, to repeat
my experiment as described, and settle that point, which has always struck
me as a very important one.'




DEFECTS OF METHOD.               341
ever carbonic acid may be contained in the air; iu' is a similar tube, filled with fragments of pumice stone moistened
with sulphuric acid; it is intended to absorb the aqueous
vapour of the air. Thus the air reaches the cylinder deprived both of its aqueous vapour and its carbonic acid. It
is now entering,-the mercury-gauge of the pump is descending, and as it enters I would beg of you to observe
the needle. If the entrance of the air diminish the radiation through the cylinder-if air be a substance which is
competent to destroy the waves of ether in any sensible
degree-this will be declared by the diminished deflection
of the galvanometer. The tube is now full, but you see no
change in the position of the needle, nor could you see any
change even if you were close to the instrument. The air
thus examined seems as transparent to radiant heat as the
vacuum itself.
By changing the screen I can alter the amount of heat
falling upon the pile; thus, by withdrawing it, I can cause
the needle to stand at 40~, 50~, 60~, 70~ and 80~ in succession; and while it occupies each position I can repeat the
experiment which I have just performed before you. In
no instance could you recognize the slightest movement of
the needle. The same is the case if I push the screen forward, so as to reduce the deflection to 20 and 10 degrees.
The experiment just made is a question addressed to
Nature, and her silence might be construed into a negative
reply. But a natural philosopher must not lightly accept a
negative, and I am not sure that we have put our question
in the best possible language. Let us analyse what we
have done, and first consider the case of our smallest deflection of 10 degrees. Supposing that the air is not perfectly diathermic; that it really intercepts a small portion
-say the thousandth part of the heat passing through the
tube-that out of every thousand rays it struck down one;
should we be able to detect this execution? This absorp



342                   LECTURE X.
tion, if it took place, would lower the deflection the thousandth part of ten degrees, or the hundredth part of one
degree, a diminution which it would be impossible for you
to see, even if you were close to the galvanometer.*  In
the case here supposed, the total quantity of heat falling
ztpon the pile is so inconsiderable, that a small fraction of
it, even if absorbed, might well escape detection.
But we have not confined ourselves to a small quantity
of heat; the result was the same when the deflection was
800 as when it was 10~. Here I must ask you to sharpen
your attention and accompany me, for a time, over rather
difficult ground. I want now to make clearly intelligible
to you an important peculiarity of the galvanometer.
The needle being at zero, let us suppose a quantity of
heat to fall upon the pile, sufficient to produce a deflection
of one degree. Suppose that I afterwards augment the
quantity of heat, so as to produce deflections of two degrees, three degrees, four degrees, five degrees; I then
know that the quantities of heat which produce these deflections stand to each other in the ratios of I: 2: 3: 4: 5;
the quantity of heat which produces a deflection of 5~ being exactly five times that which produces a deflection of
i~. But this proportionality exists only so long as the deflections do not exceed a certain magnitude. For, as the
needle is drawn more and more aside from zero, the current acts upon it at an ever augmenting disadvantage.
The case is illustrated by a sailor working a capstan; he
always applies his strength at right angles to the lever,
for, if he applied it obliquely, only a portion of that strength
would be effective in turning the capstan round. And in
the case of our electric current, when the needle is very
oblique to the current's direction, only a portion of its force
* It will be borne in mind that I am here speaking of galvanometric
not of thermometric degrees.




RELATION OF DEFLECTION TO ABSORPTION.    343
is effective in moving the needle round. Thus it happens,
that though the quantity of heat may be, and, in our case,
is, accurately expressed by the strength of the current
which it excites, still the larger deflections, inasmuch as
they do not give us the action of the whole current, but
only of a part of it, cannot be a true measure of the
amount of heat falling upon the pile.
The galvanometer now before you is so constructed
that the angles of deflection, up to 300 or thereabouts, are
proportional to the quantities of heat; the quantity necessary to move the needle from 30~ to 31~ is nearly the same
as that required to move it from 0~ to 10. But beyond 30~
the proportionality ceases. The quantity of heat required
to move the needle from 40~ to 41~ is three times that necessary to move it from 0~ to 1~; to deflect it from 50~ to
51~ requires five times the heat necessary to move it from
O0 to 1~; to deflect it from 60~ to 610 requires about ten
times the heat necessary to move it froth 0~ to 1~; to deflect it from 70~ to 71~ requires nearly twenty times, while
to move it from 800 to 81~ requires more than fifty times
the heat necessary to move it from 0~ to 10. Thus, the
higher we go, the greater is the quantity of heat represented by a degree of deflection; the reason being, that-the
force which then moves the needle is only a fraction of the
force really circulating in the wire, and hence represents
only a fraction of the heat falling upon the pile.
By a certain process, which I will not stop here to describe,* I can express the higher degrees in terms of the
lower ones; I thus learn, that while deflections of 100, 20~,
30~, respectively, express quantities of heat represented by
the numbers 10, 20, 30, a deflection of 40~ represents a
quantity of heat expressed by the number 47; a deflection
of 50~ expresses a quantity of heat expressed by the num* See Appendix to Lecture X.




344                   LEOTUIRE X.
ber 80; while the deflections 60~, 70~, 80~, express quantities of heat which increase in a much more rapid ratio
than the deflections themselves.
What is the upshot of this analysis? It will drive us,
I think, to a better method of questioning Nature. It leads
to the reflection that, when we make our angles smcall, the
quantity of heat falling on the pile is so inconsiderable, that
even if a fraction of it were absorbed, it might escape detection; while, if we make our deflections large, by employing a powerful flux of heat, the needle is in a position
from which it would require a considerable addition or
abstraction of heat, to move it. The 1,000th part of the
whole radiation in the one case would be too small, absolutely, to be measured; the 1,000th part in the other case
might be something considerable, without, however, being
considerable enough to affect the needle in any sensible degree. When, for example, the deflection is over 80~, an
augmentation or diminution of heat, equivalent to 15 or 20
of the lower degrees of the galvanometer, would be scarcely measurable.
We are now face to face with our problem; it is this,
to work with a flux of heat so large that a small fractional
part of it will not be infinitesimal, and still to keep our
needle in its most sensitive position. If we can accomplish this we shall augment indefinitely our experimental
power. If a fraction of the heat, however small, be intercepted by the gas, we can augment the absolute value of
that fraction by aucgmenting the total of which it is a fraction.
The problem, happily, admits of an effective practical
solution. You know that when we allow heat to fall upon
the opposite faces of the thermo-electric pile, the currents
generated neutralise each other more or less; and, if the
quantities of heat falling upon the two faces be perfectly
equal, the neutralisation is complete. Our galvanometer




IMPROVED APPARATUS.               345
needle is now deflected to 80~ by the flux of heat passing
through the tube; I uncover the second face of the pile,
furnish it with its conical reflector, and place a second cube
of boiling water in front of it; the needle, as you see, descends instantly.
By means of a proper adjusting screen I can so regulate
the quantity of heat falling upon the posterior face of the
pile, that it shall exactly neutralise the heat incident upon
its other face: this is now effected; and the needle points
to zero.
Here, then, we have two powerful and perfectly equal
fluxes of heat, falling upon the opposite faces of the pile,
one of which passes through our exhausted cylinder. If I
allow air to enter the cylinder, and if this air exert any appreciable action upon the rays of beat, the equality now
existing will be destroyed; a portion of the rays passing
through the tube being struck down by the air, the second
source of heat will triumph; the needle, now in its most
sensitive position, will be deflected; and from the magnitude of the deflection we can accurately calculate the absorption.
I have thus sketched, in rough outline, the apparatus by
which our researches on the relation of radiant heat to
gaseous matter must be conducted. The necessary tests
are, however, at the same time so powerful and so delicate,
that a rough apparatus like that just described would not
answer our purpose. But you will now experience no difficulty in comprehending the construction and application of
the more perfect apparatus, with which the experiments on
gaseous absorption and radiation have been actually made.
See Plate I., at the end of the volume.
Between s and s' stretches the experimental cylinder, a
hollow tube of brass, polished within; at s, and s', are the
plates of rock salt which close the cylinder air-tight; the
length from s to s', in the experiments to be first recorded,
15*




346                  LECTURE X.
is 4 feet. c, the source of heat, is a cube of cast copper,
filled with water, which is kept continually boiling by the
lamp L. Attached to the cube c by brazing is the short
cylinder F, of the same diameter as the experimental cylinder, and capable of being connected air-tight with the latter at s. Thus between the source c and the end s' of the
experimental tube, we have the front chamber F, from
which the air can be removed, so that the rays from the
source will enter the cylinder s s' unsifted. To prevent
the heat from the source c passing by conduction to the
plate at s, the chamber F is caused to pass through the
vessel v, in which a stream of cold water continually circulates, entering through the pipe i i, which dips to the bottom of the vessel, and escaping through the waste-pipe e e.
The experimental tube and the front chamber are connected, independently, with the air-pump  A, so that either of
them may be exhausted or filled without interfering with
the other. I may remark that in later arrangements the
experimental cylinder was supported apart from the pump,
being connected with the latter by a flexible tube. The
tremulous motion of the pump, which occurred when the
connection was rigid, was thus completely avoided.
r is the thermo-electric pile, placed on its stand at the
end of the experimental tube, and furnished with its two
conical reflectors. c' is the compensating cube, used to
neutralise the radiation from c; IH is the adjusting screen,
which is capable of an exceedingly fine motion to and fro.
N N is a delicate galvanometer connected with the pile P,
by the wires wt w'. The graduated tube o o (to the right
of the plate), and the appendage M K (attached to the centre of the experimental tube) shall be referred to more particularly by and by.
I should hardly sustain your interest in stating the difficulties which at first beset the investigation conducted with
this apparatus, or the numberless precautions which the




ACTION OF ATMOSPHERIC AIR.          347
exact balancing of the two powerful sources of heat, here
resorted to, rendered necessary. I believe the experiments
made with atmospheric air alone might be numbered by
tens of thousands. Sometimes for a week, or even for a
fortnight, coincident and satisfactory results would be obtained; the strict conditions of accurate experimenting
would appear to be found, when an additional day's experience would destroy the superstructure of hope, and necessitate a recommencemlent, under changed conditions, of
the whole enquiry. It is this which daunts the experimenter; it is this preliminary fight with the entanglements
of a subject, so dark, so doubtful, so uncheering; without
any knowledge whether the conflict is to lead to anything
worth possessing, that renders discovery difficult and rare.
But the experimenter, and particularly the young experimenter, ought to know, that as regards his own moral manhood, he cannot but win if he only contend aright. Even with
a negative result, the consciousness that he has gone fairly to the bottom of his subject, as far as his means allowed
-the feeling that he has not shunned labour, though that
labour may have resulted in laying bare the nakedness of
his case-reacts utpon his own mind, and gives it firmness
for future work.
But to return;-I first neglected atmospheric vapour
and carbonic acid altogether, concluding, as others did
afterwards, that the quantities of these substances being
so small, their effect upon radiant heat must be quite inappreciable; after a time, however, I found this assumption leading me quite astray. I first used chloride of calcium as a drying agent, but had to abandon it. I next used
pumice stone moistened with sulphuric acid, and had to
give it up also. I finally resorted to pure glass broken to
small fragments, wetted with sulphuric acid, and inserted
by means of a funnel into a IU tube. I found this arrangement best, but even here the greatest care was needed. It




348                  LECTURE X.
was necessary to cover each column with a layer of dry
glass fragments, for I found that the smallest particle of
dust from the cork, or a quantity of sealing wax not more
than the twentieth-part of a pin's head in size, was quite
sufficient, if it reached the acid, to vitiate the results. The
drying-tubes moreover had to be frequently changed, as
the organic matter of the atmosphere, infinitesimal though
it was, soon introduced disturbance.
To remove the carbonic acid, pure Carrara marble was
broken into fragments, wetted with caustic potash, and introduced into a U tube. These, then, are the agents for
drying the gas and removing the carbonic acid which are
used at present; but previous to their final adoption, I employed, to dry the air, the arrangement shown in Plate I.,
where the glass tubes marked Y Y, each three feet long,
were filled with chloride of' calcium, after which were
placed two U tubes n z, filled with pumice stone and sulphuric acid. IHence, the air, in the first place, had to pass
over 18 feet of chloride of calcium, and afterwards through
the sulphuric acid tubes, before it entered the experimental
tube s s'.  A gas-holder, G G, was employed for other
gases than atmospheric air. In the investigation on which
I am at present engaged, this arrangement, as I have said,
is abandoned, a simpler one being found more effectual.
My assistant has now exhausted both the front chamber
F and the experimental tube s s'. The rays are passing
from the source c through the front chamber; across the
plate of rocksalt at s, through the experimental tube, across
the plate at s', afterwards impinging upon the anterior surface of the pile P. This radiation is neutralised by that
from the compensating cube c'. The needle, you will observe, is at zero. We will commence our experiments by
applying this powerful test to dry air. It is now entering
the experimental cylinder; but, at your distance, you see
no motion of the needle, and thus our more powerful mode




AB3SORPTION OF RADIANT HEAT BY OLEFIANT GAS. 349
of experiment fails to detect any absorption on the part of
the air. Its atoms, apparently, are incompetent to shatter
a single calorific wave; it is a practical vacuum as regards
the rays of heat. Were you quite near, however, you
would see a deflection of the needle amounting to about
one degree. Oxygen, hydrogen, and nitrogen, when carefully purified, exhibit the action of atmospheric air; they
are almost neutral.
But the neutral quality of atmospheric air was thought
to extend to transparent gases' generally.  Let us see
whether this is correct. I have here a gas-holder of olefiant
gas,-common coal gas would also answer my purpose. I
discharge a little of the olefiant gas in the air, but you see
nothing; the gas is perfectly transparent. The experimental tube is exhausted, and the needle points to zero;
and now we will allow the olefiant gas to enter. Observe
the effect. The needle moves in a moment; the transparent gas strikes down the rays wholesale-the final and permanent deflection, when the tube is full, amounting to 70
degrees.
I will now interpose a metal screen between the pile P
and the end s' of the experimental tube, thus entirely cutting off the radiation through the tube. The face of the
pile turned towards the metal screen wastes its heat speedily by radiation; it is now at the temperature of this
room, and the radiation from the compensating cube alone
acts on the pile, producing a deflection of 75 degrees. But
at the commencement of the experiment the radiations from
both cubes were equal, hence the deflection 75~ corresponds
to the total radiation through the experimental tube, when
the latter is exhausted.
Taking as unit the quantity of heat necessary to move
the needle from 0~ to 10, the number of units expressed by
a deflection of 75~ is
360.




350                  LECTURE X.
The number of units expressed by a deflection of 70~ is
290.
Out of a total, therefore, of 360, olefiant gas has struck
down 290; that is about seven-ninths of the whole, or
about 81 per cent.
Does it not seem to you as if an opaque layer had been
suddenly precipitated on our plates of salt, when the gas
entered? The substance, however, deposits no such layer.
I discharge a current of the dried gas against a polished
plate of salt, but you do not perceive the slightest dimness.
The rocksalt plates, moreover, though necessary for exact
measurements, are not necessary to show the destructive
powers of this gas. HIere is an open tin cylinder which I
interpose between the pile and our radiating source; I
force olefiant gas gently into the cylinder from this gasholder and you see the needle fly up to its stops. Observe
the smallness of the quantity of gas which I shall next use.
I cleanse the open tube by forcing a current of air through
it; the needle is now at zero; and I will simply turn this
cock on and off, as speedily as I can. A mere bubble of
the gas enters the tube in this brief interval; still you see
that its presence causes the needle to swing to 70~. I next
abolish the open tube, and leave nothing but the free air
between the pile and source; from the gasometer I discharge blefiant gas into this space. You see nothing in the
air, but the swing of the needle through an arc of 60~ declares the presence of this invisible barrier to the calorific
rays.
Thus, it is shown that the ethereal undulations which
glide among the atoms of oxygen, nitrogen, and hydrogen,
without hindrance, are powerfully absorbed by the molecules of olefiant gas. WVe shall find other transparent gases
also almost immeasurably superior to air. We can limit at
pleasure the number of the gaseous atoms, and thus vary
the amount of destruction of the ethereal waves. In this




RELATION OF QUANTITY TO ABSORPTION.         351
respect gaseous bodies possess a great advantage over
liquids and solids, in experiments on radiation. Attached
to the air-pump is a barometric tube, by means of which I
can admit measured portions of the gas. The experimental cylinder is now exhausted, and turning this cock slowly
on, and observing the mercury gauge, I allow the olefiant
gas to enter, till the mercurial column has been depressed
an inch. I observe the galvanometer and read the deflection. Determining thus the absorption produced by one
inch, another inch is added, and the absorption effected by
two inches of the gas is determined. Proceeding thus we
obtain for tensions from 1 to 10 inches the following absorptions:Olefiant Gas.
Tensions
in inches                    Absorption
1,...  90
2.. 123
3..             142
4...   157....                168
6...   1T77'7    e...    182
8.. 186
9....   190
10....   193
The unit here used is the amount of heat absorbed
when a whole atmosphere of dried air is allowed to enter
the tube. The table, for example, shows that one-thirtieth
of an atmosphere of olefiant gas exercises ninety times the
absorption of a whole atmosphere of air.
The table also informs us that each additional inch of
olefiant gas produces less destruction than the preceding
one. A single inch, at the commencement, strikes down
90 rays, but a second inch strikes down only 33, while the
addition of an inch, when nine inches are already in the




352                   LECTURE X.
tube, effects the destruction of only 3 rays. This is what
might reasonably be expected. The number of rays emitted is finite, and the discharge of the first inch of -olefiant
gas amongst them has so thinned their ranks that the execution produced by the second inch is naturally less than
that of the first. This execution must diminish, as the
number of rays capable of being destroyed by the gas, becomes less; until, finally, all absorbable rays being removed,
the residual heat would pass through the gas unimpeded.
But supposing the quantity of gas first introduced to be
so inconsiderable, that the number of rays extinguished by
it is a vanishing quantity, compared with the total number
capable of being destroyed, we might then reasonably expect that, for some time at least, the quantity of execution
done would be proportional to the quantity of gas present.
That a double quantity of gas would produce a double
effect, a treble quantity a treble effect; or, in general
terms, that the absorption would, for a time, be found pro
portional to the density.
To test this idea we will make use of a portion of the
apparatus omitted in the general description. o o (Plate.J.) is a graduated glass tube, the end of which dips into the
basin of water B. The tube is closed above by means of
the stopcock r; d d is a tube containing fragments of chloride of calcium. The tube o o is first filled with water up
to the cock r, and the water is afterwards carefully displaced by olefiant gas admitted in bubbles from  below.
The gas is admitted into the experimental cylinder by the
cock r, and as it enters, the water rises in o o, each of
whose divisions represents a volume of -1'th of a cubic inch.
Successive measures of this capacity are permitted to enter
the tube, and the absorption in each particular case is determined.
In the following table the first column contains the
quantity of gas admitted into the tube; the second con



ABSORPTION BY ETHER VAPOUR.                  353
tains the corresponding absorption; the third column contains the absorption, calculated on the supposition that it is
proportional to the density.
Olefiant Gas.
Unit measure )L~th of a cubic inch.
Absorption.
Measures of Gas.          Observed.                Calculated.
1...   22...   2'2
2..           45...   4'4
3...   66...   66
4...   8'8...   88
5.   l 0..  11 0
6                     12'0           ~.  13'2'7.    ~.  14'8.       -.        15.4
8          ~ ~.  168...  176
9.     ~.  198...        19'8
10...  220...  220
11          ~ ~.  24'0...  24'2
12...  254...  264
13           ~..  29'0..  28'6
14...  30'2...  29'8
15...  33'5..  33'0
This table proves the correctness of the surmise, that,
when very small quantities of the gas are employed, the
absorption is sensibly  proportional to the density.  But
consider for a moment the tenuity of the gas with which
we have here operated.  The volume of our experimental
tube is 220 cubic inches; imagine J-6th of a cubic inch of
gas diffused in this space, and you have the atmosphere
through which the calorific rays passed in our first experiment.  This atmosphere possesses a tension not exceeding, Ith of that of ordinary air. It would depress the mercurial column connected with the air-pump not more than
-3  7th of an English inch. Its action, however, upon the
calorific rays is perfectly measurable.
But the absorptive energy of olefiant gas, extraordinary




354                   LECTURE X.
as it is shown to be by the foregoing experiments, is exceeded by that of various vapours, the action of which I
will now endeavour to illustrate. Here is a glass flask, G
(fig. 90), provided with a brass cap, into which a stopcock
can be screwed air-tight. I pour a small quantity of sulphuric ether into the flask, and completely remove, in the first place, the air which fills the
flask above the liquid. I attach the flask to
_h  the experimental tube, which is now exhausted-the needle pointing to zero-and permit
the vapour from the flask to enter the experimental tube.  The mercury of the gauge
sinks, and now that it is depressed one inch I
will stop the further supply of vapour. The
moment the vapour entered, the needle moved,
and it now points to 65~. I can add another
inch, and again determine the absorption, a
third inch and do the same. The absorptions
effected by four inches, introduced in this way, are given in
the following table. For the sake of comparison I place
the corresponding absorptions of olefiant gas in the third
column.
Sulphuric -eher.
Tensions                            Corresponding absorption
in inches.         Absorption.          of Olefiant Gas.
1....   214...    90
2.   -   v. 282                    123
3                  315...   142
4.              330...   154
For these tensions the absorption of radiant heat by the
vapour of sulphuric ether is about two and two-third times
the absorption of olefiant gas. There is, moreover, no proportionality between the quantity of vapour and the absorption.
But reflections similar to those which we have already
applied to olefiant gas are also applicable to the ether.
Supposing we make our unit measure small enough, the




RELATION OF QUANTITY TO ABSORPTION.              355
number of rays first destroyed will vanish in comparison
with the total number, and, for a time, the fact will probably manifest itself, that the absorption is directly proportional to the density. To examine whether this is the case,
the other portion of the apparatus, omitted in the general
description, was made use of. -K is one of the small flasks
already described, with a brass cap, which is closely
screwed on to the stopcock c'. Between the cocks c' and
c, which latter is connected with the experimental tube, is
the chamber N, the capacity of which Was accurately determined.  The flask k was partially filled with ether, and the
air above the liquid removed.  The stopcock c' being shut
off and c turned on, the tube s s' and the chamber ri are
exhausted.  The cock c is now shut off, and c' being turned
on, the chamber 3I becomes filled with pure ether vapour.
By turning c' off and c on, this quantity of vapour is allowed to diffuse itself through the experimental tube, where
its absorption is determined; successive measures are thus
sent into the tube, and the effect produced by each is noted.
In the following table the unit measure made use of had
a volume of  I -,th of a cubic inch.
Sulphuric Ether.
Absol ption.
Measures.               Observed.               Calculated.
1.   ~..     0...    4'6
2...  103...   92
4...  192...  184
5...  245....  23'0
6...  295...  27'0
7...      34-5..            32'2
8.            38'0..       36 8
9....  440...  414
10...  46'2....  462
11....  500...  506
12,,.  52'8....552
13....        550....  59'8
14,..  57'12....  64-4
15.,.  59'4..  69'0




356                  LECTURE X.
We here find that the proportion between density and
absorption holds sensibly good for the first eleven measures,
after which the deviation from proportionality gradually
augments.
No doubt, for smaller measures than -m th of a cubic
inch the above law holds still more rigidly true; and in
a suitable locality it would be easy to determine, with perfect accuracy, -T'th of the absorption produced by the first
measure; this would correspond to Tr-,th of a cubic inch
of vapour. But, before entering the tube, the vapour had
only the tension due to the temperature of the laboratory,
namely 12 inches. This would require to be multiplied by
2'5 to bring it up to that of the atmosphere. Hence the
O 0- th of a cubic inch would, on being diffused through a
tube possessing a capacity of 220 cubic inches, have a tension of  IX25 X,,,o->   0 =50010th of an atmosphere!
These experiments with ether and olefiant gas show that
not only do gaseous bodies, at the ordinary tension of the
atmosphere, offer an impediment to the transmission of radiant heat; not only are the interstitial spaces of such gases
incompetent to allow the ethereal undulations free passage;
but, also, that their density may be reduced vastly below
that which corresponds to the atmospheric pressure, and
still the door thus opened is not wide enough to let the undulations through. There is something in the constitution
of the individual molecules, thus sparsely scattered, which
enables them to destroy the calorific waves. The destruction, however, is merely one of form; there is no absolute
loss. Through dry air the heat rays pass without sensibly
warming it; through olefiant gas and ether vapour they
cannot pass thus freely; but every wave withdrawn from
the radiant sheaf produces its equivalent motion in the body
of the absorbing gas, and raises its temperature. It is a
case of transference, not of annihilation. I might extend
the experiments to all available volatile liquids, and show




ABSORPTION BY GASES.                   357
you that the same rule holds good for the vapours of
all.
Before changing the source of heat here made use of, I
wish to direct your attention for a moment to the action
of a few of the permanent gases on radiant heat. To
measure the quantities introduced into the experimental
tube, the mercury gauge of the air-pump was made use of.
In the case of carbonic oxide, the following absorptions
correspond to the tensions annexed to them, the action of
a full atmosphere of air, which, as you remember, produces
a deflection of 1~, being taken as unit:Carbonic Oxide.
Absorption.
Tension                 -— a            - --
in inches.               Observed.            Calculated.
0.5....    25...    25
1'0....    56...    50
1'5....    80..~        7'5
2'0...         10'0.  ~ ~       10'0
25.                 12 0...  12'5
B'0    *                 15'0. ~ o     15'0
385...           17'5..             ~.  175
As in former cases, the third column is calculated on the
assumption that the absorption is directly proportional to
the density of the gas; and we see that for seven measures,
or up to a tension of 3'5 inches, the proportionality holds
strictly good. But for large quantities this is not the case;
when, for instance, the unit measure is 5 inches, instead of
half-an-inch, we obtain the following results:
Absorption,
Tension
in inches.               Observed.            Calculated.
5..     1.                   18
10....    325...    36
15...   45..   ~   54
The case of carbonic oxide is therefore similar to that of
olefiant gas. Carbonic acid, sulphide of hydrogen, nitrous
oxide, and other gases, though differing in the energy of




358                   LECTURE X.
their absorption, and all of them exceeding carbonic oxide,
exhibit, when small and large quantities are used, a similar
deportment towards radiant heat.
Thus, then, in the case of some gases, we find an almost
absolute incompetence on the part of their atoms to be
shaken by the ethereal waves. They remain practically at
rest when the undulations speed amongst them, while the
atoms of other gases, struck by these same undulations, absorb their motion, and become themselves centres of heat.
We have now to examine what gaseous bodies are competent to do in this latter capacity; we have to enquire
whether*these atoms and molecules, which can accept motion from the ether in such very different degrees, are not
also characterised by their competency to impart motion
to the ether in different degrees; or, to use the common
language, having learned something of the power of different gases, as absorbers of radiant heat, we have now to enquire into their capacities as radiators.
I have here an arrangement, by means of which we can
put the necessary question, which has hitherto received
only a negative reply. P (fig. 91) is the thermo-electric pile
with its two conical reflectors; s is a double screen of polished tin; A is an argand burner, consisting of two concentric perforated rings; c is a copper ball, which, during the
experiments, is heated under redness; while the tube t t
leads to a gas holder. When the hot ball c is placed on
the burner it warms the air in contact with it; an ascending current is thus established, which, to some extent, acts
upon the pile. To neutralise this action a large Leslie's
cube, L, filled with water, a few degrees above the air in
temperature, is placed before the opposite face of the pile.
The needle being thus brought to zero, the gas is forced,
by a gentle water pressure, through the orifices of the
burner; it meets the ball c, glides along its surface, and
ascends, in a warm current, in front of the pile. The rays




RADIATION BY GASES.                      359
from the heated gas gush forth in the direction of the arrows against the pile, and the consequent deflection of the
Fig. 91.
galvanometer needle indicates the magnitude of the radiation.
The results of the experiments are given in the second
column of the following table, the numbers there recorded
marking'the extreme limit to which the needle swung,
when the rays from the gas fell upon the pile:
Radiation.             Absorption.
Air.                    0..  02
Oxygen..      0...  02
Nitrogen..     0...  02
Hydrogen..      0...  02
Carbonic oxide           12.. 18'0
Carbonic acid..    18... 25 0
Nitrous oxide.          29... 440
Olefiant gas..    53. 61'0




360                  LECTURE X.
In order to compare the radiation with the absorption,
I have placed in the third column the deflections due to the
absorption of the same gases, at a common tension of 5
inches. We see that radiation and absorption go hand in
hand; that the molecule which shows itself competent to
intercept a calorific flux, shows itself competent, in a proportionate degree, to generate a calorific flux. That, in
short, a capacity to accept motion from the ether, and to
impart motion to the ether, by gaseous bodies, are correlative properties.
And here, be it remarked, we are relieved from all considerations regarding the influence of cohesion, on the results. In solids and liquids the particles are more or less
in thrall, and cannot be considered as individually free.
Fig. 92.
The difference in point of radiative and absorptive power,
between alum and rocksalt, for example, might be fairly




BLACKNESS OF TRANSPARENT GASES TO HEAT. 361
regarded as due to their character as aggregates, held together by crystallising force. But the difference between
olefiant gas and atmospheric air cannot be explained in this
way; it is a difference dependent on the individual molecules of these substances, and thus our experiments with
gases and vapours probe the question of atomic constitution
to a depth, quite unattainable with solids and liquids.
I have refrained thus far from giving you as full a
tabular statement of the absorptive powers of gases and
vapours as the experiments made with the apparatus already described would enable me to do, knowing that I
had in reserve results, obtained with another apparatus,
which would better illustrate the subject. This second arrangement is the same in principle as the first; only two
changes of importance have been made in it. The first is,
that instead of making a cube of boiling water my source
of heat, I employ a plate of copper, against which a thin
steady gas-flame from a Bunsen's burner is caused to play;
the heated plate forms the back of my new front chamber,
which latter can be exhausted independently, as before.
This portion of the apparatus is sketched in fig. 92, the
chimney G being added. The second alteration is the substitution of a tube of glass of the same diameter, and 2
feet 8 inches long, for the tube of brass s s', Plate I.
All the other parts of the apparatus remain as before. The
gases were introduced in the manner already described into
the experimental tube, and from the galvanometric deflection, consequent on the entrance of each gas, its absorption
was calculated.
The following table gives the relative absorptions of
several gases, at a common tension of one atmosphere: —
Absorption at
Name.                       80 inches tension.
Air..   1
Oxygen....  1
16




362                    LECTURE X.
Absorption of
Name.                        80 inches tension.
Nitrogen...         1
Hydrogen...      1
Chlorine..                  39
Hydrochloric acid...    62
Carbonic oxide..,    90
Carbonic acid.                 90
Nitrous oxide...   355
Sulphide of hydrogen..   390
Marsh gas...          403
Sulphurous acid...   710
Olefiant gas...   970
Ammonia....  1195
The most powerful and delicate tests that I have been
able to apply have not yet enabled me to establish a difference between oxygen, nitrogen, hydrogen, and air. The
absorption of these substances is exceedingly small-probably even smaller than I have made it. The more perfectly the above-named gases are purified, the more closely
does their action approach to that of a vacuum. And who
can say that the best drying apparatus is perfect? I cannot even say that sulphuric acid, however pure, may not
yield a modicum of vapour to the gases passing through it,
and thus make the absorption by those gases appear greater than it ought. Stopcocks also must be greased, and
hence may contribute an infinitesimal impurity to the air
passing through them. But however this may be, it is
certain that if any further advance should be made in the
purification of the more feebly acting gases, it will only
serve to augment the enormous differences of absorption
exhibited by the foregoing table.
Ammonia, at the tension of an atmosphere, exerts an
absorption at least 1,195 times that of the air. If I interpose this metal screen between the pile and the experimental tube, the needle will move a little, but so little that you
entirely fail to see it. What does this experiment mean?




INFLUENCE OF ATOMIC CONSTITUTION.        363
Why, that this ammonia which, within our glass tube, is
as transparent to light as the air we breathe, is so opaque
to the heat radiating from our source, that the addition of
a plate of metal hardly augments the opacity. I have reason to believe that it does not augment it at all, and that
this light transparent gas is really as black, at the present
moment, to the calorific rays, as if the experimental tube
were filled with ink, pitch, or any other impervious substance.
In the case of oxygen, nitrogen, hydrogen, and air, the
action of a whole atmosphere is so small that it would be
quite useless to attempt to determine the action of a fractional part of an atmosphere. Could we, however, make
such a determination, the difference between them and the
other gases would come out still more forcibly than in the
last table. In the case of the energetic gases, we know
that the calorific rays are most copiously absorbed by the
portion of gas which first enters the experimental tube, the
quantities which enter last producing, in many cases, a
merely infinitesimal effect. If, therefore, instead of comparing the gases at a common tension of one atmosphere,
we were to compare them at a common tension of an inch,
we should doubtless find the difference between the least
absorbent and the most absorbent gases greatly augmented. We have already learned that for small quantities, the
heat absorbed is proportional to the amount of gas present.
Assuming this to be true for air and the other feeble gases
referred to; taking, that is, their absorption at 1 inch of
tension to be.'th of that at 30 inches, we have the following comparative effects. It will be understood that in every
case, except the first four, the absorption of 1 inch of the
gas was determined by direct experiment.
Absorption at
Name.                        1 inch tension.
Air...    1
Oxygen..   1




364                    LECTURE X.
Absorption of
Name.                         2 inch tension.
Nitrogen....   1
Hydrogen...   1
Chlorine...         60
Bromine.... 160
Carbonic oxide...750
Hydrobromic acid... 1005
Nitric oxide. 1590
Nitrous oxide... 1860
Sulphide of hydrogen.. 2100
Ammonia..'7260
Olefiant gas... 7950
Sulphurous acid... 8800
What extraordinary differences in the constitution and
character of the ultimate particles of various gases do the
above results reveal!  For every individual ray struck
down by the air, oxygen, hydrogen, or nitrogen-the ammonia strikes down a brigade of 7,260 rays; the olefiant
gas a brigade of 7,950; while the sulphurous acid destroys
8,800. With these results before us, we can hardly help
attempting to visualise the atoms themselves, trying to discern, with the eye of intellect, the actual physical qualities
on which these vast differences depend. These atoms are
particles of matter, plunged in an elastic medium, accepting
its motions and imparting their motions to it. Is the hope
unwarranted, that we may be able finally to make radiant
heat such a feeler of atomic constitution, that we shall be
able to infer from their action upon it, the mechanism of
the ultimate particles of matter themselves?
Have we even now no glimpse of the atomic qualities
necessary to form a good absorber? You remember our
experiments with gold, silver, and copper; you recollect
how feebly they radiate and how feebly they absorb. We
heated them by boiling water; that is to say, we imparted,
by the contact of the water, motion to their atoms; still
this motion was imparted with extreme slowness to the




INFLUENCE OF ATOMIC CONSTITUTION.       365
ether in which those atoms swung. That their particles
glide through the ether with scarcely any resistance may
also be inferred from the length of time which they require
to cool in vacuo. But we have seen that when the motion
which the atoms of the above bodies possess, and which
they are incompetent to transfer to the ether, is imparted,
by contact, to a coat of varnish, or to a coat of chalk or
lampblack, or even to flannel or velvet, these bodies soon
waste the motion on the ether. The same we found true
for glass and earthenware.
In what respect do those good radiators differ from the
metals referred to? In one profound particular-the metals are elements; the others are compounds. In the metals
the atoms swung singly; in the varnish, velvet, earthenware and glass, they swung in groups. And now, in bodies
as diverse from the metals as can possibly be conceived, we
find the same significant fact making its appearance. Oxygen, hydrogen, nitrogen, and air, are elements, or mixtures
of elements, and, both as regards radiation and absorption,
their feebleness is declared. They swing in the ether with
scarcely any loss of moving force. They bear the same
relation to the compound gases as a smooth cylinder turning in water does to a paddle-wheel. They create a small
comparative disturbance.
We may push these considerations still further. It is
impossible not to be struck by the position of chlorine and
bromine in the last table. Chlorine is an extremely dense
and coloured gas; bromine is a far more densely-coloured
vapour; still we find them, as regards perviousness to the
heat of our source, standing above every transparent compound gas in the table. The act of combination with hydrogen produces, in the case of each of these substances, a
transparent compound; but the chemical act, which augments the transparency to light, augments the opacity to




366                   LECTURE X.
heat; hydrochloric acid absorbs more than chlorine; and
hydrobromic acid absorbs more than bromine.
Further, I have here the element bromine in the liquid
condition; I enclose a portion of it in this glass cell; the
layer is of a thickness sufficient to extinguish utterly the
flame of a lamp or candle. But I place a candle in front
of the cell, and a thermo-electric pile behind it; the prompt
movement of the needle declares the passage of radiant
heat through the bromine. This consists entirely of the
obscure rays of the candle, for the light, as I have stated,
is utterly cut off. I remove the candle, and put in its place
our copper ball, heated not quite to redness. The needle
at once flies to its stops, showing the transparency of the
bromine to the heat emitted by the ball. I cannot use
iodine in a solid state, but, happily, it dissolves in bisulphide
of carbon. I have here the densely coloured liquid in this
glass cell. I throw the parallel electric beam upon the screen;
this solution of iodine completely cuts the light off; but
if I bring my pile into the path of the beam, the violence
of the needle's motion shows how copious is the transmission of the obscure rays. It is impossible, I think, to close
our eyes upon this convergent evidence that the free atoms
swing with ease in the ether, while when grouped to oscillating systems, they cause its waves to swell, imparting to
it, as compound molecules, an amount of motion which was
quite beyond their power to communicate, as long as they
remained uncombined.
But it will occur to you, no doubt, that lampblack,
which is an elementary substance, is one of the best absorbers and radiators in nature. Let us examine this substance a little: ordinary lampblack contains many impurities; it has various hydro-carbons condensed within it, and
these hydro-carbons are all powerful absorbers and radiators. Lampblack, therefore, as hitherto applied, can hardly
be considered an element at all. I have, however, had these




RADIATION THROUGH LAMPBLACK, ETC.        367
hydro-carbons in great part removed, by carrying through
red hot lampblack a current of chlorine gas; but the substance has continued to be both a powerful radiator and a
powerful absorber. Well, what is lampblack? Chemists
will tell you that it is an allotropic form of the diamond:
here, in fact, is a diamond reduced to charcoal by intense
heat. Now, the allotropic condition has long been defined
as due to a difference in the arrangement of a body's particles; hence, it is conceivable that this arrangement, which
causes such a marked physical difference between lampbllack
and diamond, may consist of an atomic grouping, which
causes the body to act on radiant heat as if it were a compound. I say such an arrangement of an element, though
exceptional, is quite conceivable; and I shall show you this
to be eminently the case as regards an allotropic form of
our highly ineffectual oxygen.
But, in reality, lampblack is not so impervious as you
might suppose it to be. Melloni has shown it to be transparent, in an unexpected degree, to radiant heat emanating
from a low source, and I have prepared an experiment
which will corroborate his. Here is a plate of rock-salt,
which, by holding it over a smoky lamp, has been so thickly coated with soot that it does not allow a trace of light
from the most brilliant gas jet to pass through it. I place
the plate upon its stand, and between it and this vessel of
boiling water, which is to serve as our source of heat, I
place a screen. The thermo-electric pile is at the other side
of the smoked plate. The needle is now at zero, and I
withdraw my screen; instantly the needle moves, and its
final and permanent deflection is 52~. I now cleanse the
salt perfectly, and determine the radiation through the unsmoked plate, —it is 71~. Now, the value of the deflection 52~, expressed with reference to our usual unit, is
90, and the value of 71~, or the total radiation, is about
300. Hence, the radiation through the soot is to the whole
radiation as




368                      LECTURE X.
90: 300
or as 30: 100
that is to say, 30 per cent. of the incident heat has been
transmitted by the layer of lampblack.
I have shown you the action of gases upon radiant heat,
with our glass experimental tube and. our new source of
heat; let us complete this lecture by reference to the action
of vapours.  Here, you see, I have several glass flasks, each
furnished with a brass cap, to which a stopcock can be
screwed.  Into each I pour a quantity of a volatile liquid,
reserving a flask for each liquid, so as to render admixture
of the vapours impossible.  From  each flask I remove the
air,-not only the air above the liquid, but the air dissolved
in it. This latter bubbles freely away when the flask is
exhausted; now I attach my flask to the exhausted experimental tube, and allow  the vapour to enter, without permitting any ebullition to occur. The mercury column of
the pump sinks, and when the required depression has been
obtained, I cut off the supply of vapour. In this way the
vapours of the substances mentioned in the next table have
been examined, at tensions of 0'1, 0'5, and I inch, respectively.
Absorption of Vapours
at Tensions
Name                      0O1      0'5   1'0
Bisulphide of carbon..    15       47     62
Iodide of methyl...    35      147    242
Benzol....    66      182    267
Chloroform...    85     182    236
Methylic alchohol..   109     390    590
Amylene...   182     535    823
Sulphuric Ether...   300     710    870
Alcohol...   325     622
Formic ether...   480      870   1075
Acetic ether..       590     980   1195
Propionate of ethyl..   596     970
Boracic ether,..   620




ACTION OF VAPOURS ON RADIANT HEAT.    369
These numbers refer to the absorption of a whole atmosphere of dry air as their unit; that is to say, -Lth of an
inch of bisulphide of carbon vapour does 15 times the execution of 30 inches of atmospheric air; while 3'-th of an
inch of boracic ether vapour does 620 times the execution
of a whole atmosphere of atmospheric air. Comparing air
at a tension of 0'01 with boracic ether at the same tension,
the absorption of the latter is probably 180,000 times that
of the former.
16'




APPENDIX TO LECTURE X.
I GIVE here the method of calibrating the galvanometer which
Melloni recommends, as leaving nothing to be desired as regards
facility, promptness, and precision. His own statement of the
method, translated from La Thermochrose, page 59, is as
follows:Two small vessels, v v, are half-        Fig. 98.
filled with mercury, and connected,     /
separately, by two short wires, with
the extremities G G of the galvanometer. The vessels and wires thus
disposed make no change in the action of the instrument; the thermo- ~
electric current being freely transmitted, as before, from the pile to  
the  galvanometer.    But if, by
means of a wire F, a communication
be established between the two vessels, part of the current will
pass through this wire and return to the pile. The quantity of
electricity circulating in the galvanometer will be thus diminished and with it the deflection of the needle.
Suppose, then, that by this artifice we have reduced the galvanometric deviation to its fourth or fifth part; in other words,
supposing that the needle being at 10 or 12 degrees, under the
action of a constant source of heat, placed at a fixed distance
from the pile, that it descends to 2 or 3 degrees when a portion
of the current is diverted by the external wire; I say that by
causing the source to act from various distances, and observing
in each case the total deflection, and the reduced deflection, we




CALIBRATION OF GALVANOMETER.                  371
have all the data necessary to determine the ratio of the deflections of the needle, to the forces which produce these deflections.
To render the exposition clearer, and to furnish, at the same
time, an example of the mode of operation, I will take the numbers relating to the application of the method to one of my
thermo-multipliers.
The external circuit being interrupted, and the source of heat
being sufficiently distant from the pile to give a deflection not
exceeding 5 degrees of the galvanometer, let the wire be placed
from v to v; the needle falls to 1~O5. The connection between
the two vessels being again interrupted, let the source be brought
near enough to obtain successively the deflections:
5~, 10~, 15~, 20~, 250, 30~, 350, 400, 450
Interposing after each the same wire between v and v we obtain the following numbers:l 5, 30, 405, 603, 804, 1102, 1503, 2204, 290.7.
Assuming the force necessary to cause the needle to describe
each of the first degrees of the galvanometer to be equal to unity,
we have the number 5 as the expression of the force corresponding
to the first observation. The other forces are easily obtained by
the proportions:1-5: 5=a:x=-1-.6 a-3333 a.*
where a represents the deflection when the exterior circuit is
closed. We thus obtain
5, 10, 15'2, 21, 28, 37'3.
for the forces, corresponding to the deflections,
50, 100, 150, 20~, 250, 30~.
In this instrument, therefore, the forces are sensibly proportional to the arcs, up to nearly 15 degrees. Beyond this, the proportionality ceases, and the divergence augments as the arcs increase in size.
The forces belonging to the intermediate degrees are obtained
with great ease either by calculation or by graphical construction, which latter is sufficiently accurate for these determinations.
* That is to say, one reduced current is to the total current to which
it corresponds, as any other reduced current is to its corresponding total
current.




372                APPENDIX  TO LECTURE X.
By these means we find,
Degrees.. 130, 140, 15~, 160, 17~, 18~, 19~, 20~, 21~.
Forces.. 13, 141, 15'2, 16'3, 17'4, 18'6, 19'8, 21, 22'3.
Differences. 1.1, 11, 1'1, 1'1, 1.2, 1'2, 12, 13.
Degrees.. 22~, 230~, 24~, 25~, 26~, 27~, 28~, 29~, 300.
Forces. 23'5, 24'9, 26'4, 28, 29'7, 31'5, 33'4, 353, 37-3.
Differences. 14, 15, 1'6, 1'7, 1'8, 1'9, 2.
In this table we do not take into account any of the degrees
preceding the 13th, because the force corresponding to each of
them possesses the same value as the deflection.
The forces corresponding to the first 30 degrees being known,
nothing is easier than to determine the values of the forces corresponding to 35, 40, 45 degrees, and upwards.
The reduced deflections of these three arcs are,
15~.3, 220~4, 290~7.
Let us consider them separately; commencing with the first.
In the first place, then, 15 degrees, according to our calculation,
are equal to 15'2; we obtain the value of the decimal 0'3 by multiplying this fraction by the difference 1'1 which exists between
the 15th anti 16th degrees; for we have evidently the proportion
1: 1.1=0-3: x=0'3.
The value of the reduced deflection corresponding to the 35th
degree will not, therefore, be 15~'3, but 15~'2+0~'3=150~5. By
similar considerations we find 23~'5+0~'6=24~'1, instead of
220~4, and 360~7 instead of 29~'7 for the reduced deflections of 40
and 45 degrees.
It now only remains to calculate the forces belonging to these
three deflections, 15~'5, 240.1, and 36~'7, by means of the expression 3'333 a; this gives us,
the forces, 51'7, 80'3, 122'3.
for the degrees, 35, 40, 45.
Comparing these numbers with those of the preceding table,
we see that the sensitiveness of our galvanometer diminishes considerably when we use deflections greater than 30 degrees.




LECTURE XI.
[April 3, 1862.]
ACTION OF ODOROUS SUBSTANCES UPON RADIANT HEAT-ACTION OF OZONE
UPON RADIANT HEAT-DETERMINATION OF THE RADIATION AND ABSORPTION OF GASES AND VAPOURS WITHOUT ANY SOURCE OF HEAT EXTERNAL
TO THE GASEOUS BODY-DYNAMIC RADIATION AND ABSORPTION-RADIATION THROUGH THE EARTH S ATMOSPHERE-INFLUENCE OF THE AQUEOUS
VAPOUR OF THE ATMOSPHERE ON RADIANT HEAT-CONNECTION OF THE
RADIANT AND ABSORBENT POWER OF AQUEOUS VAPOUR WITH METEOROLOGICAL PHENOMENA.
APPENDIX: FURTHER DETAILS OF THE ACTION OF HUMID AIR.
SCENTS and effluvia generally have long occupied the
attention of observant men, and they have formed favourite illustrations of the'divisibility of matter.' No
chemist ever weighed the perfume of a rose; but in radiant heat we have a test more refined than the chemist's
balance. The results brought before you in our last lecture
would enable you to hear me without surprise, were I to
assert that the quantity of volatile matter removed from a
hartshorn bottle by any person in this room, by a single act
of inhalation, would exercise a more potent action on radiant heat, than the whole body of oxygen and nitrogen
which the room contains. Let us apply this test to other
odours, and see whether they also, notwithstanding their
almost infinite attenuation, do not exercise a measurable influence on radiant heat.
I will operate in this simple way: here is a number of
small and equal squares of bibulous paper, which I roll up
thus, to form little cylinders, each about two inches in
length. I moisten the paper cylinder by dipping one end




3:74                LECTURE XI.
of it into an aromatic oil; the oil creeps by capillary attraction through the paper, and the whole of the cylinder is
now moist. I introduce the rolled paper thus into a glass
tube of such a diameter that the cylinder fills it without
being squeezed, and between my drying apparatus and the
experimental cylinder I place the tube containing the
scented paper. The experimental cylinder is now exhausted, and the needle at zero; turning this cock on, I allow
dry air to pass gently through the folds of the saturated
paper. Here the air takes up the perfume of the aromatic
oil, and carries it into the experimental tube. The absorption of an atmosphere of dry air we know to be unity; it
produces a deflection of one degree; hence, any additional
absorption which these experiments reveal, must be due to
the scent which accompanies the air.
The following table will give a condensed view of the
absorption of the substances mentioned in it; air at the
tension of one atmosphere being regarded as unity:Perfumes.
Name of Perfame             Absorption
Pachouli..... 30
Sandal Wood                  32
Geranium..... 33
Oil of Cloves                33'5
Otto of Roses               36 5
Bergamot..... 44
Neroli                       47
Lavender                     60
Lemon                        65
Portugal.....   67
Thyme.....68
Rosemary.'74
Oil of Laurel.              80
Camomile Flowers.           87
Cassia.....            109
Spikenard.....355
Aniseed..                 372




ACTION OF SCENTS ON RADIANT HEAT.       375
The number of atoms of air here in the tube must be
regarded as almost infinite in comparison with those of the
odours; still the latter, thinly scattered as they are, do, in
the case of pachouli, 30 times the execution of the air; otto
of roses does upwards of 36 times the execution of the
air; thyme, 74 times; spikenard, 355 times; and aniseed
372 times the execution of the air. It would be idle to
speculate on the quantities of matter implicated in these
results. Probably they would have to be multiplied by
millions to bring them up to the tension of ordinary air.
Thus,The sweet south
That breathes upon a bank of violets,
Stealing and giving odour,
owes its sweetness to an agent, which, though almost infinitely attenuated, may be more potent, as an intercepter of
terrestrial radiation, than the entire atmosphere from'bank' to sky.
In addition to these experiments on the essential oils,
others were made on aromatic herbs. A number of such
were obtained from  Covent Garden Market; they were
dry, in the common acceptation of the term; that is to
say, they were not green, but withered. Still I fear the
results obtained with them cannot be regarded as pure, on
account of the probable admixture of aqueous vapour. The
aromatic parts of the plants were stuffed into a glass tube
eighteen inches long and a quarter of an inch in diameter.
Previous to connecting them with the experimental tube,
they were attached to a second air-pump, and dry air was
carried over them for some minutes. They were then connected with the experimental cylinder, and treated as the
essential oils; the only difference being that a length of
eighteen inches, instead of two, was occupied by the herbs.
Thyme, thus examined, gave an action thirty-three times
that of the air which passed over it.




376                 LECTURE XI.
Peppermint exercised thirty-four tines the action of the
a'.
Spearmint exercised thirty-eight times the action of the
air.
Lavender exercised thirty-two times the action of the
air.
Wormwood exercised forty-one times the action of the
air.
Cinnamon exercised fifty-three times the action of the
air.
As already hinted, I fear that these results may be
complicated with the action of aqueous vapour: its quantity, however, must have been infinitesimal.
There is another substance of great interest to the
chemist, but the attainable quantities of which are so minute as almost to elude measurement, to which we may apply the test of radiant heat. I mean that extraordinary
substance, ozone. This body is known to be liberated at
the oxygen electrode, when water is decomposed by an
electric current. To investigate its action I had constructed three different decomposing cells. In the first, which I
shall call No. 1, the platinum plates used as electrodes had
about four square inches of surface; the plates of the second (No. 2) had two square inches of surface; while the
plates of the third (No. 3) had only one square inch of surface, each.
My reason for using electrodes of different sizes was
this: —On first applying radiant heat to the examination of
ozone, I constructed a decomposing cell, in which, to diminish the resistance of the current, very large platinum
plates were used. The oxygen thus obtained, and which
ought to have embraced the ozone, showed scarcely any of
the reactions of this substance.  It hardly discoloured
iodide of potassium, and was almost without action on radiant heat. A second decomposing apparatus, with smaller




ACTION OF OZONE ON RADIANT HEAT.         377
plates, was tried, and here I found both the action on iodide
of potassium, and on radiant heat, very decided. Being
unable to refer these differences to any other cause than the
different magnitudes of the plates, I formally attacked the
subject by operating with the three cells above described.
Calling the action of the main body of the electrolytic oxygen unity; that of the ozone which accommpanied it, in
the respective cases, is given in the following table:Number of Cell                Absorption.
No. 1... 20
No. 2.                        34
No. 3.....   47
Thus the modicum of ozone which accompanied the
oxygen, and in comparison to which it is a vanishing quantity, exerted, in the case of the first pair of plates, an action
twenty times that of the oxygen itself, while with the third
pair of plates the ozone was forty-seven times more energetic than the oxygen. The influence of the size of the
plates, or, in other words, of the density of the current
where it enters the liquid, on the production of the ozone,
is rendered strikingly manifest by these experiments.
I then cut away portions of the plates of cell No. 2, so
as to make them smaller than those of No. 3. The reduction of the plates was accompanied by an augmentation of
the action upon radiant heat; the absorption rose at once
from 34 to
65.
The reduced plates of No. 2 here transcend those of
No. 3, which, in the first experiments, gave the largest
action.
The plates of No. 3 were next reduced, so as to make
them smallest of all. The ozone now generated by No. 3,
effected an absorption of
85.




378                       LECTURE XI.
Thus we see that the action upon radiant heat advances
as the size of the electrodes is diminished.
Heat is known to be very destructive of ozone, and
suspecting the developement of heat at the small electrodes
of the cell last made use of, I surrounded the cell with a
mixture of pounded ice and salt.  Kept thus cool, the absorption of the ozone generated rose to
136.
These experiments on the action of ozone upon radiant
heat were made, bef6re I was acquainted with the researches of AIM. De la Rive, Soret, and MIeidinger, on this
substance. There is a perfect correspondence in our resuits, though there is no resemblance between our modes
of experiment. Such a correspondence is calculated to
augment our confidence in radiant heat, as an investigator
of molecular condition.*
* M. Meidinger commences his paper by showing the absence of agreement between thtory and experiment in the decomposition of water, the
difference showing itself very decidedly in a deficiency of oxygen when the
current was strong. On heating his electrolyte, he found that this difference disappeared, the proper quantity of oxygen being then liberated.
He at once surmised that the defect of oxygen might be due to the formation of ozone; but how did the substance act to produce the diminution of
the oxygen? If the defect were due to the great density of the ozone, the
destruction of this substance, by heat, would restore the oxygen to its true
volume. Strong heating, however, which destroyed the ozone, produced
no alteration of volume, hence M. Meidinger concluded that the effect
which he observed was not due to the ozone which remained mixed with
the oxygen itself. He finally concluded, and justified his conclusion by
satisfactory experiments, that the loss of oxygen was due to the formation
in the water, of peroxide of hydrogen by the ozone; the oxygen being thus
withdrawn from the tube to which it belonged. He also, as M. De la Rive
had previously done, experimented with electrodes of different sizes, and
found the loss of oxygen much more considerable when a small electrode
was used than with a large one; whence he inferred that the formation of
ozone was facilitated by augmenting the density of the current at the place
where electrode and electrolyte meet. The same conclusion is deduced from




CONSTITUTION OF OZONE.                 379
The quantities of ozone with which the foregoing experiments were made, must be perfectly unmeasurable by
ordinary means. Still its action upon radiant heat is so
energetic, as to place it beside olefiant gas, or boracic ether,
as an absorbent-bulk for bulk it might transcend either.
No elementary gas that I have examined behaves at all like
ozone. In its swing through the ether it must powerfully
disturb the medium.  If it be oxygen, it must, I think, be
oxygen atoms packed into groups. I sought to decide the
question whether it is oxygen, or a compound of hydrogen,
in the following way. /Heat destroys ozone. If it were
oxygen only, heat would convert it into the common gas;
if it were the hydrogen compound, which some chemists
consider it to be, heat would convert it into oxygen, plus
aqueous vapour. The gas alone, admitted into my tube,
would give the neutral action of oxygen, but the gas, plus
the aqueous vapour, I hoped might give a sensibly greater
action.  The. dried electrolytic gas was caused to pass
through a glass tube heated to redness, and thence direct
into the experimental tube. It was next, after heating,
made to pass through a drying tube into the experimental
tube. Hitherto I have not been able to establish, with certainty, a difference between the dried and undried gas. If,
therefore, the act of heating develope aqueous vapour, the
experimental means which I have employed have not yet
enabled me to detect it.  For the present, therefore, I hold
the above experiments on radiant heat. No two things could be more diverse than the two modes of proceeding. M. Meidinger sought for the
oxygen which had disappeared, and found it in the liquid; I examined the
oxygen actually liberated, and found that the ozone mixed with it augments in quantity as the electrodes diminish in size. It may be added that
since the perusal of M. Meidinger's paper I have repeated his experiments
with my own decomposition cells, and found that those which gave me the
greatest absorption, also showed the greatest deficiency in the amount of
oxygen liberated.




380                 LECTURE XI.
the belief, that ozone is produced by the packing of the
atoms of elementary oxygen into oscillating groups; and
that heating dissolves the bond of union, and allows the
atoms to swing singly, thus disqualifying them for either
intercepting or generating the motion, which, as systems,
they are competent to intercept and generate.
I have now to direct your attention to a series of facts
which surprised and perplexed me when I first observed
them. While experimenting last November (1861), on one
occasion I permitted a quantity of alcohol vapour, sufficient
to depress the mercury gauge 0'5 of an inch, to enter the
experimental tube; it produced a deflection of 72~. While
the needle pointed to this high figure, and previously to
pumping out the vapour, I allowed dry air to stream into
the tube, and happened, as it entered, to keep my eye upon
the galvanometer.
The needle, to my astonishment, sank speedily to zero,
and went to 25~ on the opposite side. The entry of the
almost neutral air, not only neutralised the absorption previously observed, but left a considerable balance in favour
of the face of the pile turned towards the source. A repetition of the experiment brought the needle down from 70~
to zero, and sent it to 38~ on the opposite side. In like
manner, a very small quantity of the vapour of sulphuric
ether produced a deflection of 30~; on allowing dry air to
fill the tube, the needle descended speedily to zero, and
swung to 60~ at the opposite side.
My first thought, on observing these extraordinary
effects, was, that the vapours had deposited themselves in
opaque films on the plates of rock-salt, and that the dry air
on entering had cleared these films away, and allowed the
heat from the source free transmission.
But a moment's reflection dissipated this supposition.
The clearing away of such a film could, at best, but restore
the state of things existing prior to the entrance of the




DYNAMIC RADIATION.              381
vapour. It might be conceived to bring the needle again to
0~, but it could not possibly produce the negative deflection. Nevertheless, I dismounted the tube, and subjected
the plates of salt to a searching examination. No such deposit as that above surmised was observed. The salt remained perfectly transparent while in contact with the
vapour. How, then, are the effects to be accounted for?
We have already made ourselves acquainted with the
thermal effects produced when air is permitted to stream
into a vacuum  (page 44).  We know  that the air is'warmed by its collision against the sides of the receiver.
Can it be the heat thus generated, imparted by the air to
the alcohol and ether vapours, and radiated by them against
the pile, that was more than sufficient to make amends for
the absorption?  The experimentzum crucis at once suggests itself here. If the effects observed be due to the
heating of the air on entering the partial vacuum in which
the vapour was diffused, we ought to obtain the same
effects when the sources of heat made use of hitherto are
entirely abolished. We are thus led to the consideration of
the novel and at first sight utterly paradoxical problemnamely, to determine the radiation and absorption of a gas
or vapour without any source of heat external to the gaseous
bodcy itself.
Let us, then, erect our apparatus, and omit our two
sources of heat. Here is our glass tube, stopped at one end
by a plate of glass, for we do not now need the passage of
the heat through this end; and at the other end by a plate
of rock-salt. In front of the salt is placed the pile, connected with its galvanometer. Though there is now no
special source of heat acting upon the pile, you see the needle does not come quite to zero; indeed, the walls of this
room, and the people who sit before me, are so many
sources of heat, to neutralise which, and thus to bring the
needle accurately to zero, I must slightly warm the defect



382                  LECTURE Xi.
ive face of the pile. This is done without any difficulty by
a cube of lukewarm water, placed at a distance; the needle
is now at zero.
The experimental tube being exhausted, I allow air to
enter, till the tube is filled; the horizontal column of air at
present in the tube is warmed; every atom of the air is oscillating; and if the atoms possessed any sensible power of
communicating their motion to the luminiferous ether, we
should have from each atom a train of waves impinging on
the face of the pile. But you observe scarcely any motion
of the galvanometer, and hence may infer that the quantity
of heat radiated by the air is exceedingly small. The deflection produced is 7 degrees.
But these 7~ are not really due to the radiation of the
air. To what, then? I open one of the ends of the experimental tube, and place a bit of black paper as a lining
within it; the paper merely constitutes a ring which covers
the interior surface of the tube for a length of 12 inches.
I close the tube and repeat the last experiment. The tulce
has been exhausted and the air is now entering, but mark
the needle-it has already flown through an arc of 70~.
You see here exemplified the influence of this bit of paper
lining; it is warmed by the air, and it radiates towards the
pile in this copious way. Thae interior surface of the tuibe
itself mtest cldo te same, though in a less degree, and to
the radiation from this surface, and not from the air itself,
the deflection of 7~ which we have just obtained is, I believe, to be ascribed.
Removing the bit of lining from the tube, instead of
air I allow nitrous oxide to stream into it; the needle
swings to 280, thus showing the superior radiative power
of this gas. I now work the pump, the gas within the experimental tube becomes chilled, and into it the pile pours
its heat; a swing of 20~ in the opposite direction is the consequence.




DYNAMIC RADIATION OF GASES.             383
Instead of nitrous oxide, I now allow olefiant gas to
stream into the exhausted tube. We have already learned
that this gas is highly gifted with the power of radiation.
Its atoms are here warmed, and everyone of them asserts
its power; the needle swings through an arc of 67~. Let
it waste its heat, and let the needle come to zero. I now
pump out, and the consequent chilling of the gas, within
the tube, produces a deflection of 40~ on the side of cold.
We have certainly here a key to the solution of the enigmatical effects observed with the alcohol and ether vapour.
For the sake of convenience we may call the heating of
the gas on entering the vacuum dynamic heating; its radiation I have called dynamic radiation, and its absorption,
when chilled by pumping out, dynamic absorption.  These
terms being understood, the following table explains itself.
In each case the extreme limit to which the needle swung,
on the entry of the gas into the experimental tube, is recorded.
Dynamic cRadiation of' ases.
Name                     Limit of 1st impulsion
o
Air....   7
Oxygen....   7
Hydrogen..      7
Nitrogen....
Carbonic Oxide...  19
Carbonic acid...  21
Nitrous oxide...  31
Olefiant gas...  63
We observe that the order of the radiative powers, determined in this novel way, is the same as that already obtained from a totally different mode of experiment. It
must be borne in mind that the discovery of dynamic radiation is quite recent, and that the conditions of perfect accuracy have not yet been developed; it is, how6ver, cer



384                    LECTURE XI.
tain, that the mode of experiment is susceptible of the last
degree of precision.
Let us now turn to our vapours, and while dealing with
them I shall endeavour to unite two effects which, at first
sight, might appear utterly incongruous. We have already
learned that a polished metal surface emits an extremely
feeble radiation; but that when the same surface is coated
with varnish the radiation is copious. In the communication of motion to the ether the atoms of the metal need a
mediator, and this they find in the varnish. They communicate their motion to the molecules of the varnish, and the
latter are so related to the luminiferous ether* that they
Fig. 94.
can communicate their motion to it.  You may varnish a
metallic surface by a film of a powerful gas.  I have here
* If we could change either the name given to the interstellar medium,
or that given to certain volatile liquids by chemists, it would be an advantage. It is difficult to avoid confusion in the use of the same name for
objects so utterly diverse.




DYNAMOIC RADIATION OF VAPOURS.        385
an arrangement which enables me to cause a thin stratum
of olefiant gas from the gasholder G (fig. 94) to pass through
the slit tube a b, and over the heated surface of the cube c.
The radiation from c is now neutralised by that from c';
but I allow the gas to flow over the cube c; and though
the surface is actually cooled by the passage of the gas, for
the gas has to be warmed by the metal, you see the effect
is to augment considerably the radiation: as soon as the
gas begins to flow the needle begins to move, and reaches
an amplitude of 45~.
We have here varnished a metal by a gas, but a more
interesting and subtle effect is the varnishing of one gaseous
body by another. I have here a flask containing some acetic ether; a volatile, and, as you know, a highly absorbent
substance. I attach the flask to the experimental tube, and
permit the vapour to enter the tube, until the mercury column has been depressed half an inch. There is now vapour
possessing half an inch of tension in the tube. I intend to
use that vapour as my varnish; and I intend to use the element oxygen instead of the element gold, silver, or copper,
as the substance to which my vapour varnish is to be applied. At the present moment the needle is at zero, and I
now permit dry oxygen to enter the tube: the gas is dynamically heated, and we have seen its incompetence to radiate its heat; but now it comes into contact with the acetic
ether vapour, and, communicating its motion to the vapour
by direct collision, the latter is able to send on the motion
to the pile. Observe the needle-it is caused to swing
through an arc of 70~ by the radiation from the vapour
particles. I need not insist upon the fact that in this experiment the vapour bears precisely the same relation to
the oxygen, that the varnish does to the metal in our former experiments.
Let us wait a little, and allow the vapour to pour away
the heat: it is the discharger of the calorific force gene17




386                     LECTURE XI.
rated by the oxygen-the needle is again at zero. I work
the pump, the vapour within the tube becomes chilled, and
now you observe the needle swing nearly 45~ on the other
side of zero.  In this way the dynamic radiation and absorption of the vapours mentioned in the following table
have been determined; air, however, instead of oxygen,
being the substance employed to heat the vapour. The
limit of the first swing of the needle is noted as before.
Dynamic Radiation and Absorption of Vapours.
Deflections
Radiation        Absorption
1. Bisulphide of carbon.   14..6
2. Iodide of methyl..   195...  8
3. Benzol...   30... 14
4. Iodide of ethyl..   34... 155
5. Methylic alcohol..   36
6. Chloride of amyl,.   41... 23'7. Amylene...   48
8. Alcohol...   50... 27'5
9. Sulphuric ether.   64... 34
10. Formic ether..   68'5... 38
11. Acetic ether...'70.., 43
We have here used eleven different kinds of vapour as
varnish for our air, and we find that the dynamic radiation
and absorption augment exactly in the order established by
experiments with external sources of heat. We also see
how beautifully dynamic radiation and absorption go hand
in hand, the one augmenting and diminishing with the
other.
The smallness of the quantity of matter concerned in
some of these actions on radiant heat has been often referred to; and I wish now to- describe an experiment which
shall furnish a more striking example of this kind than any
hitherto brought before you. The absorption of boracic ether




VARNISHING AIR BY VAPOUR.              387
vapour, as given at page 368, exceeds that of any other
substance there referred to; and its dynamic radiation may
be presumed to be commensurate. I exhaust the experimental tube as perfectly as possible, and introduce into it
a quantity of boracic ether vapour sufficient to depress the
mercury column -11th of an inch. The barometer stands today at 30 inches; hence the tension of the ether vapour
now in our tube is -  th of an atmosphere.
I send dry air into the tube; the vapour is warmed,
and the dynamic radiationsproduces the deflection 56~.
I work the pump until I reduce the residue of air within
it to a tension of 0'2 of an inch, or 5,th of an atmosphere.
A residue of the boracic ether vapour remains of course in
the tube, the tension of this residue being the  50th part
of that of the vapour when it first entered the tube.  I let
in dry air, and find the dynamic radiation of the residual
vapour expressed by the deflection 42~.
I again work the pump till the tension of the air within
it is 0'2 of an inch; the quantity of ether vapour now in
the tube being -, Ith of that present in the last experiment. The dynamic radiation of this residue gives a deflection of 20~.
Two additional experiments, conducted in the same
way, gave deflections of 14~ and 10~ respectively. The
question now is, what was the tension of the boracie ether
vapour when this last deflection was obtained? The following table contains the answer to this question:Dynamic ZRadiation of Boracic Ether.
Tension in parts of an atmosphere     Deflection
3o0.6
l50  300 - 46o00.      42
-ax.. I- ___              20
X _O x  X 0   675
I60  If~x x  ln -  ---— I —--        1
I       6  0- X T-, X 30=1012500000  14
x   x  16-0  MG x  gU   1876000-d0      1




388                  LECTURE XI.
The air itself, warming the interior of the tube, produces, as we have seen, a deflection of 7~; hence the entire
deflection of o10~ was not due to the radiation of the vapour. Deducting 7~, it would leave a residue of 3~. But
supposing we entirely omit the last experiment, we can
then have no doubt that at least half the deflection 14~ is
due to the residue of boracic ether vapour; this residue
we find, by strict measurement, would have to be multiplied by one thousand millions to bring it up to the tension of ordinary atmospheric air.
Another reflection here presents itself, which is worthy
of our consideration. We have measured the dynamic
radiation of olefiant gas, by allowing the gas to enter our
tube, until the latter was quite filled. What was the state
of the warm radiating column of olefiant gas in this experiment? It is manifest that the portions of the column most
distant from the pile must radiate through the gas in front
of them, and, in this forward portion of the column of gas,
a large quantity of the rays emitted by its hinder portion
will be absorbed. In fact, it is quite certain that if we
made our column sufficiently long, the frontal portions
would act as a perfectly impenetrable screen, to the radiation of the hinder ones. Thus, by cutting off the part of
the gaseous column most distant fiom the pile, we might
diminish only in a very small degree the amount of radiation which reaches the pile.
Let us now compare the dynamic radiation of a vapour
with that of olefiant gas. In' the case of vapour we use
only 0'5 of an inch of tension, hence the radiating molecules of the ether are much wider apart than those of the
olefiant gas, which have 60 times the tension; and consequently the radiation of the hinder portions of the column
of vapour will have a comparatively open door through
which to reach the pile. These considerations render it
manifest that in the case of the vapour a greater length of




EFFECT OF DISTANCE BETWEEN RADIANT CENTRES. 389
tube is available for radiation than in the case of olefiant
gas. This leads to the conclusion, that if we shorten the
tube, we shall diminish the radiation in the case of the
vapour more considerably than in the case of the gas. Let
us now bring our reasoning to the test of experiment.
We found the dynamic radiation of the following four
substances, when the radiating column was 2 feet 9 inches
long, to be represented by the annexed deflections:Olefiant gas....   63
Sulphuric ether...   64
Formic ether....   68'5
Acetic ether....   70
olefiant gas giving here the least dynamic radiation.
Experiments made in precisely the same manner with a
tube 3 inches long, or i1th of the former length, gave the
following deflections:Olefiant gas....   39
Sulphuric ether...   11
Formic ether....   12
Acetic ether....   15
The verification of our reasoning is therefore complete. It
is proved, that in the long tube the dynamic radiation of
the vapour exceeds that of the gas, while in a short one
the dynamic radiation of the gas exceeds that of the vapour.
The result proves, if proof were needed, that though diffused in air, the vapour molecules are really the centres of
the radiation.
Up to the present point, I have purposely omitted all
reference to the most important vapour of all, as far as our
world is concerned-I mean, of course, the vapour of water.  This vapour, as you know, is always diffused through
the atmosphere. The clearest day is not exempt from it:




390                 LECTURE XI.
indeed, in the Alps, the purest skies are often the most
treacherous, the blue deepening with the amount of aqueous vapour in the air. It is needless, therefore, to remind
you, that when I speak of aqueous vapour, I mean nothing
visible; it is not fog; it is not cloud; it is not mist of any
kind. These are formed of vapour which has been condensed to water; but the blue vapour with which we have
to deal is an impalpable transparent gas. It is diffused
everywhere throughout the atmosphere, though in very
different proportions.
To prove the existence of aqueous vapour in the air of
this room, I have placed in front of the table a copper vessel, which was filled an hour ago with a mixture of pounded ice and salt. The surface of the vessel was then black,
but it is now white-furred all over with hoar-frost-produced by the condensation, and subsequent congelation upon
its surface of the aqueous vapour. I can scrape off this
white substance, and collect it in my hand. As I remove
the frozen vapour, the black surface of the vessel reappears; and now I have collected a sufficient quantity to
form a respectable snow-ball. Let us go one step further.
I place this snow in a mould, and squeeze it before you
into a cup of ice-there is the cup; and thus, without quitting this room, we have experimentally illustrated the manufacture of glaciers from beginning to end. On the plate
of glass which I have used to cover the vessel the vapour
is not congealed, but it is condensed so copiously, that
when I hold the plate edgeways the water runs off it in a
stream.
The quantity of this vapour is small. Oxygen and nitrogen constitute about 991 per cent. of our atmosphere;
of the remaining 0'5, about 0'45 is aqueous vapour; the residue is carbonic acid. Had we not been already acquainted with the action of almost infinitesimal quantities of matter on radiant heat, we might well despair of being able to




ABSORPTION OF RADIANT HEAT BY HUMID AIR.  391
establish a measurable action on the part of the aqueous
vapour of our atmosphere. Indeed, I quite neglected the
action of this substance for a time, and could hardly credit
my first result, which made the action of the aqueous
vapoui' of our laboratory fifteen times that of the air in
which it was diffused. This, however, by no means expresses the true relation between aqueous vapour and dry
air.
I will make an experiment before you which shall illustrate this. Here, you see, I have resumed our first arrangement, as shown in Plate I., with a brass tube, and
with two sources of heat acting on the opposite faces of the
pile. I exhaust the experimental tube, and repeat to-day
the experiment with dry air, which I made at the commencement of the last lecture. The needle does not move
sensibly.  If close to it you would, as I have already
stated, observe a motion through about one degree. Probably, could we get our air quite pure, its action would be
even less than this. I now pump out, and allow the air of
this room to enter the experimental cylinder direct, without permitting it to pass through the drying apparatus.
The needle, you observe, moves as the air enters, and the
final deflection is 480. The needle will steadily point to
this figure as long as the sources of heat remain constant,
and as long as the air continues in the tube. These 48~
correspond to an absorption of 72; that is to say, the aqueous vapour contained in the atmosphere of this room to-day
exerts an action on the radiant heat, 72 times more powerful than that of the air itself.
This result is obtained with perfect ease, still not without due care. In comparing dry with humid air it is perfectly essential that the substances be pure. You may work
for months with an imperfect drying apparatus and fail to
obtain air, which shows this almost total absence of action
on radiant heat. An amount of organic impurity, too small




392                  LECTURE XI.
to be seen by the eye, is sufficient to augment fiftyfold the
action of the air. Knowing the effect which an almost infinitesimal amount of matter, in certain cases, can produce,
you are better prepared for such facts than I was when they
first forced themselves upon my attention. But let us be
careful in our enquiries. The experimental result which
we have just obtained will, if true, have so important an
influence on the science of meteorology, that, before it is
admitted, it ought to be subjected to the closest scrutiny.
First of all, look at this piece of rocksalt brought in from
the next room, where it has stood for some time near a
tank, but not in contact with visible moisture. The salt is
wet; it is a hygroscopic substance, and freely condenses
moisture upon its surface. Here, also, is a polished plate
of the substance, which is now quite dry; I breathe upon
it, and instantly its affinity for moisture causes the vapour
of my breath to overspread the surface in a film which exhibits beautifully the colours of thin plates. Now we
know from the table, at page 313, how opaque a solution
of rocksalt is to the calorific rays, and hence arises the
question whether, in the above experiment with undried
air, we may not in reality be measuring the action of a thin
stratum of such a solution, deposited on our plates of salt,
instead of the pure action of the aqueous vapour of the air.
If you operate incautiously, and, more particularly, if it
be your actual intention to wet your plates of salt, you may
readily obtain the deposition of moisture. This is a point
on which any competent experimenter will soon instruct
himself; but the essence of good experimenting consists in
the exclusion of circumstances which would render the pure
and simple questions which we intend to put to Nature,
impure and composite ones. The first way of replying to
the doubt here raised is to examine our plates of salt; if
the experiments have been properly conducted, no trace of
moisture is found upon the surface. To render the success




WETTING R0OCKSALT PLATES.             393
of this experiment more certain, I will slightly alter the arrangement of our apparatus. Hitherto we have had the
thermo-electric pile and its two reflectors entirely outtside
the experimental cylinder. I now take this reflector from
the pile, and removing this terminal plate of rocksalt, I
push the reflector into the cylinder. The hollow reflecting
cone is' sprung' at its base a b (fig. 95), (our former arrangement, with the single exception that one of the reflectors of the pile P is now within the tube) so that it is
Fig. 95.
held tightly by its own pressure against the inner surface
of the cylinder. The space between the outer surface of
the reflector and the inner surface of the tube I fill with
fragments of fused chloride of calcium, which are prevented from falling out by a little screen of wire gauze. I now
reattach my plate of salt, against the inner surface of which
abuts the narrow end of the reflector; bring the face of
the pile close up to the plate, though not into actual contact with it, and now our arrangement is complete.
In the first place it is to be remarked, that the plate of
salt nearest to the source of heat c is never moistened, unless the experiments are of the grossest character. Its
proximity to the source makes it the track of a flux of heat,
powerful enough to chase away every trace of humidity
17*




394                  LECTURE XIL
from its surface. The distant plate is the one in danger,
and now we have the circumferential portions of this plate
kept perfectly dry by the chloride of calcium; no moist air
can at all reach the rim of the plate; while upon its central
portion, measuring about a square inch in area, we have
converged our entire radiation. On d priori grounds we
should conclude that it is quite impossible that a film of
moisture could collect there; and this conclusion is justified
by fact. I test, as before, the dried air and the undried
air of this room, and find, as in the former instance, that
the latter produces seventy times the effect of the former.
The needle is now deflected by the absorption of the undried air; allowing this air to remain in the tube, I unscrew
my plate of salt, and examine its surface. I even use a
lens for this purpose, taking care, however, that my breath
does not strike the plate. It was carefully polished when
attached to the tube; it is perfectly polished now. Glass,
or rockcrystal, could not show a surface more exempt from
any appearance of moisture. I place a dry handkerchief
over my finger, and draw it along the surface: it leaves no
trace behind. There is not the slightest deposition of moisture; still we see that absorption has taken place. This
experiment is conclusive against the hypothesis that the
effects observed are due to a film of brine instead of to
aqueous vapour.
The doubt may, however, linger, that although we are
unable to detect the film of moisture, it may still be there.
This doubt is answered in the following way: —I detach
the experimental tube from the front chamber, and remove
the two plates of rocksalt; the tube is now open at both
ends, and my aim will be to introduce dry and moist air
into this open tube, and to compare their effects upon the
radiation from our source. And here, as in all other cases,
the practical tact of the experimenter must come into play.
The source, on the one hand, and the pile on the other, are




ABSORPTION WITHOUT ROCKSALT PLATES.    395
now freely exposed to the air; and a very slight agitation
acting upon either would disturb, and might, indeed, altogether mask the effect we seek. The air, then, must be introduced into the open tube, without producing any commotion either near the source or near the pile. The length
of the experimental tube is now 4 feet 3 inches; at c (fig.
96) is a cock connected with an India-rubber bag containing
common air, and subjected by a weight to gentle pressure;
Fig. 9G6.
Ii
at D is a second cock connected by a flexible tube, t, with
an air-pump; between the cock c and the India-rubber bag
our drying tubes are introduced; when a cock near the bag
is opened, the air is forced gently through the drying tubes
into the experimental cylinder. The air-pump is slowly
worked at the same time, and the dry air thereby drawn
towards D. The distance of c from the source s is 18
inches, and the distance of D from the pile P is 12 inches,
the compensating cube c, and the screen H, serve the same
purpose as before. By thus isolating the central portion
of the tube, we can displace dry air by moist, or moist air
by dry, without permitting any agitation to reach either
the source or the pile.
At present the tube is filled with the common air of the
laboratory, and the needle of the galvanometer points
steadily to zero. I now allow air to pass through the drying apparatus and to enter the open tube at c, the pump
being worked at the same time. Mark the effect. When
the dry air enters the needle commences to move, and the




396                  LECTURE XI.
direction of its motion shows that more heat is now passing than before. The substitution of dry air for the air of
the laboratory has rendered the tube more transparent to
the rays of heat. The final deflection thus obtained is 45
degrees. Here the needle steadily remains, and beyond
this point it cannot be moved by any further pumping in
of the dry air.
I now shut off the supply of dry air and cease working
the pump; the needle sinks, but with great slowness, indicating a correspondingly slow diffusion of the aqueous
vapour of the adjacent air into the dry air of the tube. If
I work the pump I hasten the removal of the dry air, and
the needle sinks more speedily,-it now points to zero.
The experiment may be made a hundred times in succession without any deviation from this result; on the entrance of the dry air the needle invariably goes up to 45~,
showing the augmented transparency; on the entrance of
the undried air the needle sinks to 0~, showing augmented
absorption.
But the atmosphere to-day is not saturated with moisture; hence, if I saturate the air, I may expect to get a
greater action. I remove the drying apparatus and put in
its place a U tube, which is filled with fragments of glass
moistened by distilled water. Through this tube I force
the air from the India-rubber bag, and work the pump as
before., We are now displacing the humid air of the laboratory by still more humid air, and see the consequence.
The needle moves in a direction which indicates augmented
opacity, the final deflection being 15~.
Here then we have substantially the same result as that
obtained when we stopped our tube with plates of rocksalt; hence the action cannot be referred to a hypothetical
film of moisture deposited upon the surface of the plates.
And be it remarked that there is not the slightest caprice
or uncertainty in these experiments when properly con



PROPORTION ABSORBED BY HUMID AM.         397
ducted. They have been executed at different times and
seasons; the tube has been dismounted and remounted;
the suggestions of eminent men who have seen the experiments, and whose object it was to test the results, have
been complied with; but no deviation from the effects just
recorded has been observed. The entrance of each kind
of air is invariably accompanied by its characteristic action;
the needle is under the most complete control: in short, no
experiments hitherto made with solid and liquid bodies, are
more certain in their execution, than the foregoing experiments on dry and humid air.
We can easily estimate the per centage of the entire
radiation absorbed by the common air between the points
c and D.
Introducing this tin screen between the experimental
cylinder and the pile, I shut off one of the sources of heat.
The deflection produced by the other source indicates the
total raclication.
This deflection corresponds to about 1,200 of the units
which have been adopted throughout these lectures; that
unit being the quantity of heat necessary to move the needle from 0~ to 1~. The deflection of 45~ corresponds to 50
units; out of 1,200, therefore, 50 in this instance have been
destroyed by the moist air. The following statement gives
us the absorption per hundred:
1200: 100 - 50: 4.2
An absorption of at least 4'2 per cent. was, therefore,
effected by the atmospheric vapour which occupied the
tube between c and D. Air perfectly saturated gives an
absorption of more than 5 per cent.
This absorption took place notwithstanding the partial
sifting of the heat in its passage from the source to c, and
from D to the pile. The moist air, moreover, was, probably, only in part displaced by the dry. In other experi



398                  LEOTURE XI.
ments I found, with a tube 4 feet long, and polished within,
that the atmospheric vapour, on a day of average dryness,
absorbed over 6 per cent. of the radiation from our source.
Regarding the earth as a source of heat, no doubt, at least
10 per cent of its heat is intercepted within ten feet of the
surface.* This single fact suggests the enormous influence
which this newly developed property of aqueous vapour
must have in the phenomena of meteorology.
But we have not yet disposed of all objections. It has
been intimated to me that the air of our laboratory might
be impure; and the suspended carbon particles of the London air have also been referred to, as a possible cause of the
absorption, ascribed to aqueous vapour.
I reply: 1st. The results were obtained when the apparatus was removed from the laboratory-they are obtainable in this room. 2ndly. Air was brought from the
following localities in impervious bags:-Hyde Park, Primrose Hill, Hampstead Heath, Epsom  Downs (near the
Grand Stand); a field near Newport, Isle of Wight; St.
Catharine's Down, Isle of Wight; the sea beach near
Black-gang Chine. The aqueous vapour of the air from
all these localities, examined in the usual way, exerted an
absorption seventy times that of the air in which the vapour
was diffused.
Again, I experimented thus. The air of the laboratory
was dried and purified until its absorption fell below unity;
this purified air was then led through a U tube, filled with
fragments of perfectly clean glass moistened with distilled
water. Its neutrality, when dry, showed that all prejudicial substances had been removed from it, and in passing
through the U tube, it could take up nothing but the pure
vapour of water. The vapour thus carried into the experi* Under some circumstances the absorption, I have reason to believe,
considerably exceeds this amount.




OBJECTIONS ANSWERED.               399
mental tube produced an action ninety times greater than
that of the air which carried it.
But fair and philosophic criticism does not end even
here. The tube with which these experiments were made
is polished within, and it was surmised that the vapour of
the humid air might, on entering, have deposited itself
upon the interior surface of the tube, thus diminishing its
reflective power, and producing an effect apparently the
same as absorption. But why, I would ask, should such a
deposition of moisture take place? On many of the days
when these experiments were made the air was at least 25
per cent. under its point of saturation. It can hardly be
assumed that such air would deposit its moisture on a metallic surface, against which, moreover, the rays from our
source of heat were at the time impinring. The mere consideration of the objection must deprive it of weight.
Further, the absorption is exerted when only a small fraction of an atmosphere is introduced into the tube, and it is
proportional to the quantity of air present. This is shown
by the following table, which gives the absorption, by humid air, at tensions varying from 5 to 30 inches of mercury.
Humid Air.
Absorption
Tension                     --— ~ —
in inches                  Observed       Calculated
5..   16... 16
10....   32...   32
15                        49..   48
20.....                  64..   64
25                        82..      80
30..                 98... 96
The third column of this table is calculated on the assumption that the absorption is proportional to the quantity of vapour in the tube, and the agreement of the calculated and observed results show this to be the case, within
the limits of the experiment. It cannot be supposed that




400                   LECTURE XI.
effects so regular as these, and agreeing so completely with
those obtained with small quantities of other vapours, and
even with small quantities of the permanent gases, can be
due to the condensation of.the vapour on the interior surface. When, moreover, five inches of air were in the tube,
less than 6th of the vapour necessary to saturate the space
was present. The dryest day would make no approach to
this dryness. Condensation under these circumstances is
impossible, and more especially a condensation which
should destroy, by its action upon the inner reflector, quantities of heat so accurately proportional to the quantities of
matter present.
My desire, however, was to take this important question quite out of the domain of mere reasoning, however
strong this might appear. I therefore resolved to abandon
not only the plates of rocksalt but also the experimental
tube itself, and to displace one portion of the free atmosFig. 97.
phere by another. With this view the following arrangement was made:-c (fig. 97), a cube of boiling water, is
our source of heat. Y is a hollow brass cylinder set upright, 3'5 inches wide, and 7'5 inches high. P is the ther



ABSORPTION OF AQUEOUS VAPOUR IN FREE AIR. 401
mo-electric pile, and c' a compensating cube, between which
and P is an adjusting screen, to regulate the amount of radiation falling on the posterior surface of the pile. The
whole arrangement was surrounded by a hoarding, the
space within which was divided into compartments by
sheets of tin, and these spaces were stuffed loosely with
paper or horsehair. These precautions, which required
time to be learned, were necessary to prevent the formation
of local air-currents; and also to intercept the irregular action of the external air. The effect to be measured here is
very small, and hence the necessity of removing all causes
of disturbance which could possibly interfere with its clearness and purity.
A rose-burner r was placed at the bottom of the cylinder s, and from it a tube passed to an India-rubber bag
containing air. The cylinder Y was first filled with fragments of rockcerystal, moistened with distilled water. On
subjecting the India-rubber bag to pressure, the air from it
was gently forced up among the fragments of quartz, and
having there charged itself with vapour it was discharged
in the space between the cube c and the pile. Previous to
this the needle stood at zero; but on the emergence of the
saturated air from the cylinder, the needle moved and took
up a final deflection of five degrees. The direction of the
deflection showed that the opacity of the space between
the source c and the pile was augmented by the presence
of the saturated air.
The quartz fragments were now removed, and the cylinder was filled with fragments of fresh chloride of calcium, through which the air was gently forced, exactly as
in the last experiment. Now, however, in passing through
the chloride of calcium, it was in great part robbed of its
aqueous vapour, and the air, thus dried, displaced the common air between the source and pile. The needle moved,
declaring a permanent deflection of 10 degrees; the direc



402                  LECTURE XI.
tion of the deflection showed that the transparency of the
space was augmented by the presence of the dry air. By
properly timing the discharges of the air, the swing of the
needle could be augmented to 15 or 20 degrees. Repetition showed no deviation fiom this result; the saturated
air always augmented the opacity, the dry air always augmented the transparency of the space between the source
and the pile. Not only, therefore, have the plates of rocksalt been abandoned, but also the experimental tube itself,
and the results are all perfectly concurrent as regards the
action of aqueous vapour upon radiant heat.
Were this subject less important I should not have
dwelt upon it so long. I thought it right to remove every
objection, so that meteorologists might apply, without the
faintest misgiving, the results of experiments. The applications of these results to their science must be innumerable; and here I cannot but regret that the incompleteness
of my knowledge prevents me from making the proper applications myself. I would, however, ask your permission to
refer to such points as I can now call to mind, with which
the facts just established appear to be more or less intimately connected.
And, first, it is to be remarked that the vapour which
absorbs heat thus greedily, radiates it copiously.  This
fact must, I imagine, come powerfully into play in the
tropics. We know that the sun raises from the equatorial
ocean enormous quantities of vapour, and that immediately
under him, in the region of calms, the rain, due to the condensation of the vapour, descends in deluges. Hitherto,
this has been ascribed to the chilling which accompanies
the expansion of the ascending air, and no doubt this, as a
true cause, must produce its proportional effect. But I
cannot help thinking that the radiation from the vapour itself is also influential. Imagine a column of saturated air
ascending from the equatorial ocean; for a time the vapour




TORREENTIAL RAINS OF THE TROPICS.       403
entangled in this air, is surrounded by air almost fully
saturated. Its vapour radiates, but it radiates into vapour,
and the vapour into it. To the radiation from any vapour,
a screen of the same vapour is particularly opaque. Hence,
for a time, the radiation from our ascending column is intercepted, and in great part returned by the surrounding
vapour; condensation under such circumstances cannot occur. But the quantity of aqueous vapour in the air diminishes speedily as we ascend; the decrement of tension, as
proved by the observations of Hooker, Strachy, and Welsh,
is much more speedy than that of the air; and, finally, our
vaporous column finds itself elevated beyond the protecting
screen which7 during the first portion of its ascent, was
spread out above it. It is now in the presence of pure space,
and into space it pours its heat without stoppage or requital. To the loss of heat thus endured, the condensation
of the vapour, and its torrential descent to the earth, must
certainly be in part ascribed.
Similar remarks apply to the formation of cumuli in our
own latitudes; they are the heads of columnar bodies of
vapour which rise from the earth's surface, and are precipitated as soon as they reach a certain elevation. Thus the
visible cloud forms the capital of an invisible pillar of saturated air.  Certainly the top of such a column, raised
above the vapour screen which clasps the earth, and offering itself to space must be chilled by radiation; in this action alone we have a physical cause for the generation of
clouds.
Mountains act as condensers, but how? Partly, no
doubt, by the coldness of their own masses; which coldness they owe to their elevation. Above them spreads no
vapour screen of sufficient density to intercept their heat,
which consequently gushes unrequited into space. When
the sun is withdrawn, this loss is shown by the quick and
large descent of the thermometer. This descent is not due




404                   LECTURE XI.
to radiation from the air, but to radiation from the earth,
or from the thermometer itself. Thus the difference between a thermometer which, properly confined, gives the
true temperature of the night air, and one which is permitted to radiate freely towards space, must be greater at high
elevations than at low ones. This conclusion is entirely
confirmed by observation. On the Grand Plateau of Mont
Blanc, for example, MM. Martins and Bravais found the
difference between two such thermometers to be 24~ Fahr.;
when a difference of only 10~ was observed at Chamouni.
But mountains also act as condensers by the deflection
upwards of moist winds, and their consequent expansion;
the chilling thus produced is the same as that which accompanies the direct ascent of a column of warm air into the
atmosphere; the elevated air performs work, and its heat
is correspondingly consumed. But in addition to these
causes, I think we must take into account the radiant
power of the moist air when thus tilted upwards. It is
thereby lifted beyond the protection of the aqueous layer
which lies close to the earth, and therefore pours its heat
freely into space, thus effecting its own condensation. No
doubt, I think, can be entertained, that the extraordinary
energy of water as a radiant, in all its states of aggregation, must play a powerful part in the condensation of a
mountain region. As vapour it pours its heat into space
and promotes condensation; as liquid it pours its heat into
space and promotes congelation; as snow it pours its heat
into space and thus converts the surfaces on which it falls
into more powerful condensers than they otherwise would
be. Of the numerous wonderful properties of water, not
the least important is this extraordinary power which it
possesses, of discharging the motion of heat upon the interstellar ether.
A freedom of escape similar to that from bodies of
vapour at great elevations would occur at the earth's sur



COLD OF CENTRAL ASIA, ETC.         405
face generally, were the aqueous vapour removed from the
air above it, for the body of the atmosphere in a practical
vacuum as regards the transmission of radiant heat. The
withdrawal of the sun from any region over which the atmosphere is dry must be followed by quick refrigeration.
The moon would be rendered entirely uninhabitable by beings like ourselves through the operation of this single
cause; with an outward radiation uninterrupted by aqueous
vapour, the difference between her monthly maxima and
minima must be enormous. The winters of Thibet are almost unendurable from the same cause. Witness how the
isothermal lines dip from the north into Asia, in winter, as
a proof of the low temperature of this region. Humboldt
has dwelt upon the' frigorific power' of the central portions of this continent, and controverted the idea that it
was to be explained by reference to its elevation, for there
were vast expanses of country, not much above the sea
level, with an exceedingly low temperature. But not knowing the influence which we are now studying, Humboldt, I
imagine, omitted one of the most important of the causes
which contributed to the observed result. Even the absence of the sun at night causes powerful refrigeration
when the air is dry. The removal, for a single summer
night, of the aqueous vapour from the atmosphere which
covers England, would be attended by the destruction of
every plant which a freezing temperature could kill. In
Sahara, where' the soil is fire and the wind is flame,' the
refrigeration at night is often painful to bear. Ice has been
formed in this region at night. In Australia, also, the diurnal rang1e of temperature is very great, amounting, commonly, to between 40 and 50 degrees. In short, it may be
safely predicted, that wherever the air is dry, the daily
thermomeqtric range will be great. This, however, is quite
different from saying that when the air is clear the thermometric range will be great. Great clearness to light is




406                  LECTURE XI.
perfectly compatible with great opacity to heat; the atmosphere may be charged with aqueous vapour while a
deep blue sky is overhead, and on such occasions the terrestrial radiation would, notwithstanding the' clearness,'
be intercepted.
And here we are led to an easy explanation of a fact
which evidently perplexed Sir John LesFig. 98.  lie. This celebrated experimenter constructed an instrument which he named
c      dc    ~ _ B an cethrioscope, the function of which
was to determine the radiation against
the sky. It consisted of two glass bulbs
l |      united by a vertical glass tube, so narrow that a little column of liquid was
supported in the tube by its own adhesion. The lower bulb D (fig. 98) was
protected by a metallic envelope, and
gave the temperature of the air; the upper bulb B, was blackened, and was surrounded by a metallic cup c, which protected the bulb from terrestrial radiation.' This instrument,' says its inventor,'exposed to the open air in clear weath-, -,ll   er will at all times, both during the day
and the night, indicate an impression of
cold shot downwards from the higher
regions...  The sensibility of the instrument is very
striking, for the liquor incessantly falls and rises in the
stem, with every passing cloud. But the cause of its variations does not always appear so obvious. Under a fine
blue sky the aethrioscoope will sometimes indicate a cold of
50 millesimal degrees; yet on other days, when the air
seems equally bright, the effect is hardly 30o.' This anomaly is simply due to the difference in the quantity of




CLEAR/   DAYS AND C DRY  DAYS.          407
aqueous vapour present in the atmosphere. Indeed, Leslie
himself connects the effect with aqueous vapour in these
words,'The pressure of hygrometric moisture in the air
probably affects the instrument.' It is not, however, the'pressure' that is effective; the presence of invisible vapour
intercepted the radiation from the vethrioscope, while its
absence opened a door for the escape of this radiation into
space. As regards experiments on terrestrial radiation, a
new definition will have to be given for'a clear day;' it
is manifest, for example, that in experiments with the
pyrheliometer,* two days of equal visual clearness may
give totally different results. We are also enabled to account for the fact that the radiation from this instrument
is often intercepted when no cloud is seen. Could we,
however, make the constituents of the atmosphere, its
vapour included, objects of vision, we should see sufficient
to account for this result.
Another interesting point on which this subject has a
bearing is Melloni's theory of serein.' Most authors,'
writes this eminent philosopher,' attribute to the cold, resulting from the radiation of the air, the excessively fine
rain which sometimes falls in a clear sky, during the fine
season, a few moments after sunset.''But,' he continues,'as no fact is yet known which directly proves the emissive
power of pure and transparent elastic fluids, it appears to
me more conformable,' &c., &c.  If the difficulty here
urged against the theory of s'reim be its only one, the theory will stand, for transparent elastic fluids are now proved
to possess the power of radiation which the theory assumes.
It is not, however, to radiation from the air.that the chilling can be ascribed, but to radiation from the body itself,
whose condensation produces the serein.
Let me add the remark, that as far as I can at present
* The instrument is described in Lecture XII.




408                        LECTURE XI.
judge, aqueous vapour and liquid water absorb the same
class of rays; this is another way of stating that the colour
of pure water is shared by its vapour. In virtue of aqueous
vapour the atmosphere is therefore a blue medium.  I believe it has been remarked that the colour of the firmamental blue, and of distant hills, deepens with the amount of
aqueous vapour in the air; but the substance which produces a variation of depth must be effective as an origin
of color.  Whether the azure of the sky-the most difficult
question of meteorology, —is to be thus accounted for, I
will not at present venture to enquire.
NOTE.
The fear of being led too far from my subject causes me to withhold all
speculation as to the cause of atmospheric polarisation. I may, however,
remark that the polarisation of heat was illustrated in the lectures by means
of the mica piles with which Professor (now Principal) J. D. Forbes first
succeeded in establishing the fact of polarisation.
In connexion with the investigation of the radiation and absorption of
heat by gases and vapours, it gives me pleasure to refer to the prompt and
intelligent aid rendered me by Mr. Becker, of the firm of Elliotts', 30
West Strand.
From the more energetic gases and vapours a series of very striking
class experiments may be derived, interesting alike to the chemist and the
natural philosopher. Mr. Becker has constructed a cheap form of apparatus
suitable for the experiments.  Where quantitative results are not required,
two cubes of hot water, an open tin tube, a thermo-electric pile, and a
galvanometer, magnetised, as described in the Appendix to Lecture I., will
suffice to illustrate the action of the stronger gases and vapours. A current
of air from a common bellows will carry the vapour into the tube.




APPEENDIX TO LECTURE XI.
EXTRACT FROM A PAPER IN THE' PHILOSOPHICAL TRANSACTIONS
FOR 1862,' ON THE ABSORPTION AND RADIATION OF HEAT BY
GASEOUS MATTER.' I WAS engaged in experiments on aqueous vapours when my
other duties compelled me to close this enquiry for a time. I
believe, however, I may safely say that not only is the action of
aqueous vapour on radiant heat measurable, but this action may
be made use of as ca mneasure of atmospheric moisture, the tube used
in my experimnents being thus converted into a hygrometer of surpassing delicacy. Unhappily, as in other cases touched upon in this
memoir, I have been unable to give this subject the developement
I could wish; but the results which I am in a position to record
are nevertheless interesting.' On a great number of occasions I compared the air sent
directly from the laboratory into the experimental tube with the
same air after it had been passed through the drying apparatus.
Calling the action of the dry air unity, or supposing it rather to
oscillate about unity (for the temperature of my source varied a
little from day to day), on the following days the annexed absorptions were observed with the undried air of the laboratory:Absorptions by 1Undried Air.
October 23rd.     63     November 1st.. 50
24th..   62,,  4th.. 58
29th.65              8th.   49
31st..   56,,  12th.. 62'Nearly 595ths of the above effects are due to aqueous vapour;
which, therefore, in some instances exerted nearly sixty times the
action of the air in which it was diffused.' The experiments which I have made on aqueous vapour have
been very numerous and varied. Differing as I did from so cautious and able an experimenter, I spared no pains to secure myself against error. I have experimented with air moistened in
18




410             APPENDIX TO LECTURE XI.
various ways, sometimes by allowing small bubbles of it to ascend
through water, sometimes dividing it, by sending it through the
pores of common cane immersed in water. Between the drying
apparatus -and the experimental tube I have introduced tubes
containing fragments of glass moistened with water, and allowed
the air to pass over them; large effects were in all such cases obtained, the absorption being usually more than eighty times that
of dried air. Fragments of unwetted glass, which had been
merely exposed to the air of the laboratory, had dried air led
over them into the experimental tube; the absorption was fifteen
times that of dried air. A roll of bibulous paper, taken from one
of the drawers of the laboratory, and to all appearance perfectly
dry, was enclosed in a glass tube, and dry air carried between its
leaves. The experiment was made five times in succession with
the same paper, and the following absorptions were observedc:Absorption
No. 1....     72
No. 2...     62
No. 3....     62
No. 4.               47
No. 5....     47' In fact, the action of aqueous vapour is exactly such as might
be expected from the vapour of a liquid which Melloni found to
be the most powerful absorber of radiant heat of all he had examined.'Every morning, on commencing my experiments, I had an
interesting example of the power of glass to gather a film of
moisture on its surface. Suppose the tube mounted and the air
of the laboratory removed, as far as the air-pump was capable of
removing it. On allowing dry air to enter for the first time, the
needle would move from 0~ to 50~. On pumping out it would
return to 00, and on letting in dry air a second time it would
swing almost to 40~. Repeated exhaustions would cause this
action to sink almost to nothing. These results were entirely due
to the moisture collected during the night in an invisible film on
the inner surface of the tube, and which was removed by the air
on entering, and diffused through the tube. If the dry air entered at the end of the tube nearest to the source of heat, on the
first and second admissions, and sometimes even on a third, the




ABSORPTION BY DRY AND BY MOIST AIR.    411
vapour carried from the warm end to the cold end of the tube
was precipitated as a mist upon the latter, for a distance sometimes of nearly a foot. The mist always disappeared on pumping
out. It is needless to remark that facts of this character, of
which I could cite many, were not calculated to promote incautiousness or rashness on my part. I saw very clearly how easy
it was to fall into the gravest errors, and I took due precautions
to prevent myself from doing so.'But, to place the- matter beyond all doubt, I abolished the
plates of rocksalt altogether, and operated thus: —An India-rubber bag, B (fig. 99), was filled with air, and to its nozzle a T-piece,
with the cocks q 2', was attached. The cock Q' was connected
with two tubes, u'u'V each of which was filled with fragments of
glass moistened with distilled water. The cock q was connected
with the tubes u tu, each of which was filled with fragments of
glass moistened by sulphuric acid. The other ends of these two
series of tubes were connected with the cocks o o'; and from the
T-piece between these cocks a tube led to the end E' of the open
experimental tube T. The cock A at the other end of the experimental tube was placed in connection with an air-pump. The pile
P, the screen s, and the compensating cube c' were used as in the
other experiments. E is the end of the front chamber, and c the
source of heat. In some experiments I had the end E closed by a
plate of rocksalt, in others it was allowed to remain open, a distance of about 12 inches intervening between the radiating surface
and the open end E' of the experimental tube.'Closing the cocks Q and o, and opening q' and o', gentle
pressure being applied to the bag B, a current of moist air was
slowly discharged at the end of E' of the experimental tube. The
pump in connection with A was then worked, and thus by degrees
the air was sucked into the tube T. The deflection of the galvanometer was 30~, when the moist air filled the tube as completely
as the arrangement permitted-this deflection being due to the
predominance of the compensating tube over the radiating
source c.'The cocks Q' and o' were now closed, and Q and o opened;
proceeding as before, a current of dry air was discharged at E',
and this air was drawn into the tube T in the manner just described. The moist air was thus displaced by dry; and, while




412   hPP SND'  TO ^;(T~R -
Fig. 99.''00/10f@|(
L~~~~~~~~~~~~~




ROCKSALT PLATES SUITABLE.               413
the displacement was going on, the galvanometer was observed
through the distant telescope. The needle soon commenced to
sink, and slowly went down to zero, proving that a greater quantity of heat passed through the dry than through the moist air.
The wet air was substituted for the dry, and the dry for the wet,
twenty times in succession, with the same constant result: the
entrance of the humid air always caused the needle to move from
0~ to 30~, while the entrance of dry air caused it to fall from
80~ to 0~. The air-pump was resorted to, because I found that
when I attempted to displace the air by the direct force of the
current from B, the temperature of the pile, or of the source, was
so affected by the fresh air as to confuse the result. I may remark
that not only have I operated thus for days with aqueous vapour,
but every result which I have obtained with vapours generally
has been thus confirmed, so that all doubt as to the applicability
of the rock-salt plates to researches of this nature may, I think,
be abandoned.'




LECTURE XII.
[April 10, 1862.]
DEW,-A CLEAR SKY AND CALM BUT DAMP ATMOSPHERE NECESSARY FOR
ITS COPIOUS FORMATION-DEWED SUBSTANCES COLDER THAN UNDEWED
ONES-DEWED SUBSTANCES BETTER RADIATORS THAN UNDEWED ONESDEW IS THE CONDENSATION OF THE ATMOSPHERIC VAPOUR ON SUBSTANCES
WHICH HAVE BEEN CHILLED BY RADIATION-LUNAR RADIATION-CONSTITUTION OF THE SUN-THE BRIGHT LINES IN THE SPECTRA OF TEll
METALS-AN INCANDESCENT VAPOUR ABSORBS THE RAYS WHICH IT CAN
ITSELF EMIT-KIRCHHOF'S GENERALISATION-FRAUNHOFER'S LINES-SOLAR CHEMISTRY-EMISSION OF THE SUN-HERSCHEL AND POUILLET'S EX
PERIMENTS-MAYER S METEORIC THEORY-EFFECT OF THE TIDES ON THE
EARTH S ROTATION-ENERGIES OF THE SOLAR SYSTEM-HELMHOLTZ, THOMSON, WATERSTON-RELATION OF THE SUN TO ANIMAL AND VEGETABLE
LIFE.
-5X7E have learned that our atmosphere is always more
or less charged with aqueous vapour, the condensation of which forms our clouds, hail, rain, and snow. I
have now to direct your attention to one particular case of
condensation, of great interest and beauty-one, moreover,
regarding which erroneous notions were for a long time entertained.   I refer to the phenomenon of Dew.   The
aqueous vapour of our atmosphere is a powerful radiant,
but it is diffused through air which usually exceeds its own
mass more than one hundred times. Not only, then, its
own heat, but the heat of the large quantity of air which
surrounds it, must be discharged by the vapour, before it
can sink to its point of condensation.  The retardation of
chilling due to this cause enables good solid radiators, at
the earth's surface, to outstrip the vapour in their speed of




DEW.                     415
refrigeration; and hence upon these bodies aqueous vapour
may be condensed to liquid, or even congealed to hoarfrost, while at a few feet above the surface it still maintains its gaseous state. This is actually the case in the
beautiful phenomenon which we have now to examine.
We are indebted to a London physician for a true
theory of dew. In 1818 Dr. Wells published his admirable
Essay upon this subject. He made his experiments in a
garden in Surrey, at a distance of three miles from Blackfiiars Bridge. To collect the dew he used little bundles
of wool, which, when dry, weighed 10 grains each; and
having exposed them during a clear night, the amount of
dew deposited on them was determined by the augmentation of their weight. HIe soon found that whatever interfered with the view of the sky from his piece of wool, interfered also with the deposition of dew. He supported
a board on four props; on the board he laid one of his
wool parcels, and undcer it a second similar one; during a
clear calm night, the former gained 14 grains in weight,
while the latter gained only 4. He bent a sheet of pasteboard like the roof of a house, and placed underneath it a
bundle of wool on the grass: by a single night's exposure
the wool gained 2 grains in weight, while a similar piece
of wool exposed on the grass, but quite unshaded by the
roof, collected 16 -grains of moisture.
Is it steam from the earth, or is it fine rain from the
heavens, that produces this deposition of dew? Both of
these notions have been advocated. That it does not arise
from the earth is, however, proved by the observation, that
more moisture was collected on the propped board than on
the earth's surface under it. That it is not a fine rain is
proved by the fact, that the most copious deposition occurred on the clearest nights.
Dr. Wells next exposed thermometers, as he had done
his wool-bundles, and found that at those places where the




416                 LECTURE XI.
dew fell most copiously the temperature sank lowest. On
the propped board already referred to, he found the temperature 90 lower than under it; beneath the pasteboard
roof the thermometer was 100 warmer than on the open
grass. He also found that when he laid his thermometer
upon a grass plot, on a clear night, it sank sometimes 14~
lower than a similar thermometer suspended in free air at
a height of 4 feet above the grass. A bit of cotton, placed
beside the former, gained 20 grains; a similar bit, beside
the latter, only 11 grains in weight.  The lowering of the
temperature and the deposition of the clew went hand in
hand. Not only did the shade of artificial screens interfere with the lowering of the temperature and the formation of the dew, but a cloud-screen acted in the same manner. He once observed his thermometer, which, as it lay
upon the grass, showed a temperature 12~ lower than the
air a few feet above the grass, rise, on the passage of some
clouds, until it was only 2~ colder than the air. In fact, as
the clouds crossed his zenith, or disappeared from it, the
temperature of his thermometer rose and fell.
A series of such experiments, conceived and executed
with singular clearness and skill, enabled Dr. Wells to propound a Theory of Dew, which has stood the test of all
subsequent criticism, and is now universally accepted.
It is an effect of chilling by radiation.'The upper
parts of the grass radiate their heat into regions of empty
space, which, consequently, send no heat back in return; its
lower parts, from the smallness of their conducting power,
transmit little of the earth's heat to the upper parts, which,
at the same time, receiving only a small quantity from the
atmosphere, and none from any other lateral body, must
remain colder than the air, and condense into dew its watery
vapour, if this be sufficiently abundant in respect to the
decreased temperature of the grass.'  Why the vapour
itself, being a powerful radiant, is not as quickly chilled as




THEORY OF WELLS.               417
the grass, I have already explained, on the ground that the
vapour has not only its own heat to discharge, but also that
of the large mass of air by which it is surrounded.
Dew being the result of the condensation of atmospheric vapour, on substances which have been sufficiently
cooled by radiation, and as bodies differ widely in their
radiative powers, we may expect corresponding differences
in the deposition of dew. This Wells proved to be the
case. He often saw dew copiously deposited on grass and
painted wood, when none could be observed on gravel
walks adjacent. He found plates of metal, which he had
exposed, quite dry, while adjacent bodies were covered
with dew: in all such cases the temperature of the metal
was found to be higher than that of the dewed substances.
This is quite in accordance with our knowledge that metals
are the worst radiators. On one occasion he placed a plate
of metal upon grass, and upon the plate he laid a glass
thermometer; the thermometer, after some time, exhibited
dew, while the plate remained dry. This led him to suppose that the instrument, though lying on the plate, did not
share its temperature. HIe placed a second thermometer,
with a gilt bulb, beside the first; the naked glass thermometer-a good radiator-remained 9~ colder than its
companion. To determine the true temperature of a body
is, I may remark, a difficult task: a glass thermometer, suspended in the air, will not give the temperature of the air;
its own power as a radiant or an absorbent comes into
play. On a clear day, when the sun shines, the therlometer will be warmer than the air; on a clear night, on the
contrary, the thermometer will be colder than the air.
We have seen that the passage of a cloud can raise the
temperature of a thermometer 10 degrees in a few minutes.
This augmentation, it is manifest, does not indicate a corresponding augmentation of the temperature of the air, but
18*




418                      LECTURE XI.
merely the interception and reflection, by the cloud, of the
rays of heat emitted by the thermometer
Dr. Wells applied his principles to the explanation of
many curious effects, and to the correction of many popular
errors.  Moon blindness he refers to the chill produced by
radiation into clear space, the shining of the moon being
merely an accompaniment to the clearness of the atmosphere.  The putrefying influence ascribed to the moonbeams is really due to the deposition of moisture, as a kind
of dew, on the exposed animal substances. The nipping
of tender plants by frost, even when the air of the garden
is some degrees above the freezing temperature, is also to
be referred to chilling by radiation. A cobweb screen
would be sufficient to preserve them from illjury.*
Wells was the first to explain the formation, artificially,
of ice in Bengal, where the substance is never formed naturally. Shallow pits are dug, which are partially filled
with straw, and on the straw  flat pans, containing water
which had been boiled, is exposed to the clear firmament.
The water is a poweful radiant, and sends off its heat
copiously into space. The heat thus lost cannot be supplied
from the earth-this source being cut off by the non-conducting straw.  Before sunrise a cake of ice is formed in
each vessel.  This is the explanation of Wells, and it is, no
doubt, the true on,  I think, however, it needs supplementing.  It appears, from  the description, that the con* With reference to this point we have the following beautiful passage
in the Essay of Wells:-' I had often, in the pride of half knowledge, smiled
at the means frequently employed by gardeners to protect tender plants
from cold, as it appeared to me impossible that a thin mat, or any such
flimsy substance could prevent them from attaining the temperature of the
atmosphere, by which alone I thought them liable to be injured. But
when I had learned that bodies on the surface of the earth become, during
a still and serene night, colder than the atmosphere, by radiating their heat
to the heavens, I perceived immediately a just reason for the practice which
I had before deemed useless.'




NOCTURNAL  RADIATION.               419
dition most suitable for the formation of ice, is not only a
clear air, but a dry air. The nights, says Sir Robert
Barker, most favourable for the production of ice, are those
which are clearest and most serene, and in which very little
dew appears after midcnight. I have italicised a very significant phrase. To produce the ice in abundance, the atmosphere must not only be clear, but it must be comparatively free from  aqueous vapour.  When the straw in
which the pans were laid became wet, it was always changed
for dry straw, and the reason Wells assigned for this was,
that the straw, by being wetted, was rendered more compact, and efficient as a conductor. This may have been the
case, but it is also certain that the vapour rising from the
wet straw, and overspreading the pans like a screen, would
check the chill, and retard the congelation.
With broken health Wells pursued and completed this
beautiful investigation; and, on the brink of the grave, he
composed his Essay. It is a model of wise enquiry and of
lucid exposition. He made no haste, but he took no rest
till he had mastered his subject, looking steadfastly into it
until it became transparent to his gaze. Thus he solved
his problem, and stated its solution in a fashion which renders his work imperishable.*
Since his time various experimenters have occupied
themselves with the question of nocturnal radiation; but,
though valuable facts have been accumulated, if we except a supplement contributed by Melloni, nothing of importance has been added to the theory of Wells. Mr.
Glaisher, M. Martins, and others, have occupied themselves
with the subject. The following table contains some results obtained by Mr. Glaisher, by exposing thermometers
at different heights above the surface of a grass field. The
* The tract of Wells is preceded by a personal memoir written by himself. It has the solidity of an essay of Montaigne.




420                    LECTURE XII.
chilling observed, when the thermometer was exposed on
long grass, is represented by the number 1,000; while the
succeeding numbers represent the relative chilling of the
thermometers placed in the positions indicated:
Radiation.
Long grass...      1000
One inch above the points of the grass. 671
Two inches,,,,. 570
Three inches,,,,. 477
Six inches,,,,. 282
One foot,,,,. 129
Two feet,,,,.  86
Four feet,,,,.  69
Six feet,,.  52
It may be asked why the thermometer, which is a good
radiator, is not, when suspended in free air, just as much
chilled as at the earth's surface.  Wells has answered the
question.  It is because the thermometer, when chilled,
cools the air in immediate contact with it; this air contracts, becomes heavy, and trickles downwards, thus allowing its place to be taken by warmer air. In this way the
free thermometer is prevented from  falling very low  beneath the temperature of the air.  Hence, also, the necessity of a still night for the copious formation of dew; for,
when the wind blows, fresh air continually circulates amid
the blades of grass, and prevents any considerable chilling
by radiation.
When a radiator is exposed to a clear sky it tends to
keep a certain thermometric distance, if I may use the
term, between its temperature and that of the surrounding
air. This distance will depend upon the energy of the
body as a radiator, but it is to a great extent independent
of the temperature of the air.  Thus M. Pouillet has proved
that in the month of April, when the temperature of the air
was 3~6 C., swansdown fell by radiation to — 3~5: the
whole chilling, therefore, was 7~'1. In the month of June,




MELLONI'S SUPPLEMENT TO THE THEORY OF DEW. 421
when the temperature of the air was, 7~'75 C., the temperature of the radiating swansdown was 10~'54: the chilling of
the swansdown by radiation is here 7~'21; almost precisely
the same as that which occurred in April. Thus, while the
general temperature varies within wide limits, the difierence of temperature between the radiating body and the
surrounding air, remains sensibly constant.
These facts enabled Melloni to make an important addition to the theory of dew. He found that a glass thermometer, placed on the ground, is never chilled more than 2~
C. below an adjacent thermometer, qwith silvered btul, which
hardly radiates at all. These 2~ C., or thereabouts, mark
the thermometric distance above referre$d to, which the
glass tends to preserve betweeen it and the surrounding
air. But Six, Wilson, Wells, Parry, Scoresby, Glaisher,
and others, have found differences of more than 10  C.,
between a thermometer on grass, and a second thermometer hung a few feet above the grass. How is this to be
accounted for? Very simply, according to Melloni, thus:
-The grass blades first chill themselves by radiation, 2~ C.
below the surrounding air: the air is then chilled by contact with the grass, and forms around it a cold aerial bath.
But the tendency of the grass is to keep the above constant
difference between its own temperature and that of the
surrounding medium. It therefore sinks lower. The air
sinks in its turn, being still further chilled by contact with
the grass; the grass, however, again seeks to re-establish
the former difference; it is again followed by the air, and
thus, by a series of actions and reactions, the entire stratum
of air in contact with the grass becomes lowered far below
the temperature which corresponds to the actual radiative
energy of the grass.
So much for terrestrial radiation; that of the moon
will not occupy us so long. Many futile attempts have
been made to detect the warmth of the moon's beams. No




422                  LEOTURE XII.
doubt is entertained that every luminous ray is also a heat
ray; but the light-giving power is not even an approximate
measure of the calorific energy of a beam. With a large
polyzonal lens, Melloni converged an image of the moon
upon his pile; but he found the cold of his lens far more
than sufficient to mask the heat, if such there were, of the
moon. He screened off his lens from the heavens, placed
his pile in the focus of the lens, waited until the needle
came to zero, and then suddenly removing his screen allowed the concentrated light to strike his pile. The slight
air-drafts in the place of experiment were sufficient to disguise the effect. He then stopped the tube in front of his
pile with glass screens, through which the light went
fieely to the blackened face of the pile, where it was converted into heat.  This heat could not get back throuzgh the
glass screen, and thus Melloni, following the example of
Saussure, accumulated his effects, and obtained a deflection
of 3~ or 4~. The deflection indicated warmth, and this is
the only experiment which gives us any positive evidence
as to the calorific action of the moon's rays. Incomparably
less powerful than the solar rays in the first instance, their
action is first enfeebled by distance, and, secondly, by the
fact that the obscure heat of the moon is almost wholly absorbed by our atmospheric vapour.  Even such obscure
rays as might happen to reach the earth would be utterly
cut off by such a lens as Melloni made use of. It might be
worth while to make the experiment with a metallic reflector, instead of with a lens. I have myself tried a conical
reflector of very large dimensions, but have hitherto been
defeated by the unsteadiness of the London air.
We have now to turn our thoughts to the source from
which all terrestrial and lunar heat is derived. This source
is the sun; for if the earth has ever been a molten sphere,
which is now cooling, the quantity of heat which' reaches
its surface from within, has long ceased to be sensible.




SPECTRUM ANALYSIS BY THE ELECTRIC LIGHT. 423
First, then, let us enquire what is the constitution of this
wondrous body, to which we owe both light and life.
Let us approach the subject gradually and prepare our
minds, by previous discipline, for the treatment of this
noble problem. You already know how the spectrum of
the electric light is formed. Here you have one upon the
screen, two feet wide and eight long, with all its magnificent gradations of colour, one fading into the other, without
solution of continuity. The light from which this spectrum is derived, is emitted from the incandescent carbon
points within our electric lamp. All other solids give a
similar spectrum.  When I raise this platinum wire to
whiteness by an electric current, and examine its light by a
prism, I find the same gradation of colours, and no gap
whatever between one colour and the other. But by intense heat,-by the heat of the electric lamp, for example,
--  can volatilise that platinum, and throw upon the screen,
not the spectrum of the incandescent solid, but of its incandescent vapour. The spectrum is now changed; instead
of being a continuous gradation of colours, it consists of a
series of brilliant lines, separated from each other by spaces
of darkness.
I have arranged my pieces of carbon thus:-the lower
one is now a cylinder, about half an inch in diameter, in
the top of which I have scooped a small hollow; in this
hollow I place the metal which I wish to examine-say this
piece of zinc,-and bring down upon it the upper point.
The current passes; I draw the points apart, and you see
the magnificent are that now unites them; here is its
magnified image upon the screen, a fine stream of purple
light 18 inches long. That coloured space contains the
particles of the zinc discharged across from carbon to carbon; these particles are now oscillating in certain definite
periods, and the colour which we perceive is the mixture
of impressions due to these oscillations. Let us separate,




424                  LECTURE XII.
by a prism, the coloured stream into its components; here
they are, splendid bands of red and blue. Pray remember
the character and position of these bands, as I shall have
to refer to them again immediately.
I interrupt the current; eject the zinc, and put in its
place a bit of copper. Here you see a stream of green
light between the carbons, which we will analyse as we
did the light of the zinc. You can see that the spectrum
of the copper is different from that of the zinc: here you
have bands of brilliant green, which are absent from the
zinc. We may therefore infer, with certainty, that the atoms
of copper, in the voltaic arc, swing in periods different from
those of zinc.  Let us now see whether these different
periods create any confusion, when we operate upon a substance composed of zinc and copper,-the familiar substance brass. Its spectrum is now before you, and if you
have retained the impression made by our two last experiments, you will recognise here a spectrum formed by the
superposition of the two separate spectra of zinc and copper. The alloy emits, without confusion, the rays peculiar
to the metals of which it is composed.
Every metal emits its own system of bands, which are
as characteristic of it as those physical and chemical qualities which give it its individuality. By a method of experiment sufficiently refined we can measure, accurately, the
position of the bright lines of every known metal. Acquainted with such lines we should, by the mere inspection
of the spectrum of any single metal, be able at once to
declare its name. And not only so, but in the case of a
mixed spectrum, we should be able to declare the constituents of the mixture from which it emanated.
This is true, not only of the metals themselves, but also
of their compounds, if they be volatile. I place a bit of
sodium on my lower cylinder and cause the voltaic discharge to pass from it to the upper coal-point; here is the




METALLIC VAPOURS.              425
spectrum of the sodium: a single band of brilliant yellow. If I operated with sufficient delicacy I should divide
that band into two, with a narrow dark interval between
them. I eject the sodium from the lamp and put in its
place a little common salt, or chloride of sodium. At this
high temperature the salt is volatile, and you see the exact
yellow band produced by the salt that was given by the
metal. Thus, also, by means of the chloride of strontium
I produce the bands of the metal strontium; by the chlorides of calcium, magnesium, and lithium, I produce the
spectra of these respective metals.
Here, finally, I have a carbon cylinder perforated with
holes, into which I have stuffed a mixture of all the compounds just mentioned; and there is the spectrum of the
mixture upon the screen. Surely nothing more magnificent can be imagined. Each substance gives out its own
peculiar rays, and thus they cut transversely, the whole
eight feet of the spectrum into splendid parallel bars of
coloured light.  Having previously made yourselves acquainted with the lines emitted by all the metals, you
would be able to unravel this spectrum, and to'tell me what
substances I have employed in its production.
I make use of the voltaic arc simply because its light
is so intense as to be visible to a large audience like the
present, but I might make the same experiments with a
common blow-pipe flame, which is nearly deprived of light
by the admixture of air or oxygen. The introduction of
sodium, or chloride of sodium, turns the flame yellow;
strontium turns it red; copper green, &c. The flames thus
coloured, when examined by a prism, show the exact bands
which I have displayed before you on the screen.
We have already learned that gases and vapours absorb
the rays of heat, the heat that we employed being obscure.
I have no doubt that if those rays could make an impression upon the eye-if I could spread them out before you




426                 LECTURE XII.
like the colours of the spectrum-you would find certain
classes of rays selected, in each case, for destruction, the
others being allowed free passage through the vapours.
A famous experiment of Sir David Brewster's, which I
will throw into a form suited to the lecture room, will
enable me to illustrate this power of selection in the case
of light. Into this cylinder, the ends of which are stopped
by plates of glass, I introduce a quantity of nitrous acid
gas, the presence of which is now indicated by its rich
brown colour. I project a spectrum on the screen, eight
feet long and nearly two in width, and I place this cylinder, containing the brown gas, in the path of the beam as
it issues from the lamp. You see the effect; the continuous spectrum is now furrowed by numerous dark bands,
the rays answering to which are struck down by the nitric
gas, while it permits the intervening bands of light to pass
without hindrance.
We must now take a step in advance of the principle of
reciprocity, which I have already enunciated. Hitherto
we have found in gases, liquids, and solids, that the good
absorber is the good radiator; we must now go further and
state, that a gas or vapour, absorbs those precise rays which
it can itself emit; the atoms which swing at a certain rate
intercept the waves excited by atoms swinging at the same
rate. The atoms which vibrate red light will stop red
light; the atoms that oscillate yellow will stop yellow;
those that oscillate green will stop green, and so of the
rest. Absorption, you know, is a transference of motion
from the ether to the particles immersed in it, and the absorption of any atom is exerted chiefly upon those waves
which arrive in periods that correspond with the atom's
own rate of oscillation.
Let us endeavour to prove this experimentally. We
already know that a sodium flame, when analysed, gives us
a brilliant double band of yellow. Here is a flat vessel




ABSORPTION BY SODIUM VAPOUR.          427
containing a mixture of alcohol and water; I warm the
mixture and ignite it: it gives a flame which is so feebly
luminous as to be scarcely visible. I now mix salt with
the liquid, and again ignite it; the flame, which a moment
ago was scarcely to be seen, is now a brilliant yellow. I
project a continuous spectrum upon the screen, and in the
track of the beam, as it issues from the electric lamp, I
place the yellow sodium flame. Observe the spectrum narrowly: you see a flickering gray band in the yellow of the
spectrum; sometimes it is shaded deeply enough to show
you all that the flame has, at least in part, intercepted the
yellow band of the spectrum: it has partially absorbed the
precise light which it can itself emit.
But I wish to make the effect plainer, and therefore
abandon the alcohol light, and proceed thus: here is a
Bunsen's burner, the flame of which is intensely hot,
though it hardly emits any light. I place the burner in
front of the lamp, so that the beam, whose decomposition
is to form our spectrum, shall pass through the flame. I
have here a little net of platinum wire, in which I place a
bit of the metal sodium, about the size of a pea. I also set
up a pasteboard shade, which shall cut off the light emitted
by the sodium, from the screen on which the spectrum
falls.  And now I am ready to make the experiment.
Here, then, in the first place, is the spectrum. I now introduce the platinum net in front of the lamp; the sodium
instantly colours the flame intensely yellow, and you see a
shadow coming over the yellow of the spectrum. But the
effect is not yet at its maximum. The sodium now suddenly bursts into intensified combustion, and there you see
the yellow dug utterly out of the spectrum, and a bar of
intense darkness in its place. This violent combustion will
endure for a little time. I withdraw the flame, the yellow
reappears upon the screen; I reintroduce it, the yellow
band is cut out. This I can do ten times in succession,




428                 LECTTRE XII.
and in the whole range of optics I do not think there is a
more striking experiment. Here, then, we have conclusively proved, that the light which the sodium flame absorbs is the precise light which it can emit.
Let me be still more precise in my experiment. The
yellow of the spectrum spreads over a widish interval; and
I wish now to show you that it is the particular portion
of the yellow which the sodium emits, that is absorbed by
its flame. I placera little salt solution on the ends of my
coal points; you now see the continuous spectrum with the
yellow band of the sodium brighter than the rest of the
yellow. It is thus clearly defined before your eyes. I
again place the sodium flame in front, and that particular
band which now stands out from the spectrum is cut away
a space of intense gloom occupying its place.
You have already seen a spectrum, derived from a mixture of various substances, and which was composed of a
succession of sharply defined and brilliant bars, separated
from each other by intervals of darkness. Could I take
the mixture which produced that striped spectrum, and
raise it, by means of Bunsen's burner, to a temperature
sufficiently intense to render its vapours incandescent; on
placing its flame in the path of a beam producing a continuous spectrum, I should cut out of the latter the precise
rays emitted by the components of my mixture. I should
thus, instead of furrowing my spectrum by a single dark
band, as in the case of sodium, furrow it by a series of
dark bands, equal in number to the bright bands produced,
when the mixture itself was the source of light.
I think we now possess knowledge sufficient to raise us
to the level of one of the most remarkable generalisations
of our age. When the light of the sun is properly decomposed, the spectrum is seen furrowed by innumerable dark
lines. A few of these were observed, for the first time,
by Dr. Wollaston; but they were investigated with pro



FRAUNHOFER'S LINES.              429
found skill by Fraunhofer, and called, after him, Fraunhofer's lines. It has long been supposed that these dark
spaces were caused by the absorption of the rays which
correspond to them, in the atmosphere of the sun; but nobody knew how. Having once proved that an incandescent
vapour absorbs the precise rays which it can itself emit,
and knowing that the body of the sun is surrounded by an
incandescent photosphere, the supposition at once flashes
on the mind, that this photosphere may cut off those rays
of the central incandescent orb, which the photosphere
itself can emit. We are thus led to a theory of the constitution of the sun, which renders a complete account of
the lines of Fraunhofer.
The sun consists of a central orb, liquid or solid, of exceeding brightness, which, of itself, would give a continuous spectrum, or in other words, which emits all kinds of
rays. These, however, have to pass through the photosphere, which wraps the sun like a flame, and this vaporous
envelope cuts off those particular rays of the nucleus which
it can itself emit-the lines of Fraunhofer marking the
position of these failing rays. Could we abolish the central orb, and obtain the spectrum of the gaseous envelope,
we should obtain a striped spectrum, each bright band of
which would coincide with one of Fraunhofer's dark lines.
These lines, therefore, are spaces of relative, not of absolute darkness; upon them the rays of the absorbent photosphere fall; but, these not being sufficiently intense to make
good the light intercepted, the spaces which they illuminate
are dark, in comparison to the general brilliancy of the
spectrum.
It has long been supposed that sun and planets have
had a common origin, and that hence the same substances
are more or less common to them all. Can we detect the
presence of any of our terrestrial substances in the sun? I
have said that the bright bands of a metal are character



430                  LECTURE XII.
istic of the metal; that we can, without seeing the metal,
declare its name from the inspection of the bands. The
bands are, so to speak, the voice of the metal declaring its
presence. Hence, if any of our terrestrial metals be contained in the sun's atmosphere, the dark lines which they
produce ought to coincide exactly with the bright lines
emitted by the vapour of the metal itself. In the case of
the single metal iron, about 60 bright lines have been determined as belonging to it.  When the light from the
incandescent vapour of iron, obtained by passing electric sparks between two iron wires, is allowed to pass
through one-half of a fine slit, and the light of the sun
through the other half, the spectra from both sources of
light may be placed together; and when this is done it is
found that for every bright line of the iron spectrum there
is a dark line of the solar spectrum. Reduced to actual
calculation, this means that the chances are more than 1,000,000,000,000,000,000 to I that iron is in the atmosphere of
the sun. Comparing the spectra of other metals in the
same manner, Professor Kirchhof, to whose genius we owe
this splendid generalisation, finds iron, calcium, magnesium,
sodium, chromium, and other metals, to be constituents of
the solar atmosphere, but as yet he has been unable to detect gold, silver, mercury, aluminium, tin, lead, arsenic, or
antimony.
I can imitate in a way more precise than that hitherto
employed, the solar constitution here supposed. I place in
the electric lamp a cylinder of carbon about half an inch
thick; on the top, and round about the edge of the cylinder, I place a ring of sodium, leaving the central portion
of the cylinder clear. I bring down the upper coal point
upon the middle of the cylinder's upper surface, thus producing the ordinary electric light. The proximity of this
light to the sodium is sufficient to volatilise the latter, and
thus I surround my little central sun with an atmosphere




SOLAR EMISSION.                431
of sodium vapour, as the real sun is surrounded by its
photosphere.  In the spectrum of this light you see the yellow band is absent.
Fig. 100.         The energy of solar emission
has been measured by Sir John
Herschel at the Cape of Good
Hope, and by M. Pouillet in
Paris.  The agreement between.   the measurements is very remarkable.  Sir John  Herschel
finds the direct heating effect of
a vertical sun, at the sea level, to
be competent to melt 0'00754 of
/////cl      an inch of ice per minute; while
according  to  M. Pouillet, the
quantity is 0'00703 of an inch.
The mean of the determinations
cannot be far from  the truth;
this gives 0'00728 of an inch of
ice per minute, or nearly half an
inch per hour. Before you (fig.
100) I have placed an instrument
similar in form to that used by
M. Pouillet, and called by him a
pyrheliomneter. The particular instrument which you now see is composed of a shallow cylinder of steel, ca a, which is filled with mercury. Into the cylinder this thermometer ci, is introduced, the stem of which is
protected by a piece of brass tubing. We thus obtain the
temperature of the mercury. The flat end of the cylinder
is to be turned towards the sun, and the surface thus presented is coated with lampblack.  Here is a collar and
screw, cc, by means of which the instrument may be attached to the stake driven into the ground, or into the
snow, if the observations are made at considerable heights.




432                 LECTURE XII.
It is necessary that the surface which receives the sun's
rays should be perpendicular to the rays, and this is secured
by appending to the brass tube which shields the stem of
the thermometer, a disk, e e, of precisely the same diameter
as the steel cylinder. When the shadow of the cylinder
accurately covers the disk, we are sure that the rays fall,
as perpendiculars, on the upturned surface of the cylinder.
The observations are made in the following manner:First, the instrument is permitted, not to receive the sun's
rays, but to radiate its own heat for five minutes against
an unclouded part of the firmament; the decrease of the
temperature of the mercury consequent on this radiation is
then noted. Next, the instrument is turned towards the
sun, so that the solar rays fall perpendicularly upon it for
five minutes,-the augmentation of temperature is now
noted. Finally, the instrument is turned again towards
the firmament, away from the sun, and allowed to radiate
for another five minutes, the sinking of the thermometer
being noted as before. You might, perhaps, suppose that
exposure to the sun alone would be sufficient to determine
his heating power; but we must not forget that during
the whole time of exposure to the sun's action, the blackened surface of the cylinder is also radiating into space;
it is not therefore a case of pure gain: the heat received
from the sun is, in part, thus wasted, even while the experiment is going on; and to find the quantity lost, the
first and last experiments are needed. In order to obtain
the whole heating power of the sun, we must add to his
observed heating power, the quantity lost during the time
of exposure, and this quantity is the mean of the first and
last observations. Supposing the letter i to represent the
augmentation of temperature by five minutes' exposure to
the sun, and that t and t' represent the reductions of temperature observed before and after, then the whole force
of the sun, which we may call T, would be thus expressed:




INFLUENCE OF THE EARTH'S ATMOSPHERE.   433
T=R+ -2The surface on which the sun's rays here fall is known;
the quantity of mercury within the cylinder is also known;
hence we can express the effect of the sun's heat upon a
given area, by stating that it is competent, in five minutes,
to raise so much mercury, or so much water, so many degrees in temperature. Water indeed, instead of mercury,
was used in M. Pouillet's pyrheliometer.
The observations were made at different hours of the
day, and, hence, through different thicknesses of the earth's
atmosphere; augmenting from the minimum thickness at
noon, up ot the maximum at 6 P. M., which was the time of
the latest observation. It was found that the solar energy
diminished according to a certain law, as the thickness
of the air crossed by the sunbeams increased; and from
this law M. Pouillet was enabled to infer what the atmospheric absorption of a beam would be, if directed downwards to his instrument from the zenith. This he found
to be 25 per cent. Doubtless, this absorption would be
chiefly exerted upon the longer undulations emitted by the
sun, the aqueous vapour of our air, and not the air
itself, being the main agent of absorption. Taking into
account the whole terrestrial hemisphere turned towards
the sun, the amount intercepted by the atmospheric envelope is four-tenths of the entire radiation in the direction
of the earth. Thus, were the atmosphere removed, the
illuminated hemisphere of the earth would receive nearly
twice the amount of heat from the sun that now reaches
it. The total amount of solar heat received by the earth
in a year, if distributed uniformly over the earth's surface,
would be sufficient to liquefy a layer of ice 100 feet thick,
and covering the whole earth.
Knowing thus the annual receipt of the earth, we can
calculate the entire quantity of heat emitted by the sun in
19




434                 LECTURE XII.
a year. Conceive a hollow sphere to surround the sun, its
centre being the sun's centre, and its surface at the distance of the earth from the sun. The section of the earth
cut by this surface, is to the whole area of the hollow
sphere, as 1: 2,300,000,000; hence, the quantity of solar
heat intercepted by the earth is only        - 30   of the
total radiation.
The heat emitted by the sun, if used to melt a stratum
of ice applied to the sun's surface, would liquefy the
ice at the rate of 2,400 feet an hour. It would boil, per
hour, 700,000 millions of cubic miles of ice-cold water.
Expressed in another form, the heat given out by the sun,
per hour, is equal to that which would be generated by the
combustion of a layer of solid coal, 10 feet thick, entirely
surrounding the sun; hence, the heat emitted in a year is
equal to that which would be produced by the combustion
of a layer of coal 17 miles in thickness.
These are the results of direct measurement; and
should greater accuracy be conferred on them by future
determinations, it will not deprive them of their astounding character. And this expenditure has been going on
for ages, without our being able, in historic times, to detect the loss. When the tolling of a bell is heard at a distance, the sound of each stroke soon sinks, the sonorous
vibrations are quickly wasted, and renewed strokes are
necessary to maintain the sound. Like the bell,
Die Sonne tint nach alter Weise.
But how is its tone sustained? How is the perennial
loss of the sun made good? We are apt to overlook the
wonderful in the common. Possibly to many of us —and
even to some of the most enlightened among us-the sun
appears as a fire, differing from our terrestrial fires only in
the magnitude and intensity of its combustion. But what
is the burning matter which can thus maintain itself? All




MAINTENANCE OF SOLAR POWER.          435
that we know of cosmical phenomena declares our brotherhood with the sun,-affirms that the same constituents
enter into the composition of his mass as those already
known to chemistry. But no earthly substance with which
we are acquainted-no substance which the fall of meteors
has landed on the earth-would be at all competent to
maintain the sun's combustion. The chemical energy of
such substances would be too weak, and their dissipation
would be too speedy. Were the sun a solid block of coal,
and were it allowed a sufficient supply of oyygen, to enable
it to burn at the rate necessary to produce the observed
emission, it would be utterly consumed in 5,000 years.
On the other hand, to imagine it a body originally endowed with a store of heat-a hot globe now coolingnecessitates the ascription to it of qualities, wholly different from those possessed by terrestrial matter. If we knew
the specific heat of the sun, we could calculate its rate of
cooling. Assuming this to be the same as that of waterthe terrestrial substance which possesses the highest specific heat —at its present rate of emission, the entire mass
of the sun would cool down 15,000~ Faht. in 5,000 years.
In short, if the sun be formed of matter like our own,
some means must exist of restoring to him his wasted
power.
The facts are so extraordinary, that the soberest hypothesis regarding them must appear wild. The sun we
know rotates upon his axis; he turns like a wheel once in
about 25 days: can it be the friction of the periphery
of this wheel against something in surrounding space
which produces the light and heat? Such a notion has
been entertained. But what forms the brake, and by what
agency is it held, while it rubs against the sun? The action is inconceivable; but, granting the existence of the
brake, we can calculate the total amount of heat which the
sun could generate by such friction. We khow his mass,




436                  LECTURE XII.
we know his time of rotation; we know the mechanical
equivalent of heat; and from these data we deduce, with
certainty, that the entire force of rotation, if converted into
heat, would cover more than one, but less than two centuries of emission.* There is no hypothesis involved in
this calculation.
There is another theory, which, however bold it may, at
first sight, appear, deserves our earnest attention. I have
already referred to it as the Meteoric Theory of the sun's
heat.  Solar space is peopled with ponderable objects:
Kepler's celebrated statement that' there are more comets
in the heavens than fish in the ocean,' refers to the fact
that a small portion only of the total number of comets belonging to our system, are seen from  the earth.  But
besides comets, and planets, and moons, a numerous class
of bodies belong to our system,-asteroids, which, from
their smallness, might be regarded as cosmical atoms. Like
the planets and the comets these smaller bodies obey the
law of gravity, and revolve on elliptic orbits round the sun;
and it is they, when they come within the earth's atmosphere, that, fired by friction, appear to us as meteors and
falling stars.
On a bright night, 20 minutes rarely pass at any part
of the earth's surface without the appearance of at least
one meteor. At certain times (the 12th of August and the
14th of November) they appear in enormous numbers.
During nine hours of observation in Boston, when they were
described as falling as thick as snowflakes, 240,000 meteors
were calculated to have been observed. The number falling in a year might, perhaps, be estimated at hundreds or
thousands of millions, and even these would constitute but
a small portion of the total crowd of asteroids that circulate round the sun. From  the phenomena of light and
* Meyer Dynamik des Hiimmels, p. 10.




THE ZODIACAL LIGHT.              437
heat, and by the direct observations of Encke on his comet,
we learn that the universe is filled by a resisting medium,
through the friction of which all the masses of our system
are drawn gradually towards the sun. And though the
larger planets show, in historic times, no diminution of
their periods of revolution, this may not hold good for the
smaller bodies. In the time required for the mean distance
of the earth from the sun to alter a single yard, a small
asteroid may have approached thousands of miles nearer to
our central luminary.
Following up these reflections we should infer, that
while this immeasurable stream of ponderable matter rolls
unceasingly towards the sun, it must augment in density
as it approaches its centre of convergence. And here the
conjecture naturally rises, that that weak nebulous light,
of vast dimensions, which embraces the sun-the Zodiacal
Light —may owe its existence to these crowded meteoric
masses. However this may be, it is at least proved that
this luminous phenomenon arises from matter which circulates in obedience to planetary laws; the entire mass
constituting the zodiacal light must be constantly approaching, and incessantly raining its substance down
upon the sun.
We observe the fall of an apple and investigate the law
which rules its motion. In the place of the earth we set
the sun, and in the place of the apple we set the earth, and
thus possess ourselves of the key to the mechanics of the
heavens. We now know the connection between height
of fall, velocity, and heat of the surface of the earth. In
the place of the earth let us set the sun, with 300,000 times
the earth's mass, and, instead of a fall of a few feet, let us
take cosmical elevations; we thus obtain a means of generating heat which transcends all terrestrial power.
It is easy to calculate both the maximum and the minimum velocity, imparted by the sun's attraction to an as



438                 LECTUIRE XII,
teroid circulating round him; the maximum is generated
when the body approaches the sun from an infinite distance; the entire pull of the sun being then expended upon
it; the minimum is that velocity which would barely enable the body to revolve round the sun close to his surface.
The final velocity of the former, just before striking the
sun, would be 390 miles a second, that of the latter 276
miles a second. The asteroid, on striking the sun with the
former velocity, would develope more than 9,000 times the
heat generated by the combustion of an equal asteroid of
solid coal; while the shock, in the latter case, would generate heat equal to that of the combustion of upwards of
4,000 such asteroids. It matters not, therefore, whether
the substances falling into the sun be combustible or not;
their being combustible would not add sensibly to the tremendous heat produced by their mechanical collision.
Here then we have an agency competent to restore his
lost energy to the sun, and to maintain a temperature at
his surface which transcends all terrestrial combustion.
The very quality of the solar rays-their incomparable
penetrative power-enables us to infer that the temperature of their origin must be enormous; but in the fall of
asteroids we find the means of producing such a temperature. It may be contended that this showering down of
matter must be accompanied by the growth of the sun in
size; it is so; but the quantity necessary to produce the
observed calorific emission, even if accumulated for 4,000
years, would defeat the scrutiny of our best instruments.
If the earth struck the sun it would utterly vanish from
perception, but the heat developed by its shock would
cover the expenditure of the sun for a century.
To the earth itself apply considerations similar to those
which we have applied to the sun. Newton's theory of
gravitation, which enables us, from the present form of
the earth, to deduce its original state of aggregation, re



THE TIDES AND THE EARTH IS ROTATION.    439
veals to us, at the same time, a source of heat powerful
enough to bring about the fluid state-powerful enough to
fuse even worlds. It teaches us to regard the molten condition of a planet as resulting from the mechanical union
of cosmical masses, and thus reduces to the same homogeneous process, the heat stored up in the body of the
earth, and the heat emitted by the sun.
Without doubt the whole surface of the sun displays an
unbroken ocean of fiery fluid matter. On this ocean rests
an atmosphere of glowing gas-a flame atmosphere, or
photosphere. But gaseous substances, when compared with
solid ones, emit, even when their temperature is very high,
only a feeble and transparent light. Hence it is probable
that the dazzling white light of the sun comes through the
atmosphere, from the more solid portions of the surface.*
There is one other consideration connected with the
permanence of our present terrestrial conditions, which is
well worthy of our attention. Standing upon one of the
London bridges, we observe the current of the Thames reversed, and the water poured upwards twice a-day. The
water thus moved rubs against the river's bed and sides,
and heat is the consequence of this friction. The heat thus
generated is, in part, radiated into space, and there lost, as
far as the earth is concerned. What is it that supplies
this incessant loss?  The earth's rotation. Let us look a
little more closely at this matter. Imagine the moon fixed,
and the earth turning like a wheel from west to east in its
diurnal rotation. A mountain on the earth's surface, on
approaching the moon's meridian, is, as it were, laid hold
of by the moon; forms a kind of'handle by which the
earth is pulled more quickly round. But when the meridian
is passed the pull of the moon on the mountain would be
* I am quoting here from Mayer, but this is the exact view now entertained by Kirchhof. We see the solid or liquid mass of the sun through
his photosphere.




440                  LECTURE XII.
in. the opposite direction; it now tends to diminish the
velocity of rotation as much as it previously augmented it;
and thus the action of all fixed bodies on the earth's surface is neutralised.
But suppose the mountain to lie always to the east of
the moon's meridian, the pull then would be always exerted against the earth's rotation, the velocity of which would
be diminished in a degree corresponding to the strength of
the pull.  The tidal wave occupies this position-it lies always to the east of the moon's meridian; the waters of the
ocean are, in part, dragged as a brake along the surface of
the earth, and as a brake they must diminish the velocity
of the earth's rotation. The diminution, though inevitable,
is, however, too small to make itself felt within the period
over which observations on the subject extend. Supposing, then, that we turn a mill by the action of the tide, and
produce heat by the friction of the millstones; that heat
has an origin totally different from the heat produced by
another pair of millstones which are turned by a mountain
stream. The former is produced at the expense of the
earth's rotation; the latter at the expense of the sun's ra.
diation, which lifted the millstream to its source.*
Such is an outline of the Meteoric Theory of the sun's
heat, as extracted from Mayer's Essay on Celestial Dynamics. I have held closely to his statements, and in most
cases simply translated his words. But the sketch conveys
no adequate idea of the firmness and consistency twith
which he has applied his principles. He deals with true
causes; and the only question that can affect his theory re.
fers to the quantity of action which he has ascribed to these
causes. I do not pledge myself to this theory, nor do I ask
you to accept it as demonstrated; still it would be a great
mistake to regard it as chimerical. It is a noble specula* Dynamik des Himmels. p. 38, &c.




DYNAMIC  RADIATION.                      441
tion; and depend upon it, the true theory, if this, or some
form  of it, be not the true one, will not appear less wild or
less astounding.*
Mayer published his Essay in  1848; five years afterwards Mr. Waterston  sketched, independently, a similar
theory, at the Hull Meeting of the British Association.
The Transactions of the Royal Society of Edinburgh for
1854 contain an extremely beautiful memoir, by Professor
William  Thomson, in which Mr. Waterston's sketch is developed. He considers that the meteors which are to furnish stores of energy for our future sunlight, lie principally
within the earth's orbit, and that we see them  there, as the
Zodiacal Light,' an illuminated shower, or rather tornado,
of stones' (Herschel, ~ 897).  Thus he points to the precise
source of power previously indicated by Mayer.'In conclusion, then,' writes Professor Thomson,' the source of
energy from  which  solar heat is derived is undoubtedly
meteoric... The principal source —perhaps the sole appreciable efficient source-is in bodies circulating round the
* While preparing these sheets finally for press, I had occasion to look
once more into the writings of Mayer, and the effect was a revival of the
interest with which I first read them. Dr. Mayer was a working physician
in the little German town of Heilbronn, who, in 1840, made the observation
that the venous blood of a feverish patient in the tropics was redder than
in more northern latitudes. Starting from this fact, while engaged in the
duties of a laborious profession, and apparently without a single kindred
spirit to support and animate him, Mayer raised his mind to the level
indicated by the references made to his works, throughout this book. In
1842 he published his first memoir' On the Forces of Inorganic Nature;'
in 1845, his' Organic Motion' was published; and in 1848, his' Celestial
Dynamics' appeared. After this, his overtasked brain gave way, and a
cloud settled on the intellect which had accomplished so much. The shade
however, was but temporary, and Dr. Mayer is now restored. I have never
seen him, nor has a line of correspondence ever passed between us.
Modestly and noiselessly he has done his work; and having spoken of his
merits, as accident made it my duty to speak, I confidently leave to history
the care of his fame.
19*




442                     LECTURE X1I.
sun at present inside the earth's orbit, and probably seen
in the sunlight by us called " Zodiacal Light." The store
of energy for future sunlight is at present partly dynamical
-that of the motions of these bodies round the sun; and
partly potential-that of their gravitation towards the sun.
This latter is gradually being spent, half against the resisting medium, and half in causing a continuous increase of
the former. Each meteor thus goes on moving faster and
faster, and getting nearer and nearer the centre, until some
time, very suddenly, it gets so much entangled in the solar
atmosphere as to begin to lose velocity. In a few seconds
more it is at rest on the sun's surface, and the energy given
up is vibrated across the district where it was gathered
during so many ages, ultimately to penetrate, as light, the
remotest regions of space.'
From the tables published by Prof. Thomson I extract
the following interesting data; firstly, with reference to
the amount of heat equivalent to the rotation of the sun
and planets round their axes; the amount, that is, which
would be generated, supposing a brake applied at the surfaces of the sun and planets, until the motion of rotation
was entirely  stopped:  secondly, with reference to  the
amount of heat due to the sun's gravitation-the heat,
that is, which would be developed by each of the planets
in falling into the sun. The quantity of heat is expressed in
terms of the time during which it would cover the solar
emission.
Heat of Gravitation, equal to Solar  Heat of Rotation, equal to Solar
emission for a period of      emission for a period of
Sun....   116 years  6 days
Mercury.   6 years 214 days.             15
Venus..  83,,  227,,.              99,
Earth..  94,,  303,,.              81
Mars..  12,,  252,,.               7 
Jupiter.     32240,,..    14,, 144,,
Saturn.. 9650,,..     2,, 127,,
Uranus.. 1610,,..            71,,
Neptune. 1890,,




THEORY OF HELMHIOLTZ.             443
The heat of rotation of the sun and planets, taken all
together, would cover the solar emission for 134 years;
while the heat of gravitation (that produced by falling into
the sun) would cover the emission for 45,589 years. There
is nothing hypothetical in these results; they follow directly
and necessarily from  the application of the mechanical
equivalent of heat to cosmical masses.
Helmholtz has shown that if the solar system has ever
been a nebulous mass of extreme tenuity, the mechanical
force equivalent to the mutual gravitation of the particles
of such a mass would be 454 times the quantity of mechanical force which we now possess in our system; y5,ths of
the gravitating tendency has been already satisfied and
wasted as heat. The 4T5th that remains to us would, however, if converted into heat, raise the temperature of a mass
of water, equal to the sun and planets in weight, 28 millions of degrees Centigrade. The heat of the lime light, it
may be remarked, is estimated at 2,000~ C.; of a temperature of 28,000,000~ C. we can therefore form no conception. If our entire system were pure coal, by the combustion of the whole of it only S-~Lth of the above enormous amount of heat would be generated.'But,' continues Helmholtz,'though the store of our
planetary system is so immense as not to be sensibly diminished by the incessant emission which has gone on during
the period of man's history, and though the time which
must elapse before a sensible change in the condition of
our planetary system can occur is totally incapable of
measurement, the inexorable laws of mechanics show that
this store, which can only suffer loss, and not gain, must
finally be exhausted. Shall we terrify ourselves by this
thought? Men are in the habit of measuring the greatness
of the universe, and the wisdom displayed in it, by the duration and the profit which it promises to their own race;
but the past history of the earth shows the insignificance of




444                  LECTURE XII.
the interval, during which man has had his dwelling here.
What the museums of Europe show us of the remains of
Egypt and Assyria we gaze upon with silent wonder, and
despair of being able to carry back our thoughts to a period
so remote. Still the human race must have existed and
multiplied for ages, before the pyramids could have been
erected. We estimate the duration of human history-at
6,000 years; but vast as this time may appear to us, what
is it in comparison with the period during which the earth
bore successive series of rank plants and mighty animals,
but no men? *  Periods, during which, in our own neighbourhood (K6nigsberg), the amber tree bloomed, and
dropped its costly gum on the earth and in the sea; when
in Europe and North America groves of tropical palms
flourished, in which gigantic lizards, and, after them, elephants, whose mighty remains are still buried in the earth,
found a home. Different geologists, proceeding from different premises, have sought to estimate the length of the
above period, and they set it down from one to nine millions of years. The time during which the earth has generated organic beings is again small, compared with the
ages, during which the world was a mass of molten rocks.
The experiments of Bischof upon basalt show, that for- our
globe to cool down from 2,000~ to 2000 Centigrade, would
require 350 millions of years. And with regard to the period during which the first nebulous masses condensed, so
as to form our planetary system, conjecture must entirely
cease. The history of man, therefore, is but a minute ripple in the infinite ocean of time. For a much longer period
than that during which he has already occupied this world,
the existence of a state of inorganic nature, favourable to
man's continuance, seems to be secured, so that for our* The absence of men may be doubted. See the conclusion of Lubbock's
article on the Lake Habitations of Switzerland, in the Natural IHistory
Review.




RELATION- OF THE SUN TO LIFE.         445
selves, and for long generations after us, we have nothing
to fear. But the same forces of air and water, and of the
volcanic interior, which produced former geologic revolutions, and buried one series of living forms after another,
still act upon the earth's crust. They, rather than those
distant cosmical changes of which we have spoken, will
end the human race; and, perhaps, compel us to make way
for new and more complete forms of life, as the lizard and
the mammoth have given way to us and our contemporaries.'
Grand, however, and marvellous as are those questions
regarding the physical constitution bf the sun, they are but
a portion of the wonders connected with our luminary.
His relationship to life is yet to be referred to. The earth's
atmosphere contains carbonic acid, and the earth's surface
bears living plants; the former is the nutriment of the latter. The plant apparently seizes the combined carbon and
oxygen; tears them asunder, storing up the carbon and
letting the oxygen go free. By no special force, different
in quality from other forces, do plants exercise this power,
-the real magician here is the sun. We have seen in former lectures (see Lecture V.) how heat is consumed in
forcing asunder the atoms and molecules of solids and liquids, converting itself into potential energy, which reappeared as heat, when the attractions of the separated
atoms were again allowed to come into play. Precisely
the same considerations which we then applied to heat we
have now to apply to light; for it is at the expense of the
solar light that the decomposition of the carbonic acid is
effected. Without the sun the reduction cannot take place,
and an amount of sunlight is consumed exactly equivalent
to the molecular work accomplished. Thus trees are
formed, thus the meadows grow, thus the flowers bloom.
Let the solar rays fall upon a surface of sand, the sand is
heated and finally radiates away as much as it receives; let




446                   LECTURE xT.
the same rays fall upon a forest, the quantity of heat given
back is less than that received, for the energy of a portion
of the sunbeams is invested in the building of the trees.*
I have here a bundle of cotton, which I ignite; it bursts
into flame and yields a definite amount of heat; precisely
that amount of heat was abstracted from the sun, in order
to form that bit of cotton. This is a representative case;
— every tree, plant, and flower, grows and flourishes by
the grace and bounty of the sun.
But we cannot stop at vegetable life; for this is the
source, mediate or immediate, of all animal life. In the
animal body vegetable substances are brought again into
contact with their beloved oxygen, and they burn within
us, as a fire burns in a grate. This is the source of all animal power; and the forces in play are the same, in kind,
as those which operate in inorganic nature. In the plant
the clock is wound up, in the animal it runs down. In the
plant the atoms are separated, in the animal they re-combine. And as surely as the force which moves a clock's
hands is derived from the arm which winds up the clock, so
surely is all terrestrial power drawn from the sun. Leaving out of account the eruptions of volcanoes, and the ebb
and flow of the tides, every mechanical action on the
earth's surface, every manifestation of power, organic and
inorganic, vital and physical, is produced by the sun.t His
warmth keeps the sea liquid, and the atmosphere a gas,
and all the storms which agitate both are blown by the mechanical force of the sun. He lifts the rivers and the glaciers up to the mountains; and thus the cataract and the
avalanche shoot with an energy derived immediately from
him. Thunder and lightning are also his transmuted
* Mayer' Die organisehe Bewegung,' p. 39.
F The germ, and much more than the germ, of what is here stated is
to be found in a paragraph in Sir John Herschel's Outlines of Astronomy,
published in 1833.




THE SUN THE SOURCE OF LIFE AND MOTION.  447
strength.  Every fire that burns and every flame that
glows dispenses light and heat which originally belonged
to the sun. In these days, unhappily, the news of battle is
familiar to us, but every shock, and every charge, is an application, or misapplication, of the mechanical force of the
sun. He blows the trumpet, he urges the projectile, he
bursts the bomb. And remember, this is not poetry, but
rigid mechanical truth. He rears, as I have said, the whole
vegetable world, and through it the animal; the lilies of
the field are his workmanship, the verdure of the meadows,
and the cattle upon a thousand hills. HIe forms the muscle,
he urges the blood, he builds the brain. His fleetness is in
the lion's foot; he springs in the panther, he soars in the
eagle, he slides in the snake. He builds the forest and
hews it down, the power which raised the tree, and which
wields the axe, being one and the same. The clover
sprouts and blossoms, and the scythe of the mower
swings, by the operation of the same force. The sun digs
the ore from our mines, he rolls the iron; he rivets the
plates, he boils the water; he draws the train. He not
only grows the cotton, but he spins the fibre and weaves
the web. There is not a hammer raised, a wheel turned,
or a shuttle thrown, that is not raised, and turned, and
thrown by the sun. His energy is poured freely into space,
but our world is a halting place where this energy is conditioned. Here the Proteus works his spells; the selfsame essence takes a million shapes and hues, and finally
dissolves into its primitive and almost formless form. The
sun comes to us as heat; he quits us as heat; and between
his entrance and departure the multiform powers of our
globe appear. They are all special forms of solar powerthe moulds into which his strength is temporarily poured,
in passing from its source through infinitude.
Presented rightly to the mind, the discoveries and gen.
eralizations of modern science constitute a poem more sub.




448                  LECTURE XII.
lime than has ever yet been addressed to the intellect and
imagination of man. The natural philosopher of to-day
may-dwell amid conceptions, which beggar those of Milton. Se great and grand are they, that in the contemplation of them, a certain force of character is requisite to
preserve us from bewilderment. Look at the integrated
energies of our world —the stored power of our coalfields; our winds and rivers; our fleets, armies, and guns.
What are they?  They are all generated by a portion of
the sun's energy, which does not amount to  3 o o'o-   -- oth
of the whole. This, in fact, is the entire fraction of the
sun's force intercepted by the earth, and, in reality, we
convert but a small fraction of this fraction, into mechanical energy. Multiplying all our powers by millions of millions, we do not reach the sun's expenditure. And still,
notwithstanding this enormous drain, in the lapse of human
history we are unable to detect a diminution of his store.
Measured by our largest terrestrial standards, such a reservoir of power is infinite; but it is our privilege to rise
above these standards, and to regard the sun himself as a
speck in infinite extension, —a mere drop in the universal
sea. We analyse the space in which he is immersed, and
which is the vehicle of his power. We pass to other systems and other suns, each pouring forth energy like our
own, but still without infringement of the law, which reveals immutability in the midst of change, which recognises incessant transference and conversion, but neither
final gain nor loss. This law generalises the aphorism of
Solomon, that there is nothing new under the sun, by
teaching us to detect everywhere, under its infinite variety
of appearances, the same primeval force. To Nature nothing can be added; from Nature nothing can be taken
away; the sum of her energies is constant, and the utmost
man can do in the pursuit of physical truth, or in the applications of physical knowledge, is to shift the constituents




CONSTANCY OF POWER OF NATURAL FORCES.  449
of the never-varying total, and out of one of them to form
another. The law of conservation rigidly excludes both
creation and annihilation. Waves may change to ripples, and
ripples to waves,-magnitude may be substituted for number, and number for magnitude,-asteroids may aggregate
to suns, suns may resolve themselves into fiorse and faune-,
and florL and faunak melt in air,-the flux of power is eternally the same. It rolls in music through the ages, and all
terrestrial energy,-the manifestations of life, as well as
the display of phenomena, are but the modulations of its
rhythm.




APPENDIX TO LECTURE XII.
FOR various reasons I am anxious that this book should embrace
all that I have written with regard to the relationship of Dr.
Mayer to the Dynamical Theory of Heat. Here, in the first
place, follows an abstract of a Lecture on Force, given at the
Royal Institution on the evening of Friday, June 6, 1862, and
published in the Proceedings of the Institution and in the'Philosophical Magazine.'
ON FORCE.
THE existence of the International Exhibition suggested to our
Honorary Secretary the idea of devoting the'Friday evenings after
Easter of the present year to discourses on the various agencies
on which the material strength of England is based. He wished
to make iron, coal, cotton, and kindred matters, the subjects of
these discourses: opening the series by a discourse on the Great
Exhibition itself; and he wished me to finish the series by a discourse on' Force' in general. For some months I thought over
the subject at intervals, and had devised a plan of dealing with
it; but three weeks ago I was induced to swerve from this plan,
for reasons which shall be made known towards the conclusion of
the discourse.
We all have ideas more or less distinct regarding force; we
know in a general way what muscular force means, and each of
us would less willingly accept a blow from a pugilist than have
his ears boxed by a lady. But these general ideas are not now
sufficient for us; we must learn how to express numerically the
exact mechanical value of the two blows; this is the first point
to be cleared up.
A sphere of lead weighing 1 lb. was suspended at a height
of 16 feet above the theatre floor. It was liberated, and fell by




EFFECT OF VELOCITY.                   451
gravity. That weight required exactly a second to fall to the
earth from that elevation, and the instant before it touched the
earth it had a velocity of 32 feet a second. That is to say, if at
that instant the earth were annihilated, and its attraction annulled, the weight would proceed through space at the uniform velocity of 32 feet a second.
Suppose that, instead of being pulled downward by gravity,
the weight is cast upward in opposition to the force of gravitywith what velocity must it start from the earth's surface in order
to reach a height of 16 feet? With a velocity of 32 feet a second.
This velocity imparted to the weight by the human arm, or by
any other mechanical means, would carry the weight up to the
precise height from which it had fallen.
Now the lifting of the weight may be regarded as so much
mechanical work. I might place a ladder against the wall, and
carry the weight up to a height of 16 feet; or I might draw it
up to this height by means of a string and pulley; or I might
suddenly jerk it up to a height of 16 feet. The amount of work
Clone in all these cases, as far as the raising of the weight is concerned, would be absolutely the same. The absolute amount of
work done depends solely upon two things: first of all, on the
quantity of matter that is lifted: and secondly, on the height to
which it is lifted. If you call the quantity or mass of matter m,
and the height through which it is lifted A, then the product of
m into A, or M A, expresses the amount of work clone.
Supposing now, that instead of imparting a velocity of 32
feet a second to the weight we impart twice this speed, or 64
feet a second. To what height will the weight rise? You might
be disposed to answer,' To twice the height;' but this would be
quite incorrect. But theory and experiment inform  us that the
weight would rise to four times the height; instead of twice 16,
or 382 feet, it would reach four times 16 or 64 feet. So also if we
treble the starting velocity the weight would reach nine times
the height; if we quadruple the speed at starting, we attain sixteen times the height. Thus, with a velocity of 128 feet a second
at starting, the weight would attain an elevation of 256 feet.
Supposing we augment the velocity of starting seven times, we
should raise the weight to 49 times the height, or to an elevation
of 784 feet.




452             APPENDIX TO LECTURE XII.
Now the work done-or, as it is sometimes called, the mechanical effect-as before explained, is proportional to the height, and
as a double velocity gives four times the height, a treble velocity
nine times the height, and so on, it is perfectly plain that the
mechanical effect increases as the square of the velocity.' If the
mass of the body be represented by the letter in, and its velocity
by v', then the mechanical effect would be represented by m v2.
In the case considered, I have supposed the weight to be cast
upward, being opposed in its upward flight by the resistance of
gravity; but the same holds true if I send the projectile into
water, mud, earth, timber, or other resisting material. If, for
example, you double the velocity of a cannon ball, you quadruple
its mechanical effect. Hence the importance of augmenting the
velocity of a projectile, and hence the philosophy of Sir William
Armstrong in using a 50 lb. charge of powder in his recent striking experiments.
The measure then of mechanical effect is the mass of the body
multiplied by the square of its velocity.
In firing a ball against a target the projectile, after collision,
is often found hissing hot. Mr. Fairbairn informs me that in the
experiments at Shoeburyness it is a common thing to see a flash
of light, even in broad day, when the ball strikes the target.
And if I examine my lead weight after it has fallen from a height
I also find it heated. Now here experiment and reasoning lead
us to the remarkable law that the amount of heat generated, like
the mechanical effect, is proportional to the product of the mass
into the square of the velocity. Double your mass, other things
being equal, and you double your amount of heat; double your
velocity, other things remaining equal, and you quadruple your
amount of heat,. Here then we have common mechanical motion
destroyed and heat produced.
I take this violin bow and draw it across this string. You
hear the sound. That sound is due to motion imparted to the air,
and to produce that motion a certain portion of the muscular
force of my arm must be expended. We may here correctly say,
that the mechanical force of my arm is converted into music.
And in a similar way we say that the impeded motion of our descending weight, or of the arrested cannon ball, is converted into
heat. The mode of motion changes, but it still continues motion;




CHEMICAL AFFINITY OF ATOMS.              453
the motion of the mass is converted into a motion of the atoms of the
mass; and these small motions, communicated to the nerves, produce the sensation which we call heat. We, moreover, know the
amount of heat which a given amount of mechanical force can
develop. Our lead ball, for example, in falling to the earth generated a quantity of heat sufficient to raise the temperature of its
own mass three-fifths of a Fahrenheit degree. It reached the
earth with a velocity of 32 feet a second, and forty times this velocity would be a small one for a rifle bullet; multiplying 3ths
by the square of 40, we find that the amount of heat developed by
collision with the target would, if wholly concentrated in the lead,
raise its temperature 960~. This would be more than sufficient
to fuse the lead. In reality, however, the heat developed is divided between the lead and the body against which it strikes;
nevertheless, it would be worth while to pay attention to this
point, and to ascertain whether rifle bullets do not, under some
circumstances, show signs of fusion.
From the motion of sensible masses, by gravity and other
means, the speaker passed to the motion of atoms towards each
other by chemical affinity. A collodion balloon filled with a
mixture of chlorine and hydrogen was hung in the focus of a
parabolic mirror, and in the focus of a second mirror, 20 ft. distant, a strong electric light was suddenly generated; the instant
the light fell upon the balloon, the atoms within it fell together
with explosion, and hydro-chloric acid was the result.  The
burning of charcoal in oxygen was an old experiment, but it had
now a significance beyond what it used to have; we now regard
the act of combination on the part of the atoms of oxygen and
coal exactly as we regard the clashing of a falling weight against
the earth. Ancd the heat produced in both cases is referable to
a common cause. This glowing diamond, which burns in oxygen
as a star of white light, glows and burns in consequence of the
falling of the atoms of oxygen against it. And could we measure the velocity of the atoms when they clash, and could we find
their number and weight, multiplying the weight of each atom
by the square of its velocity, and adding all together, we should
get a number representing the exact amount of heat developed
by the union of the oxygen and carbon.
Thus far we have regarded the heat developed by the clash



454             APPENDIX TO LECTURE XII.
ing of sensible masses and of atoms. Work is expended in giving
motion to these atoms or masses, and heat is developed. But
we reverse this process daily, and by the expenditure of heat execute work. We can raise a weight by heat; and in this agent
we possess an enormous store of mechanical power. This pound
of coal, which I hold in my hand, produces by its combination
with oxygen an amount of heat which, if mechanically applied,
would suffice to raise a weight of 100 lbs. to a height of 20 miles
above the earth's surface.  Conversely, 100 lbs. falling from a
height of 20 miles, and striking against the earth, would generate an amount of heat equal to that developed by the combustion
of a pound of coal. Wherever work is done by heat, heat disappears. A gun which fires a ball is less heated than one which
fires blank cartridge. The quantity of heat communicated to the
boiler of a working steam-engine is greater than that which could be
obtained from the re-condensation of the steam after it had done
its work; and the amount of work performed is the exact equivalent of the amount of heat lost. Mr. Smyth informed us in his
interesting discourse, that we dig annually 84 millions of tons of
coal from our pits. The amount of mechanical force represented
by this quantity of coal seems perfectly fabulous. The combustion
of a single pound of coal, supposing it to take place in a minute,
would be equivalent to the work of 300 horses; and if we suppose 108 millions of horses working day and night, with unimpaired strength, for a year, their united energies would enable
them to perform an amount of work just equivalent to that
which the annual produce of our coal-fields would be able to
accomplish.
Comparing the energy of the force with which oxygen and
carbon unite together, with ordinary gravity, the chemical affinity
seems almost infinite. But let us give gravity fair play; let us
permit it to act throughout its entire range. Place a body at
such a distance from the earth that the attraction of the earth is
barely sensible, and let it fall to the earth from this distance. It
would reach the earth with a final velocity of 36,747 feet in a
second, and on collision with the earth the body would generate
about twice the amount of heat generated by the combustion of
an equal weight of coal. We have stated that by falling through
a space of 16 feet our lead bullet would be heated three-fifths of




THE HEAT EMITTED BY THE SUN.              455
a degree; but a body falling from an infinite distance has already
used up 1,299,999 parts out of 1,300,000 of the earth's pulling
power, when it has arrived within 16 feet of the surface; on this
space only Tl WS-5-tths of the whole force is exerted.
Let us now turn our thoughts for a moment from the earth
towards the sun. The researches of Sir J. Herschel and M.
Pouillet have informed us of the annual expenditure of the sun
as regards heat, and by an easy calculation we ascertain the precise amount of the expenditure which falls to the share of our
planet. Out of 2,300 million parts of light and heat the earth
receives one. The whole heat emitted by the sun in a minute
would be competent to boil 12,000 millions of cubic miles of icecold water. How is this enormous loss made good? Whence is
the sun's heat derived, and by what means is it maintained?
No combustion, no chemical affinity with which we are acquainted
would be competent to produce the temperature of the sun's surface.  Besides, were the sun a burning body merely, its light
and heat would assuredly speedily come to an end. Supposing
it to be a solid globe of coal, its combustion would only cover
4,600 years of expenditure. In this short time it would burn
itself out. What agency, then, can produce the temperature and
maintain the outlay? We have already regarded the case of a
body falling from a great distance towards the earth, and found
that the heat generated by its collision would be twice that produced by the combustion of an equal weight of coal. How much
greater must be the heat developed by a body falling towards the
sun! The maximum velocity with which a body can strike the
earth is about 7 miles in a second; the maximum velocity with
which it can strike the sun is 390 miles in a second. And as the
heat developed by the collision is proportional to the square of
the velocity destroyed, an asteroid falling into the sun with the
above velocity would generate about 10,000 times the quantity
of heat generated by the combustion of an asteroid of coal of the
same weight.
Have we any reason to believe that such bodies exist in space,
and that they may be raining down upon the sun? The meteorites flashing through our air are small planetary bodies,
drawn by the earth's attraction, and entering our atmosphere
with planetary velocity. By friction against the air they are




456            APPENDIX TO LECTURE XII.
raised to incandescence and caused to emit light and heat.* At
certain seasons of the year they shower down upon us in great
numbers. In Boston 240,000 of them were observed in nine
hours. There is no reason to suppose that the planetary system
is limited to'vast masses of enormous weight;' there is every
reason to believe that space is stocked with smaller masses,
which obey the same laws as the large ones. That lenticular envelope which surrounds the sun, and which is known to astronomers as the Zodiacal light, is probably a crowd of meteors; and
moving as they do in a resisting medium they must continually
approach the sun. Falling into it, they would be competent to
produce the heat observed, and this would constitute a source
from which the annual loss of heat would be made good. The
sun, according to this hypothesis, would be continually growing
larger; but how much larger? Were our moon to fall into the
sun it would develop an amount of heat sufficient to cover one or
two years' loss; and were our earth to fall into the sun a century's loss would be made good. Still, our moon and our earth,
if distributed over the surface of the sun, would utterly vanish
from perception. Indeed, the quantity of matter competent to
produce the necessary effect would, during the range of history,
produce no appreciable augmentation in the sun's magnitude.
The augmentation of the sun's attractive force would be more
appreciable. However this hypothesis may fare as a representant
of what is going on in nature, it certainly shows how a sun might
be formed and maintained by the application of known thermodynamic principles.
Our earth moves in its orbit with a velocity of 68,040 miles an
hour. Were this motion stopped, an amount of heat would be
developed sufficient to raise the temperature of a globe of lead
of the same size as the eartfh 384,000 degrees of the Centigrade
thermometer. It has been prophesied that'the elements shall
melt with fervent heat.' The earth's own motion embraces the
conditions of fuilfillment; stop that motion, and the greater part,
if not the whole, of her mass would be reduced to vapour. If the
earth fell into the sun, the amount of heat developed by the shock
* To Mr. Joule, as stated in Lecture I., we owe this hypothesis.




IEAT DEVELOPED BY THE TIDAL WAVE.             457
would be equal to that developed by the combustion of 6,435
earths of solid coal.
There is one other consideration connected with the permanence of our present terrestrial conditions which is well worthy
of our attention. Standing upon one of the London bridges, we
observe the current of the Thames reversed, and the water poured
upward twice a-day. The water thus moved rubs against the
river's bed and sides, and heat is the consequence of this friction
The heat thus generated is in part radiated into space, and then
lost, as far as the earth is concerned. What is it that supplies
this incessant loss? The earth's rotation. Let us look a little
more closely at the matter. Imagine the moon fixed and the
earth turning like a wheel from west to east in its diurnal rotation. Suppose a high mountain on the earth's surface; on approaching the moon's meridian, that mountain is, as it were, laid
hold of by the moon, and forms a kind of handle by which the
earth is pulled more quickly round. But when the meridian is
passed, the pull of the moon on the mountain would be in the
opposite direction; it now tends to diminish the velocity of rotation as much as it previously augmented it; and thus the action
of all fixed bodies on the earth's surface is neutralised. But suppose the mountain to lie always to the east of the moon's meridian: the pull then would be always exerted against the earth's
rotation, the velocity of which would be diminished in a degree
corresponding to the strength of the pull. The tidal wave occupies
this position —it lies always to the east of the moon's meridian,
and thus the waters of the ocean are in part dragged as a brake
along the surface of the earth; and as a brake they must diminish
the velocity of the earth's rotation. The diminution, though inevitable, is, however, too small to make itself felt within the
period over which observations on the subject extend. Supposing then that we turn a mill by the action of the tide, and produce heat by the friction of the millstones; that heat has an
origin totally different from the heat produced by another mill
which is turned by a mountain stream. The former is produced
at the expense of the earth's rotation, the latter at the expense of
the sun's radiation.
The sun, by the act of vaporization, lifts mechanically all the
moisture of our air. It condenses and falls in the form of rain20




458             APPENDIX TO LECTURE XII.
it freezes and falls as snow. In this solid form it is piled upon
the Alpine heights, and furnishes materials for. the glaciers of the
Alps. But the sun again interposes, liberates the solidified liquid,
and permits it to roll by gravity to the sea. The mechanical
force of every river in the world, as it rolls towards the ocean, is
drawn from the heat of the sun. No streamlet glides to a lower
level without having been first lifted to the elevation from which
it springs by the mighty power of the sun. The energy of the
winds is also due entirely to the sun; but there is still another
work which he performs, and his connection with which is not so
obvious. Trees and vegetables grow upon the earth, and when
burned they give rise to heat, and hence to mechanical energy.
Whence is this power derived? You see this oxide of iron, produced by the falling together of the atoms of iron and oxygen;
here also is a transparent gas which you cannot now see-carbonic acid gas-which is formed by the falling together of carbon
and oxygen. These atoms thus in close union resemble our lead
weight while resting on the earth; but I can wind up the weight
and prepare it for another fall, and so these atoms can be wound
up, separated from each other, and thus enabled to repeat the
process of combination. In the building of plants carbonic acid
is the material from which the carbon of the plant is derived;
and the solar beam is the agent which tears the atoms asunder,
setting the oxygen free, and allowing the carbon to aggregate in
woody fibre. Let the solar rays fall upon a surface of sand; the
sand is heated, and finally radiates away as much heat as it receives; let the same beams fall upon a forest, the quantity of heat
given back is less than the forest receives, for the energy of a
portion of the sunbeams is invested in building up the trees in the
manner indicated. Without the sun the reduction of the carbonic acid cannot be effected, and an amount of sunlight is consumed exactly equivalent to the molecular work done. Thus
trees are formed; thus the cotton on which Mr. Bazley discoursed
last Friday is formed. I ignite this cotton, and it flames; the
oxygen again unites with its beloved carbon; but an amount of
heat equal to that which you see produced by its combustion was
sacrificed by the sun to form that bit of cotton.
But we cannot stop at vegetable life, for this is the source,
mediate or immediate, of all animal life. The sun severs the car



POWER OF THE SUN.                  459
bon from its oxygen; the animal consumes the vegetable thus
formed, and in its -arteries a reunion of the severed elements take
place, and produce animal heat. Thus, strictly speaking, the
process of building a vegetable is one of winding up; the process
of building an animal is one of running down. The warmth of
our bodies, and every mechanical energy which we exert, trace
their lineage directly to the sun. The fight of a pair of pugilists,
the motion of an army, or the lifting of his own body up mountain slopes by an Alpine climber, are all cases of mechanical
energy drawn from the sun. Not, therefore, in a poetical, but in
a purely mechanical sense, are we children of the sun. Without
food we should soon oxidize our own bodies. A man weighing
150 lbs. has 64 lbs. of muscle; but these, when dried, reduce themselves to 15 lbs. Doing an ordinary day's work, for 80 days, this
mass of muscle would be wholly oxidized. Special organs which
do more work would be more quickly oxidized: the heart, for
example, if entirely unsustained, would be oxidized in about a
week. Take the amount of heat due to the direct oxidation of a
given amount of food; a less amount of heat is developed by this
food in the working animal frame, and the missing quantity is the
exact equivalent of the mechanical work which the body accomplishes.
I might extend these considerations: the work, indeed, is
done to my hand-but I am warned that I have kept you already
too long. To whom, then, are we indebted for the striking generalisations of this evening's discourse? All that I have brought
before you is the work of a man of whom you have scarcely ever
heard. All that I have brought before you has been taken from
the labors of a German physician, named Mayer. Without external stimulus, and pursuing his profession as a town physician
in Heilbronn, this man was the first to raise the conception of the
interaction of natural forces to clearness in his own mind. And
yet he is scarcely ever heard of in scientific lectures; and even to
scientific men his merits are but partially known. Led by his
own beautiful researches, and quite independent of Mayer, Mr.
Joule published his first Paper on the' Mechanical Value of Heat,'
in 1843; but in 1842 Mayer had actually calculated the mechanical equivalent of heat from data which a man of rare originality
alone could turn to account. From the velocity of sound in air




460            APPENDIX TO LECTURIE XII.
Mayer determined the mechanical equivalent of heat. In 1845
he published his Memoir on' Organic Motion,' and applied the
mechanical Theory of Heat in the most fearless and precise manner to vital processes. He also embraced the other natural agents
in his chain of conservation. In 1853 Mr. Waterston proposed,
independently, the Meteoric Theory of the sun's heat, and in
1854 Professor William Thomson applied his admirable mathematical powers to the development of the theory; but six years
previously the subject had been handled in a masterly manner by
Mayer, and all that I have said on this subject has been derived
from him. When we consider the circumstances of Mayer's life,
and the period at which he wrote, we cannot fail to be struck
with astonishment at what he has accomplished. Here was a
man of genius working in silence, animated solely by a love of
his subject, and arriving at the most important results, some time
in advance of those whose lives were entirely devoted to Natural
Philosophy. It was the accident of bleeding a feverish patient
at Java in 1840 that led Mayer to speculate on these subjects.
He noticed that the venous blood in the tropics was of a much
brighter red than in colder latitudes, and his reasoning on this
fact led him into the laboratory of natural forces, where he has
worked with such signal ability and success. Well, you will
desire to know what has become of this man. His over-tasked
mind gave way-a result felt to be quite possible in his own case
by many a great scientific worker-and he was sent to an asylum.
In a biographical dictionary of his country it is stated that Mayer
died in the asylum: but this is incorrect. He recovered; and,
I believe, is at this moment a cultivator of vineyards in Heilbronn.
While preparing for publication my last course of Lectures on
Heat, I wished to make myself acquainted with all that Mayer
had done in connection with this subject. I accordingly wrote
to two gentlemen who above all others seemed likely to give me
the information which I needed. Both of them are Germans,
and both particularly distinguished in connection with the Dynamical Theory of Heat. Each of them kindly furnished me with
the list of Mayer's publications, and one of them was so friendly
as to order them from a bookseller, and to send them to me.
This friend, in his reply to my first letter regarding Mayer, stated




RESEARCHES OF MAYER.                  461
his belief that I should not find anything very important in
Mayer's writings; but before forwarding the memoirs to me he
read them himself. His letter, accompanying the first of these
papers, contains the following words: —' I must here retract the
statement in my last letter, that you will not find much matter
of importance in Mayer's writings; I am astonished at the multitude of beautiful and correct thoughts which they contain;' and
he goes on to point out various important subjects, in the treatment of which Mayer had anticipated other eminent writers. My
second friend, in whose own publications the name of Mayer repeatedly occurs, and whose papers containing these references
were translated some years ago by myself, was, on the 10th of last
month, unacquainted with the thoughtful and beautiful essay by
Mayer, entitled' Beitrhige zur Dynamik des Himmrels;' and in
1854, when Professor William Thomson developed in so striking
a manner the meteoric theory of the sun's heat, he was certainly
not aware of the existence of that essay, though from a recent
article in' Macmillan's Magazine' I infer that he is now aware
of it. Mayer's physiological writings have been referred to by
physiologists-by Dr. Carpenter, for example-in terms of honourable recognition.  We have hitherto, indeed, obtained fragmentary glimpses of the man partly from physicists, and partly
from physiologists; but his total merit has never yet been recognized, as it assuredly would have been had he chosen a happier
mode of publication. I do not think a greater disservice could
be done to a man of science than to overstate his claims: such
overstatement is sure to recoil to the disadvantage of him in
whose interest it is made. But when Mayer's opportunities,
achievements, and fate, are taken into account, I do not think
that I shall be deeply blamed for attempting to place him in that
honourable position which I believe to be his due.
Here, however, are the titles of Mayer's papers, the perusal
of which will correct any error of judgment into which I may
have fallen regarding their author.'Bemerkungen iiber die
Krdifte der unbeleten Natur,' Liebig's Annalen, 1842, vol. xlii.
p. 231;'Die Organische Bewegung in ihrem Zusammenhange
mit dem Stoff-wechsel;' Heilbronn, 1845;' Beitrdige zur Dynamik des Himmels,' Heilbronn, 1848;'Bemerkungen iiber das
Mechanische Equivalent der Wirme,' Heilbronn, 1851.
J. T.




462            APPENDIX TO LECTURE XII.
With reference to this Lecture, Mr. Joule published the following letter in the August number of the' Philosophical Magazine'NOTE ON THE HISTORY OF THE DYNAMICAL THEORY
OF HEAT.
BY J. P. JOULE, LL.D., F.R.S.
T'o the Editors of the Phi4osophical Magazine and Journal.
GENTLEMEN,-Will you permit me to trouble your readers
with a few remarks on the subject of my friend Professor Tyndall's Lecture at the Royal Institution, reported in your last
Number? In this Lecture he enforces the claims of M. Mayer, a
philosopher whose merit has perhaps beenoverlooked by some of
our English physicists, and unaccountably so by his fellowcountrymen. I myself was only imperfectly acquainted with his
papers, when, in good conscience and with the materials at command, I gave a sketch of the history of the Dynamical Theory
of Heat, in my paper published in the Philosophical Transactions for 1850. M. Mayer's merit consists in having announced,
apparently without knowledge of what had been done before, the
true Theory of Heat. This is no small merit, and I am the last
person who would wish to detract from it. But to give to Mayer,
or indeed to any single individual, the undivided praise of propounding the Dynamical Theory of Heat is manifestly unjust to
the numerous contributors to that *great step in physical science.
Two centuries ago, Locke said that' Heat is a very brisk agitation of the insensible parts'of the object, which produces in us
that sensation from whence we denominate the object hot; so
that what in our sensation is heat, in the object is nothing but
mnotion.' In 1798, Rumford, inquiring into the source of heat developed in the boring of cannon, observed that it was' extremely
difficult, if not quite impossible, to form any distinct idea of anything capable of being excited and communicated, in the manner
the heat was excited and communicated in these experiments, except it be motion.' In 1812, Davy wrote:'The immediate cause
of the phenomena of heat, then, is motion, and the laws of its
communication are precisely the same as the laws of the corn



LETTER OF MR. JOULE.                   463
munication of motion;'* and he confirmed his views by that
original and most interesting experiment in which he melted ice
by friction. t  In 1839, Seguin published a work entitled De
l'Influence des Chemins de Per. He shows that the theory generally adopted would lead to the absurd conclusion that a finite
quantity of heat can produce an indefinite quantity of mechanical
action, and remarks (p. 328),' I1  me parait plus naturel de supposer qu'une certaine quantite de calorique disparalt dans l'acte
mime de la, production de la force ou puissance mecanique, et reciproquement.' At p. 383 he remarks:' La force mecanique qui
apparait pendant l'abaissement de temperature d'un gaz comme
de tout autre corps qui se dilate, est la mesure et la representation
de cette diminution de chaleur.' In p. 389 he gives a Table of the
quantity of mechanical effect produced corresponding to the loss
of temperature of steam  on expanding. From  this it appears
that 1~ Cent. corresponds with 363 kilogrammes raised to. the
height of 1 metre. At p. 403 he states:' Je bornerai la mes reflexions sur un sujet dont chacun saura apprecier l'importance.
Du calorique qui est employe par l'industrie'a produire de la
force, et aux usages domestiques, une faible partie seulement est
utilisee; une autre quantite bien plus considerable, et qui pourrait suffire'a creer d'immenses valeurs et " augmenter d'autant
la richesse nationale, se trouve absolument perdue.' From  the
above extracts it will be seen that a great advance had been made
before Mayer wrote his paper in 1842. Mayer discourses to the
same effect as S6guin, but at greater length, with greater perspicuity, and with more copiousness of illustration. He adopts
the same hypothesis as the latter philosopher, viz.: that the heat
evolved on compressing an elastic fluid is exactly the equivalent
of the compressing force, and thus arrives at the same equivalent,
viz., 365 kilogrammes per 1~ Cent.
It must be remarked that at the time Seguin and Mayer wrote,
there were known no facts to warrant the hypothesis they adopted. There was no reason to assert that the heat evolved by compressing a gas was even approximately the equivalent of a compressing force. This being the case may account for the inatten* Elements of Chemical Philosophy, p. 94.
f My morning Lectures had rendered all this familiar.-J. T.




464             APPENDIX TO LECTURE XII.
tion of the scientific world to these writings. The Dynamical
Theory of Heat certainly was not established by S6guin and
Mayer. To do this required experiment; and I therefore fearlessly assert my right to the position which has been generally accorded to me by my fellow physicists as having been the first to
give a decisive proof of the correctness of this theory.
In saying this I do not wish to claim any monopoly of merit.
Even if Rumford, Mayer, and Seguin had not produced their
works, justice would still compel me to share with Thomson,
Rankine, Helmholtz, Holtzman,* Clausius, and others, whose
labours have not only given developments and applications of the
Dynamical Theory which entitle them to merit as well as their
predecessors in these enquiries, but who have contributed most
essentially in supporting it by new proofs.
Permit me to remark, in conclusion, that I applied the Dynamical Theory to vital processes in 1843; t and that in 1847, in
a popular lecture, published in the' Manchester Courier,' I explained the phenomena of shooting stars, and also stated that the
effect of the earth falling into the sun would be to increase the
temperature of that luminary. I Since that time Thomson, by
his profound investigations, has made the Dynamical Theory of
Heat, as applied to cosmical phenomena, his own.
I sincerely trust that, by the foregoing remarks, I have done
no injustice to Mayer, especially as I grieve to hear that sickness
has removed him (I hope for only a short time) from the science
to which he has contributed with so much ability. The reproduction of some of his papers in the'Philosophical Magazine,'
particularly that' On the Forces of Inorganic Nature,' would, I
am sure, interest many of your readers, and enable them to fully
appreciate his just claims.
I remain, Gentlemen,
Yours respectfully,
J. P. JOULE.
* The name of this philosopher ought to be added to those mentioned
in the Preface as the builders of the Dynamical Theory of Heat.-J. T.
t Phil. Mag. ~ 3. vol. xxiii. p. 442.
$ Ibid. vol. xxxii. p. 350; and Manchester Courier, May 12, 1847.




LETTER TO MR. JOULE.                465
I was in Switzerland when this letter appeared, and immediately after my return I published the following letter to Mr.
Joule. (Phil. Mag. Sept. 1862.)
MY DEAR JOULE,
Ox my return from  Switzerland, two days ago, I became acquainted with the note which you have published in the last
Number of the' Philosophical Magazine.' Would you allow me
to make the following remarks in connection with the subject
of it?
During the spring of the present year I gave, at the Royal
Institution, a Course of Lectures,' On Heat, regarded as a kind
of Motion.' During the early portion of the course I had engaged
a short-hand writer to report the lectures, with a view to their
subsequent publication; and from this gentleman's notes of my
second Lecture I make the following extract, which refers to the
mechanical Theory of Heat:-' It is to Mr. Joule, of Manchester,
that we are almost wholly indebted for the experimental treatment of this subject. With his mind firmly fixed upon a principle, and undismayed by the coolness with which his first labours
appear to have been received, he persisted for years in his attempts to prove the invariability of the relation between heat
and ordinary mechanical force. He placed water in a suitable
vessel, agitated it by paddles moved by measurable forces, and
determined the elevation of temperature; he did the same with
mercury and sperm oil. He also caused disks of cast iron to
rotate against each other, and measured the heat produced by
their friction. He urged water through capillary tubes, and
measured the heat thus generated. The results of his experiments
leave no doubt upon the mind that under all circumstances the
absolute amount of heat produced by the expenditure of a definite
amount of mechanical force is fixed and invariable.' Such has
been my language regarding you; and to it I still adhere. I
trust you find nothing in it which indicates a desire on my part
to question your claim to the honour of being the experimental
demonstrator of the equivalence of heat and work.
It was not my object in the Lecture to which you refer to
give a history of the mechanical Theory of Heat, but simply to
place a man of genius, to whom the fates had been singularly
20*




466             APPENDIX TO LECTURE XII.
unkind, in a position in some measure worthy of him. I was
quite aware of all that you have stated regarding Locke, Rumford, Davy, and others: you might have added Bacon to your
list-probably no great generalization was ever established without having first simmered in the minds of many thinkers. But
the writings of Mayer form an epoch in the. history of this subject;
and I certainly should not feel disposed to retract a single sentence
that I have written in his favour. I believe he deserves more
praise than I have given him. It was he who first used the term'equivalent' in the precise sense in which you have applied it;
he calculated the mechanical equivalent of heat from data which,
as I have said,' a man of rare ingenuity alone could turn to account;' and his calculation is in striking accordance with your
own experimental determinations.* You worked independently
of Mayer, and in a totally different way. You brought the mechanical theory to the test of experiment, and in this way proved
its truth.
Mayer calculated correctly the mechanical equivalent of heat;
but you say that, at the time he wrote, there were no known facts
to warrant the hypothesis which he adopted. If by this you
mean to say that he made a haphazard guess, which had no basis
of physical probability, I cannot agree with you. The known
constitution of an elastic fluid is, in my opinion, quite sufficient
to justify Mayer's proceeding. His hypothesis was this: Let the
quantity of heat required to raise the temperature of gas, preserved at a constant volume, t~, be x, and let the heat required to
raise the same gas, under constant pressure, t~, be x+y.  The
weight raised by the expanding gas in the latter case being P,
and the height to which it is raised h, then, according to Mayer,
y=Px As;
that is to say, the excess of'heat imparted in the latter case is
precisely equivalent to the mechanical work performed.
It is undoubtedly implied in this equation that the quantity
of heat y is expended wholly in external work, and that none of
it has been consumed in overcoming internal molecular attractions. This, I think, on the face of it is an extremely probable
* The corrected specific heat of air being made use of.




LETTER TO MR. JOULE.                 467
hypothesis-so probable, indeed, as to amount, in my estimation,
almost to a certainty. Clausius makes the same assumption with
no better authority than Mayer; and I believe (for I here trust to
my memory merely) that the assumption has been completely
verified by the experiments of the very philosophers who once
questioned it.' The law,' says Mayer,'." Heat =Mechanical
effect," is independent of the nature of an elastic fluid, which
only serves as the apparatus by means of which the one force is
converted into the other.'
The law of Mariotte was an old principle when Mayer wrote;
and the fact of its holding good for gases generally renders the
conclusion exceedingly probable that, in yielding to compression,
the attractions of the gaseous molecules were insensible; otherwise
it is hardly conceivable that the same results could have been
obtained with gases so differently constituted: the attractions of
the hydrogen atoms, for example, would in all probability be
different from those of oxygen. Mayer was further justified in
his hypothesis as to the absence of interior work in the case of a
true gas, by the experiments of (Ersted and Despretz, which
showed that the law of Mariotte was departed from by the liquefiable gases-the amount of departure depending on the proximity of the gas to its point of condensation. Where, therefore, no
departure from the law had been observed (in the case of air for
instance), Mayer, I submit, was perfectly warranted in assuming
that the molecular attractions were insensible, and that the quantity of heat (y) before referred to was entirely expended in raising
the weight, and had its true nmechanical equivalent in the weight
so raised.
With reference to the application of the mechanical Theory of
Heat to cosmical phenomena, if it were not a liberty, I would ask
whether you have read the Essay of Mayer entitled,'Beitrdge
zur Dynamik des Hirmmels'? If so, then I have good reason to
suspect my competence to come to a correct conclusion as to what
constitutes a scientific right.
Knowing that the original memoirs of Mayer would be the
true court of appeal in connection with this subject, I some
months ago urged the responsible editor of the'Philosophical
Magazine' to publish translations of them. This I hope he will
do; for I quite agree with you in thinking that they would in



468             APPENDIXX TO LECTURE XI:.
terest many of the readers of the magazine. Let me add, in conclusion, that I do not think the public estimate of your labours
can be in the least affected by any recognition which may be accorded to Mayer. There is room for both of you on this grand
platform. Certainly, had Mayer never written a syllable on the
mechanical Theory of Heat, I should not deem your work a whit
nobler than I now hold it to be.
Believe me, yours, &c.,
JOHN TYNDALL.
ROYAL INSTITUTION:
August, 1862.
The public is now in possession of all that I have written with
reference to the claims of Dr. Mayer. The whole of it being
placed thus together will facilitate future reference.




INDEX.
A                    Air, passage of sound through, 261
- cooling effect of, 253
Absolute zero of temperature, 90      - thermometer uninfluenced by heat
Absorber, qualities necessary to form a   that has passed through glass, 318
good, 364                           -  not warmed  by passage of heat
Absorption of heat by coats of whiting   through, 319
and tin, 304                        - dry, absorption of heat by, 341
- - takes place within a body, 314   - - dynamic radiation of, 383
--- bydifferent thicknesses of glass,     - -    when varnished by va315                                   pours, 385
- - by gases, mode of experiment, -  difficulties in obtaining  perfectly
338, et seq.                          pure, 391
- -   by olefiant gas, 349, 351, 353  - saturated with moisture, absorption
- - -proportional to density of gas   of, 396, 410, 413
in small quantities, 353            - humid, table of absorption by at dif-   by sulphuric ether, 355          ferent tensions, 399, 409
- - - the transference of motion, not - cause of slow nocturnal cooling of,
annihilation, 356                     414
—... by ammonia, 362                Alcohol, expansion of by heat, shown, 91
- -- - by vapours, 368               -- evaporation of, produces cold, 168
-- -- — by aqueous vapour, 391, et seq. - absorption of heat by vapour of, 368
-- and radiation of heat, reciprocity of, Alps, formation and motion of glaciers
305                                   on, 198, et seq.
-- - -. -- by gases and vapours de. Alum, powerful absorption and radiatermined without external heat, 381    tion of, 311
-- ---    -dynamic, table of gases, - number of luminous and obscure rays
383                                   transmitted by, 317
Acetic ether, absorption of heat by va- America, extreme cold of E. coast of, 193
pour of, 368                        Ammonia, absorption of heat by, 361
Acoustic experiments, 292             Amylene, absorption of heat by vapour
Actual energy defined, 151              of, 368
iErolites, velocity of, 23            Ancient glaciers, evidences of, 203,.Ethrioscope, 407                       et seq.
Agassiz, bM.,movement of glaciers, 208   Aniseed, absorption of heat by perfume
-  on the cause of bubbles in ice, 329   of, 374
Aggregation, change of state of, in  Animal substances, table of conductive
bodies by heat, 161                   power of, 242
Air, compressed, chilled by expansion, Angular velocity of reflected ray ex27                                    plained, 276
- from bellows, heated, 28            Aqueous vapour, precipitated by rare-effect of stoppage of motion of, 44   faction of air, 46
- compression of, containing bisulphide - - cause of precipitation of in Engof carbon, 43'                        land, 189
-expanded by heat, 79                 - - use of in our climate, 189
-expansion of, under constant pressure, - - precipitation of less, east of Ire80                                    land, 190
- expansion of, under constant volume, - - definition of, 389
82                                  - - amount of in atmosphere, 390
- heated, ascends, illustrations of and  - - action of on radiant heat, 391,
experiments to prove, 182             et seq.




470                               INDEXo
Aqueous vapour, absorption of in air ob- Blood, heat of, why so constant in all
tained from various places, 398         climates, 230
-     -objections to experiments on an-  Body, cause of its resisting high temswered, 398                             peratures, 230
- decrease of in higher regions of at-  Boiling of water, by friction, Rumford's
mosphere, 401                           experiments, 24
- -cause of copious precipitation of in  -      - - fully described, 69
tropics, 401                          - - - to what due, 128
- effect of removal of from English  - point of water raised by being freed
atmosphere, 405                         of air, 126
- - absorbs same class of rays as  - - true definition of, 129
water, 407                            - - lowered by ascending, 130
Asbestos, cause of bad conduction of - - on summit of Mt. Blanc, Mt. Rosa,
heat by, 247                            &c., 120
Asia, cause of coldness of central parts  - - depends on external pressure, 121
of, 405                               - - why selected as a standard point of
Astatic needle described, with prepara-   temperatutre, 106
tion of, 33                           Boiler explosions, 127, 178
Atlantic, Europe  the  condenser of, Boracic ether, large absorption of heat
194                                     by vapour of, 368
Atmosphere, effect. of its pressure on  -  -  table of dynamic radiation of
boiling point, 129, et seq.             vapour of, 388
- diminution of its pressure lowers  Boutigny, M., his experiments on the
boiling point, 130                      spheroidal state of liquids, 178
- amount of aqueous vapour in, 390      - - - water first frozen in a red-hot
-  action of aqueous vapour in on ra-    crucible by, 179
diant heat, 391                       Brass,expansion of, by heat, 97, et seq.
- use of aqueous vapour in, 405         Breeze, land and sea, how produced, 87
- absorption of solar heat by, 319, 433    British Isles, cause of dampness of, 189
Atoms, collision of carbon and oxygen, Bromine, opacity of to light, but trans.
59                                      parency to heat proved, 366
- when separated, heat consumed, 154  Bunsen, Prof., description of his burner,
- enormous attractions of, 153            62
- absorb and emit same rays, 426        - - his determination of the temper.
Atomic oscillations of a body increased    ature of Geysers, 138
by heat, 74                           - - his Geyser theory, 139
- motion, how propagated, 76            Burner, Bunsen's description of, 62
- forces, power of, 93.
- constitution, influence of on absorption, 361                                               C
B                     Calibration of the galvanometer, Melloni's method of, 370
Bacon, extract from 2nd Book of Novum   Calms, the region of, 188
Organum, 66                           - cause of torrents of rain in region
Balloon, fire, experiment with, 80        of, 401
Bark of trees, bad conductive power of, Caloric proved not to exist by Davy,
242                                     107
Bell struck by hammer, motion not lost,  Calorific power of a body, Rumford's
41                                      estimation of, 163
Beeswax, contraction of in cooling, 118    - conduction, three axes of in wood,
Bengal, cause of formation of ice in, by    241
nocturnal radiation, 418              - - of liquids, 313
Benzol, absorption of heat by vapour of, - rays, sifting of by glass, water, &c.,
368                                     316
Bibulous paper, absorption of heat by  Candle, combustion of, 60
moisture contained iln, 410           Cannon, origin of heat produced by
Bisulphide of carbon, vapour of ignited    boring. 67
by compression, 43                    Capacity for heat, different in different
----   note on compression of air   bodies, 38
containing, 71                        - - - explained, means of determin- --- -    absorption of heat by, 368     ing, 158
- --  -transparency of, to heat, 309, Carbon, light of lamps due to solid par313                                    ticles of, 62
Bismuth, expansion of in cooling, 95   - atoms, collision of with oxygen, 59
Blagden and Chasitrey, their exposure of - amount of heat generated by its cornthemselves in heated ovens, 229        bination with oxygen, 163




INDEX.                                471
Carbonic acid, how produced by corn-  Compression, heat generated by, 19
bustion, 60                           Compressed air, expansion of, produces
- - solid, propel-ties of, 171            cold, 29
- - power of radiation and absorption  Condensation, congelation and combinapossessed by, 359, et seq.              tion, mechanical value of each in the
Carbonic oxide, table of absorption of   case of water, 162, et seq.
heat by, at difierent tensions, 357   Condensation of aqueous vapour in trop- - power of radiation and absorption    ics, cause of, 401
possessed by, 359, et seq.            - -- - by mountains, ditto, 402
Celestial dynamics, essay on Mayer on, - and congelation promoted by water. 85, 440                                 in its different states, 403
Celsius, his thermometer, 106           Conduction of heat defined and illusChantrey and Blagden, their exposure of   trated, 219
themselves in heated ovens, 229       - - - not the same in every substance,
Chemical harmonica, how produced, 279    220
Chilling, when produced, 273            - - - by metals, 221, 224
- an effect of rarefaction, 45          - --  experiments of Ingenhausz, 223
- by radiation, dew an effect of, 401      - - Despretz's method of observing,
Chlorinle, absorption of, 362             223
- and hydrogen, effect of their combi-  - - - by different metals determined
nation on radiant heat, 365             by MMi. Wiedemann and Franz, 224
Chloroform, absorption of heat by vapour   -  - by crystals and wood, 231, et
of, 368                                 seq.
Climate, cause of dampness of English,  - - - by wood in different directions,
189                                     table of, 240
- mildness of European, 193               - -  -importance of knowing specific
Clothing, conductivity of materials used    heat in experiments on, 243
in, 246                                - - - by hydrogen gas, 253, 255
Clouds, cause of generation of, 402     -- -  of cold, illustrations of, 228
- composition of, 195                   - power of not always the same in
Coal mines, cause of explosions in, 251   every direction, 231
Co-efficient of expansion of a gas, 81  - of woollen textures, imperfect, use
-- - linear, superficial and cubic, ex-    of, 246
plained, 104                           Conductors, good and bad, defined, 220,
- -  of various substances, table of,   et seq.
105                                   Conductivity of various substances, 224,
Cohesion not the cause of the sticking  - - of crystals and wood, 231, et seq.
of two piece's of ice, 335            Contraction generally the result of soli-  force of, lessened by heat, 75         dification, 118
- of water increased by removal of air, - of India-rubber by heat, 102
126                                   Conservation of force shown in steam
Cold, effect of on thermo-electric pile, 16    engine, 133
- produced by rarefaction, 44           Convection of heat defined, 191
- produced by the stretching of wire, -- - examples of, 192
101                                   - - - by hydrogen, 255
- of snow and salt, 166                 Cooling a loss of motion, 258
- generated in passing fiom the solid to  - effect of air and hydrogen on heated
the liquid state, 166                   bodies, 254
-- -  from the liquid to the gaseous  - hastened by coating substance with
state, 168                              good radiators, 303
--. by stream of carbonic acid, 170  Copper wire, iron found in, 34
- conduction of, 228                    - -  simple mode of ascertaining the
- apparent reflection of rays of, 282     purity of, 35
Collision of atoms, heat and light pro-  --   influence of silk covering of, 35
duced by, 64                          Crevasses in glaciers, to what due,,
Colour, physical cause of, 272            214
- of sky, possible cause of, 408        - - - proof of the non-viscosity of ice,
- influence of on radiation, 302          214, et seq
Combustion, effect of height on, 63     Cryophorus, or ice carrier, 169
- Dr. Frankland's memoir on, 64         Crystals, expansion of, 101
- theory of, 64                         - of snow, 196
- of bisulphide of carbon by compres-  Crystals, difference of conductivity in
sion of its vapour, 71                  different directions, 232
- of gases in tubes, sounds produced  Crystallisation, temperature of constant,
by paper on, 284                        under same pressure, 105
Compounds good absorbers and radia-  Cumberland, traces of ancient glaciers
tors, cause of, 365                     in, 204




472                               INDEX.
Current of electricity, direction of, 32    Earth, all the energies of due to the sun,
Currents, eerial, how produced, 182       445, et seq.
- upper and lower in atmosphere, 183   Earthquake at Caraccas, 186
Elastic bodies, their difference from inelastic, 73
Elective power possessed by bodies with
D                       regard to rays of light and heat, 308,
et seq.
Davy, Sir H., his views of heat, 24     Electricity and heat, relationship of
- -  experiment on the liquefaction    shown in the conduction of by various
of ice by friction, 40, 107, et seq.    bodies, 224
-- -    first scientific memoir, 41, 107    - current of increased by cooling con-- -- chemical philosophy referred    ducting wire, 227
to, 46                                Elements bad absorbers and radiators,.- - - -investigation of flame, 59        365...- - - - discovery of the safety lamp, Emission theory of Newton, 263
251                                   Energy, mechanical, converted to heat,
-      experiment on the passage of    21
heat through a vacuum, 258            - potential or possible, defined, 151
De la Rive and De Candolle on conduc-  - dynamic or actual, ditto, 151
tion of wood, 232                     - potential and dynamic, sum of con.
Density, point of maximum, in water,   stant, 152
93                 X- all terrestrial due to the sun, 446, et
Despretz, his experiments on the con-   seq.
duction of heat, 223                  England, cause of even temperature of,
Dew, Dr. Wells' experiments on, and    189
theory of, 415, et seq.               Equatorial current, Europe overflowed
- cause of deposition of, 416             by, 188
- a still night necessary for the forrma   - ocean, winds from, cause of moisture
tion of, 420                            of Enlgland, 189
Diamond, Newton's opinion of, 58        Equivalent, mechanical, of heat, 54, et
- combustion of in oxygen, 59             seq.
Dlathermancy  explained  and  illus- -- - -    how calculated 83, et seq.
trated, 310                           Ether, sulphuric, evaporation of, pro- not a test of transparency. 321         duces cold, 168
Dilatation of gases affected without chill  - — and solid carbonic acid, 179
ing, 88                               - - absorption of heat by vapour of, at
Dissolving of nitreo salt, &c,, produces    different tensions, 354, 368
cold, 165                                           by diffbrent measures of
Drying tubes, difficulties in selecting    vapour of, 355
and obtaining, 347                    - the luminiferous, mode of transmis.
Dynamic energy defined, 151               sion of heat by, 300
-radiation and absorption, discovery of,  - -- fills all space and penetrates all
380                                     bodies, 307
- - - - of gases, table of, 383         Ether, the luminiferous, the power of
-- - - - - vapours, ditto) 386            imparting motion to and accepting mo- - - - - boracic ether vapour, table   tion from are proportional, 355
of, 387                               Europe the condenser of the Atlantic,
- - -- -- in different lengths of tube,   194
389                                   - cause of mildness of climate of, 194
Dynamical theory of heat, 39            Evaporation produces cold, 168
- water frozen )y, 169
Exchanges, Prevost's theory of, 274
Expansion of volume, 74
- - gases by heat, amount of, 79
Earth, amount of heat that would be  - - - co-efficient of, 81
generated by stoppage of its motion, - - - without performing work, 88
57                                    - - bismuth in cooliiing, 95
falling into the  - - water in freezing, 93
sun, 57, 443                          -            use of in Nature, 94
-    resisting the ro- -   alcohol by heat 91
tation of, 443                        - - of water by heat, 92
- crust of thicker than generally sup   -          cold, 73
posed, 119                            - - solid bodies by heat, 96
- its rotation, effect of on trade winds, - - crystals, 100
183                                   - - lead, curious effect of, 99
- time required to cool down, 444      Expansive force of heat, 78, 99




INDEX.                                   473
Explosions of steam boilers, 127          Fusible alloy liquefied by rotation in
- -     - possibly due to the spheroidal    magnetic field, 51
state, 178                              Fusion, point of, effect of pressure on,
- in coal mines, cause of, 251              119, 147
G
F                      Galvanometer described, 15
- coirstructlon of, 32
Fahrenheit, his thermometer, 106           - peculiarity of in high deflections, 342
Faraday, his discovery of maguetoelec-  - method of calibration of, 369, et seq.
tricity, 50                              Gas, carbonic acid, from soda water, con-   explanation of the Trevelyan in.  sumes heat, 28
strumenlt, 142                          - combustion of, 60
- - regelation of ice, 199                 - illuminating power of. 63
- - experiments ont singing flames, and  - co-efficient of expansion of, 81
theory of, 284                           - a feeble, varnished by a powerful one,
- mercury first frozen in a red-hot cru.   383
ci ble by, 179                          - absorbs those rays which it emits, 426
Fibre of wood, power of conduction of  Gases, constitution of, 76
heat by, 240                             Gases, different powers of accepting moFire produced by friction, 22               tion from  the ether, or difference in
- balloon, 80                               absorption possessed by, 351, et seq.
- screeins of glass, action of, 319       - different powers of iniparting motion
Flame, constitution of, 59                  to the ether or difference in radiation
- cause of its inability to pass through    possessed by, 358, et seq.
lwire gauze, 250                        - table of dynamic radiation of, 368
Flames, singing, paper on, 284             Gaseous condition of matter, 75
-  -Count Schaffgotsch, experiments  Gassiot, iron cylinders burst by, 94, note
on, 292                                  Gauze wire, cause of its stopping pasFluorescence of sulphate of quinine in    sage of flame, 250
the invisible spectrum, 269              Geyser, the Great, of Iceland, descripFoot pounds, explaniation of, 54            tion of, 133
Forbes, Prof. J. D., on the Trevelyan  - Bunsen's theory of, 136
instrument, 143                          Gilbert, Professor, on the vibration of the
-- viscous theory of ice, 203, 214           Trevelyan instrument, 142
- - law of movement of glaciers, 208       Glaciers, formation of, 198
- - observations on the  Glacier du  - motion of described, 198, et seq.
Gdant, 208                              - viscous theory of, 198
- - merits as an investigator, 209        - regelation ditto, 199, et seq.
- -   surmise that the motion of glaciers  - ancient, evidences of in various places,
was retarded. 211                         203
Force, lecturie on, 450                   - cold alone cannot produce, 206
- of heat in expanding bodies, 98          - central portions move quickest, 209
- vital, supposed conservative action  - point of swiftest motion shifts, 210
of, 230                                   - -- maximum motion of determined,
Forces, molecular. energy of, 93, 153       211
- -  polar, heat required to overcome,  - motion of retarded, cause of, 211
160                                     - irate of motion, 213
Formic ether, absorption of heat by va-  Glass, why cracked by hot water, 98
pour of, 368                          - broken by a grain of quartz, 99
Frankland, Dr., his experiments on com. - opacity of to heat, 310
bustion, 63                             - absorption of heat by different thickFraunhofer's lines, 429                     nesses of, 315
Freezing, effect of on water pipes, 94    - filie-screens, use and philosophy of,
- point of thermometers, 106                319
- together of pieces of ice, 147           Gmelin, his definition of heat, 37
- point lowered by pressure, 148          Gore, his experimnents on revolving balls,
- of water produced by its own evapora    117
tion, 169                               Gravity, velocity imparted to a body by,
-- planes of in ice, how recognised, 324    56
Friction, generation of heat by, 18       Grease, use of on wheels and axles, 21
- against space, heat developed by, 48    Green silk, magnetic, 35
Frost, means of preserving plants from,  Gulf-stream, 193
418                                      Gypsum, powdered, bad conduction of
- cause of their preservation, 418          heat by, 248




474                               INDEX.
Heat, distinction between it and ordinary
H.                     motion, 218
- conduction of defined and illustrated,
Harmonica, chemical, 293                  219
-condition of flame producing, 295     --  - n- ot equally possessed by everyHeat and cold, opposite ef'ects upon    body, 220
thermo-electric pile, 16              - method of determining the conducti- generated by mechanical processes, 17   bility of bodies obr, 223
Heat generated by friction, 18         - and electricity, relationship of, 225
~- - - - - — compression, 19           - motion of interferes with the motion
- -- -- percussion, 19                   of electricity, 226
-- -  falling of mercury or water, 20   - conversion of into potential energy,
- consumption of in work, 26             230
-nature of, 37, et seq                 - difference of conductivity of in crys- a motion of ultimate particles, 39     tals and wood, 231, et seq.
- developed when air compressed, 41  — transmission of through wood, 233,
et seq.                                 240
- -- - motion of air stopped, 28, 45      -- - influenced by the mechanical
-- by rotation in magnetic field, 51      state of the body, 247
- mechanical equivalent of, 54, 83, et - conduction of by hydrogen gas, 255
seq.                                  - passage of through a vacuum, 258
- proportional to height through which  - to what motion of imparted, 265
the body falls, 55                    - radiant, 265
- relation of to velocity, 56           - rays beyond visible spectrum, 268
- Bacon's conception of, 66             - obeys the same laws as light, 277 et
- Rumford's essay on, 67                  seq.
-- production of inexhaustible, 70      - action of on oxygen and hydrogen,281
- a motion, Rumford's conception of, -law of inverse squares applied to, 298
24, 70                                - transversal undulation of waves of,
— a rectilinear motion, 76               300
- expansion of a gas by, 79             - motion of, more impeded by groups
— imparted to a gas under constant   of than by single atoms, 301
-pressure, 80                           - quality of, 317
- - - - at constant volume, 82       - rays of sifted by absorbent bodies,
- produced by stretching India-rubber,   317
101                                   - transmission of through opaque bod- - -friction of ice, 108                ies, 321
-a repulsive motion, 110               - effect of on ice, 324, et seq.
- conversion of into mechanical energy, - absorption of by gases, mode of ex112                                     periment, 338
----   sound, 117                         - - -  -      means of detecting minute
- developed by electricity, 117, 226      amount of, 342
- performance of work by in steam- - radiant, apparatus for researches on
engine, 132                             described, 345
- power of in expanding bodies, 154     - free passage of through dry air, oxy- two kinds of motion produced in    gen, hydrogen, and nitrogen, 348
bodies by, 155                        - absorption of by gases and vapours,
- interior work performed by, 155         349, et seq.
- consumed in forcing atoms asunder. - absorption and radiation of a gas or
155                                     vapour determined without external
- generated by drawing atoms together,   beat, 350
156                                   - absorption of by aqueous vapour, 390,
- quantity yielded up by different bod-   et seq.
ies in cooling, 157                   - nocturnal radiation of, the cause of
- specific, 158                           dew, 414, et seq.
- causes change of state of aggregation  - amount of generated by collision of
in bodies, 161                          meteors with the sun, 436
— latent, of water, steam, and aqueous  - developed by friction of tidal wave,
vapor, 162, et seq. 206                 440
-  defiiition of, 162                   - source of this heat, 440
- generated in passing from  liquid to  Height, influence of on combustion, 63
solid state, 166                      Helmholtz, his remarks on the exhaus-cause of niore equal distribution of,   tion of the mechanical force of our
189, 193                               system, 443
- convection of, 193                   - his calculation of heat that would be
- necessary for the production of gla-   caused by stoppage of earth's motion,
ciers, 206                             57




nNDEX.                                  475
Herschel, Sir William, his discovery of Ireland, traces of ancient glaciers in, 204
the obscure rays of the spectrum, 268   Iron, bottle burst by frieezing water, 93
- Sir John, note on rock-salt, 340       - expansion of by heat, 97, et seq.
- - -measurements of solar radiation,  - presence of in sun proved, 430
431                                    Isothermal line runs north and south in
Herbs, aromatic, action of their odours    England, 193
on radiant heat, 374                   Ivory, bad conductivity of, 242
Humboldt on the cold of Central Asia,
405
tnuyghens, his theory of light, 263                          J
Hydrochloric acid, absorption of heat
by, 362                                Joule, his experiments on the mechanicHydrogen, collision of atoms of with    al equivalent of heat, 25, 86
oxygen, 60                             - —.-  heat and work, 52
- amount of heat generated by cornm-  - - -    magneto-electricity, 86
bining with oxygen, to form  water, -    --  the  shortening  of India163                                      rubber by heat, 101
- cooling effect of on heated bodies, 254  - letter of Dr., 462
- low power of absorption possessed by,
359, et seq.
Hygrometer, use of radiant heat as, 409                     K
Kopp, Professor, his determination of
I                       the cubic coefficients of expansion, 105
Ice liquefied by friction, 40, 108
- why it swims on water, 93                                 L
- liquefied by pressure, 121, 148, 328
- structure and.beauty of, 122           Lampblack, powerful absorption  and
- dissected by heat, 123                   radiation of, 366
-- flowers, 122, et seq.; 324 et seq.    - radiation -of heat through, 367
- memoir on physical properties of, Land breeze, how produced, 187
146, 324                               Latent heat of water, 40, 162
- carrier or cryophorus, 169             - --  liquids, 165
- viscous theory of, 198                 - - - vapour, 168
- regelation ditto, 199, et seq.         Lead ball heated by collision, 55
- proofs of non-viscosity of, 214        Lead, curious efbect of expansion of, 99
- power of absorbing heat by, 320        - carrier in Trevelyan's instrument, 115
- planes of freezing in, how recognised, - low specific heat of, 157
324                                    Lecture on force, 450
- not homogeneous, 328                   Leidenfrost, first observer of the sphe- theory of melting interior of by con.   roidal state of liquids, 177
duction of heat, 332                   Letter of Dr. Joule, 462
- examination of waterblebs in, 334      - - Priof. Tyndall, 465
- cause of freezing of two pieces of, 335  Light produced by firiction of quartz, 23
-  artificial formation of by nocturnal - of lamps, to what due, 60
radiation, 418                         - of gas destroyed when mnixed with
- this theory supplemented, 419            air, 62
- amount melted per minute by solar  -theories of, 263
radiation; Herschel and Pouillet's  - popalation alnd sensation of, 264
measurements, 431                      - reflection of, 275
- amount melted per hour by total - action of on chlorine and hydrogen,
emission of sun, 434                     280
Iceland, Geysers of, 134                 - law of diminution of with distance,
India-rubber, stretching  of produces    298
heat, 101                              - undulations of-transversal, 300
- - contraction of by heat, 102          Liquids, calorific transmission of, MelIngenhausz, his experiments on the con-   loni's table, 313
duction of heat, 223                   - condition of matter, 75
Interior work performed by heat, 155    - chianging to solid produces heat, 166
-- different kinds of, 160               - the spheroidal state of, 172, et seq.
Iodide of methyl, absorption of heat by  Liquefaction of ice by friction, 40, 108
vapour of, 368                         - - - - pressure, 121, 148, 328
Iodine, dissolved in bisulphideof carbon, Linear co-efficient of expansion, 104
diathermancy of, 366                   Lloyd Dr., his tables of rainfall in IreIleland, more rain on west side than on    land, 190
east, 190                              Locke, his view of heat, 39




476                               xNDEX.
Meteors, zodiacal light supposed to be,
eM                      58, 437
- number of seen in Boston, 436
Magnus, Professor, pure copper obtained  - amount of heat generated by collision
by, 34                                  of with sun, 438
---  his experiments on gaseous con- - sun's light and heat, kept up by, 58,
duction, 253                            436
- - - - - the conductivity of hydro. Meteorology, absorption of aqueous vagen, 255                                pour applied to phenomena of, 401, et
Magnetic axes of astatic needles cross   seq.
each other, 33                        Methylic alcohol, absorption of heat by
- field, apparent viscosity of, 49        vapour of, 368
Magnetism of copper wire, cause of, 35   Mitscherlich, Professor, his experiments
Marsh gas, absorption of, 361             on the expansion of crystals, 100
Material theory of heat, 37             Molecular motion, heat a, 41, 74, 218
Matter, liquid condition of, 75         - vibration of a body more intense when
- gaseous ditto, 75                       heated, 74
Mayer, Dr., the relation found by be-  - force irresistible, 93
tween heat and work, 52               - - power of, 154
- -  his calculation of the heat that  -- - -  calculated, 159
would be produced by stoppage of  - action in wood, effect of, 241
earth's motion, 57                    Mooni blindness, cause of, 418
- --  mechanical equivalent of heat, - beams, cause of putrefying power of,
85                                      418
--  essay  on celestial dynamics,  - warmth of rays of, Melloni's experiquoted from, 440                        ments on, 422
-  his meteoric theory of sun's heat, - obscure heat of cut off by our atmo440                                     sphere, 422
Mechanical processes, generation of heat Moraines the means of tracing the tribu.
by, 17                                  taries of glaciers, 208
- work, consumption of heat in, 26      Mosely, Rev. Canon, curious effect of ex- theory of heat, 26                      pansion noted by, 99
- equivalent of heat, 54, 83, el seq., 87    Motion, heat an expansive, Bacon, 67
-- work, gas expanding without per-  - - considered to be, by Rumford, 70
forming, 88                           - - - - - - Locke, 39
— force, amount formerly and now pos- - transference of from  mass to moleosessed in our system, 443               cules, 74
Meidinger, M, his experiments on ozone, - point of maximum in a glacier, 210
378                                   Mountains good conidensers, cause of,
Melloni, his suggestion for obtaining    402 et seq
pure copper wire, 34                  Moving force, amount of heat produced
- - mode of proving the diminution of   by destruction of, 57
heat as the square of distance, 299   - - produced by steam, 133
-- researches on radiant heat, 310      - - or dynamic energy, defined, 151
- - table of the transmission of heat
through solids, 311
- - ditto, ditto, liquids, 313
- - theory of s6rein, 407                                 N
-- - addition to the theory of dew, 421
— experiments on the warmth of the  N6v6, the feeder of the glacier, 198
lunar rays, 422                       Newton, his opinion of the dianlond, 58
Mercury, low specific heat of, 157      - - enmission theory of light, 263
- frozen by solid carbonic acid, 171    Nitre, production of cold by dissolving
- -in red-hot crucible, 179               of, 165
Mer de-Glace, abstract of discourse on, Nitrogen, absorption and radiation of,
208                                     360, et seq.
Metals good conductors of heat, 221     Nitrous oxide, absorption and radiation
- proofs of difference in conduction by,   of, 360, e! seq.
221, et seq.                          - - dynamic radiation of, 383
- bad radiators, 301                    - acid gas, bands produced by spectrum
- - absorbers, 306                        of, 426
- effect of their bad radiation, 417    Nocturnal radiation, experiments on by
- bands seen in spectra of their vapours    Wells, Glaisher, and others, 420
424                                   - - artificial folrmation of ice by, 418
- presence of terrestrial, in sun proved,  Novum  Organum, extract from  2ud
430                                     book of, 66




INDEX.                                 477
0                     Pressure lowers freezing point, 148
Propionate of ethyl, absorption of heat
Obscure heat, rays of, obey same laws    by vapor of, 368
as light, 277                          Pulse sonorous, how produced, 271
--  ratio of luminous to obscure rays  Pyrheliometer, use and description of,
from different sources, 323              431
Ocean, influence of on temperature, 160  Pyrometers, 97
Olefiant gas, athermancy of, 349, et seq.
- - table of absorption of at different                     Q
tensions, 351
by various measures, Quartz, clear and smoky, transmit equal
353                                      amounts of heat, 312
- - radiation of, 360                    Quality of heat, definition of, 317
- - dynlamic radiation of, 383
---- varnishing metal by, 384, et seq.
Organic motion, paper by Mayer on, 85                      R
- structures, table of conductivity of, 242
Oxygen, collision of atoms of with car-  Rain, cause of the torrents of in the
bon, 59                                  tropics, fall, 188, 401
-  small absorption of heat by, 360, et - fall greater on -west than on east
seq.                                     coast of Ireland, 190
Ozorne, action of on radiant heat, 376   - - Dr. Lloyd's table of, in Ireland, 190
- increase of, by reduction in size of - - places where greatest occur, 190, et
electrodes, 377                         seq.
- experiments on means of producing  - - upon what dependant, 190
incr'eased quantity of, 377            Radiant heat emitted by all bodies, 273
- probable constitution of, 379          - - and light, analogy between, 274
- - laws the same as those of' light, 277
- - reflection and convergence of rays
P                       of, 278
-- law of inverse squares applied to,
Particles of matter, space between, 110    298, et seq.
- impact of, causes sensation of heat, - - apparatus for researches on de77,                                      scribed, 345.
-ultimate, motion of produces heat, 39  - - action of perfumes on, 373, et seq.
Parabolic mirrors, reflection of light and  - -- absorption of ty gases, 48, et seq.
heat from, 279, et seq.                - - -- - - vapours, 368
Percussionl, heat generated by, 20       Radiation, effect of colour on, 302
Perfumes, how propagated, 76             - and absorption, reciprocity of, 304
- table of absorlition of heat by, 374   - obscure, 323
Perspiration, use of in lhot climates, 230   - by gases, 360
Photosphere of sun, action of on solar  -and absorption of a gas or vapour derays, 429                                ternlined without external heat, 381
Physical properties of ice, memoir on, - - - dynanic,   table of gases, 383
324                                    - - - by gaseous matter, paper on, 409
Pile, thermo-electric, construction and  - dew an effect of chilling by, 416
use of, 14, 30                         - nocturnal, Giaisher's table of chilling
Pipes, water, how burst, 94                by, 420.
Pitchl of note, upon what dependant, 271  - -artificial formation of ice by, 418
Planets, orbital velocity of sone, 23    Radiating body and air, difference be- heat that would be developed by their   tween constant, 421
falling into sun, or by resisting revo-  Rarefiaction, chilling effect of, 44
lution of, 443                         - will not by itself lower mean tempePolar forces, heat required to overcome,    rature, 88
160                                    Reaumur, his thermometer, 106
Potential or possible energy defined, 151  Rectilinear motion, atoms of gases move
Pouillet, M., his experiments on  the    with, 76.
temperature of air and swan's down,  Refrigeration by expansion of a gas, 85
421                                    Reflection of light and heat obey sanme
- -- measurement of solar radiation,   laws, 274
431                                    Revelation, discovery of by Faraday.
- - pyrheliometer, 431                    199
Pressure, relation to heating of gases, - of snow granules, note on, 216
80, 84                                 Rendu, his plastic theory of ice, 203
- effect of on poitnt of fusion, 119     Repulsive motion of heat, 41, 110
- --- crust of earth, 119                Resistance, heat of electric current pro- liquefaction of ice by, 121              portional to, 117,225




478                                INDEX.
Revolving balls, Gores experiments on,  Snow, carbonic acid, 170
117                                    - beauty of, 194
Rifle ball, amount of heat generated by  - crystals, 196
stoppage of its motion, 56            - line, the, 195
Rivers, point of swiftest motion in, - formation of glaciers from, 197
209                                   - ball, cause of adherence of, 200
Rocker used in the Trevelyan instru    - bridges, how crossed, 201
ment, 113                             - squeezed to ice, 201
Rock-salt, transparency of, to heat, 310   - granules, note on the regelation of,
- hygroscopic character of, 393, 410      213
- deposition of moisture on avoided,  Sodium, yellow bands emitted and ab394                                     sorbed by vapour of, 428, 431
Rotation of silver medal stopped by  Solar spectrum, cause of dark lines in,
magnet, 50                              430. See also Sun.
— earth, effect of on trade winds,  Solids, expansion of by heat, 97
183                                   -- calorific transmission of, Melloni's
~ - -   - --   -climate of England,   table, 311
189                                   Solidification  accompanied  by expan.
Rumford, Count, his experiments on    sion, 93, et seq.
heat produced by friction, 23         - - - contraction, 118
-- - overthrow of the material theory  Sound produced by Trevelyan instruof heat, 39                             ment, 113
- - abstract of his essay on heat, 67    - mode of its transmission through air,
-- his estimation  of the  calorific    260
power of a body, 162                  - produced by flame, 261
- -- experiments on the conductivity  - undulation of waves of, longitudinal,
of clothing, 246                        300
- - - on the conductivity of liquids  Sounds, inaudible, 273
and gases, 252                        - musical, produced by gas flame in
Rupert's drops, 99                        tubes, 284, et seq.
Specific heat of bodies, how determined,
S                       159
Specific heat of water the highest, conSafety lamp, explanation and use of,   sequences, 160
251                                   - - masking the conductive power of
Salt and sugar, dissolving of produces    a body, 243
cold, 65                               Spectra of zinc, copper, &c., 425, et seq,
- common, yellow bands emitted and  Spectrum, invisible, proved, 268, et seq.
absorbed by vapour of, 42S            - solar, cause of dark lines in, 430
Scents, action of on radiant heat, 373, et - of carbon, 423
seq.                                  - of solids, similar to, 423
Schemnitz, machine for compression of  Spherical form  of earth, effect of on
air at, 46                              winds, 187
Schaffgotsch, Count, musical notes obh.  Spheroidal state of liquids, 172, et seq.
trained from coal gas by, 287         - condition, first observer of, 177
--  his  paper on  acoustic  expert-  Spheroid, floating of, in its vapour, 172.
ments, 292                            - not in conitact, proved, 175
Schlagilltweit, Messrs., their observa-  Springs, boililng, of Iceland, described,
tions on the bubbles in ice, 329        134
Schwartz, his observation of sound pro-  Steam, how produced, 128
duced by cooling silver, 112           - elastic force of increased by heating,
Sea warmer after a storm, 20              132
- breeze, how produced, 187             - latent heat of, 162
Selenite, absorption of heat by different  Strokkur, the, imitation of, 139
thicknesses of, 316                    Storms produced by heated air, 182
&Srein, Melloni's theory of, 407        Sulphate of soda, cold produced by disShooting stars, theory of, 23             solving, 166
Silica, water of Geysers contains and  - --- heat produced by crystallising,
deposits, 134                           167.
- as crystal, high conductive power of,  Sulphuric acid used for drying gases,
242                                     350
- powder, low ditto, 247                - ether, absorption of heat by vapour
Singing flames, 261, et seq.              of, 354, et seq., 368
- - paper on, 284                       Sulphurous acid, absorption of, 362
Sky, colour of, 408                     Sulphide of hydrogen, absorption of, 362
Snow, shower of produced by issuing of Sunl, cause of continuance of heat and
compressed air, 46                      light of, 58, 436




INDEX.                                  479
Sun, production of winds by heat of, 182                    U
- does not heat dry air sensibly, 319
- constitution of, 429.                  Ultimate particles, motion of produces
- and planets, supposed common origin    heat, 39
of, 430                                Undulation-theory, 263
— heating power of, measurements by
Herschel and Pouillet, 431
- mode of determining the radiation of,                     V
432
- atmospheric  absorption of heat of, Vacuum in centre of ice flowers, 124
433                                     - passage of heat through, 258
- total amount of heat emitted by, 434   Vacuum, dry air similar to, with regard
- flame atmosphere surroundinig, 439       to radiant heat, 348
-  all organic and inorganic energy re-  Vapour of water condensed by rarefacferred to 445, et seq.                   tion of air, 45
- small fraction of its heat that pro-  - production of consumes heat, 168, 206
duces all terrestrial energy, 448      - phenomena attending production of,
Switzerland, evidences of ancient glaciers    172
in, 203                                 - supporting of spheroid by, 173
Sereno, singing  flame  produced  by  — of water, condensation promoted by,
sounding, 286                            40
--  metals, spectrum of, 424. et seq.
- absorbs those rays which it emits, 426
T                      Vapours, table of absorption of heat by,
368
Temperature, upon what dependant, 111  - - - dynamic radiation and absorp- absolute zero of, 90                     tion of, 386
- high, how endured, 229                  Vaporous condition of matter, 75
dew caused by lowering of, 416           Varnishing a metal or feeble gas by a
- difficulties in ascertaining the true,    powerful one, 385
417                                     Velocity of planets and aSrolites, 23'eneriffe, Peak of, two currents blow   - heat augments as the square of, when
on, 188                                   a body stopped, 56
Thermal effects of air, 43                Vibratory motion, heat thought by Davy
- - - a body falling friom a height, 54     to )be, 111
Thermo-electric pile, 14, 30              Vibration, of heated metal, 114
-   - used diflerentially in researches  - - bodies having different temperaon radiant heat, 344                     tures, abstract of lecture on, 142
electricity, discovery of, 32          - - sounding disks, 260
Thermometer, construction of, 105         Viscous theory of ice, 198
Thomson, Professor WVm., on earth's  Viscosity, test of, 214
crust, 94                               Vital force, supposed conservative action
- -- his suggestion that India-rubber    of, 230
would shorten by heat, 102              Volume of a gas augmented by heat, 79,.-.. -  theory of the sun, 440              et seq
-- - - tables of energy, 442
- Professor James, on the influence of
pressure(^ on fusion, 120                                 W
- --  his explanation of the freezing
together of two pieces of ice, 146      Water boiled by friction, 24, 269, et seq.
Tidal wave, velocity of earth's rotation  - expanded by heat, 92
diminished by, 440                      --- cold, 92
Trade winds, upper and lower, 183        - maximum density of, 93
Transparency of bodies, cause of, 307     - contraction of by heat, 93
- not a test for diathermancy, 321       - ppes, why burst, 94
Transmission of heat through  solids,  -cohesion of increased by removing air
Melloni's table, 311                     from, 125, el seq.
-- - -   liquids, ditto, 313             - hammer, 125
Tr6laporte, squeezing of glaciers through  - effects of, when in a highly cohesive
valley of, 212                           condition, 126
Trevelyan, Mr. A., his instrument, 112    - formerly regarded as incompressible,
---   cause of vibrationls of, 115          153
- - -notes of lecture on, 142            - Bacon's experiment on the compresTropics, flow of air from and to, 183       sion of, 154
- the region of calms or rains, 188      - has the highest specific heat, 158
- cause of the torrents of rain in, 401   - amount of heat yielded by, in cooling,
Tyndall's letter to Dr Joule, 465          10, 157.




480                               INDEX.
Water, specific heat of, how determined, Winds, direction of influenced by earth's
159                                     rotation, 183
- amount of work equal to heating of - lesser, cause of, 187
1~, 160                               Wollaston, Dr., lines in solar spectrum
- effect of high specific heat of, 161    observed by, 429
- latent heat of, 163                   - - his cryophorus, 169
- mechanical value of combination, con. Wood, bad conductibility of, 229
densation and congelation of, 164, et - diff'erence of conductivity in, 232
seq.                                  -  apparatus for ascertaining condue- evaporation of produces cold, 168      tivity of, 233
- frozen by its own evaporation, 169    - three axes of conductive power in, 241
-- i — ll red-hot crucible, 179         Woollen textures, imperfect conduction
-opacity of to heat, 309                  of, 246
- distilled, colour of, 314             Work, constant proportion between it
its power of sifting heat, 321         and heat, 53
effects of its energy as a radiant, in all - interior, 155
its states, 404
-  as liquid, absorbs same rays as its                    Y
vapour, 407
- amount of would be boiled by the Young, Dr. Thos., his theory of light, 23
total emission of sun, 434            -- - - establishment of the undulaWaves of sound, 271                       tion theory, 263
- light, 272
- - heat and sound, difference between,
300                                                     Z
Wells, Dr., his theory of dew, 415, et seq,
- - many curious effects explained by, Zero, absolute, of temperature, 90
418                                   Zinc, bands seen in spectrum of vapour
Winds, extinction of light of gas by, 63  of, 424
- produced by sun, 182                  Zodiacal light, probable cause of, 58, 435
Winlds, trade, 183                       442