No. 5 —University Series.
SCIENTIFIC ADDRESSES
BY
PROF. 7OHN  TYNDALL, LL. D., F. R. S.,
Royal Institution.
1. ON THE METHODS AND TENDENCIES OF PHYSICAL INVESTIGATION.
2. ON HAZE AND DUST.
3. ON THE SCIENTIFIC USE OF THE IMAGINATION.
NEW HA VEN, CONN.:
CHARLES C. CHATFIELD & CO.
i870.




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V.-SCIENTIFIC ADDRESSES:- I. On the Methods and
Tendencies of Physical Investigation. 2. On Haze and Dust. 3. On
the Scientific Use of the Imagination. By Prof. JOHN TYNDALL,
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No. 5 —-University Series.
SCIENTIFIC ADDRESSES
BY
PROF. 7OHVN TYNDALL, LL. D., F. R. S.,
Royal Institution.
i. On the Methods and Tendencies'of Physical Investigation.
2. On Haze and Dust.
3. On the Scientific Use of the Imagination.
NEW HAVEN, CONN.:
CHARLES C. CHATFIELD & CO,
1870.




NEW HAVEN, CONN.:
THE COLLEGE COURANT PRINT.
Electrotyped by E. B. Sheldon, New ffaven, Conn.




PUBLISHER'S NOTE.
The first of the addresses of Professor Tyndall here published,
i. e., that on the " Methods and Tendencies of Physical Investigation,". was delivered at Norwich on the Igth of August, I868,
before the. Physical Section of the British Association for the
Advancement of Science, of which section Professor Tyndall was
the President.  The second, on "Haze and Dust," was delivered
as a lecture before the Royal Institution of Great Britain —in which
Institution Professor Tyndall is professor of physics-on  the
2ist of January, I87o.  A note by the author, supplementing it
in certain directions, was published in " Vature," vol. I, page 449,
March 17, I870.  The third address, on the "Scientific Use of
the Imagination," was delivered before the British Association at
its meeting held in Liverpool, in September last.








TYNDALL S ADDRESSES.
I.
On  the Methods and  Tendencies of Physical Investigation.
The celebrated Fichte, in his lectures on the "Vocation of the Scholar," insisted on a culture for the scholar
which should not be one-sided, but all-sided.  His intellectual nature was to expand spherically, and not in a
single direction. In one direction, however, Fichte required' that the scholar should apply himself directly to
nature, become a creator of knowledge, and thus repay,
by original labors of his own, the immense debt he owed
to the labors of others. It was these which enabled him
to supplement the knowledge derived from his own researches, so as to render his culture rounded, and not
one-sided.
Fichte's idea is to some extent illustrated by the constitution and the labors of the British Association.  We
have here a body of men engaged in the pursuit of natural knowledge, but variously engaged. While sympathizing with each of its departments, and supplementing his culture by knowledge drawn from all of them,




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each student amongst us selects one subject for the exercise of his own original faculty-one line along which
he may carry the light of his private intelligence a little
way into the darkness by which all knowledge is surrounded. Thus, the geologist faces the rocks; the biologist fronts the conditions and phenomena of life; the
astronomer, stellar masses and motions; the mathematician the properties of space and number; the chemist
pursues his atoms, while the physical investigator has
his own large field in optical, thermal, electrical, acoustical, and other phenomena.  The British Association,
then, faces nature on all sides, and pushes knowledge
centrifugally outwards, while, through circumstance or
natural bent, each of its working members takes up a
certain line of research in which he aspires to be an
original producer, being content in all other directions
to accept instruction from his fellow-men. The sum of
our labors constitutes what Fichte might call the sphere
of natural knowledge. In the meetings of the Association it is found necessary to resolve this sphere into its
component parts, which take concrete form under the
respective letters of our sections.
This section (A) is called the Mathematical and Physical section. Mathematics and Physics have been long
accustomed to coalesce, and hence this grouping. For
while mathematics, as a product of the human mind, is
self-sustaining and nobly self-rewarding,-while the pure
mathematician may never trouble his mind with considerations regarding the phenomena of the material universe, still the form of reasoning which he employs, the
power which the organization of that reasoning confers,
the applicability of his abstract conceptions to actual
phenomena, render his science one of the most potent




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instruments in the solution of natural problems.  Indeed, without mathematics, expressed or implied, our
knowledge of physical science would be friable in the
extreme.
Side by side with the mathematical method, we have
the method of experiment.  Here, from a starting-point
furnished by his own researches or those of others, the
investigator proceeds by combining intuition and verification.  He ponders the knowledge he possesses and
tries to push it further, he guesses and checks his guess,
he conjectures and confirms or explodes his conjecture.
These guesses and conjectures are by no means leaps in
the dark; for knowledge once gained casts a faint light
beyond its own immediate boundaries. There is no discovery so limited as not to illuminate something beyond
itself. The force of intellectual penetration into this
penumbral region which surrounds actual knowledge is
not dependent upon method, but is proportional to the
genius of the investigator. There is, however, no genius
so gifted as not to need control and verification. The
profoundest minds know best that nature's ways are not
at all times their ways, and that the brightest flashes in
the world of thought are incomplete until they have
been proved to have their counterparts in the world of
fact. The vocation of the true experimentalist is the
incessant correction and realization of his insight; his
experiments finally constituting a body, of which his
purified intuitions are, as it were, the soul.
Partly through mathematical, and partly through experimental research, physical science has of late years
assumed a momentous position in the world. Both in
a material and in an intellectual point of view it has produced, and it is destined to produce, immense changes,




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vast social ameliorations, and vast alterations in the
popular conception of the origin, rule, and governance
of things. Miracles are wrought by science in the physical world, while philosophy is forsaking its ancient metaphysical channels, and pursuing those opened or indicated by scientific research. This must become more and
more the case as philosophic writers become more deeply
imbued with the methods of science, better acquainted
with the facts which scientific men have won, and with
the great theories which they have elaborated.
If you look at the face of a watch, you see the hour
and minute-hands, and possibly also a second-hand,
moving over the graduated dial. Why do these hands
move, and why are their relative motions such as they
are observed to be? These questions cannot be answered without opening the watch, mastering its various
parts, and ascertaining their relationship to each other.
When this is done, we find that the observed motion of
the hands follows of necessity from the inner mechanism
of the watch when acted upon by the force invested in
the spring.
This motion of the hands may be called a phenomenon of art, but the case is similar with the phenomena
of Nature. These also have their inner mechanism, and
their store of force to set that mechanism going. The
ultimate problem of physical science is to reveal this
mechanism, to discern this store, and to show that from
the combined action of both, the phenomena of which
they constitute the basis must of necessity flow.
I thought that an attempt to give you even a brief and
sketchy illustration of the manner in which scientific
thinkers regard this problem would not be uninteresting
to you on the present occasion; more especially as it




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will give me occasion to say a word or two on the tendencies and limits of modern science, to point out the
region which men of science claim as their own, and
where it is mere waste of time to oppose their advance,
and also to define, if possible, the bourne between this
and that other region to which the questionings and
yearnings of the scientific intellect are directed in vain.
But here your tolerance will be needed. It was the
American Emerson, I think, who said that it is hardly
possible to state any truth strongly without apparent injury to some other truth.  Under the circumstances, the
proper course appears to be to state both truths strongly,
and allow each its fair share, in the formation of the resultant conviction.  For truth is often of a dual character, taking the form  of a magnet with two poles; and
many of the differences which agitate the thinking part
of mankind are to be traced to the exclusiveness with
which different parties affirm' one half of the duality in
forgetfulness of the other half. But this waiting for the
statement of the two sides of a question implies patience. It implies a resolution to suppress indignation if
the statement of the one half should clash with our convictions, and not to suffer ourselves to be unduly elated
if the half-statement should chime in with our views.
It implies a determination to wait'calmly for the statement of the whole before we pronounce judgment either
in the form of acquiescence or dissent.
This premised, let us enter upon our task. There
have been writers who affirmed that the pyramids of
Egypt were the productions of nature; and in his early
youth Alexander Von Humboldt wrote an essay with
the express object of refuting this notion. We now regard the pyramids as the work of men's hands, aided




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probably by machinery of which no record remains.
We picture to ourselves the swarming workers toiling at
those vast erections, lifting the inert stones, and, guided
by the volition, the skill, and possibly at times by the
whip of the architect, placing the stones in their proper
positions.  The blocks in this case were moved by a
power external to themselves, and the final form of the
pyramid expressed the thought of its human builder.
Let us pass from this illustration of building power to
another of a different kind. When a solution of common salt is slowly evaporated, the water which holds the
salt in solution disappears, but the salt itself remains
behind.  At a certain stage of concentration, the salt
can no longer retain the liquid form; its particles, or
molecules, as they are called, begin to deposit themselves as minute solids, so minute, indeed, as to defy all
microscopic power.  As evaporation continues solidification goes on, and we finally obtain, through the clustering together of innumerable molecules, a finite mass
of salt of a definite form. What is this form? It sometimes seems a mimicry of the architecture of Egypt.
We have little pyramids built by the salt, terrace above
terrace from base to apex, forming thus a series of steps
resembling those up which the Egyptian traveler is
dragged by his guides.  The human mind is as little disposed to look at these pyramidal salt-crystals without
further question as to look at the pyramids of Egypt
without inquiring whence they came.  How, then, are
those salt pyramids built up?
Guided by analogy, you may suppose that, swarming
among the constituent molecules of the salt, there is an
invisible population, guided and coerced by some invisible master, and placing the atomic blocks in their posi



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tions.  This, however, is not the scientific idea, nor do
I think your good sense will accept it as a likely one.
The scientific idea is that the molecules act upon each
other without the intervention of slave labor; that they
attract each other and repel each other at certain
definite points, and in certain definite directions; and
that the pyramidal form is the result of this play of attraction  and  repulsion.  While, then, the blocks of
Egypt were laid down by a power external to themselves, these molecular blocks of salt are self-posited,
being fixed in their places by the forces with which they
act upon each other.
I take common salt as an illustration, because it is so
familiar to us all; but almost any other substance would
answer my purpose equally well.  In fact, throughout
inorganic nature, we have this formative power, as
Fichte would call it-this structural energy ready to
come into play, and build the ultimate particles of matter into definite shapes. It is present everywhere. The
ice of our winters and of our polar regions is its handwork, and so equally are the quartz, feldspar, and mica
of our rocks.  Our chalk-beds are for the most part
composed of minute shells, which are also the product
of structural energy; but behind the shell, as a whole,
lies the result of another and more subtle formative act.
These shells are built up of little crystals of calc-spar,
and to form these the structural force had to deal with
the intangible molecules of carbonate of lime.  This tendency on the part of matter to organize itself, to grow
into shape, to assume definite forms in obedience to the
definite action of force, is, as I have said, all-pervading.
It is in the ground on which you tread, in the water you
drink, in the air you breathe. Incipient life, in fact,




manifests itself throughout the whole of what we call
inorganic nature.
The forms of minerals resulting from  this play of
forces are various, and exhibit different degrees of complexity. Men of science avail themselves of all possible
means of exploring this moleculer architecture. For
this purpose they employ in turn as agents of exploration, light, heat, magnetism, electricity, and sound.
Polarized light is especially useful and powerful here:
A beam of such light, when sent in among the molecules of a crystal, is acted on by them, and from this action we infer with more or less of clearness the manner
in which the molecules are arranged.  The difference,
for example, between the inner structure of a plate of
rock-salt and a plate of crystalized sugar or sugar-candy
is thus strikingly revealed. These differences may be
made to display themselves in phenomena of color of
great splendor, the play of molecular force being so regulated as to remove certain of the colored constituents
of white light, and to leave others with increased intensity behind.
And now let us pass from what we are accustomed to
regard as a dead mineral to a living grain of corn.
When it is examined by polarized, light, chromatic phenomena similar to those noticed in crystals are observed.
And why? Because the architecture of the grain resembles in some degree the architecture of the crystal.
In the corn the molecules are also set in definite positions, from which they act upon the light.  But what
has built together the molecules of the corn? I have
already said, regarding crystalline architecture, that you
may, if you please, consider the atoms and molecules to
be placed in position by a power external to themselves.




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The same hypothesis is open to you now. But, if in the
case of crystals you have rejected this notion of an external architect, I think you are bound to reject it now,
and to conclude that the molecules of the corn are selfposited by the forces with which they act upon each
other.  It would be poor philosophy to invoke an external agent in the one case and to reject it in the other.
instead of cutting our grain into thin slices and subjecting it to the action of polarized light, let us place it
in the earth and subject it to a certain degree of warmth.
In other words, let the molecules, both of the corn and
of the surrounding earth, be kept in a state of agitation;
for warmth, as most of you know, is, in the eye of
science, tremulous molecular motion.  Under these circumstances, the grain and the substances.which surround
it interact, and a molecular architecture is the result of
this interaction.  A bud is formed; this bud reaches
the surface, where it is exposed to the sun's rays, which
are also to be regarded as a kind of vibratory motion.
And as the common motion of heat with which the grain
and the substances surrounding it were first endowed,
enable the grain and these substances to coalesce, so the
specific motion of the sun's rays now enables the green
bud to feed upon the carbonic acid and the aqueous
vapor of the air, appropriating those constituents of
both for which the blade has an elective attraction, and
permitting the other constituent to resume its place in
the air. Thus forces are active at the root, forces are
active in the blade, the matter of the earth and the
matter of the atmosphere are drawn towards the plant,
and the plant augments in size. We have in succession,
the bud, the stalk, the ear, the full corn in the ear.  For
the forces here at play act in a cycle, which is completed




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by the production of grains similar to that with which
the process began.
Now there is nothing in this process which necessarily
eludes the power of mind as we know it.  An intellect
the same kind as our own, would, if only sufficiently expanded, be able to follow the whole process from beginning to end. No entirely new intellectual faculty would
be needed for this purpose. The duly expanded mind
would see in the process and its consummation an instance of the play of molecular force. It would see
every molecule placed in its position by the specific attractions and repulsions exerted between it and other
molecules. Nay, given the grain and its environment,
an intellect the same in kind as our own, but sufficiently
expanded, might trace out a priori every step of the process, and by the application of mechanical principles
would be able to demonstrate that the cycle of actions
must end, as it is seen to end, in the reproduction of
forms like that with which the operation began. A similar necessity rules here to that which rules the planets
in their circuits round the sun.
You will notice that I am stating my truth strongly,
as at the beginning we agreed it should be stated. But
I must go still further, and affirm that in the eye of
science the animal body is-just as much the product of
molecular force as the stalk and ear of corn, or as the
crystal of salt or sugar. Many of its parts are obviously
mechanical.  Take the human heart,'for example, with
its exquisite system of valves, or take the eye or the
hand.  Animal heat, moreover, is the same in kind as
the heat of a fire, being produced by the same chemical
process.  Animal motion, too, is as directly derived
from the food of the animal, as the motion of Treve



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thyck's walking-engine from the fuel in its furnace. As
regards matter, the animal body creates nothing; as regards force, it creates nothing. Which of you by taking thought can add one cubit to his stature? All that
has been said regarding the plant may be re-stated with
regard to the animal. Every particle that enters into
the composition of the muscle, a nerve, or a bone, has
been placed in its position by molecular force. And
unless the existence of law in these matters be denied,
and the element of caprice be introduced, we must conclude that, given the relation of any molecule of the
body to its environment, its position in the body might
be predicted. Our difficulty is not with the quality of
the problem, but with its complexity; and this difficulty
might be met by the simple expansion of the faculties
which man now possesses.  Given this expansion, and
given the necessary molecular data, and the chick might
be deduced as rigorously and as logically from the egg
as the existence of Neptune was deduced from the disturbances of Uranus, or as conical refraction was deduced from the undulatory theory of light.
You see I am  not mincing matters, but avowing
nakedly what many scientific thinkers more or less distinctly believe. The formation of a crystal, a plant, or
an animal, is in their eyes a purely mechanical problem,
which differs from the problems of ordinary mechanics in
the smallness of the masses and the complexity of the
processes involved.  Here you have one half of our
dual truth; let us now glance at the other half. Associated with this wonderful mechanism of the animal
body we have phenomena no less certain than those of
physics, but between which and the mechanism we discern no necessary connection. A man, for example,




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can say I feel, I think, I love; but how does consciousness infuse itself into the problem? The human brain
is said to be the organ of thought and feeling; when
we are hurt the brain feels it, when we ponder it is the
brain that thinks, when our passions or affections are
excited it is through the instrumentality of the brain.
Let us endeavor to be a little more precise here.  I
hardly imagine that any profound scientific thinker who
has reflected upon the subject exists, who would not admit the extreme probability of the hypothesis, that for
every fact of consciousness, ivhether in the domain of
sense, of thought, or of emotion, a certain definite
molecular condition is set up in the brain; that this relation of physics to consciousness is invariable, so that,
given the state of the brain, the corresponding thought
or feeling might be inferred; or, given the thought or
feeling, the corresponding state of the brain might be
inferred. But how inferred? It is at bottom not a case
of logical inference at all, but of empirical association.
You may reply that many of the inferences of science
are of this character; the inference, for example, that
an electric current of a given direction will deflect a
magnetic needle in a definite way; but the cases differ
in this, that the passage from the current to the needle,
if not demonstrable, is thinkable, and that we entertain
no doubt as to the final mechanical solution of the problem; but the passage from the physics of the brain to
the corresponding facts of consciousness is unthinkable.  Granted that a definite thought and a definite
molecular action in the brain occur simultaneously, we
do not possess the intellectual organ, nor, apparently,
any rudiment of the organ, which would enable us to
pass by a process of reasoning from the one phenome



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non to the other. They appear together, but we do not
know why. Were our minds and senses so expanded,
strengthened, and illuminated as to enable us to see and
feel the very molecules of the brain; were we capable
of following all their motions, all their groupings, all
their electric discharges, if such there be; and were we
intimately acquainted with the corresponding states of
thought and feeling, we should be as far as ever from
the solution of the problem. " How are these physical
processes connected with the facts of consciousness?"
The chasm  between the two classes of phenomena
would still remain intellectually impassable.  Let the
consciousness of love, for example, be associated with
a right-handed spiral motion of the molecules of the
brain, and the consciousness of hate with a left-handed
spiral motion.  We should then know  when we love
that the motion is in one direction, and when we hate
that the motion is in the other; but the "WHY?" would
still remain unanswered.
In affirming that the growth of the body is mechanical, and that thought, as exercised by us, has its correlative in the physics of the brain, I think the position
of the " Materialist" is stated as far as that position is
a tenable one.  I think the materialist will be able
finally to maintain this position against all attacks; but
I do not think, as the human mind is at present constituted, that he can pass beyond it. I do not think he is
entitled to say that his molecular groupings and his
molecular motions explain everything. In reality they
explain nothing. The utmost he can affirm is the association of two classes of phenomena of whose real bond
of union he is in absolute ignorance. The problem of
the connection of the body and soul is as insoluble in




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its modern form  as it was in the pre-scientific ages.
Phosphorus is known to enter into the composition of
the human brain, and a courageous writer has exclaimed,
in his trenchant German, "Ohne phosphor kein gedanke."  That may or may not be the case; but even if
we knew it to be the case, the knowledge would not
lighten our darkness. On both sides of the zone here
assigned to the materialist he is equally helpless. If
you ask him whence is this " matter " of which we have
been discoursing, who or what divided it into molecules,
who or what impressed upon them this necessity of running into organic forms, he has no answer.  Science
also is mute in reply to these questions.  But if the
materialist is confounded, and science rendered dumb,
who else is entitled to answer? To whom has the
secret been revealed? Let us lower our heads and acknowledge our ignorance, one and all.  Perhaps the
mystery may resolve itself into  knowledge at some
future day. The process of things upon this earth has
been one of amelioration. It is a long way from the
Iguanodon and his contemporaries to the president and
members of the British Association. And whether we
regard the improvement from the scientific or from the
theological point of view as the result of progressive
development, or as the result of successive exhibitions
of creative energy, neither view entitles us to assume
that man's present faculties end the series-that the
process of amelioration stops at him. A  time may
therefore come when this ultra-scientific region by which
we are now enfolded may offer itself to terrestrial, if
not to human investigation.  Two-thirds of the rays
emitted by the sun fail to arouse in the eye the sense of
vision. The rays exist, but the visual organ requisite




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for their translation into light does not exist. And so
from this region of darkness and mystery which surrounds us, rays may now be darting which require but
the development of the proper intellectual organs to
translate them into knowledge as far surpassing ours as
ours does that of the wallowing reptiles which once
held possession of this planet. Meanwhile the mystery
is not without its uses. It certainly may be made a
power in the human soul; but it is a power which has
feeling, not knowledge,.for its base. It may be, and
will be, and we hope is turned to account, both in steadying and strengthening the intellect, and in rescuing man
from that littleness to which, in the struggle for existence or for precedence in the world, he is continually
prone.




II.
On  Haze and Dust.
Solar light in passing through a dark room reveals its
track by illuminating the dust floating in the air. "The
sun," says Daniel Culverwell, "discovers atomes, though
they be invisible by candle-light, and makes them dance
naked in his beams."
In my researches on the decomposition of vapors by
light, I was compelled to remove these " atomes" and
this dust.  It was essential that the space containing
the vapors should embrace no visible thing; that no
substance capable of scattering the light in the slightest
sensible degree should, at the outset of an experiment,
be found in the "experimental tube" traversed by the
luminous beam.
For a long time I was troubled by the appearance
there of floating dust, which, though invisible in diffuse
daylight, was at once revealed by a powerfully condensed
beam.  Two tubes were placed in succession in the
path of the dust: the one containing fragments of glass
wetted  with concentrated sulphuric acid; the other,
fragments of marble wetted with a strong solution of
caustic potash. To my astonishment it passed through
both. The air of the Royal Institution, sent through
these tubes at a rate sufficiently slow to dry it and to remove its carbonic acid, carried into the experimental
tube a considerable amount of mechanically-suspended
matter, which was illuminated when the beam passed
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through the tube.  The effect was substantially the
same when the air was permitted to bubble through the
liquid acid and through the solution of potash.
Thus, on the 5th of October, I868, successive charges
of air were admitted through the potash and sulphuric
acid into the exhausted experimental tube. Prior to the
admission of the air the tube was optically empty; it contained nothing competent to scatter the light. After
the air had entered the tube, the conical track of the
electric beam was in all cases clearly revealed. This,
indeed, was a daily observation at the time to which I
now refer.
I tried to intercept this floating matter in various
ways; and on the day just mentioned, prior to sending
the air through the drying apparatus, I carefully permitted it to pass over the tip of a spirit-lamp flame.
The floating matter no longer appeared, having been
burnt up by the flame. It was, therefore, organic matter.
When the air was sent too rapidly through the flame, a
fine blue cloud was found in the experimental tube.
This was the smoke of the organic particles. I was by
no means prepared for this result; for I had thought,
with the rest of the world, that the dust of our air was,
in great part, inorganic and non-combustible.
Mr. Valentin had the kindness to procure for me a
small gas-furnace, containing a platinum tube, which
could be heated to vivid redness. The tube also contained a roll of platinum  gauze, which, while it permitted the air to pass through it, insured tlhe practical
contact of the dust with the incandescent metal. The
air of the laboratory was permitted to enter the experimental tube, sometimes through the cold, and sometimes through the heated tube of platinum. The rapid



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ity of admission was also varied. In the first column
of the following table the quantity of air operated on is
expressed by the number of inches which the mercury
gauge of the air-pump sank when the air entered.  In
the second column the condition of the platinum tube is
mentioned, and in the third the state of the air which
entered the experimental tube.
Quantity of Air.  Stite of Platinum Tube.  State of Experimental Tube.
15 inches..  Cold...  Full of particles.
I5   "     ~.     Red-hot..  Optically empty.
15   "     ~.  Cold...Full of particles.
I5   "..  Red-hot..  Optically empty.
I5   "..  Cold... Full of particles.
I5   "..  Red-hot.   Optically empty.
The phrase "optically empty" shows that when the
conditions Af perfect combustion were present, the floating matter totally disappeared.  It was wholly burnt up,
leaving not a trace of residue. From spectrum analysis,
however, we know that soda floats in the air; these organic dust particles are, I believe, the rafts that support
it, and when they are removed it sinks and vanishes.
When the passage of the air was so rapid as to render imperfect the combustion of the floating matter, instead of optical emptiness a fine blue cloud made its appearance in  the experimental tube.   The following
series of results illustrate this point:
Quantity.       Platinum Tube.    Experimental Tube.
15 inches, slow. Cold.. Full of particles.
I5   "           ". Red-hot.. Optically empty.
15    "   quick.      ".. A blue cloud.
c5 "  ". Intensely hot  A fine blue cloud.
The optical character of these clouds was totally different from that of the dust which produced them. At
right angles to the illuminating beam  they discharged




( 23 )
perfectly polarized light. The cloud could be utterly
quenched by a transparent Nicol's prism, and the tube
containing it reduced to optical emptiness.
The particles floating in the air of London being thus
proved to be organic, I sought to burn them up at the
focus of a concave reflector. One of the powerfully
convergent mirrors employed in my experiments on
combustion by dark rays was here made use of, but I
failed in the attempt. Doubtless the floating particles
are in part transparent to radiant heat, and are so far
incombustible by such heat. Their rapid motion through
the focus also aids their escape.  They do not linger
there sufficiently long to be consumed. A flame it was
evident would burn them up, but I thought the presence
of the flame would mask its own action among the particles.
In a cylindrical beam, which powerfully illuminated
the dust of the laboratory, was placed an ignited spiritlamp.  Mingling with the flame, and round its rim, were
seen wreaths of darkness resembling an intensely black
smoke. On lowering the flame below the beam  the
same dark masses stormed upwards. They were at times
blacker than the blackest smoke that I have ever seen
issuing from the funnel of a steamer, and their resemblance to smoke was so perfect as to lead the most practiced observer to conclude that the apparently pure
flame of the alcohol lamp required but a beam of sufficient intensity to reveal its clouds of liberated carbon.
But is the blackness smoke? The question presented
itself in a moment. A red-hot poker was placed underneath the beam, and from  it the black wreaths also
ascended. A large hydrogen flame was next employed,
and it produced those whirling masses of darkness far




(24)
more copiously than either the spirit-flame or poker.
Smoke was, therefore, out of the question.
What, then, was the blackness? It was simply that
of stellar space; that is to say, blackness resulting from
the absence from the track of the beam of all matter
competent to scatter its light. When the flame was
placed below the beam the floating matter was destroyed
in situ; and the air, freed from this matter, rose into the
beam, jostled aside the illuminated particles and substituted for their light the darkness due to its own perfect
transparency.   Nothing could more forcibly illustrate
the invisibility of the agent which renders all things visible. The beam crossed, unseen, the black chasm formed
by the transparent air, while at both sides of the gap
the thick-strewn particles shone out like a luminous solid
under the powerful illumination.
But here a difficulty meets us. It is not necessary to
burn the particles to produce a stream  of darkness.
Without actual combustbn, currents may be generated
which shall exclude the floating matter, and therefore
appear dark amid the surrounding brightness.  I noticed
this effect first on placing a red-hot copper ball below
the beam, and permitting it to remain there until its
temperature had fallen below that of boiling water.
The dark currents, though much enfeebled, were still
produced.  They may also be produced by a flask filled
with hot water.
To study this effect a platinum wire was stretched
across the beam, the two ends of the wire being connected with the two poles of a voltaic battery. To regulate the strength of the current a rheostat was placed
in the circuit.  Beginning with a feeble current the
temperature of the wire was gradually augmented, but




( 25)
before it reached the heat of ignition, a flat stream of
air rose from  it, which when looked at edgeways appeared darker and sharper than one of the blackest
lines of Fraunhofer in the solar spectrum. Right and
left of this dark vertical band the floating matter rose
upwards, bounding definitely the non-luminous stream
of air. What is the explanation?  Simply this. The
hot wire rarefied the air in contact with it, but it did not
equally lighten the floating matter. The convection
current of pure air therefore passed upwards among tke
particles, dragging them after it right and left, but forming between them an impassable black partition.  In
this way we render an account of the dark currents produced by bodies at a temperature below that of combustion.
Oxygen, hydrogen, nitrogen, carbonic acid, so prepared as to exclude all floating particles, produce the
darkness when poured or blown into the beam.  Coalgas does the same. An ordiitry glass shade placed in
the air with its mouth downwards permits the track of
the beam to be seen crossing it. Let coal-gas or hydrogen enter the shade by a tube reaching to its top, the
gas gradually fills the shade from the top downwards.
As soon as it occupies the space crossed by the beam,
the luminous track is instantly abolished. Lifting the
shade so as to bring the common boundary of gas and
air above the beam, the track flashes forth. After the
shade is full, if it be inverted, the gas passes upwards
like a black smoke among the illuminated particles.
The air of our London rooms is loaded with this organic dust, nor is the country air free from its pollution.
However ordinary daylight may permit it to disguise
itself, a sufficiently powerful beam  causes the air in




( 26 )
which the dust is suspended to appear as a semi-solid
rather than as a gas. Nobody could, in the first instance, without repugnance place the mouth at the
illuminated focus of the electric beam and inhale the
dirt revealed there. Nor is the disgust abolished by the
reflection that, although we do not see the nastiness, we
are churning it in our lungs every hour and minute of
our lives. There is no respite to this contact with dirt;
and the wonder is, not that we should from time to time
suffer from its presence, but that so small a portion of
it would appear to be deadly to man.
And what is this portion?  It was some time ago the
current belief that epidemic diseases generally were propagated by a kind of malaria, which consisted of organic matter in a state of mnotor-decay; that when such
matter was taken into the body through the lungs or
skin, it had the power of spreading there the destroying
process whic: had attacked itself.  Such a spreading
power was visibly exerted it the case of yeast. A little
leaven was seen to leaven the whole lump, a mere speck
of matter in this supposed state of decomposition being
apparently competent to propagate indefinitely its own
decay. Why should not a bit of rotten malaria work in
a similar manner within the human frame?  In I836 a
very wonderful reply was given to this question. In
that year Cagniard de la Tour discovered the yeast plant,
a living organism, which, when placed in a proper
medium, feeds, grows, and reproduces itself, and in this
way carries on the process which we name fermentation.
Fermentation was thus proved to be a product of life
instead of a process of decay.
Schwann, of Berlin, discovered the yeast plant independently, and in February, I837, he also announced the




( 27 )
important result, that when a decoction of meat is effectually screened from ordinary air, and supplied solely
with air which has been raised to a high temperature,
putrefaction never sets in. Putrefaction, therefore, he
affirmed to be caused by something derived from the air,
which something could be destroyed by a sufficiently
high temperature.  The experiments of Schwann were
repeated and confirmed by Helmholtz and Ure.  But
as regards fermentation, the minds of chemists, influenced probably by the great authority of Gay-Lussac,
who ascribed putrefaction to the action of oxygen, fell
back upon the old notion of matter in a state of decay.
It was not the living yeast plant, but the dead or dying
parts of it, which, assailed by oxygen, produced the fermentation. This notion was finally exploded by Pasteur.
He proved that the so-called " ferments" are not such;
that the true ferments are organized beings which find
in the reputed ferments their necessary food.
Side by side with these rsearches and discoveries, and
fortified by them and others, has run the germ theory of
epidemic disease.  Tihe notion was expressed by Kircher,
and favored by Linnaeus, that epidemic diseases are due
oggerms which float in the atmosphere, enter the body,
and produce disturbance by the development within the
body of parasitic life. While it was still struggling
against great odds, this theory found an expounder and
a defender in the President of this Institution.  At a
time when most of his medical brethren considered
it a wild dream, Sir Henry Holland contended that
some form. of the germ theory was probably true. The
strength of this theory consists in the perfect parallelism
of the phenomena of contagious disease with those of
life. As a planted acorn gives birth to an oak compe2




(28)
tent to produce a whole crop of acorns, each gifted with
the power of reproducing its parent tree, and as thus
from a single seedling a whole forest may spring, so
these epidemic diseases literally plant their seeds, grow,
and shake abroad new germs, which, meeting in the
human bodytheir proper food and temperature, finally
take possession of whole.. populations.  Thus Asiatic
cholera, beginning in a small way in the Delta of the
Ganges, contrived in seventeen years to spread itself
over nearly the whole habitable world. The development from an infinitesimal speck of the virus of smallpox of a crop of pustules, each charged with the original poison, is another illustration. The reappearance
of the scourge, as in the case of the Dreadnought at
Greenwich, reported on so ably by Dr. Budd and Mr.
Busk, receives a satisfactory explanation from the theory
which ascribes it to the lingering of germs about the infected place.
Surgeo-.as.. have long known the danger of permitting
air to enter an open abscess. To prevent its entrance
they employ a tube called a cannula, to which is attached a sharp steel point called a trocar.  They puncture with the steel point, and by gentle pressure they
force the pus through the cannula. It is necessary to
be very careful in cleansing the instrument; and it is
difficult to see how it can be cleansed by ordinary
methods in air loaded with organic impurities, as we
have proved our air to be.  The instrument ought, in
fact, to be made as lot-t" as —its- -temper will bear.  But
this is not done, and hence, notwithstanding all the surgeon's care, inflammation often sets in after the first operation, rendering necessary a second and  a third.
Rapid putrefaction is found to accompany this new in



'29)
flammation.  The pus, moreover, which was sweet at
first, and showed no trace of animal life, is now fetid,
and swarming with active little organisms called vibrios.
Prof. Lister, from whose recent lecture this fact is derived, contends,.with every show of reason, that this
rapid putrefaction and this astounding development of
animal life are due to the entry of germs into the abscess
during the first operation, and their subsequent nurture
and development under favorable conditions of food and
temperature.  The celebrated physiologist andphysicist,
Helmholtz, is attacked annually by hay-fever.  From
the 20th of May to the end of June he suffers from a
catarrh of the upper air-passages; and he has found
during this period, and at no other, that his nasal secretions are peopled by these vibrios.  They appear to
nestle by preference in the cavities and recesses of the
nose, for a strong sneeze is necessary to dislodge them.
These statements sound uncomfortable  but by disclosing our enemy they lhable us to fight him.  When
he clearly eyes his quarry the eagle's strength is doubled,
and his swoop is rendered sure.  If the germ theory be
proved true, it will give -a definiteness to our efforts to
stamp out disease which they could not previously possess.  And it is only by definite effort under its guidance that its truth or falsehood can be established,  It
is difficult for an outsider like myself to read without
sympathetic emotion such papers as those of Dr. Budd,
of Bristol, on cholera, scarlet-fever, and small-pox.  He
is a man of strong imagination, and may occasionally
take a flight beyond his facts; but without this dynamic
heat of heart, the stolid inertia of the free-born Briton
cannot be overcome.  And as long as the heat is employed to warm up the truth without singeing it over



(30)
much; as long as this enthusiasm  can overmatch its
mistakes by unequivocal examples of success, so long
am I disposed to give it a fair field to work in, and to
wish it God speed.
But let us return to our dust. It is needless to remark that it cannot be blown away by an ordinary bellows; or, more correctly, the place of the particles
blown away is in this case supplied by others ejected
from the bellows, so that the track of the beam remains
unimpaired.  But if the nozzle of a good bellows be
filled with cotton wool not too tightly packed, the air
urged through the wool is filtered of its floating matter,
and it then forms a clean band of darkness in the illuminated dust. This was the filter used by Schroeder in
his experiments on spontaneous generation, and turned
subsequently to account in the excellent researches of
Pasteur.  Since i868 I have constantly employed it
myself.
But by far the most interesting and important illustration of this filtering process is furnished by the human breath.  I fill my lungs with ordinary air and
breathe through a glass tube across the electric beam.
The condensation of the aqueous vapor of the breath is
shown by the formation of a luminous white cloud of
delicate texture. It is necessary to abolish this cloud,
and this may be done by drying the breath previous to
its entering into the beam; or still more simply, by
warming the glass tube. When this is done the luminous track of the beam is for a time uninterrupted. The
breath impresses upon the floating matter a transverse motion, but the dust from the lungs makes good the particles
displaced. But after some time an obscure disc appears
upon the beam, the darkness of which increases, until




(3')
finally, towards the end of the expiration, the beam is,
as it were, pierced by an intensely black hole, in which
no particles whatever can be discerned. The air, in
fact, has so lodged its dirt within the lungs as to render
the last portions of the expired breath absolutely free.
from suspended matter. This experiment may be repeated any number of times with the same result. It
renders the distribution of the dirt within the lungs as
manifest as if the chest were transparent.
I now empty my lungs as perfectly as possible, and
placing a handful of cotton wool against my mouth and
nostrils, inhale through it. There is no difficulty in
thus filling the lungs with air. On expiring this air
through the glass tube, its freedom from floating matter
is at once manifest. From the very beginning of the
act of expiration the beam is pierced by a black aperture. The first puff from the lungs abolishes the illuminated dust and puts a patch of darkness in its place,
and the darkness continues throughout the entire course
of the expiration. When the tube is placed below the
beam and moved to and fro, the same smoke-like appearance as that obtained with a flame is observed. In
short, the cotton wool, when used in sufficient quantity,
completely intercepts the floating matter on its way to
the lungs.
And here we have revealed to us the true philosophy
of a practice followed by medical men, more from instinct than from actual knowledge. In a contagious atmosphere the physician places a handkerchief to his
mouth and inhales through it. In doing so he unconciously holds back the dirt and germs of the air. If the
poison were a gas it would not be thus intercepted.
On showing this experiment with the cotton wool to Dr.




(3  )
Bence Jones, he immediately repeated it with a silk
handkerchief.  The result was substantially the same,
though, as might be expected, the wool is by far the
surest filter. The application of these experiments is
obvious. If a physician wishes to hold back from the
lungs of his patient, or from his own, the germs by
which contagious disease is said to be propagated, he
will employ a cotton wool respirator. After the revelations of this evening, such respirators must, I think,
come into general use as a defence against cohtagion.
In the crowded dwellings of the London poor, where
the isolation of the sick is difficult, if not impossible,
the noxious air around the patient may, by this simple
means, be restored to practical purity. Thus filtered,
attendants may breathe the air unharmed. In all probability  the  protection of the lungs will be protection  of the  entire  system.  For it is exceedingly
probable that the germs which  lodge  in  the  airpassages, and which, at their leisure, can work their
way across the mucous membrane, are those which sow
in the body epidemic disease.  If this be so, then
disease can certainly be warded off by filters of cotton
wool. I should be most willing to test their efficacy in
my own person.  And time will decide whether in lung
diseases also the woolen respirator cannot abate irritation, if not arrest decay. By its means, so far as the
germs are concerned, the air of the highest Alps may
be brought into the chamber of the invalid.




III.
Scientific Use of the Imagination.
I carried with me to the Alps this year the heavy burden of this evening's work. In the way of new investigation I had nothing complete enough to be brought
before you; so all that remained to me was to fall back
upon such residues as I could find in the depths of consciousness, and out of them to spin the fiber and weave
the web of this discourse.  Save from memory I had no
direct aid upon the mountains; but to spur up the emotions, on which so much depends, as well as to nourish
indirectly the intellect and will, I took with me two
volumes of poetry, Goethe's "Farbenlehre," and the work
on " Logic" recently published by Mr. Alexander Bain.
The spur, I am sorry to say, was no match for the integument of dullness it had to pierce.
In Goethe, so glorious otherwise, I chiefly noticed the
self-inflicted hurts of genius, as it broke itself in vain
against the philosophy of Newton. For a time Mr.
Bain became my principal companion.  I found him
learned and practical, shining generally with a dry light,
but exhibiting at times a flush of emotional strength,
which proved that even logicians share the common fire
of humanity. He interested me most when he became
the mirror of my own condition. Neither intellectually
nor socially is it good for man to be alone, and the
griefs of thought are more patiently borne when we find
that they have been experienced by another. From cer(33)




(34)
tain passages in his book I could infer that Mr. Bain
was no stranger to such sorrows.  Take this passage as
an illustration. Speaking of the ebb of intellectual
force which we all from time to time experience, Mr
Bain says: "The uncertainty where to look for the next
opening of discovery brings the pain of conflict and the
debility of indecision."  These words have in them the
true ring of personal experience.
The action of the investigator is periodic.  He grapples with a subject of inquiry, wrestles with it, overcomes it, exhausts, it may be, both himself and it for
the time being. He breathes a space, and then renews
the struggle in another field.  Now this period of halting between two investigations is not always one of pure
repose. It is often a period of doubt and discomfort,
of gloom and ennui.  "The uncertainty where to look
for the next opening of discovery brings the pain of conflict and the debility of indecision."  Such was my precise condition in the Alps this year; in a score of words
Mr. Bain has here sketched my mental diagnosis; and
it was under these evil circumstances that I had to
equip myself for the hour and the ordeal that are now
come.
Gladly, however, as I should have seen this duty in
other hands, I could by no means shrink from it.  Disloyalty would have been worse than failure.  In some
fashion or other-feebly or strongly, meanly or manfully,
on the higher levels of thought, or on the flats of commonplace-the task had to be accomplished.  I looked
in various directions for help and furtherance; but without me for a time I saw only "antres vast," and within
me "deserts idle."  My case resembled that of a sick
doctor who had forgotten his art, and sorely needed the




(35)
prescription of a friend.  Mr. Bain wrote one for me.
He said: " Your present knowledge must forge the links
of connection between what has been already achieved
and what is now required."
In these words he admonished me to review the past
and recover from it the broken ends of former investigations.  I tried to do so.  Previous to going to Switzerland I had been thinking much of light and heat, of
magnetism  and electricity, of organic germs, atoms,
molecules, spontaneous generation, comets and skies.
With one or another of these I now sought to re-form
an alliance, and finally succeeded in establishing a kind
of cohesion between thought and light. The wish grew
within me to trace, and to enable you to trace, some of
the more occult operations of this agent. I wished, if
possible, to take you behind the drop-scene of the senses,
and to show you the hidden mechanism of optical
action. For I take it to be well worth the while of the
scientific teacher to take some pains, and even great
pains, to make those whom he addresses co-partners of
his thoughts. To clear his own mind in the first place
from all haze and vagueness, and then to project into
language which shall leave no mistake as to his meaning-which shall leave even his errors naked-the definite ideas he has shaped.
A great deal is, I think, possible to scientific exposition conducted in this way. It is possible, I believe,
even before an audience like the present, to uncover to
some extent the unseen things of nature, and thus to
give, not only to professed students, but to others with
the necessary bias, industry and capacity, an intelligent
interest in the operations of science. Time and labor
are necessary to this result, but science is the gainer
from the public sympathy thus created.




(36-)
How then are those hidden things to be revealed?
How, for example, are we to lay hold of the physical
basis of light, since, like that of life itself, it lies entirely
without the domain of the senses? Now, philosophers
may be right in affirming that we cannot transcend experience. But we can, at all events, carry it a long way
from its origin. We can also magnify, diminish, qualify,
and combine experiences, so as to render them fit for
purposes entirely new. We are gifted with the power of
imagination, combining what the Germans called Anschauungsgabe and Einbildungskraft, and by this power
we can lighten the darkness which surrounds the world
of the senses.
There are tories even in science who regard imagination as a faculty to be feared and avoided rather than
employed. They had observed its action in weak vessels and were unduly impressed by its disasters. But
they might with equal justice point to exploded boilers
as an argument against the use of steam. Bounded and
conditioned by co6perant reason, imagination becomes
the mightiest instrument of the physical discoverer.
Newton's passage from a falling apple to a falling moon
was a leap of the imagination. When William Thomson tries to place the ultimate particles of matter between his compass points, and to apply to them a scale
of millimeters, it is an exercise of the imagination.
And in much that has been recently said about protoplasm and life, we have the outgoings of the imagination
guided and controlled by the known analogies of science.
In fact, without this power our knowledge of nature
would be a mere tabulation of coexistences and sequences.
We should still believe in the succession of day and
night, of summer and winter; but the soul of force




(37)
would be dislodged from our universe; casual relations
would disappear, and with them  that science which is
now binding the parts of nature to an organic whole.
I should like to illustrate by a few simple instances
the use that scientific men have already made of this
power of imagination, and to indicate afterwards some
of the further uses that they are likely to make of it.
Let us begin with the rudimentary experiences. Observe
the falling of heavy rain drops into a tranquil pond.
Each drop as it strikes the water becomes a center of
disturbance, from which a series of ring ripples expands
outwards. Gravity and inertia are the agents by which
this wave motion is produced, and a rough experiment
will suffice to show that the rate of propagation does
not amount to a foot a second.
A series of slight mechanical shocks is experienced
by a body plunged in the water as the wavelets reach it
in succession.  But a finer motion is at the same time
set up and propagated.  If the head and ears be immersed in the water, as in an experiment of Franklin's,
the shock of the drop is communicated to the auditory
nerve  the  tick  of the  drop  is heard.  Now  this
sonorous impulse is propagated, not at the rate of a
foot a second, but at the  rate  of 4,700oo   feet a
second.  In this case it is not the gravity but the
elasticity of the water that is the urging force. Every
liquid particle pushed against its neighbor delivers up
its motion with extreme rapidity, and the pulse is propagated as a thrill. The incompressibility of water, as
illustrated by the famous Florentine experiment, is a
measure of its elasticity, and to the possession of this
property in so high a degree the rapid transmission of,a sound-pulse through water is to be ascribed.




(38)
But water, as you know, is not necessary to the conduction of sound; air is its most common vehicle. And
you know  that when the air possesses the particular
density and elasticity corresponding to the temperature
of freezing water, the velocity of sound in it is i,o90 feet
a second. It is almost exactly one-fourth of the velocity in water; the reason being that though the greater
weight of the water tends to diminish the velocity, the
enormous molecular elasticity of the liquid far more
than atones for the disadvantage due to weight. By
various contrivances we can compel the vibrations of
the air to declare themselves; we know the length and
frequency of sonorous waves, and we have also obtained
great mastery over the various methods by which the
air is thrown into vibration. We know the phenomena
and laws of vibrating rods, of organ pipes, strings,
membranes, plates, and bells.  We can abolish one
sound by another. We know the physical meaning of
music and noise, of harmony and discord. In short, as
regards sound we have a very clear notion of the external physical processes which correspond to our sensations.
In. these phenomena of sound we travel a very little
way from downright sensible experience. Still the imagination is to some extent exercised. The bodily eye,
for example, cannot see the condensations and rarefactions of the waves of sound.  We construct them in
thought, and we believe as firmly in their existence as
in that of the air itself. But now our experience has to
be carried into a new region, where a new use is to be
made of it.
Having mastered  the cause and  mechanism  of
sound, we desire to know  the cause and mechanism




(39)
of light. We wish to extend our inquiries from the auditory nerve to the optic nerve. Now there is in the
human intellect a power of expansion-I might almost
call it a power of creation-which is brought into play
by the simple brooding upon facts. The legend of the
Spirit brooding over chaos may have originated in a
knowledge of this power. In the case now before us it
has manifested itself by transplanting into space, for
the purposes of light, an adequately modified form of
the mechanism of sound. We know intimately whereon
the velocity of sound depends. When we lessen the
density of a medium  and preserve its elasticity constant, we augment the velocity. When we highten the
elasticity and keep the density constant, we also augment the velocity.  A small density, therefore, and
a great elasticity are the two things necessary to rapid
propagation.
Now light is known to move with the astounding
velocity of I85,000 miles a second. How is such a
velocity to be obtained? By boldly diffusing in space
a medium of the requisite tenuity and elasticity. Let
us make such a medium our starting point, endowing it
with one or two other necessary qualities; let us handle
it in accordance with strict mechanical laws; give to
every step of your deduction the surety of the syllogism;
carry it thus forth from the world of imagination to the
world of sense, and see whether the final outcrop of the
deduction be not the very phenomena of light which
ordinary knowledge and skilled experiment reveal. If
in all the multiplied varieties of these phenomena, including those of the most remote and entangled description, this fundamental conception always brings us face
to face with the truth; if no contradiction to ourdeduc



(40)
tions from it be found in external nature; if, moreover,
it has actually forced upon our attention phenomena
which no eye had previously seen, and which no mind
had previously imagined; if by it we are gifted with a
power of prescience which has never failed when
brought to an experimental test; such a conception,
which never disappoints us, but always lands us on the
solid shores of fact, must, we think, be something more
than a mere figment of the scientific fancy.  In forming
it that composite and creative unity in which reason and
imagination are together blent, has, we believe, led us
into a world not less real than that of the senses, and
of which the world of sense itself is the suggestion and
justification.
Far be it from me, however, to wish to fix you immovably in this or in any other theoretic conception. With
all our belief of it, it will be well to keep the theory
plastic and capable of change.  You.may, moreover,
urge that although the phenomena occur as if the medium  existed, the absolute demonstration of its existence is still wanting. Far be it from me to deny to this
reasoning such validity as it may fairly claim. Let us
endeavor by means of analogy to form  a fair estimate
of its force.
You believe that in society you are surrounded by
reasonable beings like yourself.  You are, perhaps, as
firmly convinced of this as of anything. What is your
warrant for this conviction? Simply and solely this, your
fellow-creatures behave as if they were reasonable; the
hypothesis, for it is nothing more, accounts for the facts.
To take an eminent example, you believe that our president is a reasonable being. Why?  Thereis no known
method of superposition by which any one of us can




(4T)
apply himself intellectually to another so as to demonstrate coincidence as regards the possession of reason.
If, therefore, you hold our president to be reasonable,
it is because he behaves as if he were reasonable.  As
in the case of the ether, beyond the "as if" you cannot
*go. Nay, I should not wonder if a close comparison of
the data on which both inferences rest caused many respectable persons to conclude that the ether had the.best of it.
This universal medium, this light-ether as it is called,
is a vehicle, not an origin of Wave motion.  It receives
and transmits, but it does not create. Whence does it
derive the motions it conveys? For the most part from
luminous bodies. By this motion of a luminous body I
do not mean its sensible motion, such as the flicker of a
candle, or the shooting out of red prominences from the
limb of the sun.  I mean an intestine motion of the
atoms or molecules of the luminous body.  But here a
certain reserve is necessary.  Many chemists of the
present day refuse to speak of atoms and molecules as
real things. Their caution leads them to stop short of
the clear, sharp, mechanically intelligible atomic theory
enunciated by Dalton, or any form  of that theory, and
to make the doctrine of multiple proportions their intellectual bourne.  I respect the caution, though I think it
is here misplaced.  The chemists who recoil from these
notions of atoms and molecules accept without hesitation the undulatory theory of light. Like you and me
they one and all believe in an ether and its light-producing waves.  Let us consider what this belief involves.
Bring your imaginations once more into play and
figure a series of sound waves passing through air.




(42)
Follow them up to their origin, and what do you there
find? A definite, tangible, vibrating body. It may be
the vocal chords of a human being, it may be an organ
pipe, or it may be a stretched string.  Follow in the
same manner a train of ether waves to their source, remembering at the same time that your ether is matter,
dense, elastic, and capable of motions subject to and
determined by mechanical laws. What then do you expect to find as the source of a series of ether waves?
Ask your imagination if it will accept a vibrating multiple proportion-a numerical ratio in a state of oscillation? I do not think it will. You cannot crown the
edifice by this abstraction.  The scientific imagination,
which is here authoritative, demands as the origin and
cause of a series of ether waves a particle of vibrating
matter quite as definite, though it may be excessively
minute, as that which gives origin to a musical sound.
Such a particle we name an atom or a molecule.  I
think the imagination when focused so as to give definition without penumbral haze is sure to realize this image at last.
To preserve thought continuous throughout this discourse, to prevent either lack of knowledge or failure of
memory from producing any rent in our picture, I here
propose to run rapidly over a bit of ground which is
probably familiar to most of you, but which I am anxious to make familiar to you all.
The waves generated in the ether by the swinging
atoms of luminous bodies are of different lengths and
amplitudes. The amplitude is the width of swing of
the individual particles of the wave. In water waves
it is the hight of the crest above the trough, while the
length of the wave is the distance between two con



(43)
secutive crests.  The aggregate of waves emitted by the
sun may be broadly divided into two classes, the one
class competent, the other incompetent, to excite vision.
But the light-producing waves differ markedly among
themselves in size, form, and force. The length of the
largest of these waves is about twice that of the smallest, but the amplitude of the largest is probably a hundred times that of the smallest. Now the force or energy
of the wave, which, expressed with reference to sensation, means the intensity of the light, is proportional to
the square of the amplitude. Hence the amplitude
being one hundred-fold, the energy of the largest lightgiving waves would be ten thousand-fold that of the
smallest. This is not improbable. I use these figures,
not with a view to numerical accuracy, but to give you
definite ideas of the differences that probably exist
among the light-giving waves.  And if we take  the
whole range of solar radiation into account-its nonvisual as well as its visual waves-I think it probable
that the force, or energy of the largest wave is a million
times that of the smallest.
Turned into their equivalents of sensation, the different light waves produce different colors. Red, for example, is produced by the largest waves, violet by the
smallest, while green is produced by a wave of intermediate length and amplitude. On entering from air into
more highly refracting substances, such as glass or water
or the sulphide of carbon, all the waves are retarded,
but the smallest ones most.  This furnishes a means of
separating the different classes of waves from each
other-in other words, of analyzing the light.  Sent
through a refracting prism, the waves of the sun are
turned aside in different degrees from their direct course,




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the red least, the violet most. They are virtually pulled
asunder, and they paint upon a white screen placed to
receive them "the solar spectrum."
Strictly speaking, the spectrum embraces an infinity
of colors, but the limits of language and of our powers
of distinction cause it to be divided into seven segments:
Red, orange, yellow, green, blue, indigo, violet. These
are the seven primary or prismatic colors. Separately,
or mixed in various proportions, the solar waves yield
all the colors observed in nature and employed in art.
Collectively they give us the impression of whiteness.
Pure unsifted solar light is white; and if all the wave
constituents of such light be reduced in the same proportion, the light, though diminished in intensity, will
still be white.  The whiteness of Alpine snow with the
sun shining upon it is barely tolerable to the eye. The
same snow under an overcast firmament is still white.
Such a firmament enfeebles the light by reflection, and
when we lift ourselves above a cloud-field-to an Alpine
summit, for instance, or to the top of Snowdon-and
see, in the proper direction, the sun shining on the
clouds, they appear dazzlingly white. Ordinary clouds,
in fact, divide the solar light impinging on them into
two parts-a reflected part and a transmitted part, in
each of which the proportions of wave motion which
produce the impression of whiteness are sensibly preserved.
It will be understood that the conditions of whiteness
would fail if all the waves were diminished equally, or
by the same absolute quantity. They must be reduced
proportionately instead of equally.  If by the act of reflection the waves of red light are split into exact halves,
then, to preserve the light white, the waves of yellow,




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orange, green, and blue must also be split into exact
halves. In short, the reduction must take place, not by
absolutely equal quantities, but by equal fractional parts.
In white light the preponderance as regards energy of
the larger over the smaller waves must always be
immense.  Were the case otherwise, the physiological
correlative, blue, of the smaller waves would have the
upper hand in our sensations.
My wish to render our mental images complete, causes
me to dwell briefly upon these known points, and the
same wish will cause me to linger a little longer among
others. But here I am disturbed by my reflections. When
I consider the effect of dinner upon the nervous system,
and the relation of that system to the intellectual powers I
am now invoking; when I remember that the universal
experience of mankind has fixed upon certain definite
elements of perfection in an after-dinner speech, and
when I think how conspicuous by their absence these
elements are on the present occasion, the thought is not
comforting to a man who wishes to stand well with his
fellow-creatures in general, and with the members of the
British Association in particular. My condition might
well resemble that of the ether, which is scientifically
defined as an assemblage of vibrations. And the worst
of it is that, unless you reverse the general verdict regarding the effect of dinner, and prove in your own persons that a uniform experience need not continue uniform-which will be a great point gained for some
people-these tremors of mine are likely to become
more and more painful.  But I call to mind the comforting words of an inspired, though uncanonical writer,
who admonishes us in the Apocrypha that fear is a bad




(46)
counsellor. Let me then cast him out, and let me trustfully assume that you will one and all postpone that
balmy sleep, of which dinner might, under the circumstances, be regarded as the indissoluble antecedent, and
that you will manfully and womanfully prolong your investigations of the ether and its waves into regions
which have been hitherto crossed.by the pioneers of
science alone.
Not only are the waves of ether reflected by clouds,
by solids, and by liquids, but when they pass from light
air to dense, or from dense air to light, a portion of the
wave-motion is always reflected. Now our atmosphere
changes continually in density from top to bottom. It
will help our conceptions if we regard it as made up of
a series of thin concentric layers or shells of air, each
shell being of the same density throughout, and a small
and sudden change of density occuring in passing from
shell to shell. Light would be reflected at the limiting
surfaces of all these shells, and their action would be
practically the same as that of the real atmosphere.
And now I would ask your imagination to picture this
act of reflection.  What must become of the reflected
light? The atmospheric layers turn their convex surfaces towards the sun; they are so many convex mirrors of feeble power, and you will immediately perceive
that the light regularly reflected from these surfaces
cannot reach the earth at all, but is dispersed in space.
But though the sun's light is not reflected in this
fashion from the aerial layers to the earth, there is indubitable evidence to show that the light of our firmament
is reflected light. Proofs of the most cogent description could be here adduced; but we need only consider
that we receive light at the same time from all parts of




(47)
the hemisphere of heaven.  The light of the firmament
comes to us across the direction of the solar rays, and
even against the direction of the solar rays; and this
lateral and opposing rush of wave-motion can only be
due to the rebound of the waves from the air itself, or
from something suspended in the air. It is also evident
that, unlike the action of clouds, the solar light is not
reflected by the sky in the proportions which produce
white.  The sky is blue, which indicates a deficiency
on the part of the larger waves. In accounting for the
color of the sky, the first question suggested by analogy
would undoubtedly be, is not the air blue? The blueness of the air has, in fact, been given as a solution of
the blueness of the sky.  But reason basing itself on
observation asks in reply, How, if the air be blue, can
the light of sunrise and sunset, which travels through
vast distances of air, be yellow, orange, or even red?
The passage of the white solar light through a blue medium  could by no possibility redden the light.  The
hypothesis of a blue air is therefore untenable. In fact,
the agent, whatever it is, which sends us the light of the
sky, exercises in so doing a dichroitic action. The light
reflected is blue, the light transmitted is orange or red.
A marked distinction is thus exhibited between the matter of the sky and that of an ordinary cloud, which latter exercises no such dichroitic action.
By the force of imagination and reason combined we
may penetrate this mystery also.  The cloud takes no
note of size on the part of the waves of ether, but reflects
them all alike. It exercises no selective action. Now
the cause of this may be that the cloud particles are so
large in comparison with the size of the waves of ether
as to reflect them all indifferently. A broad cliff re



(48)
flects an Atlantic roller as easily as a ripple produced
by a sea bird's wing; and in the presence of large reflecting surfaces the existing differences of magnitude
among the waves of ether may disappear. But supposing the reflecting particles, instead of being very large,
to be very small, in comparison with the size of the
waves.  In this case, instead of the whole wave being
fronted and in great part thrown back, a small portion
only is shivered off. The great mass of the wave passes
over such a particle without reflection.  Scatter then, a
handful of such minute foreign particles in our atmosphere, and set imagination to watch their action upon
the solar waves. Waves of all sizes impinge upon the
particles, and you see at every collision a portion of the
impinging wave struck off by reflection. All the waves
of the spectrum, from the extreme red to the extreme
violet, are thus acted upon. But in what proportions
will the waves be scattered?  A clear picture will enable
us to anticipate the experimental answer. Remembering that the red waves are to the blue much in the relation of billows to ripples, let us consicer whether those
extremely small particles are competent to scatter all
the.waves in the same proportion. If they be not-and
a little reflection will make it clear to you that they are
not-the production of color must be an incident of the
scattering.  Largeness is a thing of relation; and the
smaller the wave the greater is the relative size of any
particle on which the wave impinges, and the greater
also the ratio of the reflected portion to the total wave.
A pebble placed in the way of the ring-ripples produced by our heavy rain-drops on a tranquil pond will
throw back a large fraction of the ripple incident upon
it, while the fractional part of a larger wave thrown back




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by the same pebble might be infinitesimal.  Now we
have already made it clear to our minds that to preserve
the solar light white, its constituent proportions must
not be altered; but in the act of division performed by
these very small particles we see that the proportions
are altered; an undue fraction of the smaller waves is
scattered by the particles, and, as a consequence, in the
scattered light blue will be the predominant color. The
other colors of the spectrum must, to some extent, be
associated with the blue. They are not absent, but deficient.' We ought, in fact, to have them all, but in diminishing proportions, from the violet to the red.
We have here presented a case to the imagination,
and assuming the undulatory theory to be a reality, we
have, I think, fairly reasoned our way to the conclusion
that, were particles, small in comparison to the size of
the ether waves, sown in our atmosphere, the light scattered by those particles would be exactly such as we observe in our azure skies. When this light is analyzed
all the colors of the spectrum are found; but they are
found in the proportions indicated by our conclusion.
Let us now turn our attention to the light which passes
unscattered among the particles. How must it be finally
affected? By its successive collisions with the particles,
the white light is more and more robbed of its shorter
waves; it therefore loses more and more of its due proportion of blue. The result may be.anticipated. The
transmitted light, where short distances are involved,
will appear yellowish. But as the sun sinks towards the'horizon, the atmospheric distances increase, and consequently the number of the scattering particles. They
abstract, in succession, the violet, the indigo, the blue,
and even disturb the proportions of green. The trans



( 5)
mitted light under such circumstances must pass from
yellow  through orange to red.  This also is exactly
what we find in nature. Thus, while the reflected light
gives us at noon the deep azure of the Alpine skies, the
transmitted light gives us at sunset the warm crimson of
the Alpine snows.  The phenomena certainly occur as
ifour atmosphere were a medium rendered slightly turbid by the mechanical suspension of exceedingly small
foreign particles.
Here, as before, we encounter our skeptical "as if."
It is one of the parasites of science, ever at hand, and
ready to plant itself and sprout, if it can, on the weak
points of our philosophy.  But a strong constitution
defies the parasite, and in our case, as we question the
phenomena, probability grows like growing. health, until
in the end the malady of doubt is completely extirpated.
The first question that naturally arises is, Can small
particles be really proved to act in the manner indicated?
No doubt of it.  Each one of you can submit the question to an experimental test. Water will not dissolve
resin, but spirit will, and when spirit which  holds
resin in solution is dropped into water the resin immediately separates in solid particles, which render the
water milky.  The coarseness of this precipitate depends on the quantity of the dissolved resin. You can
cause it to separate in thick clots or in exceedingly fine
particles.  Professor Briicke has given us the proportions which produce particles particularly suited to our
present purpose.  One gramme of clean mastic is dissolved in eighty-seven grammes of absolute alcohol, and
the transparent solution is allowed to drop into a beaker
containing clear water kept briskly stirred. An exceedingly fine precipitate is thus formed, which declares its




(51)
presence by its action upon light. Placing a dark surface
behind the beaker, and permitting the light to fall into it
from the top or front, the medium is seen to be distinctly
blue. It is not, perhaps, so perfect a blue as I have seen on
exceptional days, this year, among the Alps, but it is a
very fair sky blue. A trace of soap in water gives a tint
of blue. London, and I fear Liverpool milk, makes an
approximation to the same color through the operation
of the same cause; and Helmholtz has irreverently disclosed the fact that a blue eye is simply a turbid medium.
Numerous instances of the kind might be cited. The
action of turbid media upon light was fully and beautifully illustrated by Goethe, who, though unacquainted
with the undulatory theory, was led by his experiments
to regard the blue of the firmament as caused by an
illuminated turbid medium with the darkness of space
behind it. He describes glasses showing a bright yellow
by transmitted, and a beautiful blue by reflected light.
Professor Stokes, who was probably the first to discern
the real nature of the action of small particles on the
waves of ether, describes a glass of a similar kind.
What artists call "chill" is no doubt an effect of this
description.  Through the action of minute particles,
the browns of a picture often present the appearance of
the bloom of a plum.  By rubbing the varnish with a
silk handkerchief optical continuity is established and
the chill disappears.
Some years ago I witnessed Mr. Hirst experimenting
at Zermatt on the turbid water of the Visp, which was
charged with the finely divided matter ground down by
the glaciers. When kept still for a day or so the grosser
matter sank, but the finer matter remained suspended,
and gave a distinctly blue tinge to the water, No doubt
3




(52)
the blueness of certain Alpine lakes is in part due to
this cause. Professor Roscoe has noticed several striking cases of a similar kind. In a very remarkable paper
the late Principal Forbes showed that steam  issuing
from the safety valve of a locomotive, when favorably observed, exhibits at a certain stage of its condensation
the colors of the sky. It is blue by reflected light, and
orange or red by transmitted light.  The  effect, as
pointed out by Goethe, is to some extent exhibited by
peat smoke.
More than ten years ago I amused myself at Killarney, by observing on a calm day, the straight smoke columns rising from the chimneys of the cabins. It was
easy to project the lower portion of a column against
a bright cloud.   The  smoke in  the  former case
was blue, being seen mainly by reflected light; in
the  latter case it was reddish, being seen mainly
by transmitted light.  Such smoke was not in  exactly the condition to give us the glow of the Alps,
but it was a step in this direction. Briicke's fine precipitate above referred to looks yellowish by transmitted
light, but by duly strengthening the precipitate you may
render the white light of noon as ruby colored as the
sun when seen through Liverpool smoke or upon Alpine
horizons.
I do not, however, point to the gross smoke arising
from coal as an illustration of the action of small particles, because such smoke soon absorbs and destroys the
waves of blue instead of sending them to the eyes of the
observer.
These multifarious facts, and numberless others which
cannot now be referred to, are explained by reference to
the single principle that where the scattering particles




(53 )
are small in comparison to the size of the waves, we
have in the reflected light a greater proportion of the
smaller waves, and in the transmitted light a greater proportion of the larger waves, than existed in the original
white light. The physiological consequence is that in the
one light blue is predominant, and in the other light orange
or red. And now let us push our inquiries forward. Our
best microscopes can readily reveal objects not more
than.-5    of an inch in diameter.  This is less than
the length of a wave of red light.  Indeed, a first-rate
microscope would enable us to discern objects not exceeding in diameter the length of the smallest waves of
the visible spectrum.  By the microscope, therefore, we
can submit our particles to an experimental test. If
they are as large as the light-waves they will infallibly
be seen; and if they are not seen it is because they are
smaller.
I placed in the hands of our president a bottle containing Briicke's particles in greater number and coarseness than those examined  by Briicke himself.  The
liquid was a milky blue, and Mr. Huxley applied to it
his highest microscopic power. He satisfied me at the
time that had particles of even Ivoluo  of an inch in
diameter existed in the liquid they could not have
escaped detection. But no particles were seen.  Under
the microscope the turbid liquid was not to be distinguished from distilled water. Briicke, I may say, also
found the particles to be of ultra microscopic magnitude.
But we have it in our power to imitate far more closely
than we have hitherto done the natural conditions of
this problem. We can generate in air, as many of you
know, artificial skies, and prove their perfect identity with




(54)
the natural one as regards the exhibition of a number
of wholly unexpected phenomena.  By a continuous
process of growth, moreover, we are able to connect
sky matter, if I may use the term, with molecular matter on the one side, and with molar matter, or matter in
sensible masses, on the other.,
In illustration of this, I will take an experiment described by M. Morren, of Marseilles, at the last meeting of the British Association.  Sulphur and oxygen
combine to form sulphurous acid gas. It is this choking gas that is smelt when a sulphur match is burnt in
air. Two atoms of oxygen and one of sulphur constitute the molecule of sulphurous acid.  Now it has been
recently shown in a great number of instances that
waves of ether issuing from a strong source, such as the
sun or the electric light, are competent to shake asunder
the atoms of gaseous molecules. A chemist would call
this " decomposition" by light; but it behooves us, who
are examining the power and function of the imagination,
to keep constantly before us the physical images which
we hold to underlie our terms. Therefore I say, sharply
and definitely, that the components of the molecules
of sulphurous acid are shaken asunder by the ether,
waves. Inclosing the substance in a suitable vessel,
placing it in a dark room, and sending through it a
powerful beam of light, we at first see nothing; the vessel containing the gas is as empty as a vacuum.  Soon,
however, along the track of the beam a beautiful skyblue color is observed, which is due to the liberated
particles of sulphur. For a time the blue grows more
intense; it then becomes whitish; and from a whitish blue
it passes to a more or less perfect white. If the action
be continued long enough, we end by filling the tube




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with a dense cloud of sulphur particles, which by the
application of proper means may be rendered visible.
Here, then, our ether waves untie the bond of chemical affinity, and liberate a body-sulphur-which at ordinary temperatures is a solid, and which therefore soon
becomes an object of the senses. We have first of all
the free atoms of sulphur, which are both invisible and
incompetent to stir the retina sensibly with scattered
light.  But these atoms gradually coalesce and form
particles, which grow larger by continual accretion until
after a minute or two they appear as sky matter. In
this condition they are invisible themselves, but competent to send an amount of wave motion to the retina
sufficient to produce the firmamental blue. The particles continue, or may be caused to continue, in this condition for a considerable time, during which no microscope can cope with them. But they continually grow
larger, and pass by insensible gradations into the state of
cloud, when they can no longer elude the armed eye.
Thus, without solution of continuity, we start with matter in the molecule, and end with matter in the mass,
sky matter being the middle term of the series of transformations.
Instead of sulphurous acid we might choose from a
dozen other substances, and produce the same effect
with any of them.  In the case of some-probably in
the case of all-it is possible to preserve matter in the
skyey condition for fifteen or twenty minutes under the
continual operation of the light.  During these fifteen or
twenty minutes the particles are constantly growing
larger, without ever exceeding the size requisite to the
production of the celestial blue. Now when two vessels are placed before.you, each containing sky matter,




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it is possible to state with great distinctness which vessel contains the largest particles.
The eye is very sensitive to differences of light, when,
as here, the eye is in comparative darkness, and when
the quantities of wave motion thrown against the retina
are small. The larger particles declare themselves by
the greater whiteness of their scattered light. Call now
to mind the observation, or effort at observation, made by
our president when he failed to distinguish the particles
of resin in Brficke's medium, and when you have done
so follow me. I permitted a beam of light to act upon
a certain vapor. In two minutes the azure appeared,
but at the end of fifteen minutes it had not ceased to
be azure. After fifteen minutes, for example, its color
and some other phenomena pronounced it to be a blue
of distinctly smaller particles than those sought for in
vain by Mr. Huxley.  These particles, as already stated,
must have been less than 1TYoo  of an inch in diameter.
And now I want you to submit to your imagination
the following question: Here are particles which have
been growing continually for fifteen minutes, and at the
end of that time are demonstrably smaller than those
which defied the microscope of Mr. Huxley. What
must have been the size of these particles at the beginning of their growth? What notion can you form  of
the magnitude of such particles? As the distances of
stellar space give us simply a bewildering sense of vastness without leaving any distinct impression on the mind,
so the magnitudes with which we have here to do impress us with a bewildering sense of smallness. We
are dealing with infinitesimals compared with which the
test objects of the microscope are literally immense.




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From their perviousness to stellar light, and other
considerations, Sir John Herschel drew some startling
conclusions regarding the density and weight of comets.
You know that these extraordinary and mysterious bodies sometimes throw out tails ioo,000,0ow  of miles in
length, and 50,ooo miles in diameter. The diameter of
our earth is 8,0oo miles. Both it and the sky, and a
good portion of space beyond the sky, would certainly
be included in a sphere Io,ooo miles across. Let us fill
this sphere with cometary matter, and make it our unit
of measure. An easy calculation informs us that to
produce a comet's tail of the size just mentioned, about
300,000 such measures would have to be emptied into
space. Now suppose the whole of this stuff to be swept
together, and suitably compressed, what do you suppose
its volume would be? Sir John Herschel would probably tell you that the whole mass might be carted away
at a single effort by one of your dray-horses. In fact, I
do not know that he would require more than a small
fraction of a horse-power to remove the cometary dust.
After this you will hardly regard as monstrous a notion
I have sometimes entertained concerning the quantity
of matter in our sky. Suppose a shell, then, to surround the earth at a hight above the surface which
would place it beyond the grosser matter that hangs in
the lower regions of the air-say at the hight of the
Matterhorn or Mont Blanc. Outside this shell we have
the deep blue firmament.. Let the atmospheric space
beyond the shell be swept clean, and let the sky matter
be properly gathered up. What is its probable amount?
I have sometimes thought that a lady's portmanteau
would contain it all. I have thought that even a gentleman's portmanteau-possibly his snuff-box-might take it




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in. And whether the actual sky be capable of this amount
of condensation or not, I entertain no doubt that a sky
quite as vast as ours, and as good in appearance, could
be formed from  a quantity of matter which might be
held in the lpllow of the hand.
Small in mass, the vastness in point of number of the
particles of.our sky may be inferred from the continuity
of its light. It is not in broken patches nor at scattered
points that the heavenly azure is revealed. To the observer on the summit of Mont Blanc the blue is as uniform and coherent as if it formed the surface of the most
close-grained solid. A marble dome would not exhibit
a stricter continuity.  And Mr. Glaisher will inform you
that if our hypothetical shell were lifted to twice the
hight of Mont Blanc above the earth's surface, we
should still have the azure overhead.  Everywhere
through the atmosphere those sky particles are strewn.
They fill the Alpine valleys, spreading like a delicate
gauze in front of the slopes of pine. They sometimes
so swathe the peaks with light as to abolish thefr definition. This year I have seen the Weisshorn thus dissolved in opalescent air.
By proper instruments the glare thrown from the sky
particles against the retina may be quenched, and then
the mountain which it obliterated starts into sudden
definition. Its extinction in front of a dark mountain
resembles exactly the withdrawal of a veil. It is the
light then taking possession of the eye, and not the
particles acting as opaque bodies, that interfere with the
definition.
By day this light quenches the stars; even by moonlight it is able to exclude from vision all stars between
the fifth and the eleventh magnitude. It may be likened




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to a noise, and the stellar radiance to a whisper drowned
by the noise. What is the nature of the particles which
shed this light? On points of controversy I will not
here enter, but I may say that De la Rive ascribes the
haze of the Alps in fine weather to floating organic
germs. Now the possible existence of germs in such
profusion has been held up as an absurdity. It has
been affirmed that they would darken the air, and on
the assumed impossibility of their existence in the
requisite numbers, without invasion'of the solar light, a
powerful argument has been based by believers in spontaneous generation.
Similar arguments have been used by the opponents
of the germ theory of epidemic disease, and both parties have triumphantly challenged an appeal to the
microscope and the chemist's balance to decide the question. Without committing myself in the least to De la
Rive's notion, without offering any objection here to
the doctrine of spontaneous generation, without expressing any adherence to the germ theory of disease, I
would simply draw attention to the fact that in the atmosphere we have particles which defy both the microscope and the balance, which do not darken the air, and
which exist, nevertheless, in multitudes sufficient to reduce to insignificance the Israelitish hyperbole regarding the sands upon the seashore.
The varying Judgments of men on these and other
questions may perhaps be, to some extent, accounted for
by that doctrine of relativity which plays so important
a part in philosophy. This doctrine affirms that the impressions made upon us by any circumstance, or combination of circumstances, depends upon our previous
state. Two travelers upon the same peak, the one hav



(6o)
ing ascended to it from the plain, the other having descended to it from a higher elevation, will be differently
affected by the scene around them. To the one nature
is expanding, to the other it is contracting, and feelings
are sure to differ which have two such different antecedent states.
In our scientific judgments the law of relativity may
also play an important part. To two men, one educated
in the school of the senses, who has mainly occupied
himself with observation, and the other educated in the
school of imagination as well, and exercised in the con-.ception of atoms and molecules to which we have so
frequently referred, a bit of matter, say -51-0  of aninch
in diameter, will present itself differently. The one descends to it from his molar hights, the other climbs'to
it from his molecular lowlands.  To the one it appears
small, to the other large. So also as regards the appreciation of the most minute forms of life revealed by the
microscope.  To one of these men they naturally appear conterminous with the ultimate particles of matter,
and he readily figures the molecules from which they directly spring; with him there is but a step from the
atom to the organism. The other discerns numberless
organic gradations between both.  Compared with his
atoms, the smallest vibrios and bacteria of the microscopic field are as behemoth and leviathan.
The law of relativity may to some extent explain the
different attitudes of these two men with regard to the
question of spontaneous generation.  An amount of
evidence which satisfies the one entirely fails to satisfy
the other; and while'to the one the last bold defense
and startling expansion of the doctrine will appear perfectly conclusive, to the other it will present itself as im



(61)
posing a profitless labor of demolition on subsequent investigators. The proper and possible attitude.of these
two men is that each of them should work as if it were
his aim and object to establish the view entertained by
the other.
I trust, Mr. President, that you-whom untoward circumstances have made a biologist, but who still keep
alive your sympathy with that class of inquiries which
nature intended you to pursue and adorn-will excuse
me to your brethren if I say that some of them seem to
form an inadequate estimate of the distance which separates the microscopic from  the molecular limit, and
that, as a consequence, they sometimes employ a phraseology which is calculated to mislead.
When, for example, the contents of a cell are described as perfectly homogeneous, as absolutely structureless, because the microscope fails to distinguish any
structure, then I think the microscope begins to play a
mischievous part. A little consideration will make it
plain to all of you that the microscope can have no voice
in the real question  of germ  structure.   Distilled
water is more perfectly homogeneous than the contents
of any possible organic germ. What causes the liquid
to cease contracting at 39~ F.,.and to grow bigger until
it freezes?  It is a structural process of which the
microscope can take no note, nor is it likely to do so
by any conceivable extension of its powers. Place this
distilled water in the field of an electro-magnet, and
bring a microscope to bear upon it.  Will any change
be observed when the magnet is excited? Absolutely
none; and still profound and complex changes have
occurred.
First of all, the particles of water are rendered dia



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magnetically polar; and secondly, in virtue of the structure impressed upon it by the magnetic strain of its
molecules, the liquid twists a ray of light in a fashion
perfectly determinate both as to quantity and direction.
It would be immensely interesting to both you and me
if one here present, who has brought his brilliant imagination to bear upon this subject, could make us see as
he sees the entangled molecular processes involved in
the rotation of the plane of polarization by magnetic
force. While dealing with this question he lived in a
world of matter and of motion to which the microscope
has no passport, and in which it can offer no aid. The
cases in which similar conditions hold are simply numberless. Have the diamond, the amethyst, and the
countless other crystals formed in the laboratories of
nature and of man, no structure? Assuredly they have,
but what can the microscope make of it? Nothing. It
cannot be too distinctly borne in mind that between the
microscopic limit and the true molecular limit there is
room for infinite permutations and combinations. It is
in this region that the poles of the atoms are arranged,
that tendency is given to their powers, so that when
these poles and powers have free action and proper
stimulus in a suitable environment, they determine first
the germ and afterwards the complete organism. This
first marshaling of the atoms on which all subsequent
action depends baffles a keener power than that of the
microscope. Through pure excess of complexity, and
long before observation can have any voice in the matter, the most highly trained intellect, the most refined
and disciplined imagination, retires in bewilderment
from the contemplation of the problem. We are struck
dumb by an astonishment which no microscope can re.




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lieve, doubting not only the power of our instrument,
but even whether we ourselves possess the intellectual
elements which will ever enable us to grapple with the
ultimate structural energies of nature.
But the speculative faculty, of which imagination
forms so large a part, will nevertheless wander into
regions where the hope of certainty would seem to be
entirely shut out. We think that though the detailed
analysis may be, and may ever remain, beyond us, general notions may be attainable.  At all events, it is plain
that beyond the present outposts of microscopic inquiry
lies an immense field for the exercise of the imagination.
It is only, however, the privileged spirits who know how
to use their liberty without abusing it, who are able to
surround imagination by the firm frontiers of reason,
that are likely to work with any profit here. But freedom to them is of such paramount importance that, for
the sake of securing it, a good deal of wildness on the
part of weaker brethren may be overlooked. In more
senses than one Mr. Darwin has drawn heavily upon
the scientific tolerance of his age. He has drawn heavily upon time in his development of species, and he has
drawn adventurously upon matter in his theory of pangenesis. According to this theory, a germ already microscopic is a world of minor germs.  Not only is the
organism as a whole wrapped up in the germ, but every
organ of the organism has there its special seed.
This, I say, is an adventurous draft on the power of
matter to divide itself and distribute its forces. But,
unless we are perfectly sure that he is overstepping the
bounds of reason, that he is unwittingly sinning against
observed fact or demonstrated law-for a mind like that
of Darwin can never sin wittingly against either fact or




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law-we ought, I think, to be cautious in limiting his
intellectual horizon. If there be the least doubt in the
matter, it ought to be given in favor of the freedom of
such a mind. To it a vast possibility is in itself a
dynamic power, though the possibility may never be
drawn upon.
It gives me pleasure to think that the facts and
reasonings of this discourse tend rather towards the
justification of Mr. Darwin than towards his condemnation, that they tend rather to augment than to diminish
the cubic space demanded by this soaring speculator;
for they seem to show the perfect competence of matter
and force, as regards divisibility and distribution, to bear
the heaviest strain that he has hitherto imposed upon
them.
In the case of Mr. Darwin, observation, imagination,
and reason combined have run back with wonderful
sagacity and success over a certain length of the line of
biological succession.  Guided by analogy, in his "Origin of Species " he placed as the root of life a primordial germ, from which he conceived the amazing richness and variety of the life that now is upon the earth's
surface, might be deduced.  If this were true it would.
not be final. The human imagination would infallibly
look behind the germ, and inquire into the history of its
genesis.
Certainty is here hopeless, but the materials for an
opinion may be attainable.  In this dim twilight of
speculation the inquirer welcomes every gleam, and seeks
to augment his light by indirect incidences. He studies
the methods of nature in the ages and the worlds within
his reach, in order to shape the course of imagination
in the antecedent ages and worlds. And though the




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certainty possessed by experimental inquiry is here shut
out, the imagination is not left entirely without guidance.
From the examination of the solar system, Kant and
Laplace came to the conclusion that its various bodies
once formed parts of the same undislocated mass; that
matter in a nebulous form preceded matter in a dense
form; that as the ages rolled away heat was wasted,
condensation followed, planets were detached, and that
finally the chief portion of the fiery cloud reached, by
self-compression, the magnitude and density of our sun.
The earth itself offers evidence of a fiery origin; and
in our day the hypothesis of Kant and Laplace receives
the independent countenance of spectrum  analysis,
which proves the same substances to be common to the
earth and sun. Accepting some such view of the construction of our system as probable, a desire immediately
arises to connect the present life of our planet with the
past. We wish to know something of our remotest ancestry.
On its first detachment from the central mass, life, as
we understand it, could hardly have been present on the
earth. How then did it come there? The thing to be
encouraged here is a reverent freedom-a freedom preceded by the hard discipline which checks licentiousness
in speculation-while the thing to be repressed, both in
science and out of it, is dogmatism. And here I am in
the hands of the meeting-willing to end, but ready to
go on. I have no right to intrude upon you, unasked,
the unformed notions which are floating like clouds or
gathering to more solid consistency in the modern speculative scientific mind. But if you wish me to speak
plainly, honestly, and undisputatiously, I am willing to
do so. On the present occasion
You are ordained to call, and I to come.




(66.)
Two views, then, offer themselves to us. Life was
present potentially in matter when in the nebulous form,
and was unfolded from it by the way of natural development, or it is a principle inserted into matter at a later
date. With regard to the question of time, the views of
men have changed remarkably in our day and generation; and I must say as regards courage also, and a
manful willingness to engage in open contest, with fair
weapons, a great change has also occurred.
The clergy of England-at all events the clergy of
London-have nerve enough to listen to the strongest
views which any one amongst us would care to utter;
and they invite, if they do not challenge, men of the
most decided opinions to state and stand by those opinions in open court.  No theory upsets them. Let the
most destructive hypothesis be stated only in the language current among gentlemen, and they look it in the
face. They forego alike the thunders of heaven and the
terrors of the other place, smiting the theory, if they do
not like it, with honest secular strength.  In fact, the
greatest cowards of the present day are not to be found
among the clergy, but within the pale of science itself.
Two or three years ago in an ancient London college
-a clerical institution-I heard a very remarkable lecture by a very remarkable man. Three orfour hundred
clergymen were present at the lecture.  The orator
began with the civilization of Egypt in the time of
Joseph; pointing out that the very perfect organization
of the kingdom, and the possession of chariots, in one
of which Joseph rode, indicated a long antecedent
period of civilization. He then passed on to the mud
of the Nile, its rate of augmentation, its present thickness, and the remains of human handiwork found therein;




(67 )
thence to the rocks which bound the Nile valley, and
which team with organic remains. Thus, in his own
clear and admirable way, he caused the idea of the
world's age to expand itself indefinitely before the mind
of his audience, and he contrasted this with the age
usually assigned to the world..During his discourse he seemed to be swimming
against a stream; he manifestly thought that he was opposing a general conviction. He expected resistance;
so did I. But it was all a mistake; there was no adverse current, no opposing conviction, no' resistance,
merely here and there a half humorous but unsuccessful attempt to entangle him in his talk. The meeting
agreed with all that had been said regarding the antiquity of the earth and of its life. They had, indeed,
known it all long ago, and they good-humoredly rallied
the lecturer for coming amongst them with so stale a
story. It was quite plain that this large body of clergymen, who were, I should say, the finest samples of their
class, had entirely given up the ancient landmarks, and
transported the conception of life's origin to an indefinitely distant past.
In fact, clergymen, if I might be allowed a parenthesis to say so, have as strong a leaning towards sclentific truth as other men, only the resistance to this bent
-a resistance due to education-is generally stronger
in their case than in others. They do not lack the positive element, namely, the love of truth, but the negative
element, the fear of error, preponderates.
The strength of an electric current is determined by
two things-the electro-motive force, and the resistance
that force has to overcome. A fraction, with the former
as numerator and the latter as denominator, expresses




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the current-strength.  The "current-strength" of the
clergy towards science may also be expressed by making the positive element just referred to the numerator,
and the negative one the denominator of a fraction.
The numerator is not zeronor is it even small, but the denominator is large; and hence the current strength is
such as we find it to be. Slowness of conception, even
open hostility, may be thus accounted for.  They are
for the most part errors of judgment, and not sins
against truth. To most of us it may appear very simple, but to. a few of us it appears transcendently wonderful, that in all classes of society truth should have
this power and fascination. From the countless modifications that life has undergone through natural selection and the integration of infinitesimal steps, emerges
finally the grand result that the strength of truth is
greater than the strength of error, and that we have
only to make the truth clear to the world to gain the
world to our side. Probably no one wonders more at
this result than the propounder of the law of natural
selection himself. Reverting to an old acquaintance of
ours, it would seem, on purely scientific grounds, as if
a Veracity were at the heart of things; as if, after ages of
latent working, it had finally unfolded itself in the life of
man; as if it were still destined to unfold itself, growing in
girth, throwing out stronger branches and thicker leaves,
and tending more and more by its overshadowing presence to starve the weeds of error from the intellectual
soil.
But this is parenthetical; and the gist of our present
inquiry regarding the introduction of life is this: Does
it belong to what we call matter, or is it an independent
principle inserted into matter at some suitable epoch



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say when the physical conditions become such as to
permit of the development of life?  Let us put the
question with all the reverence due to a faith and culture in which we all were cradled-a faith and culture,
moreover, which are the undeniable historic antecedents
of our present enlightenment. I say, let us put the
question reverently, but let us also put it clearly and
definitely.
There are the strongest grounds. for believing that
during a certain period of its history the earth was not,
nor was it fit to be, the theater of life. Whether this
was ever a nebulous period, or merely a molten period,
does not much matter; and if we revert to the nebulous
condition, it is because the probabilities are really on its
side.  Our question is this: Did creative energy pause
until the nebulous matter had condensed, until the earth
had been detached, until the solar fire had so far withdrawn from the earth's vicinity as to permit a crust to
gather round a planet? Did it wait until the air was isolated, until the seas were formed, until evaporation,
condensation, and the descent of rain had begun, until
the eroding forces of the atmosphere had weathered and
decomposed the molten rocks so as to form soils, until
the sun's rays had become so tempered by distance and
by waste as to be chemically fit for the decompositions
necessary to vegetable life?  Having waited through
those aeons until the proper conditions had set in, did
it send the fiat forth, "Let life be!"? These questions
define a hypothesis not without its difficulties, but the
dignity of which was demonstrated by the nobleness of
the men whom it sustained.
Modern scientific thought is called upon to decide between this hypothesis and another; and public thought




( 70 )
generally will afterwards be called upon to do the same.
You may, however, rest secure in the belief that the
hypothesis just sketched can never be stormed, and that
it is sure, if it yield at all, to yield to a prolonged siege.
To gain new territory, modern argument requires more
time than modern arms, though both of them move with
greater rapidity than of yore.
But however the convictions of individuals here and
there may be influenced, the process must be slow and
secular which commends the rival hypothesis of natural
evolution to the public mind. For what are the core
and essence of this hypothesis? Strip it naked and
you stand face to face with the notion that not alone the
more ignoble forms of. animalcular or animal life, not
alone the nobler forms of the horse and lion, not alone
the.exquisite and wonderful mechanism of the human
body, but that the human mind itself-emotion, intellect, will, and all their phenomena-were once latent in
a fiery cloud.  Surely the mere statement of such a
motion is more than a refutation. But the hypothesis
would probably go even further than this. Many who
hold it would probably assent to the position that at the
present moment all our philosophy, all our poetry, all
our science, and all our art-Plato, Shakespeare, Newton, and Raphael-are potential in the. fires of the sun.
We long to learn something of our origin.  If the
evolution hypothesis be correct, even this unsatisfied
yearning must have come to us across the ages which
separate the unconscious primeval mist from the consciousness of to-day. I do not think that any holder of
the evolution hypothesis would say that I overstate it or
overstrain it in any way. r merely strip it of all vagueness, and bring before you, unclothed and unvarnished,
the notions by which it must stand or fail




(7 )
Surely these notions represent an absurdity too monstrous to be entertained by any sane mind.  Let us,
however, give them fair play. Let us steady ourselves
in front of the hypothesis, and, dismissing all terror and
excitement from our minds, let us look firmly into it with
the hard, sharp eye of intellect alone. Why are these
notions absurd, and why should sanity reject them?
The law of relativity, of which we have previously
spoken, may find its application here. These evolution
notions are absurd, monstrous, and fit only for the
intellectual gibbet in relation to the ideas concerning
matter which were drilled into us when young. Spirit
and matter have ever been presented to us in the rudest
contrast, the one as all noble, the other as all vile. But
is this correct? Does it represent what our mightiest
spiritual teacher would call the eternal fact of the universe? Upon the answer to this question all depends.
Supposing, instead of having the foregoing antithesis
of spirit and matter presented to our youthful minds, we
had been taught to regard them as equally worthy and
equally wonderful; to consider them, in fact, as two opposite faces of the self-same, mystery. Supposing that
in youth we had been impregnated with the notion of
the poet Goethe, instead of the notion of the poet
Young, looking at matter, not as brute matter, but as
"the living garment of God;" do you not think that
under these altered circumstances the law of relativity
might have had an outcome different from its present
one? Is it not probable that our repugnance to the
idea of primeval union between spirit and matter might
be considerably abated? Without this total revolution
of the notions now prevalent the evolution hypothesis
must stand condemned; but in many profoundly




(72-)
thoughtful minds such a revolution has already taken
place.  They degrade neither member of the mysterious duality referred to; but they exalt one of them
from its abasement, and repeal the divorce hitherto existing between both.  In substance, if not in words,
their position as regards spirit and matter is: "What
God hath joined together let not man put asunder."
I have thus led you to the outer rim of speculative
science, for beyond the nebula scientific thought has
never ventured hitherto, and have tried to state that
which I considered ought, in fairness, to be outspoken.
I do not think this evolution hypothesis is to be flouted
away contemptuously; I do not think it is to be denounced as wicked.  It is to be brought before the bar
of disciplined reason, and there justified or condemned.
Let us hearken to those who wisely support it, and to
those who wisely oppose it; and let us tolerate those,
and they are many, who foolishly try to do neither of
these things.
The only thing out of place in the discussion is dogmatism on either side. Fear not the evolution hypothesis. Steady yourselves in its presence upon that faith
in the ultimate triumph of truth which was expressed by
old Gamaliel when he said: " If it be of God, ye cannot
overthrow it; if itbe of man, it will come to naught."
Under the fierce light of scientific inquiry this hypothesis is sure to be dissipated if it possess not a core of
truth. Trust me, its existence as an hypothesis in the
mind is quite compatible with the simultaneous existence of all those virtues to which the term Christian
has been applied. It does not solve-it does not profess to solve-the ultimate mystery untouched.  At bottom it does nothing more than "transport the conception of life's origin to an indefinitely distant past."




(73)
For, granting the nebula and its potential life, the
question, whence came they? would still remain to
baffle and bewilder us. And with regard to the ages of
forgetfulness which lie between the conscious life of the
nebula and the conscious life of the earth, it is but an
extension of that forgetfulness which preceded the birth
of us all. Those who hold the doctrine of evolution
are by no means ignorant of the uncertainty of their
data, and they yield no more to it than a provisional
assent. They regard the nebular hypothesis as probable, and in the utter absence of any evidence to prove
the act illegal, they extend the method of nature from
the present into the past. Here the observed uniformity of nature is their only guide. Within the long range
of physical inquiry they have never discerned in nature
the insertion of caprice. Throughout this range the
laws of physical and intellectual continuity have run
side by side.  Having thus determined the elements of
their curve in this world of observation and experiment,
they prolong that curve into an antecedent' world, and
accept as probable the unbroken sequence of development from the nebula to the present time.
You never hear the really philosophical defenders of
the doctrine of uniformity speaking of impossibilities in
nature.  They  never say, what they are constantly
charged with saying, that it is impossible for the builder
of the universe to alter His work. Their business is
not with the possible, but the actual; not with a world
which might be, but with a world which is. This they
explore with a courage not unmixed with reverence, and
according to methods which, like the quality of a tree,
are tested by their fruits. They have but one desireto know the truth. They have but one fear-to believe




(74)
a lie. And if they know the strength of science, and
rely upon it with unswerving trust, they also know the
limits beyond which science ceases to be strong. They
best know that questions offer themselves to thought
which science, as now prosecuted, has not even the tendency to solve.  They keep such questions open, and
will not tolerate any unlawful limitation of the horizon
of their souls. They have as little fellowship with the
atheist who says there is no God as with the theist who
professes to know the mind of God.
"Two things," said Immanuel Kant, "fill me with
awe: the starry heavens and the sense of moral responsibility in man."  And in his hours of health and
strength and sanity, when the stroke of action has
ceased and the pause of reflection has set in, the scientific investigator finds himself overshadowed by the
same awe. Breaking contact with the hampering details of earth, it associates him with a power which gives
fulness and tone to his existence, but which he can
neither analyze nor comprehend.




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