j/01 ,jfV> ,j0' fyxmll mmvmtg Hitog Al.3.9.... Cornell University Library The original of this book is in the Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31924098489606 THE INTERNATIONAL SCIENTIFIC SERIES. VOLUME VI. I. II. III. IV. V. VI. VII. viii. IX. X. XI. XII. XIII. XIV. XV. XVI. XVII. XVIII. XIX. xx. XXI. XXII. THE INTERNATIONAL SCIENTIFIC SERIES. Works already Published. FORMS OF WATER, in Clouds, Rain,. Rivers, Ice, and Glaciers. By Prof. John Tvndall, LL. D., F. R.S. ivol. Cloth. Price, $1.50. PHYSICS AND POLITICS; or, Thoughts on the Application of the Principles of "Natural Selection" and " Inheritance " to Political Society. By Walter Bagehot, Esq., author of "The English Constitution." 1 vol. Cloth. Price, $1.50. FOODS. By Edward Smith, M. D., LL. B., F. R. S. 1 vol. Cloth. Price, $1.75. „ , MIND AND BODY: the Theories of their Relations. By Alex. Bain, LL. 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THE NEW CHEMISTRY. u m. •■/.•) ET JOSIAH P: COOKE, Je., EUTINQ PBOFESSOR OP CHEMISTBT AND M I N E K A L O T IN HARVARD UNIVERSITY. ^CORNELL UNIVERSITYi LIBRARY NEW YORK : D. APPLETON AND COMPANY, 649 & 551 BROADWAY. 1878. Enteeed, according 1 to Act of Congress, in the year 1S7S, by D. APPLETON & CO., In the Office of the Librarian of Congress, at Washington. TO JOH2*Sk. LOWELL, LL.D., BOSTON, MASSACHUSETTS. Deae Sie : From the early lectures of the Lowell In- stitute I derived, when a boy, my taste for the science which became the occupation of my after-life, and it has since often been my privilege to illustrate before the intelligent audiences — which, for more than thirty winters, the Institute has gathered under your direc- tion — the results of the studies that I there began. Allow me, then, to dedicate to you this volume, as an expression of my indebtedness to the foundation you have so long and so ably administered. With great respect, Your obedient servant, JOSIAH P. COOKE, Je. PREFACE The lectures now published were delivered before the Lowell Institute, in Boston, in the autumn of 1872. They aimed to present the modern theories of chem- istry to an intelligent but not a professional audience, and to give to the philosophy of the science a logi- cal consistency, by resting it on the Jaw of Avogadro. Since many of the audience had studied the elements of chemistry, as they were formerly taught under the dualistic system, it was also made an object to point out the chief characteristics by which the new chemistry differed from the old. The limitations of a course of popular lectures necessarily precluded a full presenta- tion of the subject, and ouly the more prominent and less technical features of the new system were discussed. In writing out his notes for the press, the author has retained the lecture style, because it is so well adapted for the popular exposition of scientific subjects ; but he 6 PKKFACE. is painfully conscious that any description of experi- ments must necessarily fall far short of giving that force of impression which the phenomena of Nature produce when they speak for themselves, and, in weighing the arguments presented, he must beg his readers to make allowances for this fact. Cambridge, September 6, 1873. CONTENTS, *LE0TTTBE PAGE I. Molecules and Avoqadeo's Law 9 II. The Molecular Condition of the Three States op Mat- ter — the Gas, the Liquid, and the Solid . . 37 III. How Molecules are weighed 63 IV. Chemical Composition — Analysis and Synthesis — The Atomic Theory 84 V. Elementary Substances and Combining Proportions . 104 VI. Atomic Weights and Chemical Symbols . . . 122 VII. Chemical Reactions . . . 149 VIII. Chemical Changes classified 175 IX. The Theory of Combustion 195 X. Gunpowder and Nitro-glycerine .... 216 XI. Quantiyalence and Metathesis — Alkalies and Acids 238 XII. Electro-chemical Theory 265 XIII. Isomerism, and the Synthesis of Organic Compounds . 296 THE NEW CHEMISTRY. LECTURE I. MOLECULES AND AYOGADEo's LAW. In every physical science we have carefully to dis- tinguish between the facts which form its subject-mat- ter and the theories by which we attempt to explain these facts, and group them in our scientific systems. The first alone can be regarded as absolute knowledge, and such knowledge is immutable, except in so far as subsequent observation may correct previous error. The last are, at best, only guesses at truth, and, even in their highest development, are subject to limitations, and liable to change. But this distinction, so obvious when stated, is often overlooked in our scientific text-books, and not without reason, for it is the sole aim of these elementary treatises to teach the present state of knowledge, and they might fail in their object if they attempted, by a too critical analysis, to separate the phenomena from the systems by which alone the facts of Nature are correlated and rendered intelligible. When, however, we come to study the history of science, the distinction between fact and theory ob- trudes itself at once upon our attention. We see that, while the prominent facts of science have re- 10 MOLECULES AND AVOGADRO'S LAW. mained the same, its history has been marked by very frequent revolutions in its theories or systems. The courses of the planets have not changed since they were watched by the Chaldean astronomers, three thou- sand years ago ; but how differently have their motions been explained — first by Hipparchus and Ptolemy, then by Copernicus and Kepler, and lastly by Newton and Laplace ! — and, however great our faith in the law of universal gravitation, it is difficult to believe that even this grand generalization is the final result of astronomical science. Let me not, however, be imderstood to imply a be- lief that man cannot attain to any absolute scientific truth ; for I believe that he can, and I feel that every great generalization brings him a step nearer to the promised goal. Moreover, I sympathize with that beautiful idea of. Oersted, which he expressed in the now familiar phrase, " The laws of Nature are the thoughts of God;'''' but, then, I also know that our knowledge of these laws is as yet very imperfect, and that our human systems must be at the best but very partial expressions of the truth. Still, it is a fact, wor- thy of our profound attention, that in each of the physi- cal sciences, as in astronomy, the successive great gen- eralizations which have marked its progress have in- cluded and expanded rather than superseded those which went before them. Through the great revolutions which have taken place in the forms of thought, the elements of truth in the successive systems have been preserved, while the error has been as constantly eliminated ; and so, as I believe, it always will be, until* the last generalization of all brings us into the presence of that law which is indeed the thought of God. There is also another fact, which has an important ANTICIPATION IN SCIENCE. n bearing on the subject we are considering. Almost all the great generalizations of science have been more or less fully anticipated, at least in so far that the gen- eral truth which they involve has been previously conceived. The Copernican theory was taught, sub- stantially, by the disciples of Pythagoras. The law of gravitation was suggested, both by Hooke and Cassini, several years before Newton published his " Principia ; " and the same general fact has been recently very markedly illustrated in the discovery of the methods of spectrum analysis, every principle of which had been previously announced. The history of science shows that the age must be prepared before really new scientific truths can take root and grow. The barren premonitions of science have been barren because these seeds of truth fell upon unfruitful soil ; and, as soon as the fullness of the time was come, the seed has taken root and the fruit has ripened. No one can doubt, for example, that the law of gravitation would have been discovered before the close of the seventeenth century if Newton had not lived ; and it is equally true that, had Newton lived before Galileo and Kepler, he never could have mastered the difficult problems it was his privilege to solve. We justly honor with the greatest veneration the true men who, having been called to occupy these distinguished places in the history of science, have been equal to their position, and have acquitted themselves so nobly before the world ; but every student is surprised to find how very little is the share of new truth which even the greatest genius has added to the previous stock. Science is a growth of time, and, though man's cultivation of the field is an essential condition of that growth, the de- velopment steadily progresses, independently of any in^ 12 MOLECULES AND AVOGADRO'S LAW. dividual investigator, however great his mental power. The greatest philosophical generalizations, if prema- ture, will fall on barren soil, and, when the age is ripe, they are never long delayed. The very discovery of law is regulated by law, or, as we rather believe, is directed by Providence ; but, however we may prefer to represent the facts, this natural growth of knowl- edge gives us the strongest assurance that the growth is sound and the progress real. Although the foun- dations of science have been laid in such obscurity, its students have worked under the direction of the same guiding power which rules over the whole of Nature, and it cannot be that the structure they have reared with so much care is nothing but the phantom of a dream. Still it is true that, beyond the limits of direct observation, our science is not infallible, and our theo- ries and systems, although they may all contain a ker- nel of truth, undergo frequent changes, and are often revolutionized. Through such a revolution the theory of chemistry has recently passed, and the system which is now uni- versally accepted by the principal students of the sci- ence is greatly different from that which has been taught in our schools and colleges until within a few years. I have, therefore, felt that the best service I could render in this course of lectures would be to ex- plain, as clearly as I am able, the principles on which the new philosophy is based, and to show in what it differs from the old. I have felt that there were many who, having studied what we must now call the old chemistry, would be glad to bridge over the gulf which separates it from the new, and to become acquainted with the methods by which we now seek to group to' gether and explain the old facts. STARTING-POINT OF THE NEW CHEMISTRY. 13 Those who studied the science of chemistry twenty- years ago, as it was taught, for example, in the works of the late Dr. Turner, were greatly impressed with the simplicity of the system and the beauty of its no- menclature. Until recently the study of the new chemis- try has been far less inviting; since the science has been passing through a process of reconstruction, and dis- played the imperfections of any half-built edifice ; but it has now reached a condition in which it can be pre- sented with the unity of a philosophical system. Our starting-point in the exposition of the modern chemis- try must be the great generalization which is now known as the law of Avogadro, or Ampere. This law was first stated by Amedeo Avogadro, an Italian physicist, in lsll, and was reproduced by Ampere, a French physicist, in 1814. But, although attained thus early in the history of our science, this grand conception remained barren for nearly half a century. Now, however, it holds the same place in chemistry that the law of gravitation does in astronomy, though, unlike the latter, it was announced half a century be- fore the science was sufficiently mature to accept it. The law of Avogadro may be enunciated thus : Equal volumes of all substances, when in the state of gas, and under like conditions, contain the same numbee of molecules (Avogadro, 1811— Ampere, 1814). The enunciation of this law is very simple, but, be- fore we can comprehend its meaning, we must under- stand what is meant by the term molecule. This word is the one selected by Avogadro in the enuncia- tion of his law. It is obviously of Latin origin, and means simply a little mass of matter. Ampere used in 14 MOLECULES AND AVOGADKO'S LAW. its place the word particle, in precisely the same sense. Both words signify the smallest mass into which any substance is capable of being subdivided by physical processes ; that is, by processes which do not change its chemical nature. In many of our text-books it is denned as the smallest mass of any substance which can exist by itself, but both definitions are in essence the same. As this is a very important point, it must be fully illustrated. In the first place, we recognize in Nature a great variety of different substances. Indeed, on this fact the whole science of chemistry rests ; for, if Nature were made out of a single substance, there could be no chemistry, even if there could be intel- ligences to study science at all. Chemistry deals exclusively with the relations of different substances. Now, these substances present themselves to us under three conditions : those of the solid, the liquid, and the gas. Some substances are only known in one of these conditions, others in only two, while very many may be made to assume all three. Charcoal, for example, is only known in the solid state ; alcohol has never been frozen, but can easily be volatilized ; while, as every one knows, water can most readily be changed both into solid ice and into aeriform steam. Let me begin with this most familiar of all substances to illustrate what I mean by the word molecule. "When, by boiling under the atmospheric pressure, water changes into steam, it expands 1,800 times ; or, in other words, one cubic inch of water yields one cubic foot of steam, nearly. Now, two suppositions are possible as modes of explaining this change. The first is, that, in expanding, the material of the water becomes diffused throughout the cubic foot, so as to fill the ■ space completely with the substance we PARTICLES SEPARATED IN STEAM. 15 Fio. 1. call water, the resulting mass of steam being absolutely homogeneous, so that there is no space within the cubic foot, however minute, which does not contain its prop- er proportion of water. The second is, that the cubic inch of water consists of a certain number of definite particles, which, in the process of boiling, are not subdivided, so that the cubic foot of steam contains the same number of the same particles as the cubic inch of water, the conversion of the one into the other depending simply on the action of heat in separating these particles to a greater dis- tance. Hence the steam is not absolutely homogene- ous ; for, if we consider spaces sufficiently minute, we can distinguish between such as contain a particle of water and those which lie between the particles. Now, the small masses of water, whose isolation we here as- sume, are what Avogadro calls molecules, and, follow- 16 MOLECULES AND AVOGADRO'S LAW. Fig. 2. ing his authority, we shall designate them hereafter ex- clusively by this word. The rude diagrams before you will help me to make clear the difference between the two suppositions I have made. In the first (Fig. 1), we assume that the material of this cubic inch is uniformly expanded through the cubic foot. In the other (Fig. 2), we have in both volumes a definite number of molecules, the only difference being that these dots, which we have used to represent the molecules, are more widely separa- ted in the one case than in the other. Kow, which of these suppositions is the more probable ? Let us sub- mit the question to the test of experiment. We have here a glass globe, provided with the ne- cessary mountings — a stop-cock, a pressure-gauge, and a thermometer — and which we will assume has a capacity of one cubic foot. Into this globe we will first pour one INTERSPACES IN VAPORS. 17 cubic inch of water, and, in order to reduce the condi- tions to the simplest possible, we will connect the globe with our air-pump, and exhaust the air, al- though, as it will soon appear, this is not necessary for the success of our experiment. Exposing, next, the globe to the temperature of boiling water, all the liquid will evaporate, and we shall have our vessel filled with ordinary steam. If, now, that cubic foot of space is really packed close with the material we call water — if there is no break in the continuity of the aqueous mass — we should expect that the vapor would fill the space,, to the exclusion of every thing else, or, at least, would fill it with a certain degree of energy which must be overcome before any other vapor could be forced in. Now, what is the case ? The stop-cock of the globe is so arranged that we can introduce into it an additional quantity of any liquid on which we desire to experiment, without otherwise opening the vessel. If, then, by this means, we add more water, the additional quantity thus added will not evaporate, pro- vided that the temperature remains at the boiling-point. Let us next, however, add a quantity of alcohol, and what do we find \ Why, not only that this immedi- ately evaporates, but we find that just as much alcohol- vapor will form as if no steam were present. The presence of the steam does not interfere in the least degree with the expansion of liquid alcohol into alco- hol-vapor. The only difference which we observe is, that the alcohol expands more slowly into the aque- ous vapor than it would into a vacuum. If, now that the globe is filled with aqueous vapor and alcohol- vapor at one and the same time, each acting, in all re- spects, as if it occupied the space alone, we add a quan- tity of ether, we shall have the same phenomena re- 18 MOLECULES AND AVOGADRO'S LAW. peated. The ether will expand and fill the space with its vapor, and the globe will hold just as much ether- vapor as if neither of the other two were present ; and so we might go on, as far as we know, indefinitely. There is not here a chemical union between the sev- eral vapors, and we cannot in any sense regard the space as filled with a compound of the three. It con- tains all three at the same time, each acting as if it were the sole occupant of the space ; and that this is the real condition of things we have the most unques- tionable evidence. You know, for example, that a vapor or gas exerts a certain very considerable pressure against the walls of the containing vessel. Now, each of these vapors exerts its own pressure, and just the same pressure as if it occupied the space alone, so that the total pressure is exactly the sum of the three partial pressures. Evidently, then, no vapor completely fills the space which it occupies, although equally distributed through it ; and we can give no satisfactory explanation of the phenomena of evaporation except on the assumption that each substance is an aggregate of particles, or units, which, by the action of heat, become widely separated from each other, leaving very large intermolecular spaces, within which the particles of an almost indefi- nite number of other vapors may find place. Pass now to another class of facts, illustrating the same point. The three liquids, water, alcohol, and ether, are ex- panded by heat like other forms of matter, but there is a striking circumstance connected with these phenom- ena, to which I wish to direct your observation. I have, therefore, filled three perfectly similar thermometer- bulb tubes, each with one of those liquids. The tubes are mounted in a glass cell standing before the con- UNEQUAL EXPANSION IN LIQUIDS. 19 denser of a magic lantern, and you see their images projected on the screen. You also notice that the liquids (which have been colored to make them visible) all stand at the same height; and, since both the bulbs and the tubes are of the same dimensions, the relative change in volume of the inclosed liquids will be indicated by the rise or fall of the liquid columns in the tubes. We will now fill the cell with warm water, and notice that, as soon as the heat begins to penetrate the liquids, the three columns begin to rise, indicating an increase of volume ; but notice how unequal is the expansion. The ether in the right-hand tube expands more than the alcohol in the centre, and that again far more than the water on the left. What is true of these three liquids is true in general of all liquids. Each has its own rate of expansion, and the amount in any case does not appear to depend on any peculiar physical state or condition of the liquid, but is con- nected with the nature of the substance, although, in what way, we are as yet wholly ignorant. But you may ask: What is there remarkable in this ? Why should wc not expect that the rate of ex- pansion would differ with different substances % Cer- tainly, there is no reason to be surprised at such a fact. But, then, the remarkable circumstance connected with this class of phenomena has yet to be stated. Raise the temperature of these liquids to a point a little above that of boiling water, and we shall convert all three substances into vapor. We thus obtain three gases, and, on heating these aeriform bodies to a still higher temperature, we shall find that, in this new con- dition, they expand far, more rapidly than in the liquid state. But we shall also find that the influence of the nature of the substance on the phenomenon lias wholly 20 MOLECULES AND AVOGADRO'S LAW. disappeared, and that, in the aeriform condition, these substances, and in general all substances, expand at the same rate under like conditions. Why, now, this difference between the two states of matter ? If the material fills space as completely in the aeriform as it does in the liquid condition, then we cannot conceive why the nature of the substance should not have the same influence on the phenomena of ex- pansion in both cases. If, however, matter is an ag- gregate of definite small masses or molecules, which, while comparatively close together in the liquid state, become widely separated when the liquids are con- verted into vapor, then it is obvious that the action of the particles on each other, which might be consider- able in the first state, would become less and less as the molecules were separated, until at last it was inap- preciable ; and if, further, as Avogadro's law assumes, the number of these particles in a given space is the same for all gases under the same conditions, then it is equally obvious that, there being no action between the particles, all vapors may be regarded as aggregates of the same number of isolated particles similarly placed, and we should expect that the action of heat on such similar masses would be the same. Thus these phenomena of heat almost force upon us the conviction that the various forms of matter we see around us do not completely fill the spaces which they appear to occupy, but consist of isolated particles separated by comparatively wide intervals. There are many other facts which might be cited in support of the same conclusion ; and among these two, which are more especially worthy of your attention, because they aid us in forming some conception of the size of the molecules themselves. INTERSTICES IN SOLIDS. 21 If this mass of glass is perfectly homogeneous — if the vitreous substance completely fills its allotted space, and there is no break whatever in the continuity of the material — then you would expect that its physical relations would not depend at all on the size of the surface affected. Suppose you wished to penetrate it with a fine wire. The point of this wire, however small, would not detect any difference at different points of the surface. Assume, however, that it con- sists of masses separated by spaces, like, for example, this sheet of wire netting. Then, although the surface would seem perfectly homogeneous to a bar large enough to cover a number of meshes, it woidd not be found to be by any means homogeneous to a wire which was small enough to penetrate the meshes. If, now, there are similar interstices in this mass of glass, we should expect that, if our wire were small enough (that is, of dimensions corresponding to the interstices), it would detect differences in the resistance at different points of this glass surface. Make, now, a further supposition. Assume that we have a number of these wires of different sizes, the largest being twice as stout as the smallest. It is ob- vious that, if the interstices we have assumed were, say, several thousand times larger than the largest wire, all the wires would meet with essentially the same oppo- sition when thrust at the glass. If, however, the inter- stices were only four or five times larger than the wires, then the larger would encounter much greater resist- ance from the edges of the meshes than the smaller. It is unnecessary to say that no physical point can detect an inequality in the surface of a plate of glass, but we have, in what we call a beam of light, an agent which does find a passage through its mass. Now, it 22 MOLECULES AND AVOGADKO'S LAW. is perfectly true that we have no absolute knowledge of the nature of a beam of light. We have a very plausible theory that the phenomena of light are the effects of waves transmitted through a highly-elastic medium we call ether, and that, in the case of our plate of glass, the motion is transmitted through the ether, which fills the interstices between the molecules of this transparent solid ; but we have no right to assume this theory in our present discussion. Indeed, I cannot agree with those who regard the wave-theory of light as an established principle of science. That it is a theory of the very highest value I freely admit, and that it has been able to predict the phases of unknown phenomena, which experiment has subsequently brought to light, is a well-known fact. All this is true ; but then, on the other side, the theory requires a combination of qualities in the ether of space, which I find it difficult to believe are actually realized. For instance, the rapidity with which wave-motion is transmitted depends, other things being equal, on the elasticity of the medium. Assuming that two media have the same density, their elasticities are proportional to the squares of the velocities with which a wave trav- els. The velocity of the sound-wave in air is about 1,100 feet a second or \ of a mile, that of the light- wave about 192,000 miles a second, or about one million times greater; and, if we take into account certain causes, which, though they tend to increase the velocity of sound, can have no effect on the luminiferous ether, the difference would be even greater than this. Now, were the density of the ether as great as that of the atmosphere (say \ of a grain to the cubic inch), its elasticity or power of resisting pressure would be a million square, or a million million times that of the DIFFICULTIES WITH THE ETHEK. 23 atmosphere. But, as you well know, the atmosphere can resist a pressure of about fifteen pounds to the square inch ; hence the ether, when equally dense, would re- sist a pressure of fifteen million million pounds to the square inch, or, making the correction referred to above, seventeen million million pounds to the square inch. Of course, such numbers convey no impression, except that of vast magnitude ; and you will obtain a clearer idea of the power when I tell you that this pressure is about the weight of a cubic mile of granite rock. Here is a glass cylinder filled with air, and here a piston which just fits it. The area of the piston is about a square inch — we will assume that it is exactly that. If we put a weight of fifteen pounds on the top of the piston, it will descend just half-way in the tube, and the air will be condensed to twice its normal density. Now, if we had a cylinder and piston, ether- tight as this is air-tight, and of sufficient strength, and, if we put on top of it a cubic mile of granite rock, it would only condense the ether to about the same den- sity as that of the atmosphere at the surface of the earth. Of course, the supposition is an absurdity, for it is assumed that the ether pervades the densest solids as readily as water does a sponge, and could not, there- fore, be confined ; but the illustration will give you an idea of the nature of the medium which the undulatory theory assumes. It is a medium so thin that the earth, moving in its orbit 1,100 miles a minute, suffers no per- ceptible retardation, and yet endowed with an elasticity in proportion to its density a million million times greater than air. Whether, however, there are such things as waves of ether or not, there is something concerned in the phenomena of light which has definite dimensions, that 24 MOLECULES AND AVOGADRO'S LAW. have been measured with as much accuracy as the di- mensions of astronomy, although they are at the oppo- site extreme of the scale of magnitude. We represent these dimensions to our imagination as wave-lengths, that is, as the distances from crest to crest of our as- sumed ether-waves, and we shall find it difficult to think clearly upon the subject without the aid of this wave-theory, and every student of physics will bear me out in the statement that, though our theory may be a phantom of our scientific dreaming, these magnitudes must be the dimensions of something. Here they are : Dimensions of Light-waves. OOIOES. Number of waves in one inch. Number of oscillations in one second. Red 89,000 42,000 44,000 47,000 51,000 54,000 57,000 477,000,000,000,000 506,000,000,000,000 535,000,000,000,000 577,000,000,000,000 622,000,000,000,000 658,000,000,000,000 699,000,000,000,000 Yellow Blue Violet You know that the sensation we call white light is a very complex phenomenon, and is produced by rays of all colors acting simultaneously on the eye. A very pretty experiment will illustrate this point. I have projected on the screen the image of a circular disk made of sectors of gelatine-paper, variously colored. By means of a very simple apparatus, I can revolve the disk, and thus cause the several colors to succeed each other at the same point with great rapidity, and you notice that the confused effect of the different colors produces the impression you call white, or, at least, nearly that. The sunbeam produces the same impression, be- MAGNITUDES OF ETHER-WAVES. 25 cause it contains all these colored rays ; and, if we pass it through a prism, the several rays, being bent un- equally by the glass, diverge on emerging, so that, if we receive the beam thus divided on a screen placed at a sufficient distance, we obtain that magnificent band of blending hues we call the solar spectrum. To each of the colored rays which fall along the line of the spectrum corresponds a definite wave- length. In the diagram, we have given the wave- lengths, corresponding to only a few selected points, one in each color, and marked in the solar spectrum itself by certain remarkable dark lines by which it is crossed.^ These values always create a smile with a popular audience, which makes it evident that, by those unfamiliar with the subject, they are looked upon as unreal if not absurd. But this is a prejudice. In our universe the very small is as real as the very great ; and if science in astronomy can measure dis- tances so great that this same swift messenger, light, traveling 192,000 miles a second, requires years to cross them, we need not be surprised that, at the other end of the scale, it can measure magnitudes like these. Let not, then, these numbers impair your confidence in our results ; but remember that the microscope re- veals a universe with dimensions of the same order of magnitude. Moreover, the magnitudes with which we are here dealing are not beyond the limits of mechani- cal skill. It is possible to rule lines on a plate of glass so close together that the bands of fine lines thus ob- tained cannot be resolved even by the most powerful microscopes ; and I am informed that the German opti- cian, Nobert, has ruled bands containing about 224,000 lines to the inch. He regularly makes plates with bands consisting of from about 11,000 to 112,000 lines 26 MOLECULES AND AVOGADRO'S LAW. to the inch. These bands are numbered from the 1st to the 19th, and are used for microscopic tests. I am indebted to our friend Mr. Stodder for the opportu- nity of exhibiting to you a beautiful photograph of the 19th band, containing over 112,000 lines to the inch (Fig. 3). The photograph was made with one of Tolles's Fig. 8.— Nobert's 19th Band. microscopes, and any microscopist will tell you that to resolve this band is a great triumph of art, and that you could have no better evidence of the skill of our eminent optician than this photograph affords. In projecting the image on the screen, some of the sharp- ness is lost, but I think the separate lines of the band must be distinctly visible to all who are not too far off. Now, the distance between the lines on the original plate is not very different from one-half of the mean length of a wave of violet light, or one-third of a wave- length of red light ; and, what is still more to the pur- pose, these very bands give us the means of measuring the dimensions of the waves of light themselves. Evi- dently, then, the dimensions with which we are dealing are not only conceivable, but wholly within the range THE INTERSPACES IN GLASS. 27 of our perceptions, aided as they have been by the ap- pliances of modern science. But, to return to my argument: these values, if they are not wave-lengths, are real magnitudes, which differ from each other in size just as the above measure- ments show. Moreover, we have reason to believe that the various color-giving rays differ in nothing else, and it is certain from astronomical evidence that they all pass through the celestial spaces with the same velocity. Now, when a beam of light enters a mass of glass, not only does its velocity diminish, but, what is more re- markable, the different rays assume at once different velocities, and, according to the well-known principles of wave-motion, the unequal bending that results is the necessary effect of the unequal change in velocity which the rays experience. But, if the material of the glass were perfectly homogeneous throughout, it is im- possible to conceive, either on the wave theory or any other theory of light we have been able to form, how a mere difference in size in what we now call the luminous waves should determine this unequal velocity with the accompanying difference of refrangibility, and the fact that such a difference is produced is thought by many to be strong evidence that there is not an ab- solute continuity in the material ; in fine, that there are interstices in the glass, although they are so small that it requires the tenuity of a ray of light to detect them. Still we cannot make our conceptions the measure of the resources of Nature, and I, therefore, do not attach much value to this additional evidence of the molecular structure of matter. But the importance of these optical phenomena lies in this, that, assuming the other evidence sufficient, they give us a rough measure of the size of the molecules. . For, as is evident Missing Page HOW TO MAKE SOAP-BUBBLES. 29 inent for yourselves. And, first, I must tell you how to prepare the soap-suds. Procure a quart-bottle of clear glass and some of the best white castile-soap (or, still better, pure palm-oil soap). Cut the soap (about four ounces) into thin shav- ings, and, having put them into the bottle, fill this up with distilled or rain-water, and shake it well together. Repeat the shaking until you get a saturated solution of soap. If, on standing, the solution settles perfectly clear, you are prepared for the next step ; if not, pour off the liquid and add more water to the same shav- ings, shaking as before. The second trial will hardly fail to give you a clear solution. Then add to two volumes of soap-solution one volume of pure, con- centrated glycerine. Those who are near can see what grand soap-bubbles we can blow with this preparation. The magnificent colors which are seen playing on this thin film of water are caused by what we call the interference of light. The color at any one point depends on the thickness of the film, and by varying the conditions we can show that this is the case, and make these effects of color more regular. For this purpose I will pour a little of the soap-solution into a shallow dish, and dip into it the open mouth of a common tumbler. By gently raising the tumbler it is easy to bring away a thin film of the liquid covering the mouth of the glass. You can all easily make the experiment, and study at your lei- sure the beautiful phenomena which this film presents. To exhibit them to a large audience is more difficult, but I hope to succeed by placing the tumbler before the lantern in such a position that the beam of light will be reflected by the film upon the screen, and then, on interposing a lens, we have at once a distinct image 30 MOLECULES AND AVOGADEO'S LAW. of the film. Success now depends on our keeping perfectly still, as the slightest jar would be sufficient to break this wonderfully delicate liquid membrane. See ! the same brilliant hues which give to the soap- bubble its beauty are beginning to appear on our film, but notice that they appear in regular bands, crossing the film horizontally. As I have already stated, the color at any point depends on the thickness of the film, and, as it is here held in a vertical position, it is evident that the effect of gravity must be to stretch the liquid membrane, constantly thinning it out, be- ginning from the upper end — which, however, it must be remembered, appears on the screen at the lower end, since the lens inverts the image — and notice that, as the film becomes thinner and thinner, these bands of color which correspond to a definite thickness move downward, and are succeeded by others corresponding to a thinner condition of the film, which give place to still others in their turn. These colors are not pure colors, but the effect is produced by the over- lapping of very many colored bands, and, in order to r»,duee the conditions to the simplest possible, we must use pure colored light — monochromatic light, as we call it. Such a light can be produced by placing a plate of red glass (colored by copper) in front of the lantern. At once all the particolors vanish and we have merely alternate red and dark bands. Watch, now, the bands as they chase each other, as it were over the film, and notice that already new bands cease to appear, and that a uniform light tint has spread over the upper half (lower in the image) of the surface. Now comes the critical point of our experiment. If the film is in the right condition so that it can be stretched to a sufiieient degree of tenuity, this light OPTICAL EFFECTS OF SOAP-BUBBLES. 31 tint will be succeeded by a gray tint, .... and there it appears in irregular patches at the upper border. But in an instant all has vanished, for the film has broken, as it always breaks, soon after the gray tint appears. *' &$£$&? ^QH H .;?; ■i^i ».:.rS;.T. V- 1 _jm FiQ. 4.— Bands on Soap-film. Having now seen the phenomena, you will be bet- ter prepared to appreciate the strength of the ar- gument to which I now have to ask your careful attention. You know that the red and dark bands seen in the last experiment, when we used the red glass, are caused by the interference of the rays of light, which are reflected from the opposite surfaces of the film. It is evident that the path of the rays re- flected from the back surface must be longer than that of those reflected from the front surface by just twice the thickness of this film of water ; and, as Prof. Tyndall has so beautifully shown you in the course of lectures just finished, whenever this difference of path brings the crests of the waves of one set of rays over the troughs of the second set, we obtain this won- derful result — that the union of the two beams of light produces darkness. It would, at first sight, seem that 32 MOLECULES AND AVOGADKO'S LAW. such a result must be produced in the case of our film whenever its thickness is equal to J, f , £, J, or any- odd number of fourths of the length of a wave of red light, and this would be the case were it not for the circumstance that, in consequence of certain mechani- cal conditions, the rays of light reflected from the back of the film lose one-half of a wave-length in the very ae£ of reflection. But, without entering into details, which have been so recently and so beautifully illus- trated in this place, let me call your attention to this diagram, which tells the whole story : Okdee of Bands. Gray film , Light film First dark band. . . , First light band. . . Second dark band.. Second light band. , Third dark band. . . Third light band.., Fourth dark band. , Fourth light band . , Retardation of rays reflected from back- surface of film. i ware-length. 1 " 2 2£ 3 " 3£ 4 " 4i Thickness of film in waves of red light suiso of an inch. Less than £ wave-length. i i or £ I 1 or i li " f H " f If " I 2 " f 2J- " f You thus see that the theory of light enables us to measure the thickness of the film, and we know that where that gray tint appeared in our experiment the thickness of the film was less than \ of the length of a wave of red light, or less than 1S ^ 060 of an inch, and no wonder that the film broke when it reached such a degree of tenuity as that. But, having followed me thus far, and being assured, as I hope you are, that we are on safe ground, and talking about what we do know, your curiosity will lead you to inquire whether we can stretch the film any farther. The facts are that, after the appearance of the gray tint, although the film evidently stretches to a limited SEPARATING THE MOLECULES OP WATER. 33 extent, it very soon breaks. Practically, then, we can- not stretch it beyond this point to any great extent ; but why not ? Theoretically, if the material of water is perfectly homogeneous, there would seem to be no good reason why it should not be capable of an in- definite extension, and why this film could not be stretched to an indefinite degree of attenuation. As- sume, however, that water consists of molecules ot a definite size, then it is evident that a limit would be reached as soon as the thickness of the film was re- duced to the diameter of a single molecule. Obvi- ously we could not stretch the film beyond this with- out increasing the distance between the molecules, and thus increasing the total volume of the water. Now, there is evidence that, when the gray tint appears, we are approaching a limit of this sort. It is hardly necessary to say that we cannot separate, to any con- siderable extent, the molecules of water from each other — that is, increase the distance between them — without changing the liquid into a gas, or, in other words, converting the water into steam, and the only way in which we can produce this effect is by the application of heat. The force required is enormous, but the force exerted by heat is adequate to the work, and it is one of the triumphs of our modem science that we have been able to measure this force, and re- duce it to our mechanical standard. In order to pull apart the molecules of a pound of water, that is, con- vert it into steam, we must exert a mechanical power which is the equivalent of 822,600 foot-pounds, that is, a power which would raise nearly four tons to the height of one hundred feet, and, as we can readily esti- mate the weight of say one square-inch of our film, we know the force which would be required to pull apart the molecules of which it consists. 34 MOLECULES AND AVOGADRO'S LAW. Again, on the other hand, singular as it may seem, we have been able to calculate the force which is re- quired to stretch the film of water. This calculation is based on the theory of capillary action, of which the soap-bubble is an example. Moreover, to a certain limit, we are able to measure experimentally the force required to stretch the film, and we find that, as far as our experiments go, the theory and the experiments agree. Our experiments necessarily stop long before we reach the limit of the gray film ; but our theory is not thus limited, and we can readily calculate how great a force would be required to stretch the film until the thickness was reduced to the j-^^^.tto^ of an inch ; that is, the ^V? of the thickness of the light film, or the ts.tots °^ a wave-length. Now, the force required to do this work is as great as that required to pull apart the molecules of the water and convert the liquid into vapor. It is therefore probable that, before such a degree of tenuity can be attained, a point would be reached where the film had the thickness of a single molecule, and that, in stretching it further, we should not reduce its thickness, but merely draw the molecules apart, and, thus overcoming the cohesion which deter- mines its liquid condition, and gives strength to the film, convert the liquid into a gas. There are many other physical phenomena which point to a similar limit, and, unless there is some fal- lacy in our reasoning, this limit would be reached at about the -^^^ h>,Tnm of an inch. Moreover, it is wor- thy of notice that all these phenomena point to very nearly the same limit. I have great pleasure in refer- ring you, in this connection, to a very remarkable pa- per of Sir William Thompson, of Glasgow, on this sub- ject, which, appearing first in the English scientific DIMENSIONS OP MOLECULES. 35 weekly called Nature, was reprinted in Silliman's Journal of July, 1870. He fixes the limits at between tne snr. ooVooo and tne t. ooo . Joo . ooo of an incn » an( i, in order to give some conception of the degree of coarse- grainedness (as he calls it) thus indicated by the struct- ure, he adds that, if we conceive a sphere of water as large as a pea to be magnified to the size of the earth, each molecule being magnified to the same extent, the magnified structure would be coarser-grained than a heap of small lead shot, but less coarse-grained than a heap of cricket-balls. These considerations will, I hope, help to show you how definite the idea of the molecule has become in the mind of the physicist. It is no longer a metaphysical abstraction, but a reality, about which he reasons as confidently and as successfully as he does about the plan- ets. He no longer connects with this term the ideas of infinite hardness, absolute rigidity, and other in- credible assumptions, which have brought the idea of a limited divisibility into disrepute. His molecules ai-e definite masses of matter, exceedingly small, but still not immeasurable, and they are the points of applica- tion to which he traces the action of the forces with which he has to deal. These molecules are to the physi- cist real magnitudes, which are no further removed from our ordinary experience on the one side, than are the magnitudes of astronomy on the other. In regard to their properties and relations, we have certain defi- nite knowledge, and there we rest until more knowledge is reached. The old metaphysical question in regard to the infinite divisibility of matter, which was such a sub- ject of controversy in the last century, has nothing to do with the present conception. "Were we small enough to be able to grasp the molecules, we might be able to 36 MOLECULES AND AYOGADRO'S LAW. split them, and so, were we large enough, we might be able to crack the earth ; but we have made sufficient advance since the days of the old controversy to know that questions of this sort, in the present state of knowl- edge, are both irrelevant and absurd. The molecules are to the physicist definite units, in the same sense that the planets are units to the astronomer. The ge- ologist tears the earth to pieces, and so does the chem- ist deal with the molecules, but to the astronomer the. earth is a unit, and so is the" molecule to the physicist. The word molecule, which means simply a small mass of matter, expresses our modern conception far better than the old word atom, which is derived from the Greek a, privative, and rifiva, and means, therefore, in- divisible. In the paper just referred to, Sir W. Thomp- son used the word atom in the sense of molecule, and this must be borne in mind in reading his article. "We shall give to the word atom an utterly different signifi- cation, which we must be careful not to confound with that of molecule. In our modern chemistry, the two terms stand for wholly different ideas, and, as we shall see, the atom is the unit of the chemist in the same sense that the molecule is the unit of the physicist. But we will not anticipate. It is sufficient for the pres- ent if we have gained a clear conception of what the word molecule means, and I have dwelt thus at length on the definition because I am anxious to give you the same clear conviction of their existence which I have myself. As I have said before, they are to me just as much real magnitudes as the planets, or, to use the words of Thompson, " pieces of matter of measurable dimensions, with shape, motion, and laws of action, in- telligible subjects of scientific investigation." * 1 See Lecture on Molecules, by Prof. Maxwell, Nature, Sept. 25, 1873. LECTURE H. THE MOLECULAR CONDITION OF THE THREE STATES IF MATTEK THE GAS, THE LIQUID, AND THE SOLID. In my first lecture I endeavored to give you some conception of the meaning of the word molecule, and this meaning I illustrated by a number of phenomena, which not only indicate that molecules are real magnitudes, but which also give us some idea of their absolute size. Avogadro's law declares that all gases contain, un- der like conditions of temperature and pressure, the same number of molecules in the same volume ; and, if we can rely on the calculations of Thompson, which are based on the well-known theorem of molecular me- chanics deduced by Clausius, this number is about one hundred thousand million million million, or 10 23 to a cubic inch. Of course, as the volume of a given quan- tity of gas varies with its temperature and pressure, the number of molecules contained in a given volume must vary in the same way; and the above calculation is based on the assumption that the temperature is at the freezing-point, and the pressure of the air, as indicated by the barometer, thirty inches. The law only holds, moreover, when the substances are in the condition of perfect gases. It does not apply to solids or liquids, and not even to that half-way state between liquids and gases which Dr. Andrews has recently so admirably 38 THE THREE STATES OF MATTER. defined. In the state of perfect gas, it is assumed that the molecules are so widely separated that they exert no action upon each other, but the moment the gas is so far condensed that the molecules are brought within the sphere of their mutual attraction, then, although the aeriform state is still retained, we no longer find that the law rigidly holds ; and when, by the condensation, the state of the substance is changed to that of a liquid or a solid, all traces of the law disappear. In order that you may gain a clear conception of this relation, I shall ask your attention in this lecture to the explana- tion which our molecular theory gives of the char- acteristic properties of the three conditions of matter, the gas, the liquid, and the solid. We begin with the gas, because its mechanical condition is, theoretically at least, by far the simplest of the three. Every one of my audi- ence must be familiar with the fact that every gas is in a state of constant tension, tending to expand indefi- nitely into space. In the case of our atmosphere, this tension is so great that the air at the level of the sea exerts a pressure of between Fig. 5.— Barometer. EXPANSIVE ENERGY IN GASES. 39 fourteen and fifteen pounds on every square inch of surface — about a ton on a square foot. It is this pressure which sustains the column of mercury in the tube of a barometer (Fig. 5) ; and since, by the laws of hydrostatics, the height of this column of mercury depends on the pressure of the air, rising and falling in the same proportion as the pressure in- creases or diminishes, we use the barometer as a meas- ure of the pressure, and, instead of estimating its amount as so many pounds to the square inch, we more fre- quently describe it by the height in inches (or centi- metres) of the mercury-column, which it is capable of sustaining in the tube of a barometer. The tension of the air is balanced by the force of gravitation, in con- sequence of which the lower stratum of the air in which we live is pressed upon by the whole weight of the su- perincumbent mass. The moment, however, the ex- ternal pressure is relieved, the peculiar mechanical con- dition of the gas becomes evident. Hanging under this large glass receiver is a small rubber bag (a common toy balloon), partially dis- tended with air (Fig. 6). The air confined within the bag is exerting the great tension of which I have spo- ken, but the mass remains quiescent, because this ten- sion is exactly balanced by the pressure of the atmos- phere on the exterior surface of the bag. You see, how- ever, that, as we remove, by means of this air-pump, the air from the receiver, and thus relieve the external pressure, the bag slowly expands, until it almost com- pletely fills the bell. There can, then, be no doubt that there exists within this mass of gas a great amount of energy, and since this energy exactly balances the at- mospheric pressure, it must be equal to that pressure. But I wish to show you more than this, for not only 40 THE THREE STATES OF MATTER. is it true that the bag expands as the pressure is relieved, but it is also true that the gas in the bag expands in exactly the same proportion as the external pressure Fig. 6. — Expanding Bag under Air-pump. diminishes. In order to prove this, I will now place under this same glass one of those small gasometers, which are used by the itinerant showmen in our streets for measuring what they call the volume of the lungs, while under this tall bell at the side I have arranged a barometer-tube for measuring the external pressure. The two receivers are connected together by rubber hose, so as to form essentially one vessel, and both are connected with the air-pump. We will begin by blowing air into the gasometer until the scale marks 100 cubic inches, and, noticing after adjusting the apparatus that the barometer stands at 30 inches, we will now proceed to exhaust the air, at the same time carefully watching the barometer. . . . It has now fallen to 15 inches ; that is, the pressure on LAW OF MAEIOTTB. 4! the outside of the gasometer has been reduced to one- half, and the scale of the instrument shows me that the volume of the air in the interior has become 200 cubic inches ; that is, has doubled. But let us continue the exhaustion. . . . The barometer now marks 10 inches, showing that the pressure has been reduced to one- third. The gasometer now contains 300 cubic inches of gas. The volume, then, has trebled. . . . Pushing the experiment still further, we have now the barome- ter stariding at 7£ inches, and the scale of the gasome- ter shows that the volume of the inclosed air has be- • come 400 cubic inches. The pressure has been reduced to one-fourth, and the volume of the air has quadrupled ; and so we might go on. . . . Let, now, the atmosphere reenter the apparatus, and at once the air in the gas- ometer shrinks to its original volume, while the barome- ter goes back to 30 inches. "We might next take a condensing-pump, and, ar- ranging our apparatus so as to resist the ever-increasing pressure, as the air was forced into the receivers, we should find that, when the barometer marked 60 inches, the scale of the gasometer would show 50 cubic inches, and that, when the mercury column had risen to 120 inches, the air in the gasometer would have shrunk to 25 cubic inches ; and so on. There are, however, ob- vious mechanical difficulties, which make this phase of the experiment unsuitable for a large lecture-room, and what we have seen is sufficient to illustrate the general principle which I wished to enforce. The principle, in a few words, is this : The volume of a confined mass of gas is inversely pro- portional to the pressure to which it is exposed : the smaller the pressure the larger the volume, and the. greater the pressure the less the volume. 42 THE THREE STATES OF MATTEE. This principle holds true not only with air, but also with every kind of aeriform matter. If, instead of using that mixture of oxygen and nitrogen we call air, we had introduced into the gasometer 100 cubic inches of pure oxygen or of pure nitrogen, or of any other true gas, we should have obtained precisely the same effect. The results of the experiment are not in the least degree influenced by the nature of the gas employed ; and, assuming that we start with the same gas-volumes, the resulting volumes are the same at each stage of the experiment. In every case the volume varies inversely as the pressure. The principle thus developed is one of the most important laws of physical science. It was discovered by the chemist Boyle in England in 1662, and verified by the Abbe Mariotte in France somewhat later, and is by some called the law of Mariotte, and by others the law of Boyle. This law of Mariotte or Boyle is most closely related to the law of Avogadro. The one law is found to hold just as far as the other, and any deviation from the one is accompanied by a corresponding deviation from the other. So close, indeed, is the connection, that we cannot resist the conviction that the two laws are merely different phases of one and the same condition of matter ; and our molecular theory explains this con- nection in the following way : The molecules of a body are not isolated masses in a fixed position, all at rest, but, like the planets, they are in constant motion. In a gas this motion is sup- posed to take place in straight lines, the molecules hurrying to and fro across the containing vessel, strik- ing against its walls, or else encountering their neigh- bors, rebounding and continuing on their course in a new direction, according to the well-known laws which MOLECTTLAR MOTION IN GASES. 43 govern the impact of elastic bodies. Of course, in such a system, all the molecules are not moving with the same velocity at the same time ; but they have a cer- tain mean velocity, which determines what we call the temperature of the body, and the higher the tempera- ture the greater is this mean velocity ; moreover, the mean velocity of the molecules of each substance is always the same at the same temperature. It varies, however, for different substances, and, for any given temperature, the less the density of the gas the greater is this velocity, although, as we shall hereafter see, the velocities of the molecules of two different gases are inversely proportional, not simply to their densities, but to the square roots of these quantities. "We are able to calculate for each gas at least approximately what this velocity must be for any temperature, and, in the case of hydrogen gas, the value at the temperature of freezing water is about 6,097 feet per second. The internal energy, therefore, in a pound of hydrogen gas at the freezing-point is as great as that of a pound-ball moving 6,097 feet per second, and the energy in an equal volume (a little over 6.6 cubic yards when the barometer is at 30 inches) of any*other true gas is equally great under the same conditions; a greater molecular weight compensating in every case for a less molecular velocity. Let us now bring together the two remarkable results already reached in this lecture. One cubic inch of every gas, when the barometer marks 30 inches, and the thermometer 32° Fahr., con- tains 10 23 molecules. Mean velocity of hydrogen molecules, under same conditions, 6,097 feet per second. ,3 It is evident, then, that every mass of gas must contain a large amount of internal energy, and this 44 THE THREE STATES OF MATTER. energy is made manifest in many ways, especially in what we call the permanent tension of the gas. Every surface in contact with a mass of gas is be- ing constantly bombarded by the molecules, and hence the great pressure which results. Now, it can easily be seen that, if the volume of the gas is diminished — that is, if the same number of molecules are crowded into a less space — they will strike more frequently against a given surface,/ and therefore exert a greater pressure. Moreover, it can readily be proved, although the mathematical demonstration would be out of place in a popular lecture, that the pressure must be inversely as the volume ; in other words, that the law of Mariotte is the necessary result of the molecular condition we have described. Another effect of molecular motion is that condi- tion of matter which the word temperature, just used, denotes. There are few scientific terms more difficult to define than this common word temperature. In ordinary language we apply the terms hot or cold to other bodies according as they are in a condition to impart heat to, or abstract it from, our own, and the various degrees of"hot or cold are what we call, in gen- eral, temperature. Two bodies have the same temper- ature if, when placed together, neither of them gives or loses heat ; and, when, under the same conditions, one body loses while the other gains heat, that body which gives out heat is said to have the higher temperature. Increased temperature tested in this way is found to be accompanied by an increase of volume, and we employ this change of volume as the measure of tem- perature. This is the simple principle of a thermome- ter. The essential part of this instrument is a glass bulb, connected with a fine tube, and filled with mer- WHAT THERMOMETERS TELL US. 45 cury to a variable point in the stem. The least change in the volume of the mercury is indicated by the rise of the column in the tube. Primarily, the thermome- ter is a very delicate measure of the change of volume of the inclosed liquid ; secondarily, it becomes a meas- ure of temperature. You know how the thermometer is graduated. We plunge it into a mass of melting ice and mark the point to which the mercury falls, and then we immerse it in free steam, and mark the point to which the column rises. We now divide the distance between these fixed points into an arbitrary number of equal spaces, and' continue the divisions of the same size above and below our two standard points. In our common Fahrenheit scale this distance is di- vided into 180 parts, the freezing-point is marked 32°, and the boiling, of course, 212° ; the zero of this scale be- ing placed at the thirty-second division below the freez- ing-point. In our laboratories we generally use a scale in which this distance is divided into 100 parts, and the freezing-point marked 0°, the divisions below freez- ing being distinguished with a mmus-sign. All this, however, is purely arbitrary, and the instrument mere- ly gives us the means of comparing temperatures. Here, for example, are two bodies. We apply the thermometer first to one and then to the other. It rises in each case to 50°. The only information we have obtained is, that both bodies are at the same tem- perature corresponding to a certain volume of the mer- cury in our thermometer, a temperature which we have agreed to call 50° ; and we can predict that, if the two bodies are brought together, no heat will pass from one to the other. We now apply the thermometer to a third body, and it rises to 100°. We thus learn, further, that the third body is at a higher temperature 46 THE THREE STATES OF MATTER. than the other two, and in a condition to transfer to them a part of its heat. We cannot, however, say- that its temperature is twice as high, or that it has any- definite relation to that of the other two bodies. There is, however, a theoretical way of measuring temperature, which appears to lead to something more than a mere arbitrary comparison. Let us assume that we have a cylindrical tube, closed below, but open above (Fig. ?). Let us further assume that the air B46 1 373' 273' 273° 200° 150° 100° 50° 0° -60° -100° -150° -200° 0° LJ-273° Fig. T. 982 ■i I 671 491° 459° 523° 392° 302° 212° 122° 32° 0° -88° -148° -238° ■328° 0° |_U59° Fig. 7, bis. in the tube is confined by a piston, which has no weight, and moves without friction. As the tempera- ture rises or falls, of course our assumed piston would rise or fall in the tube, following the expanding or con- tracting of the confined air. Let us mark the point to which the piston falls at the temperature of freezing AN ABSOLUTE SCALE OF TEMPERATURE. 47 water, 0°, and the point to which it rises at the temperature of boiling water, 100°. Lastly, let us divide the distance between these two points, as in a centigrade thermometer, into one hundred equal parts, and continue the divisions of the same size above 100° and below 0°. We shall find that we can make almost exactly 273 such divisions before reaching the closed bottom of our tube. Transfer, now, the zero of our scale to this lowest point or bottom of our tube, so that our old zero, or freezing-point of water, will be at 273° of the new scale, and the boiling-point of water at 373°. We shall then have what is probably very nearly an absolute scale of temperature, such a one that we can say, for example, that the temperature at 500° is twice as great as that at 250°. Moreover, this is a scale such that the volume of any gas, under the same pressure, is exactly proportional to the temperature : for example, the volume of a given mass of air at 600° is twice as great as the volume at 300°. That this must be the case for air is evident from the construc- tion of our theoretical thermometer ; and it is equally true of any other perfect gas, for there would be no dif- ference in effect whatever if the tube were filled with hydrogen, oxygen, or nitrogen, instead of air. It is very easy to refer degrees of our ordinary thermometer to degrees of this absolute scale. If the degrees are centigrade, we have merely to add 273 ; if they are Fahrenheit, we must add 459 (see Fig. 7, bis) ; and, for many purposes, it is exceedingly convenient to measure temperature in this way. Suppose, for example, we have 100 cubic inches of gas, at 4° centigrade, and we wish to know what would be its volume at 281°. Converting these values into absolute degrees by adding 273, we 48 THE THKEE STATES OP MATTER. obtain 277° and 554°. Then, since the volume of a gas is exactly proportional to the absolute temperature, we have 277 : 554 = 100 : answer, 200 cubic inches. But the chief value of this method of measuring temperature is to be found in the simplicity with which it presents to us the property of gases we have been studying. The volume of a gas depends solely on two conditions : its pressure and its absolute temperature. As I before showed, it is inversely proportional to the pressure, and it now appears that it is directly proportional to the absolute temperature. "We must then qualify the law of Mariotte by a second principle, equally funda- mental and important : The volume of a given mass of gas, under a constant pressure, varies directly as the absolute temperature. This we call the law of Charles. The molecular theory of gases explains the law of Charles very much in the same way as it explained the law of Mariotte. The pressure of a gas, as we have seen, is due to its molecular energy. If, by any means, we increase that energy, we must also increase the pressure in the same proportion ;■ or, if the gas is free to expand under a constant pressure, we must increase the volume. In other words, the effect of increased energy must be the same as the effect which we know follows increased temperature. What more natural than to infer that the unknown condition, to which we have given the name of temperature, is simply molecular energy ? Here, then, is our theoretical ex- planation of the law of Charles. The temperature of a body is the moving power of its molecules. At the 0° of our absolute scale the molecules would be re- duced to a state of rest, and, at other temperatures, the molecular energy is directly proportional to the de- THE LAW OF CHARLES. 49 grees of this scale ; so that, for example, the molecules of air, at 273° (the 0° of centigrade), have only one- half of the energy which the same molecules possess when the temperature is raised to 546°. As the press- ure exerted by the air must be proportional to the molecular energy, the increased temperature will, if the air is confined, double this pressure, or, if the air is free to expand under the constant pressure of the atmosphere, it will double the volume. It would lead me too far to attempt to develop here at any greater length the dynamical theory of heat, and I regret that I am not able to do more than to give this bare outline of the remarkable properties of gases, which it so beautifully explains ; but I take great pleas- ure in referring all who are interested in the subject to the very excellent work of Prof. Clerk Maxwell on the theory of heat. It is not a popular work, or one which is easy reading, but it contains a most ele- gant exposition of the modern theory of heat, in as simple a form as is consistent with accuracy and con- ciseness. There is only one other point, in connection with the molecular theory of gases, to which it is important for me to refer in these lectures. We have seen that all gases have two essential characteristics : 1. Their volume is inversely proportional to the pressure to which they are exposed ; and, 2. Their volume is directly proportional to the absolute temperature. Now, if we assume the molecular theory of gases as true, it can be proved, mathematically, that all gases at the same temperature and pressure must have the same number of molecules in the same volume. I do not give the proof, because it would be out of place here, and be- cause all who are interested will find it in the work of 50 THE THREE STATES OF MATTER. Prof. Maxwell, to which I have referred. It would be more satisfactory to enter into details, but I shall have accomplished the first object of this lecture if I have been able to leave with you a clear idea of the three laws which may be said to define the aeriform condition of matter, and which all true gases obey — The Law of Mariotte, The Law of Charles, The Law of Avogadko. The first two are independent of any theory, and simply declare that the volume of every gas varies in- versely as the pressure, and directly as the absolute temperature. The third is based on the molecular theory. It is more general, and includes the other two. It declares that equal volumes of all gases, under the same conditions of temperature and pressure, contain the same number of molecules. -' Liquids are distinguished from gases chiefly in hav- ing a definite surface. Their particles have the same freedom of motion, but this motion is limited to the mass of the liquid. The particles of the air, if uncon- fined, would move off indefinitely into space ; but the particles of this water, although moving with equal freedom within the liquid mass, cannot, as a rule, rise above what we call the surface of the water. ; Again, if we introduce a quantity of air, however small, into a vacuous vessel, it will instantly expand until it com- pletely fills the vessel. A quantity of water, under the same conditions, will fall to the bottom of the vessel, and will be separated by a distinct surface from the vapor which forms above it. Lastly, if a gas is sub- jected to pressure, it is compressed in the exact pro- portion to the pressure, while with a liquid the com- MOLECULAR STRUCTURE OF LIQUIDS. 51 pression is barely perceptible, even when the press- ure is exceedingly great. Hence, gases are frequently called compressible and liquids incompressible fluids. The explanation which the molecular theory gives of this difference of relations is very simple. In the gas the molecules are separated beyond the sphere of each other's influence, and move through space wholly free from the effects of the mutual attraction. In a liquid, on the other hand, this attraction, which we call cohesion, is very sensible, and restrains the individual molecules within the mass, although they are free to move among themselves. Tou can easily understand, by referring again to the diagram (Fig. 2, on page 16), how this attractive force would act. A molecule, in the midst of the mass, moves freely, because the attractions are equal in all directions, but a molecule near the surface is in a very different con- dition. As it approaches the surface, the attraction toward the mass of the liquid becomes greater than the attraction toward the surface, and when it reaches the surface the whole force of the inward attraction is pulling it back, and, unless the moving power of the molecule is sufficiently great to overcome this force, its motion is arrested, and it turns back on its course. It may happen, however, especially when heat is entering the liquid, that some of the molecules, through the effects of their mutual collisions, acquire sufficient energy to fly off from the liquid mass, and hence result the well- known phenomena of evaporation. Thus our theory defines the liquid condition of matter, and explains how the liquid is converted by heat into the gas. In all theoretical discussions, it is always highly sat- isfactory when, in following out our theoretical concep- tions to their consequences, we find that these conse- 62 THE THREE STATES OF MATTER. quences are actually realized in natural phenomena, and such satisfaction we can have in the present case. Consider what must be the form which a mass of liquid molecules isolated in space would necessarily take. Re- member that these molecules are moving with perfect freedom within the body, but that the extent of the motion of each molecule is limited by the attraction of the mass of the liquid. Remember also that, accord- ing to the well-known principles of mechanics, this at- traction may be regarded as proceeding from a single point, called the centre of gravity. Remember, fur- ther, that the molecules have all the same moving power, and you will see that the extreme limits of their excursions to and fro through the liquid mass must be on all sides at the same distance from the central point. Hence the bounding surface will be that whose points are all equally distant from the centre. I need not tell you that such a surface is a sphere, nor that a mass of liquid in space always assumes a spherical form. The rain-drops have taught every one this truth. Still, a less familiar illustration may help to enforce it. I have therefore prepared a mixture of alcohol-and-water, of the same specific gravity as olive-oil, and in it I have suspended a few drops of the oil. By placing the liquid in a cell, between parallel plates of glass, I can readily project an image of the drops on the screen, and I wish you to notice how perfectly spherical they are. And I would have you, moreover, by the aid of your imagina- tion, look within this external form, and picture to yourselves the molecules of oil moving to and fro through the drops, but always slackening their motion where they approach the surface, and on every side coming to rest and turning back at the same distance from the centre of motion. MOLECULAR STRUCTURE OF SOLIDS. 53 Neither liquids nor gases present the least trace of structure. They cannot even support their own weight, much less sustain any longitudinal or shearing stress. A solid, on the other hand, has both tenacity and struct- ure, and resists, with greater or less energy, any force tending to alter its form, as well as change its volume. The tenacity and peculiar forms of elasticity which solids exhibit are characteristics which are familiar to every one, but the evidences of structure are not so conspicuous. The structure of solids is most frequently manifested by their crystalline form, and this form is one of the most marked features of the solid state. But although, under definite conditions, most substances as- sume a fixed geometrical form, yet, to ordinary expe- rience, these forms are the exceptions, and not the rule. I will therefore make the crystallization of solid bodies the subject of a few experimental illustrations. For the first experiment, I have prepared a concen- trated solution of ammonic chloride (sal-ammoniac), and with this I will now smear the surface of a small glass plate. Placing this before our lantern, and using a lens of short" focus, so as to form a greatly-enlarged image on the screen, let us watch the separation of the solid salt as the solution evaporates. . . . Notice that, first, small particles appear, and then from these nuclei the crystals shoot out and ramify in all directions, soon covering the plate with a beautiful net-work of the fila- ments of the salt. We cannot here, it is true, distin- guish any definite geometrical form ; but it can be " shown that these very filaments are aggregates of such forms, and their structure is made evident by a fact, to which I would especially call your attention — that, as the crystalline shoots ramify over the plate, the sprays keep always at right angles to the stem, or else branch 54 THE THREE STATES OF MATTER. at an angle of 45°, which is the half of a right angle (Fig. 8). For a further illustration of the process of crystal- lization I have prepared a solution in alcohol of a solid Fig. 8. — Crystallization of Sal- Ammoniac. Fig. 9. — Crystallization of Urea. substance called urea, with which we will experiment in precisely the same way as before. . . . The process of crystallization, which is here so beautifully exhibited, is one of the most striking phenomena in the whole range of experimental science. It is, of course, not so wonderful as the development of a plant or an animal from its germ, but then organic growth is slow and gradual, while here beautiful, symmetrical forms shape themselves in an instant out of this liquid mass, reveal- ing to us an architectural power in what we call lifeless matter, whose existence and controlling influence but few of us have probably realized. The general order of the phenomena in this experiment is the same as in the last ; but notice how different the details. We do not see here that tendency to ramify at a definite angle, but the crystals shoot out in straight lines, and cover the plate with bundles of crystalline fibres, which meet or in- CRYSTALLINE STRUCTURE OF ICE. 55 tersect each other irregularly as the accidental directions of the several shoots may determine (Fig. 9). As before, we cannot recognize the separate crystals ; indeed, large isolated crystals, such as you may see in collections of minerals, cannot be formed thus rapidly. They are of slow growth, and only found where the conditions have fa- vored their development. But all the mineral substances, of which the rocks of our globe consist, have a crystal- line structure, and are aggregates of minute crystals like the arborescent forms whose growth you have witnessed. The external form is but one of the indications of crystalline structure, and by various means this structure may frequently be made manifest when the body appears wholly amorphous. Nothing could appear externally more devoid of structure than a block of transparent ice. Yet it has a most beautiful symmetrical structure, which can easily be made evident by a very simple ex- periment, originally devised, I believe, by Prof. Tyn- dall. For this purpose I have prepared a plate of ice about an inch in thickness, whose polished surfaces are parallel to the original plane of freezing. I will now place this plate in front of the condenser of my lantern, and, placing before it a lens, we will form on the curtain an image of the ice-plate, some twenty times as large as the plate itself. The rays of heat which accompany the light-rays of our lantern soon begin to melt the ice ; but, in melting it, they also dissect it, and reveal its structure. . . . Notice those symmetrical six-pointed stars which are appearing on the wall (Fig. 10). Prof. Tyndall calls them, very appropriately, ice-flowers, for, as the flower shows forth the structure of the plant, so these hexagonal forms disclose the six-sided structure of ice. You can hardly fail to notice the similarity of these forms to those of the snow-flake. The six petals 56 THE THREE STATES OF MATTER. of the ice-flowers on our screen make with each other an angle of 60°, and, if you examine, with a magnifier, flakes of fresh-fallen snow (Fig. 11), or the arborescent Fig. 10. — Ice-Flowers. forms which crystallize on the window-panes in frosty weather, you will find that, in all cases, the crystalline shoots ramify at this angle, which is as constant a char- acter of the solid condition of water as is the right an- gle of sal-ammoniac. There are other solids whose crystalline structure, like that of ice, becomes evident during melting ; hut a far more efficient means of discovering the structure of solids, when transparent, is furnished by polarized light. It would be impossible for me, without devoting a great deal of time to the subject, either to explain the nature of what the physicists call polarized light, or to give any clear idea of the manner in which it brings out the structure of the solid. I can only show you a few experiments, which will make evident that such is the fact. "We have now thrown on the screen a lumi- nous disk, which is illuminated by polarized light. To the unaided eye it does not appear differently from INDICATIONS BY POLAKIZED LIGHT. 57 ordinary light; but there is this peculiarity in the beam. I have here a prism of well-known construc- tion, made of Iceland-spar, and called a Nicol prism. Fig. 11.— Snow-Crystals. The spar is as translucid as glass, and, with ordinary light, it transmits, as you see, the beam equally well, whether it is placed in one position or another. But, with the polarized beam, we shall have a very different result. In one position, as you notice, it allows the light to pass freely ; but, on turning it round through an angle of 90°, almost all the light is intercepted : the beam of light seems to have sides, which stand in a different relation to the prism in one position from that which they bear to it in the other. To describe this condition of the beam, the early experimenters adopted the wokI polarized, which was not, however, a happy designation ; for the term now implies an opposition of relations very unlike the difference which we recognize between the sides of such a beam of light. Placing now the Nicol prism in the posi- tion in which it intercepts the polarized beam, I will first place between it and the source of light a plate 58 THE THREE STATES OF MATTER. of glass. Yon notice that there is no difference of effect. Besides the arrangement for polarizing the light and the Nicol prism there is no other apparatus here except a lens, which would form on the screen an image of the glass plate or of any thing depicted upon it, were it not for the circumstance that the Nicol prism cuts off the light. By turning the Eicol so that the polarized light can pass, and putting a glass photograph in the place of the glass plate, you see at once the photograph projected on the screen. Having turned back the Eicol until the light is again intercepted, I will remove the photograph, and put in its place a thin sheet of gypsum. . . . See this brilliant display of colors. The plate of gypsum is as colorless and transparent as the glass, and the gorgeous hues result from the decomposition of the polarized light produced by the crystalline structure of the gypsum. I will next turn round the film of gypsum, and you notice that the colors gradually fade out and finally disappear. As we turn farther they reappear, and so on. Evidently, the colors are only produced in a definite position of the gypsum plate with reference to our polarizing apparatus. Moreover, as I can readily show you, the tint of color depends on the thickness of the film. I have here a simple geometrical design formed of plates of gypsum of different thicknesses, and you notice that each plate assumes a different hue. On turning, however, our Nicol prism 90% these colors are suddenly exchanged for their complementary tints. It is obvious that any colored designs might be re- produced in this way by combining gypsum plates cut to the required thickness and form, as in mosaic work ; and I will now show you a number of beautiful illus- trations of this peculiar form of art. ... But you can- EFFECTS OF GYPSUM PLATES. 59 not appreciate the wonder of these experiments without bearing in mind that these gypsum mosaics show no color whatever in ordinary light, consisting, as they do, of plates which appear like colorless glass. Let me now substitute for the gypsum designs the glass plate on which we recently crystallized urea, and notice that the crystals of this substance, which we saw form on the glass, yield similar brilliant hues. The experiment becomes still more striking, if we crys- tallize the salt under these conditions. I will, there- fore, take another glass plate, and, having smeared it as before with the solution of urea, I will place it in the focus of my lens before the polarizer. The field is now perfectly dark, but, as soon as the crystals begin to form, you see these colored needles shoot out on the dark ground, presenting a phenomenon of wonderful beauty. Now, all this indicates a definite structure, and, to those familiar with these phenomena, they point to a definite conclusion in regard to this structure. I wish I could fully develop the argument before yon, but this would require more time than the plan of my lectures allows, and I must be content if I have been able to impress upon your minds the single general truth which these experiments suggest. You saw the urea crystallize, that is, assume a definite structure, and you now see that this structure so modifies the polarized light as to produce these gorgeous hues. You have seen similar hues, but still more brilliant, produced by -a plate of gypsum, and I can only add that the conclu- sion which the analogy suggests is legitimate, and sus- tained by the most conclusive evidence. The trans- parent plates of gypsum have as definite a structure as the crystals of urea, and to the student of optics these colors reveal that structure just as clearly as it is mani- 60 THE THREE STATES OF MATTER. fested, even to the uninstructed eye, by the processes of crystallization, which we have witnessed this evening. "Would, however, that I could convey to you a more ' ■:■*'■■<«''*-- 1 '. -r.' : ' w KSBtlH^ ^^P^ slS^lllfa'J; ..■"■;.";.: - - . ■ J: 1/J.'/;' §§*/*% mm m Fig. 12.— Magnetic Curves, one pole. Fig. IS.— Magnetic Curves, two poles. definite idea of the nature of that structure, for our theory gives us a very clear conception of what we suppose to be the relations of the molecules in these solid bodies ! But the subject is a difficult one, and it would require a long time to make the matter intelli- gible. Still, by the aid of a few parallel experiments, I may be able to give you, at least, a glimpse of the manner in which, as we suppose, the structure of solid bodies is produced. Everybody knows that a magnetic needle, when free to move, assumes a definite position, pointing, in general, north and south. Now, a magnetic needle is a needle of steel (hardened iron) in a condition which we call polarized ; and, what is true of it, is true of every polarized body, to a greater or less extent. So, also, if we have a collection of such polarized bodies, they will always arrange themselves in some definite position with reference to each other — will form, in a word, a definite structure. Further, it is well known that a magnet polarizes all masses of iron in its neighborhood, and this circum- STRUCTURE PRODUCED BY MAGNETISM. 61 stance enables us to illustrate the truth of the principle just stated, in a most striking manner : If we bring a bar -magnet near some iron filiDgs sprinkled over a Tib. 14.— Kings, Uniaxial Crystals. Fig. 15.— Rings, Biaxial Crystals. plate of glass, these little bits of iron become at once polarized by induction ; and, if then we gently tap the glass, the iron particles will swing round on its smooth surface, and arrange themselves in the most wonderful way. By means of my vertical lantern I can show you this effect most beautifully. I first sprinkle the filings on the glass stage of our lantern, and then, having pro- tected them by a thin covering-glass, I bring near the glass one of the poles of a bar-magnet. . . . Notice how, on tapping the glass, the filings spring into posi- tion, arranging themselves on lines radiating from this pole (Fig. 12). Here, evidently, we have a definite structure produced. Let us now clear our stage, and ar- range for a second experiment. This time, however, we will lay the bar-magnet on the covering-glass, so that the bits of iron shall be brought under the influence of both of its poles at the same time. - . . See what a beautiful set of curves results on tapping the glass (Fig. 13), and let me beg you to try to carry in your mind for a moment the general aspect of this structure, as well as of the first. 62 THE THREE STATES OF MATTER. Now, we suppose that, in solid bodies, the structure depends on the polarity of the molecules, and that the molecules, like the bits of iron in our experiment, take up the relative position which the polar forces require. And, next, I will show you that a beam of polarized light develops in some solids an evidence of structure not very unlike that you have just seen. Returning, then, to our polariscope, I place in the beam of light a plate of Iceland-spar cut in a definite manner. . . . See those radiating lines, and those iris- colored circles (Fig. 14). Does not that remind you of the structure we developed around a single magnetic pole ? Next, I will use a similar plate cut from a crys- tal of nitre ; . . . . and, see, we have almost the repro- duction of the curves about the double pole (Fig. 15). It is the form of the curves as indicating a certain struct- ure, not the brilliant colors, to which I would direct your attention. The iris hues are caused simply by the breaking up of the white light we are using ; for the crystal decomposes it to a greater or less extent, like a prism. If, by interposing a plate of red glass, we cut off all the rays except those of this one color, the varied tints disappear, but, in the black curves which now take' their place, the analogy I am endeavoring to present becomes still more marked. Certainly, you could have no more striking analogy than this. I can add nothing by way of commentary to the experiments without entering into unsuitable details, and I will only, say, further, that I am persuaded that the resemblances we have seen have a profound significance, and that the structure, which the polarized beam reveals in these solid bodies, is really analogous to that which the mag- net produces from the iron filings. LECTTJEE III. HOW MOLECULES AJKE WEIGHED. In order that we may make sure Cf the ground we have thus far explored, let me recapitulate the charac- teristic qualities of the three conditions of matter which I sought to illustrate in the last lecture. A gas always completely fills the vessel by which it is inclosed. It is in a state of permanent tension, and con- forms to the three laws of Mariotte, of Charles, and of Avogadro. A liquid has a definite surface. It can be only very slightly compressed, and obeys neither of these three laws. A solid has a definite structure, and resists both longitudinal and shearing stresses to a lim- ited extent. Having now presented to you "the molecular theory as fully as I can without entering into mathematical details, I come back again to the great law of Avoga- dro, which is at the foundation of our modern chem- istry : When in the condition of a perfect gas, all sub- stances, under like conditions of temperature and press- ure, contain in equal volumes the same number of mole- cules. I have already shown you that, if we assume the general truth of the molecular theory (in other words, 64 HOW MOLECULES ARE WEIGHED. if we assume that a mass of gas is an aggregate of iso- lated moving molecules), then the law of Avogadro follows as a necessary consequence from the known properties of aeriform matter, and may, therefore, in a certain limited sense, be said to be capable of proof. As yet, however, we have only considered the purely physical evidence in favor of the law. "We come next to the chemical evidence which may be adduced in sup- port of its validity, and this is equally strong. It would be impossible at the present stage of our study to make the force of this evidence apparent, be- cause, so far as chemistry is concerned, the law of Avo- gadro is a generalization from a large mass of facts, and the proof of its validity is to be found solely in the cir- cumstance that it not only explains the known facts of chemistry, but that it is constantly leading to new dis- coveries. This law, as I have intimated, bears about the same relation to modern chemistry that the law of gravitation does to modern astronomy. Modern astron- omy itself is the proof of the law of gravitation ; mod- ern optics the proof of the undulatory theory of light ; and so the whole of modern chemistry, and nothing less, is the proof of the law of Avogadro. I do not say that this great law of chemistry stands as yet on as firm a basis as the law of gravitation ; but I do say that it is based on as strong foundations as the undulatory theory of light, and is more fully established to-day than was the law of gravitation more than a century after it was announced by Newton. I have already briefly referred to the history of the law. The original memoir was published by Amedeo Avogadro in the Journal de Physique, July, 1811. In this paper the Italian physicist " enunciated the opinion that gases are formed of material particles, sufficiently PROGRESS OF THE INQUIRY. 65 removed from one another to be free from all recipro- cal attraction, and subject only to the repulsive action of heat ; " and, from the facts, then already well estab- lished, that the same variations of temperature and press- ure produce in all gases nearly the same changes of vol- ume, he deduced the conclusion that equal volumes of all gases, compound as well as simple, contain, under like conditions, the same number of these molecules. This conception, simple and exact as it now appears, was at the time a mere hypothesis, and was not ad- vanced even with the semblance of proof. The discov- ery of Gay-Lussac, that gases combine in very simple proportions by volume, was made shortly after, and, had its important bearings been recognized at once, it would have been seen to be a most remarkable confir- mation of Avogadro's doctrine. But the new ideas passed almost unnoticed, and were reproduced by Am- pere in 1814, who based his theory on the experiments of Gay-Lussac, and defended it with far weightier evi- dence than his predecessor. Still, even after it was thus reaffirmed, the theory seems to have received but little attention either from the physicists or the chem- ists of the period. The reason appears to have been that the integrant molecules of Avogadro and the par- ticles of Ampere were confused with the atoms of Dal- ton, and, in the sense which the chemists of the old school attached to the word atom, the proposition ap- peared to be true for only a very limited number even of the comparatively few aeriform substances which were then known. Moreover, the atomic theory itself was rejected by almost all the German chemists ; and, in physics, the theory of a material caloric then pre- vailing was not enforced by the new doctrine. In a word, this beautiful conception of Avogadro and Am- 66 HOW MOLECULES AEE WEIGHED. pere came before science was ripe enough to benefit- by it. A half-century, however, has produced an im- mense change. The development of the modern the- ory of chemistry has made clear the distinction between molecules and atoms, while the number of substances known in their aeriform condition has been vastly in- creased. It now appears that, with a few exceptions, all these substances conform to the law, and these ex- ceptions can, for the most part at least, be satisfactorily explained. On the other side, in the science of physics, more exact notions of the principles of dynamics have become general, and the dynamical theory of heat necessarily involves the law of equal molecular vol- umes. Thus, this theory of Avogadro and Ampere, which remained for half a century almost barren, has come to stand at the diverging-point of two great sci- ences, and is sustained by the concurrent testimony of both. It is not, then, without reason that we take this law as the basis of the modern system of chemistry ; and, starting from it, let us see to what it leads : In the first place, then, it gives us the means of de- termining directly the relative weight of the molecules of all such substances as are capable of existing in the aeriform condition. For, it is obvious, if equal volumes of two gases contain the same number of molecules, the rel- ative weights of these molecules must be the same as the relative weights of the equal gas-volumes. Thus, a cubic foot of oxygen weighs sixteen times as much as a cubic foot of hydrogen under the same conditions. If, then, there are in the cubic foot of each gas the same number of molecules, each molecule of oxygen must weigh six- teen times as much as each molecule of hydrogen. It is much more convenient in all chemical calcula- tions to use the French system of weights and meas- FRENCH SYSTEM OF WEIGHTS AND MEASURES. 67 ures; and since, through modern school-books, the names of these measures have become quite familiar to almost every one, I think I can refer to them with out confusion. The accompanying table will serve to refresh your memory, and may be useful for reference : The metre is approximately the TT>, 6 6 6 , 6<><> P ar t °f a quadrant of a meridian of the earth measured from the pole to the equator. The metre equals 10 decimetres or 100 centimetres. The cubic metre, or stere, equals 1,000 cubic decime- tres or litres. The cubic decimetre, or litre, equals 1 ,000 cubic cen- timetres. The gramme is the weight, in vacuo, of one cubic centimetre of water at i° centigrade (the point of maxi- mum density). The Mlogramme equals 1,000 grammes, and is, there- fore, the weight of one cubic decimetre or litre of water under the same conditions. The crith is tlie weight, in vacuo, of one litre of hydrogen gas at 0° centigrade (tlie freezing-point of water), and at 76 centimetres (the normal height of the larometer). It equals 0.09 of a gramme very nearly. The metre is equal to Z\feet nearly. The litre is equal to l%pint nearly. The gramme is equal to 15£ grains nearly. The kilogramme is equal to 2\ pounds nearly. The convenience of the French system depends not at all on any peculiar virtue in the metre (the standard of length on which the system is based), but upon the two circumstances — 1. That all the standards are divided decimally so as to harmonize with our decimal arithme- tic ; and, 2. That the measures of length, volume, and weight, are connected by such simple relations that any 68 HOW MOLECULES AKE WEIGHED. one can be most readily reduced to either of the other two. In order to make clear these last relations, I must ask you to distinguish between two terms which are constantly confounded in the ordinary use of language, namely, density and specific gravity. The density of a substance is the amount of matter in a unit-volume of the substance. In the English sys- tem it is the weight in grains of a cubic inch, and in the French system the weight in grammes of a cubic centi- metre. Thus the density of wrought-iron is 1,966 grains English, or 7.788 grammes French. So also the density of water at 4° centigrade (the point of maxi- mum density) is 252.5 grains, or 1 gramme. The specific gravity of a substance is the ratio be- tween the weight of the substance and that of an equal volume of some other substance taken as a standard. For liquids and solids, water is always the standard selected, and the specific gravity, therefore, expresses how many times heavier the substance is than water. It can evidently be found by dividing the density of the substance by the density of water, because, as we have just seen, these densities are the weights of equal volumes. Hence the specific gravity of iron equals — 1966 grains 7.788 grammes H ,_ DO » g „ , -. — or — ; = 7.788 252.5 grams 1 gramme Of course, the specific gravity of a substance will be expressed by the same number in all systems ; and, fur- ther, in the French system, as the example just cited shows, this number expresses the density as well as the specific gravity. Density, however, is a weight, while specific gravity is a ratio, and the two sets of numbers are identical in the French system only because in that system the cubic centimetre of water has been selected as the unit of weight. SIMPLICITY OF THE FRENCH SYSTEM. 69 In the French system, then, the same numher ex- presses both the specific gravity and also the weight of one cubic centimetre of the substance in grammes ; and, since both 1,000 grammes = 1 kilogramme, and 1,000 cubic centimetres = 1 litre, it expresses also the weight of one litre in kilogrammes. These relations are shown in the following table : The specific gravity of a liquid or solid shows how many times heavier the tody is than an equal volume of water at 4° centigrade. The same number expresses also the weight of one cubic centimetre of the substance in grammes, or of one litre in kilogrammes. Gold. Alcohol. Water. Sulphur. Iron. | --= "= S|H ^^m - ^3 ■^^B = Sp. Gr., 0.8 1. 2.1 7.8 19.8 Density, 0.8 gram. 1. gram. 2.1 gram. 7.8 gram. 19.8 gram. The black squares are supposed to represent cubic centimetres. If assumed to represent cubic decimetres, then the weights which measure the densities would be in kilogrammes instead of grammes. It will now be seen how simple it is in the French system to calculate weight from volume. "When the specific gravity of a substance is given, we know the weight both of one cubic centimetre and of one litre of that substance, and we have only to multiply this weight by the number of cubic centimetres, or of litres, to find the weight of the given volume. Thus the weight of a wrought-iron boiler-plate <£ centimetre thick, and measuring 120 cen- timetres by 75, would be — 0.5 x 120 x 75 x 7.788 = 35,046 grammes. In general — W.=V.xSp. Gr. 70 HOW MOLECULES ARE WEIGHED. "When V. is given in cubic centimetres, the resulting weight will be in grammes ; when in litres, the weight will be kilogrammes. In estimating the specific gravity of gases, we avoid large and fractional numbers, by selecting, as our stand- ard, hydrogen gas, which is the lightest form of mat- ter known ; but we thus lose the advantage gained by having the unit-volume of our standard the unit of weight. It is no longer true that W.=V. xSp. Gr. In order to preserve this simple relationship, it has been found convenient to use in chemistry, for estimat- ing the weight of aeriform substances, another unit called the crith. The crith is the weight, in vacuo, of one litre of hydrogen gas at 0° centigrade, and with a tension of 76 centimetres. It is equal to 0.09 of a gramme nearly. "We may now define the density of a gas as the weight of one litre of the substance in criths, and its specific gravity as a number which shows how many times heavier the aeriform substance is than an equal volume of hydrogen under the same condi- tions of temperature and pressure. "We always esti- mate the absolute weight of a gas under what we call the standard condition, namely, when the centigrade thermometer marks 0°, and the barometer stands at 76 centimetres. But, in determining the specific gravity of a gas, the comparison with the standard gas may be made at any temperature or pressure, since, as all gases are affected alike by equal changes in these conditions, the relative weights of equal volumes will not be altered by such changes. The subject may be made more clear by the following table : The specific gravity of a gas shows how many times heavier the aeriform substance is than an equal volume of hydrogen gas under the same conditions of tempera- DENSITY AND SPECIFIC GRAVITY. ture and pressure. The same number also expresses tlie weight in criths of one litre of the gas under the stand- ard conditions. Hydrogen. Nitrogen. Oxygen. Chlorine. 8p. Gr., 1 Density, 1 crith. Now we have again W. =V. x Sp. Gr., only wo must remember that W. here stands for a certain num- ber of criths, V. for a certain number of litres, and Sp. Gr. for the specific gravity of the gas referred to hy- drogen, a number which also expresses the weight of one litre of the gas in criths. To return now to the subject of molecular weights. If one litre of hydrogen weighs one crith, and one litre of oxygen sixteen criths, and if both contain the same number of molecules, then each molecule of oxygen must weigh sixteen times as much as each molecule of hydrogen. Or, to put it in another way, represent by n the constant number of molecules, some billion bill- ion, which a litre of each and every gas contains, when under the standard conditions of temperature and pressure. Then the weight of each molecule of hydro- gen will be - of a crith, and that of each molecule of 1 fi oxygen — of a crith, and evidently 1.15 = 1:16; n n that is, again, the weights of the molecules have the same relation to each other as the weights of the equal 4 72 HOW MOLECULES ARE WEIGHED. gas-volumes. Excuse such an obvious demonstration, but it is so important that we should fully grasp this conception that I could not safely pass it by with a few words. It is so constantly the case that the simplest processes of arithmetical reasoning appear obscure when the objects with which they deal are not familiar. Since, then, a molecule of any gas weighs as much more than a molecule of hydrogen, as a litre of the same gas weighs more than a litre of hydrogen, it is obvious that, if we should select the hydrogen-molecule as the unit of molecular weights, then the number rep- resenting the specific gravity of a gas would also ex- press the weight of its molecules in these units. For example, the specific gravity of oxygen gas is 16, that is, a litre of oxygen is sixteen times as heavy as a litre of hydrogen. This being the case, the molecule of oxygen must weigh sixteen times as much as the mole- cule of hydrogen, and, were the last our unit of molec- ular weights, the molecule of oxygen gas would weigh 16. So for other aeriform substances. In every case the molecular weight would be represented by the same number as the specific gravity of the gas referred to hydrogen. Unfortunately, however, for the simplicity of our system, but for reasons which will soon appear, it has been decided to adopt as our unit of molecular weight not the whole hydrogen-molecule, but the half-mole- cule. Hence, in the system which has been adopted, the molecule of hydrogen weighs 2 ; the molecule of oxygen, which is sixteen times heavier, 16 times 2, or 32 ; the molecule of nitrogen, which is fourteen times heav- ier, 14 times 2, or 28 ; and, in general, the weight of the molecule of any gas is expressed by a number equal to twice its specific gravity referred to hydrogen. Noth- THE UNIT EMPLOYED. 73 ing, then, can be simpler than the finding of the mo- lecular weight of a gas or vapor on this system. "We have only to determine the specific gravity of the aeri- form substance with reference to hydrogen gas, and double the number thus obtained. The resulting prod- uct is the molecular weight required in terms of the unit adopted, namely, the half-molecule of hydrogen. Perhaps there may be some one who, having lost one or more of the steps in the reasoning, wishes to ask the question, Why do you double the specific gravity in this method ? Let me answer by recapitulating. It all depends on the unit of molecular weights we have adopt- ed. Had we selected the whole of a hydrogen-molecule as our unit, then the number expressing the specific grav- ity of a gas would also express its molecular weight ; but, on account of certain relations of our subject, not yet explained, which make the half- molecule a more convenient unit, we use for the molecular weights a set of numbers twice as large as they would be on what might seem, at first sight, the simpler assumption. In order to give a still greater definiteness to our conceptions, I propose to call the unit of molecular weight we have adopted a microcrith, even at the risk of coining a new word. We already have become familiar with the crith, the weight of one litre of hy- drogen, and I have now to ask you to accept another unit of weight, the half hydrogen-molecule, which we will call for the future a microcrith. Although a unit of a very different order of magnitude, as its name im- plies, the microcrith is just as real a weight as the crith or the gramme. We may say, then, that A molecule of hydrogen weighs 2 microeriths. " oxygen " 32 " nitrogen " 28 chlorine " 71 " 74 HOW MOLECULES ARE WEIGHED. Now, what I am most anxious to impress upon your minds is the truth that, if the molecules, as we believe, are actual pieces of matter, these weights are real magnitudes, and that we have the same knowl- edge in regard to them that we have, for example, in re- gard to the weights of the planets. The planets are visi- ble objects. We can examine them with the telescope ; and, when we are told Jupiter weighs 320 times as much as the earth, the knowledge seems more real to us than the inference that the oxygen-molecule weighs 32 microcriths. But you must remember that your knowledge of the weight of Jupiter depends as wholly on the law of gravitation as does your knowledge of the weight of the molecules of oxygen on the law of Avogadro. You cannot, directly, weigh either the large or the small mass. Your knowledge in regard to the weight is in both cases inferential, and the only question is as to the truth of the general principle on which your inference is based. This truth admitted, your knowledge in the one case is just as real as it is in the other. Indeed, there is a striking analogy between the two. The units to which the weights are respectively referred are equally beyond the range of our experience only on the opposite sides of the com- mon scale of magnitude ; for what more definite idea can we acquire of the weight of the earth than of the molecule of hydrogen, or its half, the microcrith ? It is perfectly true that, from the experiments of Maskelyne, Cavendish, and the present Astronomer-Koyal of Eng- land, we are able to estimate the approximate weight of the earth in pounds, our familiar standard of weight ; and so, from the experiments of Sir W. Thompson, we are able to estimate approximately the weight of the hydrogen -molecule, and hence find the value of the MOLECULAR WEIGHTS REAL MAGNITUDES. 75 microcrith in fractions of the crith or gramme. 1 It is true that the limit of error in the last case is very much larger than in the first, but this difference is one which future investigation will in all probability remove. I have dwelt thus at length on the definition of molecular weight, because, without a clear conception of this order of magnitudes, we cannot hope to study the philosophy of chemistry with success. Our the- ory, I grant, may all be wrong, and there may be no such things as molecules ; but, then, the philosophy of every science assumes similar fundamental principles, of which the only proof it can offer is a certain har- mony with observed facts. So it is with our science. The new chemistry assumes as its fundamental pos- tulate that the magnitudes we call molecules are reali- ties ; but this is the only postulate. Grant the postu- late, and you will find that all the rest follows as a necessary deduction. Deny it, and the " New Chemis- try " can have no meaning for you, and it is not worth your while to pursue the subject further. If, therefore, we would become imbued with the spirit of the new philosophy of chemistry, we must begin by believing in molecules ; and, if I have succeeded in setting forth in a clear light the fundamental truth that the mole- cules of chemistry are definite masses of matter, whose weight can be accurately determined, our time has been well spent. Before concluding this portion of my subject, it only remains for me to illustrate the two most important practical methods by which the molecular weights of substances are actually determined. It is evident from 1 According to Thompson, one cubic inch of any perfect gas contains, under standard conditions, 10 23 molecules. Hence, one litre contains 61 X 10 M molecules and 1 crith = 122 x 10 23 microcriths. 76 HOW MOLECULES ARE WEIGHED. what has been said that we can easily find the molecu- lar weight of any substance capable of existing in the state of gas or vapor, by simply determining experi- mentally the specific gravity of such gas or vapor with reference to hydrogen. Twice the number thus ob- tained is the molecular weight required in microcriths. Now, the specific gravity of an aeriform substance is found by dividing the weight of a measured volume of the substance by the weight of an equal volume of hydrogen gas under the same conditions. This simple calculation implies, of course, a knowledge of two quantities : first, the weight of a measured volume of the substance, and, secondly, the weight of an equal volume of hydrogen gas under the same conditions. Of these two weights, the last can always be calculated (by the laws of Mariotte and Charles) from the weight which a cubic decimetre of hydrogen, under the stand- ard conditions, is known to have, namely, 0.0896 gramme or 1 crith ; so that the method practically re- solves itself into weighing a measured volume of the gas or vapor and observing the temperature and press- ure of the substance at the time. There are always at least four quantities to be observed : first, the volume of the gas or vapor ; secondly, its weight ; thirdly, its tem- perature ; fourthly, its tension ; and, lastly, the weight of an equal volume of hydrogen, under the same condi- tions, is to be calculated from the known data of science. The most common case that presents itself is that of a substance which, though liquid or even solid at the ordinary temperature of the air, can be readily converted into vapor by a moderate elevation of tem- perature ; such a substance, for example, as alcohol. Now, we can find the weight of a measured volume of such a vapor at an observed temperature and tension DUMAS' METHOD. ) from the molecule of sodic carbonate (Na 2 C0 3 ) unite each with an atom of chlorine (CI) from the two molecules of hydrochloric acid (2HC1), and there are thus formed two molecules of common salt (2NaCl). Meanwhile, the original molecules hav- ing been broken up, the other atoms group themselves together to form a molecule of water (H 2 0) and a mole- cule of carbonic dioxide (C0 2 ). In a word, the chemi- cal change consists in the breaking up of the old mole- cules and the rearrangement of the atoms to form others, and you will notice how perfectly our system of symbols enables us to follow the steps of the process. In saying that this equation represents the pro- cess, we assume the truth of the principle, already so often reiterated, that what is true of the molecules is true of the substances. Our equation merely repre- sents the reaction between one molecule of sodic car- bonate and two of hydrochloric acid. Of course, there were billions on billions of molecules in our glass jar, but then the action here represented was simply so many billion of times repeated. There is only one other point in connection with this experiment to which I wish to call your special at- tention before closing the lecture. We used a great deal of water in the process, and the experiment would not have succeeded without it. Now, what part does the water play ? An essential part — and this point has a most important bearing on our theory of molecules. The reaction we have been studying takes place, as we have said, between molecules. But, in order that the molecules of the one body shoidd act on those of the other, it is obviously necessary that they should have a certain freedom of motion. If the molecules had been rigidly fixed in the material of the two substances, it 146 CHEMICAL SYMBOLS. would obviously have been impossible for them to mar- shal themselves in the manner we have described, two of one substance associating with one of the other in the resulting chemical process. Now, in a solid body, the molecules are to a great extent fixed, and hence no chemical action is possible between such substances, except to a limited extent. There "are, in general, two ways by which the required freedom of motion can be obtained : One is to convert the substance into vapor, when, as we have seen, the molecules become com- pletely isolated, and move with great velocity through space, their motion being only limited by the walls of the containing vessel ; but this method is only appli- cable to volatile bodies. The second method is to dis- solve the solid in some solvent, when the molecules, as before, become isolated, and move freely through the mass of the liquid. The last is the method generally used, and water, being such a universal solvent, is the common vehicle employed to bring substances together, and for that reason it enters into a very great number of chemical changes. Such was its office in the process we have been studying. We dissolved both the sodic carbonate and the hydrochloric acid in water, in order that their molecules might readily coalesce. An experi- ment will enforce the principle I have been enunciating:. There are a great many substances which will act on sodic carbonate like hydrochloric acid ; for example, almost all the so-called acids or acid salts, and, among others, that white solid with which you are familiar un- der the name of cream-of-tartar. Here we have cream- of-tartar and sodic carbonate, both in fine powder, and we have been carefully mixing them together in this mortar. Tou see, there is no action whatever ; and, in a dry place, we can keep the mixture indefinitely with- CONDITION OF SOLUTION REPRESENTED. 147 out change. If, however (placing the mixture in this glass vessel), we pour water over it, we have at once a brisk effervescence, and carbonic dioxide is evolved as before. It required the water to bring the molecules together. Since, then, the water plays such an important part in the reaction, I prefer to indicate its presence, and this may be done by using the symbol Aq. as previously described. (Na 2 C0 3 + 2H01 + Aq.) = (2NaCl + H 2 + Aq.) + COT. Solution of Sodic Carbonate Solution of Common Salt and Hydrochloric Acid. This indicates not only that both of the factors are in solution, but also that we have, as one of the prod- ucts, a solution of common salt. That the second prod- uct, carbonic dioxide, is a gas, I sometimes indicate by a line drawn over the symbol, as above. The second reaction is equally simple, but cream- of-tartar has a vastly more complex molecule than HC1. Its symbol is HKC 4 H 4 6 , that is, each molecule con- sists of four atoms of carbon, six atoms of oxygen, one atom of potassium, and five atoms of hydrogen. I write one of the atoms of hydrogen apart from the rest, because it has a very different relation to the molecule — a relation which I shall hereafter explain. The reaction would be written thus : (Na a C0 3 + 2HKO.H4O0 + Aq.) = (2NaKC4HiO, + H 2 + Aq.) + C0 2 . Solution of Rochelle Salts. With this reaction many of my audience must be familiar, as a mode of raising dough in the process of making bread. The first member of the equation in- dicates that the two substances are used in solution. There is formed, as the product of the reaction, be- sides the carbonic dioxide gas, which puffs up the 148 CHEMICAL SYMBOLS. dough, the solution of a salt, whose molecule has the complex constitution I have indicated, and which is a well-known medicine under the name of Rochelle-salts. When soda and cream-of-tartar are used in making bread, this salt remains in the loaf. The amount formed is too small to be injurious, but I cannot but think, although it may be a prejudice, that chemicals had better be kept out of the kitchen. LECTUEE VII. CHEMICAL REACTIONS. To master the symbolical language of chemistry, so as to understand fully what it expresses, is a great step toward mastering the science ; and so important is this part of my subject that I propose to occupy the hour this evening with a number of illustrations of the use of symbols for expressing chemical changes. First, I will recur to the experiment of the last lecture, for we have not yet learned all that it is cal- culated to teach. Let us again write on the black-board the symbols which represent the chemical process : (Na„00 8 + 2H01 + Aq.) = (2NaCl + H 2 + Aq.) + C07. Sodic Hydrochloric Common Water. Carbonic Carbonate. Acid. Salt. Dioxide Gas. "We bring together a solution of sodic carbonate and hydrochloric acid ; and there are formed as prod- ucts a solution of common salt, water, and carbonic dioxide gas. I need not refer again to the circum- stance that the state of solution is an essential condi- tion of the change, for this point was fully discussed at the time ; but, before we pass on to another experi- ment, I wish to call your attention to the fact that the several terms in this equation stand for absolutely defi- 150 CHEMICAL REACTIONS. nite weights of the quantities they represent. Each symbol stands for the known weights of the atoms which are tabulated in this diagram (table, page 112), and the weights of the molecules, which the several terms represent, are found by simply adding up the weights of the several atoms of which they consist. "When the substance is capable of existing in the aeri- form condition, its molecular weight can be found, as I have shown, from its specific gravity ; but these sym- bols assume that either by this or by some other method the constitution of the molecule has been determined; and, now that the result is expressed in symbols, noth- ing is easier than to interpret what they have to tell us. To begin with the sodic carbonate, Na 3 C0 3 . The •weight of this moleciue is 2x23 + 12 + 3x16=46 + 12 + 48 = 106 m.c. The weight of the molecule HC1 is 1 + 35.5 = 36.5, and two such molecules would weigh 73 m.c. Next, for the products, we have NaCl = 23 + 35.5 = 58.5, and 2NaCl = 117.0, also C0 2 = 12+32 = 44, and H 2 = 2 + 16 = 18. Hence the terms of our equation stand for the weights written over them below : 106 73 117 18 44 (Na 2 OO s + 2HC1 + Aq.) = (2NaCl + H 2 + Aq.) + CO,. We leave out of the account the water represented by Aq., for this, being merely the medium of the reac- tion, is not changed. Now we can prove our work ; because, if we have added correctly, the sum of the weights of the factors must exactly equal the sum of the weights of the products — and so it is 106 + 73 = 179, and 117 + 18+44 = 179. Besides the information which the equation gives us in regard to the manner in which the chemical change takes place, the symbols also inform us that 106 parts by weight of sodic car- bonate are acted upon by 73 parts by weight of hydro- CHEMICAL ARITHMETIC. 151 chloric acid, and that the yield is 117 parts of common salt, 18 parts of water, and 44 parts of carbonic-dioxide gas. "We learn from this, in the first place, the exact proportion in which the sodic carbonate and hydro- chloric acid can be most economically used ; for, if the least excess of one or the other substance over the pro- portions indicated is taken, that excess will be wasted. It will not enter into the chemical change, but will be left behind with the salt and water. Assume, then, that we have 500 grammes of sodic •carbonate, and we wish to know what amount of hy- drochloric acid to use, we simply make the proportion as 106 : 73 = 500 : a; = 344$) 6 ¥ . Again, suppose we wish to know how much common salt would be pro- duced from these amounts of sodic carbonate and acid, we write a similar proportion — 106 : 117 = 500 : x = 552, nearly. So, then, in any process, after we have written the reaction as above, if the weight of any factor or prod- uct is given, we can calculate the weight of any other factor or product by this simple rule : As the total molecular weight of the substance given is to the total molecular weight of the substance required, so is the given weight to the required weight. By total molecular weight we mean, evidently, not the weight of a single molecule, but the weight of the number of molecules which the equation indicates. This may be called the golden rule of chemistry. In the laboratory we never mix our materials at random, but always weigh out the exact proportions found by this rule. "When one of the products is a gas, as in the present case, a simple modification of the 152 CHEMICAL REACTIONS. rule enables us to calculate the volume of the resulting gas. Suppose, for example, we wished to calculate what volume of carbonic-dioxide gas could be obtained from 500 grammes of sodic carbonate. We should first find the weight by the above rule : 106 : 44 = 500 : x = 207i, nearly. The answer is 207£ grammes of carbonic dioxide. To find the corresponding volume in litres, we have merely to divide this value by the weight of one litre of the gas. Now, there are tables, in which the weight of one litre of each of the common gases is given ; but. such tables, although convenient, are not necessary, when, as in a written reaction, we know the molecular weights of the substances with which we are dealing. You remember that the molecular weight is always twice the specific gravity with reference to hydrogen. Half the molecular weight is, then, the specific gravity with reference to hydrogen. For example, the molecu- lar weight of carbonic dioxide (C0 2 ) is 44, and its spe- cific gravity with reference to hydrogen 22 — in other words, a litre of carbonic dioxide weighs 22 times as much as a litre of hydrogen. Now, a litre of hydro- gen, under the normal pressure of the atmosphere, and at the freezing-point of water, weighs one crith, or 0.0896 gramme, or, near enough for common purposes, 0.09 gramme. If, then, a litre of carbonic dioxide is 22 times as heavy, its weight is 22 criths, or 22 x 0.09 = 1.98 gramme. Our total product, above, be- ing 207£ grammes, the number of litres will be 207i -5- 1.98, or very nearly 104 litres. A litre, as I have said, is very nearly If pint, but we always use these French weights and measures in the laboratory, so that the values are as significant to the chemist as are pounds DECOMPOSITION OF CARBONIC DIOXIDE. 153 and pints to the trader. The general rule, then, is this : We first find the weight of one litre of the gas in grammes, by simply multiplying one-half of its molec- ular weight by jfo, and then we reduce the weight of the gas in grammes to litres by dividing the weight by this product. Let us pass, now, to another case of chemical change, and the example which I have selected is closely related to the last. One of the products of that reaction was carbonic-dioxide gas, and here we have a jar of that aeriform substance. On the other hand, I have in this bottle an elementary substance, called sodium. It belongs- to the class of metals, and is one of the constituents of sodic carbonate, which we used in the former experiment. I now propose to cause these two substances to act chemically upon each other ; but, as before, no chemical action will result unless the molecules have sufficient freedom of motion. Those of the carbonic-dioxide gas are already as free as the wind, moving with immense velocity through this jar. But not so with those of the sodium. In the usual solid, condition of this metal, the motion of its mol- ecules is restricted within very narrow limits. Before, we gave freedom to the molecules of sodic carbonate and hydrochloric acid by dissolving the substances in water. That method is not applicable here, for sodium acts chemically on water, and with great violence ; but we can reach a similar result by melting the sodium, and heating the molten metal until it begins to volatilize. Then, on introducing the crucible containing the seeth- ing metal into the gas, the molecules of the sodium, as they are forced up by the heat, will come into contact with those of the carbonic dioxide, and a violent chemi- cal action will be the result. 154 CHEMICAL REACTIONS. This action is made evident to you by the brilliant light evolved, and the sodium, as you would say, is burning in the carbonic-dioxide gas. Let us now rep- resent this chemical change by our symbols. Beginning with the factors, the molecule of carbonic dioxide, as already stated, is represented by the symbol C0 2 . The weight of the molecule of sodium has not yet been accurately determined ; and, in the absence of exact information, we will assume, as is most prob- able, that the molecular weight is twice the atomic weight, or, in other words, that the molecules consist of two atoms, Na-Na. Passing, next, to the products, we find only two, charcoal, and a substance called sodic oxide. As regards the last, we have every rea- son to believe that its molecules consist of two atoms of sodium united to a single atom of oxygen, Na^O. About the charcoal molecules, we have absolutely no knowledge whatever ; and we will, therefore, as is usual in such cases, represent them as consisting of sin- gle atoms. Hence, writing the products after the fac- tors, we have — CO, Na-Na ' Na 2 0. Carbonic Dioxide. Sodium. Carbon. Sodic Oxide. Eemembering, now, that the number of atoms on the two sides must be the same, it is evident that the amount of oxygen in a molecule of C0 2 will yield 2]STa 2 ; and, further, that, to form two molecules of Na 2 0, two molecules of N a-Na are necessary. Hence our reaction must be written : C0 2 + 2Na-]Sra = + 2Na 2 0. By this we learn that, from one molecule of carbonic dioxide (C0 2 ) and two molecules of sodium (2Na-Na), there are formed two molecules of sodic oxide (2Na 2 0) CHEMICAL RELATIONS OF CARBON. 155. and one atom of carbon (C). It is probable that the atoms of carbon group themselves into molecules ; but, as we know nothing about their constitution, we can- not express it by our symbols. Both of the products of this process are solids, and will be found, at the close of the experiment, in the small iron crucible in which the sodium was melted and introduced into the jar of carbonic-dioxide gas. The sodic oxide is a white solid, which is very soluble in water, or, rather, combines with water to form what is called caustic soda, which dissolves in the liquid ; and caustic soda, as you well know, is a very important chemical agent. But the chief interest in this experi- ment centres about the other product. Charcoal is one of the forms of carbon ; and the peculiar chemical re- lations of this element, which are illustrated by our ex- periment, are not only highly interesting in themselves, but have an important bearing on the subject of these lectures. I shall, therefore, digress for a moment from my immediate topic, in order to bring these facts to your notice. Carbon, as you probably know, is one of the most remarkable of the chemical elements. In the first place, it is most protean in the outward aspects which it assumes. These brilliant crystals of diamond, the hardest of all bodies ; this black graphite, as extreme in softness as is the diamond in hardness ; these still more familiar lumps of coal, are all formed of the same elementary substance. In the second place, the various forms of fuel used on the earth also consist chiefly of this element, which is, therefore, the great source of our artificial light and heat, and the reservoir of that en- ergy which, by the aid of the steam-engine, man uses with such effect. 156 CHEMICAL REACTIONS. All carbonaceous materials used as fuel, whether ■wood, coal, oil, or gas, if not themselves visibly organ- ized, were derived from organized structures, chiefly plants ; and all the light, all the heat, all the power, which they are capable of yielding, were stored away during the process of vegetable growth. The origin of all this energy is the sun, and it is brought to the earth by the sun's rays. Coal is the charred remains of a former vegetation, and the energy of our coal-beds was accumulated during long periods in the early ages of the geological history of the earth. Wonderful as the truth may appear, it is no less certain that the energy which drives our locomotives and forces our steamships through the waves came from the sun, than that the water, which turns the wheels of the Lowell factories, came from the springs of the New-Hampshire hills. How it comes, how there can be so much power in the gentle influences of the sunbeam, is one of the great mysteries of Nature. We believe that the effect is in some way connected with the molecular structure of matter ; but our theories are, as yet, unable to cope with the subject. That the power comes from the sun, we know ; and, moreover, we are able to put our finger on the exact spot where the mysterious action takes place, and where the energy is stored ; and that spot, singular as it may appear, is the delicate leaf of a plant. This same carbonic dioxide, on which we are here experimenting, is the foo'd of the plant, and, indeed, the chief article of its diet. The plant absorbs the gas from the air, into which it is constantly being poured from our chimneys and lungs, and the sun's rays, act- ing upon the green parts of the leaf, decompose it. The oxygen it contains is restored to the atmosphere, while the carbon remains in the leaf to form the struct- LATENT ENERGY IN COAL. 157 ure of the growing plant. This change may be repre- sented thus : CO, = + 0=0. Carbonic Dioxide. Carbon. Oxygen. Now, to tear apart the oxygen atoms from the carbon requires the expenditure of a great amount of energy, and that energy remains latent until the wood is burned ; and then, when the carbon atoms again unite with oxygen, the energy reappears undi- minished in the heat and light, which radiate from the glowing embers. Just as, when a clock is wound up, the energy which is expended in raising the weight re- appears when the weight falls ; so the energy, which is expended by the sun in pulling apart the oxygen and carbon atoms, reappears when those atoms again unite. This is one of the most wonderful and mysterious ef- fects of Nature ; for, although the process goes on so silently and unobtrusively as to escape notice, it accom- plishes an amount of work compared with which most of the noisy and familiar demonstrations of power are mere child's-play. It is one of the greatest achieve- ments of modern science, that it has been able to meas- ure this energy in the terms of our common mechanical unit, the foot-pound ; and we know that the energy exerted by the sun and rendered latent in each pound of carbon, which is laid away in the growing wood, would be adequate to raise a weight of five thousand tons one foot. The chief interest connected with the experiment before us is to be found in the fact that it is almost the parallel to the process which is going on in the leaf of every plant that waves in the sunshine. Compare the two > reactions as they are here written, the one over the other : 158 CHEMICAL EE ACTIONS. 00 2 + 2Na-Na = C + 2NaO. OO s = + 0=0. In the first, the cause of the breaking up of the C0 2 molecule is evident. The molecules of the sodium have what is called an intense affinity for the atoms of oxygen, and attract them with such power as to tear them away from the atom of carbon. Now, when you remember that the atoms of carbon and oxygen are united by such a force that it requires the great energy I have described to tear them apart, and in the light of this knowledge study the second reaction, you will fail to find in the symbols any adequate explanation of the effect. And they cannot explain it ; for the sun's energy cannot be expressed by a chemical formula. But, yet, this energy does here precisely the same work which the sodium accomplishes in our crucible. More- over, there is another striking analogy between the two processes, which must not be overlooked. The carbonic dioxide is decomposed in a vegetable leaf; and, of the two products of the reaction, the oxy- gen gas escapes into the air, while the carbon is depos- ited in the vegetable tissue. This relation between .the two products depends on the aeriform condition of oxygen on the one hand, and the great fixity of carbon on the other. Carbon is peculiar in this respect : In all its conditions, whether of diamond, graphite, or coal, it is one of the most fixed solids known. Even when ex- posed to the highest artificial heat, it never loses its solid condition, and so the molecules of carbon, as they form in the leaf, assume their native immobility, and become a part of the skeleton of the growing plant. To fully appreciate this remarkable relation of carbon to organic structures, you must recall the fact that the only other three elementary substances, of which ani- INFUSIBILITY OF CARBON. 159 mals and plants chiefly consist — oxygen, hydrogen, and nitrogen — are not only aeriform, but they are gases, which no amount of pressure or cold is able to reduce to the liquid or solid condition. All organized beings may be said to be skeletons of carbon, which have con- densed around the carbon atones the elements of water and of air. This point is one of such interest that a familiar illustration of it may be acceptable. "When a piece of wood is heated out of contact with the air, the volatile elements, hydrogen, oxygen, and nitrogen, are driven off in various combinations, while the carbon molecules are left behind, retaining the same relative position they had in the tree ; and, if we examine the charcoal with a microscope, we shall find that it has preserved the forms and markings of the cells, and the rings of an- nual growth ; and, in fact, all those details of structure which marked the kind of wood from which it was made. My assistant has projected on the screen a magni- fied image of a thin section of wood, which has been thoroughly carbonized, and you see how strikingly the facts I have stated appear. Now, just as the non-volatile carbon is deposited from the carbonic dioxide in the cell of the plant, so in our experiment is it deposited in the crucible. Both of the products of the reaction are to a great extent fixed, but the carbon by far the most so ; and, in this experiment, all, or, at least, a great part, of the carbonic dioxide, which previously filled the jar, has deposited the carbon it contained in the iron crucible. In the plant the carbonic dioxide, which passes through the structure in the process of plant-life, leaves its carbon in the leaf or stalk ; and so here, the carbonic dioxide, which is brought by the currents in the jar in contact 160 CHEMICAL REACTIONS. with the heated sodium, leaves its carbon in the cruci- ble. In order to show you that carbon has been thus formed, I will now remove the crucible, and quench it with water. The sodic oxide (Na 2 0) dissolves, and the charcoal is set free, and you see that the water in this jar is black with the particles of floating charcoal. Let us now pass on to study a remarkable series of chemical changes, in which carbonic dioxide also plays an important part. The first of the series is one with which you are all so familiar, that it is perhaps not im- portant to repeat it in this place ; but, as I am anxious that yon should have the processes we are studying presented to you in visible form, I will make the trivial experiment of slaking some common lime. The action is very violent, and great heat is devel- oped. As we shall hereafter see, the evolution of heat is an indication of chemical combination, and, in the case before us, the lime unites with the water. Let us try to represent this change by our symbols. Lime is a compound of a metal we call calcium and oxygen. It is, in a word, a metallic ore ; and I have a small bit of the metal which it contains in this tube. By projecting an image of the tube on the screen, you can see almost all that I can, save only that the metal has a brilliant lustre and ruddy tint, like bismuth. A molecule of lime is formed of two atoms, one of this metal and the other of oxygen. Hence the symbol CaO. A molecule of water, as we know, is represented by H 3 0. The product of the reaction is a light, white powder we familiarly call slaked lime, and its analysis, interpreted by its chemical relations, shows that it has the constitution Ca0 2 H 2 . The chemical name is calcic hydrate, and the change by which it was produced we can now express thus : PRODUCTION OF CHALK. 161 CaO + H 2 = CaO a H a . Lime. Water. Calcic Hydrate. In this reaction, as you see, two molecules unite to form a third, which consists of the atoms of the other two. If, now, we mix this slaked lime with a larger body of water, the result is an emulsion called milk-of- lime, and consisting merely of particles of the hydrate suspended in water. A part of the hydrate actually dissolves ; and, if we employ as much as 700 times its volume of water, the whole dissolves, forming a trans- parent solution. This milk-of-lime, then, is a solu- tion of calcic hydrate, containing a large excess of the solid hydrate in suspension. But there is a very sim- ple means of separating the solid from the solution. We use for the purpose a circular disk of porous paper, called a filter, which we fold in the shape of a cone, and place in a glass funnel. On pouring the tur- bid liquid into the paper cone, the clear solution will trickle through the pores of the paper, but the solid sediment will be retained on the upper surface. Having now obtained a clear solution of calcic hy- drate (CaO a II 3 + Aq), I propose to show you next the action of carbonic dioxide, upon it. For that purpose we will prepare some more of the gas, and, having poured our clear solution into this jar, we will pour in after it a quantity of carbonic dioxide, which, although a gas, is so heavy that we can handle it very much like a liquid. The gas is now resting on the solution, but the action is exceedingly slow ; for, although the particles of the calcic hydrate are free to move in the liquid, and those of the carbonic dioxide in the space above the liquid, yet each is restricted to those spaces, and the two sets of molecules cannot come in contact, except at the surface of separation. 162 CHEMICAL REACTION'S. But, let us shake up the liquid, so as to bring the mole- cules of both liquid and gas in contact, and you see that, at once, we have a very marked change. The liquid becomes turbid, and, after a while, a quantity of a wliite powder will fall to the bottom, which, if collected and examined, will be found to be identical with chalk. Now that you are acquainted with our method of no- tation, I can best explain to you this change by writing at once the reaction : (Ca0 2 H 2 + C0 3 + Aq.) = OaOQ 3 + (H 2 + Aq.). Calcic Hydrate. Calcic Carbonate. The symbols of the factors of the reaction you will at once recognize, and you will also interpret the meaning of Aq., used to indicate that the calcic hy- drate and carbonic dioxide come together in solution. Among the products of the reaction, the first symbol represents one molecule of calcic carbonate, the mate- rial of chalk. This body, being insoluble in water, drops out of the solution, and forms what is called a precipitate, a condition which we indicate arbitrarily by drawing a line under the symbol. The only other product of the reaction is water, which, of course, min- gles with the great mass of water present, and this we express by H 2 0+Aq. I need not tell you that this white powder is not only the material of chalk, but the material of the limestone-rocks, which form so great a part of the rocky crust of our globe. Not only the rough moun- tain limestones, but the fine marbles, and that beauti- ful, transparent, crystalline mineral we call Iceland- spar, are aggregates of molecules, having the same con- stitution as those which have formed in this experi- ment. The differences of texture may, doubtless, be referred to differences of molecular aggregation ; but CHALK DISSOLVES IN SODA-WATER. 163 we have not yet been able to discover, either what the difference is, or on what it depends. In order to produce the last reaction, we poured the gas upon the solution of calcic hydrate ; and the chalk was only produced as fast as the gas dissolved in the liquid. We shall obtain the reaction more promptly, if, instead of taking the gas itself, we employ a solution of the gas in water, previously prepared. Moreover, this form of the experiment will enable me to show you a phase of the process which might otherwise es- cape your notice. I need not tell you that we can easily obtain such a solution ready-made to our hands. That beverage, which we persist in miscalling soda- water, is simply an over-saturated solution of carbonic dioxide in water, made by forcing a large excess of the gas into a strong vessel filled with water. At the or- dinary pressure of the air, water will dissolve its own volume of this gas ; but, when forced in by pressure, the water dissolves an additional volume for every additional atmosphere of pressure. As soon, how- ever, as this solution is drawn out into the air, the ex- cess of gas above one volume escapes, causing the effer- vescence with which we are so familiar. Carbonic di- oxide is formed in the process of fermentation by which beer and wine are prepared ; and it is the es- cape of the excess of this gas, dissolved under pressure, which causes the effervescence of bottled beer and champagne. The solution in water (soda-water) is now supplied to the market in bottles called siphons, which are convenient for&ur purpose. Notice that, as I permit the solution to flow into the lime-water, the same white powder appears as be- fore ; but, now, notice further that, as I continue to add the solution of carbonic dioxide, this white solid 164 CHEMICAL REACTIONS. redissolves, and we have a beautifully clear solution. It is generally believed that, under these conditions, in presence of a great excess of carbonic dioxide, the molecule of calcic carbonate combines with additional atoms of carbon, oxygen, and hydrogen, to form the very complex molecule H 2 CaC 2 6 , which is assumed to be soluble in water ; but, as this point is one of doubt, I prefer to present the phenomenon to you as simply one of solution, and as illustrating a remarkable point in our chemical philosophy — the fact that the produc- tion of a given compound is frequently determined by the circumstance of its insolubility. The calcic carbon- ate forms, in the first instance, because this compound is insoluble ; but, when a proper solvent like the aerated water is present in sufficient excess, no such compound results, or, at least, we have no evidence of its forma- tion. Most of my audience will be more interested, how- ever, in this solution of chalk in soda-water (for such it is), from the fact that it plays a very important part in Nature, and is a common feature of domestic experi- ence. Such a solution as this is what we call hard water, and spring-water is frequently in this condition. Such water is said to kill soap, and is disagreeable when used in washing, because the lime in solution forms with the fatty constituent of the soap an insoluble, sticky mass, which adheres to the hands or cloth. Moreover, when such water is boiled, the carbonic dioxide is driven off, and the water loses its power of holding the chalk in solution, which is deposited sometimes as a loose powder, but at other times as a hard crust on the sides of the boiler. I cannot readily show you the reprecipitation un- der these conditions ; but I have here a crust, which HOW LIMESTONES MAY BE FORMED. 165 was formed in a steam-boiler in the manner I have de- scribed. A precisely similar action gives rise to the formation of stalactites in lime-caverns, and of a form of lime-rock called travertine. Some of the finest mar- bles have been formed in this way. -Thus it is that we have been imitating here the production of chalk, limestone, and marble, at least so far as the chemical process is concerned. The mole- cule of all these substances has the same constitution, expressed by the symbol CaC0 3 . Now, it is evident that CaC0 3 = CaO + C0 2 . Calcic Carbonate. Lime. Carbonic Dioxide. I mean simply by this, that it is theoretically possi- ble to form, from one molecule of calcic carbonate, one molecule of lime and one molecule of carbonic dioxide ; but it does not follow from this that it is practically pos- sible to break up the molecule of calcic carbonate in this way ; and we must avoid the error, not unfrequently made by chemical students, of being led astray by our notation. These equations, which we call reactions, are not like the equations of algebra. Any thing that can be deduced from an algebraic equation, according to the rules of the science, must be true ; but it by no means follows that any combinations we may form with our symbols can be realized. "We cannot deduce facts from chemical symbols. They are merely the language by which we express the results of experiment; and for this reason I have been, and shall be, very careful to show you the facts before I attempt to express them in chemical language. But, in the case before us, our caution is needless, for we can break up the molecule in the precise way which our assumed reaction indi- cates; and I will show you, lastly, two additional 166 CHEMICAL REACTIONS. chemical processes, which will bring back our material to the condition of lime and carbonic dioxide, the sub- stances from which we started. The first is a reaction, identical with the one I have just written. Since the beginning of the lecture, I have been strongly heating some lumps of chalk in this platinum crucible. The process is a slow one ; and it was necessary to begin the experiment early, in order that I might show you the result. The chemical change is identical, however, with that which may be observed in any lime-kiln, where lime is made by burn- ing limestone. Each molecule of chalk, CaC0 3 , looses a molecule of carbonic dioxide, C0 2 , and we have left a molecule of lime, CaO. But the change in the ap- pearance of the white mass produced by burning is so slight that I must bring in the aid of experiment to prove that any change has taken place ; and, first of all, I must show you the test I am going to use. In the first of these two jars I have an emulsion of chalk, and in the second milk-of-lime. Notice that this piece of paper, colored by a vegetable dye called turmeric, remains unchanged when dipped in the emul- sion of chalk, but. turns red in the milk-of-lime. Let us test, now, the contents of our crucible. "We will first empty it into some water. The white lumps almost instantly become slaked, and render the water milky. We will now dip in a sheet of turmeric-paper, and you see that, although we began with inactive chalk, we have obtained a material which acts on the turmeric-paper like caustic lime. Thus, then, we have regenerated the lime. Let us next see if we can regenerate the carbonic dioxide : In the last experiment, carbonic dioxide was pro- DECOMPOSITION OF CHALK. 167 duced, but it escaped so slowly, and in such small quan- tities, as entirely to escape notice. Where, however, limestone is burned on a large scale, the current of gas from th£ kiln is frequently very perceptible ; and more than one poor vagrant, who has sought a night's lodg- ing under the shelter of the stack, has been suffocated by the stream. But we can make evident the produc- tion of carbonic dioxide from chalk without the aid of such a sad illustration. Pro. 22.— Pneumatic Trough, with Two-necked Gas-bottle. In this bottle we have some bits of chalk. One of the two necks of the bottle is closed by a cork, through which passes tightly an exit-tube, to conduct away any gas that may be formed. The other is also corked, and through the cork passes a funnel-tube, by which I can introduce any liquid reagent into the bottle (Fig. 22). On pouring in some muriatic acid, a violent efferves^ cence ensues, and a gas is formed which, flowing from the exit-tube, displaces the water in this glass bell. The bell stands in what we call a pneumatic trough, and this simple apparatus for collecting gases must, I think, be familiar to all of my audience. The open 168 CHEMICAL REACTIONS. mouth of the bell rests on the shelf of the trough un- der water, and the liquid is sustained in it by the press- ure of the air. Let me, while the experiment is going on, write out the reaction : OaCO.3 + (2HC1 + Aq.) = (CaCl* + Aq.) + 00^ Chalk. Hydrochloric Acid. Calcic Chloride. We already know the symbols of all the factors, and we may, therefore, confine our attention to the products. The products are, first, carbonic-dioxide gas ; and, secondly, a solution in water of a compound whose molecule consists of calcium and chlorine, and which we call calcic chloride. And, now that the jar is filled, I can easily show that we have regenerated car- bonic dioxide. Eemoving the jar from the trough, we will first lower into it this lighted candle, and then pour into it some lime-water. The candle is instantly extinguished, and the lime-water rendered turbid. Thus we end the torture of these molecules. You have seen how easily we have formed them, and how readily we have broken them up. "We began with lime and carbonic dioxide, which we united to form chalk. We dissolved the chalk in a solution of C0 2 , and learned how, in Nature, various forms of limestone could be crystallized from this solution. Lastly, we have recovered from the chalk the lime and carbonic dioxide with which we begun. I hope you have been able to follow these changes, and to understand the language in which they are expressed. If so, we have taken another step in advance, and, at the next lecture, shall be able to go on and classify these reactions, and thus prepare the way by which we may reach still fur- ther truth in regard to this wonderful microcosm of molecules and atoms. NOMENCLATURE OP CHEMISTRY. 169 Before, however, closing my lecture, I will embrace the opportunity offered by this division of my subject to explain, as briefly as I can, the principles of our chemical nomenclature. This nomenclature originated in 1787 with a committee of the French Academy of Sciences, a committee of which the great chemist La- voisier was the ruling spirit. It was an attempt to in- dicate the composition of a substance by its name, and, for half a century after its adoption, it served most admirably the purpose for which it was devised, and exerted a marked influence on the development of chemistry. The nomenclature was based, however, on the dualistic theory, of which Lavoisier was the father, and, when at last our science outgrew this theory, the old names lost much of their significance and appropri- ateness. Within the last few years attempts have been made to modify the old nomenclature, so as to better adapt the names to our modern ideas. Unfortunately, the result, like most attempts to piece out an old gar- ment, is far from satisfactory, and reviewers revel in the absurdities to which the nomenclature leads when applied to many of the products of modern chemical investigation. Fortunately, however, chemical symbols now supply to a great extent the place of philosophical names, and hence the nomenclature is a far less im- portant feature in the new chemistry than it was in the old. I shall not, therefore, enter into much detail in regard to it, but limit myself to the statement of a few rules which will give you the key to the significance of the more common chemical terms. The names of elementary substances are necessarily arbitrary. Those which were known before 1787 retain their old names, such as sulphur, pkosphoms, iron, gold, and several others, including all the useful metals. Most 170 CHEMICAL REACTIONS. of the more recently-discovered elements have been named in allusion to some prominent property, or some circumstance connected with their history: as oxygen, from of-in ryevvda) (acid generator) ; hydrogen, from vhop yevvda (water generator) ; chlorine, from ^Twapos (green) ; iodine, from laStf*; (violet) ; bromine, from fipupos (fetid odor). The names of the newly-discovered metals have a common termination, um, as potassium, sodium, plati- num ; and, the names of several of the non-metallic ele- ments end in ine, as chlorine, bromine, iodine, fluorine. Passing next to binary compounds — that is, com- pounds of only two elements — we notice, first, that the simple compounds of the other elements with oxygen are all called oxides, and that, in order to distinguish the different oxides, we use adjectives formed from the name of the element with which the oxygen is com- bined, preferring however, in many cases, the Latin name to the English, both for the sake of euphony and in order to secure more general agreement in different languages. Thus we have — Argentic oxide AgjO Plumbic oxide PbO Stannic oxide Sn0 2 When the same element forms with oxygen two compounds the termination ic is retained for the higher oxide, while the termination ous is given to the lower. Thus— Ferrous oxide FeO Ferric oxide Fe a Oa Sulphurous oxide SOa Sulphuric oxide S0 3 If there are more than two oxides, or if, in any case, there are objections to the use of the termination ous, the necessary distinctions are made by means of Greek numeral prefixes : NOMENCLATURE OF CHEMISTRY. 171 Nitrous oxide N 2 Nitric oxide NO Dinitric trioxide N 2 3 Nitric dioxide N0 2 Dinitric pentoxide N 2 5 Carbonic oxide CO Carbonic dioxide ' COa The names of the binary compounds of the other elements are formed like those of the oxides. Compounds of Chlorine are called Chloride*. " Bromine " Bromides. " Iodine " Iodides. " Fluorine " Fluorides. " Sulphur " Sulphides. " Nitrogen " Nitrides. " Phosphorus " Phosphides. " Arsenic '' Arsenides. " Antimony " Antimonides. " Carbon " Carbonides. Moreover, the specific names in the several classes of compounds also follow the analogy of the oxides, thus : Stannous chloride SnCl s Stannic chloride SnCh Diferrous sulphide Fe 2 S Ferrous sulphide FeS Ferric sulphide Fe 2 S 3 Ferric disulphide FeSa And here, before we pass on to the names 'of compounds of a higher order, let me ask you to carefully fix in your memory the fact that the termination ide always indi- cates a compound containing only two elements. Of compounds of three or more elements the most prominent class is that of the acids, bodies originally so called on accoiint of their sharp or acrid taste. Now, the greater part of the inorganic or mineral acids are 172 CHEMICAL REACTIONS. composed of the two elements hydrogen and oxygen, united to some third element, which is the characteristic constituent in each case ; and, from this third element the acid takes its name, the terminations ic and ous being used as in the case of binaries to indicate a greater or less amount of oxygen in the compound. Thus we have — Nitrous acid HN0 2 Nitric acid HN0 3 Sulphurous acid H2SO3 Sulphuric acid H2SO4 Phosphorous acid H 3 P0 3 Phosphoric acid H a P0 4 In every acid we can by various chemical processes replace the hydrogen it contains with different metallic elements, and we thus obtain a very large class of com- pounds called salts. The generic name of the salts of each acid is formed by changing the termination ic, of the name of the acid, into ate, or the termination ous into ite, thus : Sulphurous acid forms Sulphites, Sulphuric acid " Sulphates, Phosphorous acid " Phosphites, ' Phosphoric acid " Phosphates, Carbonic acid '" , Carbonates, Silicic acid " Silicates, and the different salts of the same acid are distinguished by adjectives as before. For example : Nitrio acid HN0 3 Sodic nitrate NaNOs Potassic nitrate ENOa Argentic nitrate AgNOs So also : Sulphuric acid H2SO4 Potassic sulphate KaS04 NOMENCLATURE OF CHEMISTRY. 173 Calcic sulphate CaSO* Mercurows sulphate Hg 2 S0 4 Mercuric sulphate HgS0 4 Ferrous sulphate FeS0 4 Ferric sulphate Fe 3 (S0 4 )3 The terminations ous and ic, used in the names of these salts, indicate the same difference in the condition of the metallic element which determines the union of the metal with more or less oxygen. Ferrous and ferric sulphates, for example, correspond to ferrous and ferric oxides. The nature of this difference will be discussed in the chapter on quantivalence. There is an important class of compounds which bears to water a relation similar to that which salts sus- tain to their respective acids. This class of compounds is called the hydrates, and may be regarded as derived from water, by replacing one-half of its hydrogen. Thus we have — Potassic hydrate KOH from HOH Calcic hydrate CaOsH,, " 2HOH Bismuthic hydrate Bi0 3 H a " 3HOH Silicic hydrate Si0 4 H 4 " 4HOH So also : Ferrous hydrate FeOjHa Ferric hydrate FesOeHo The very interesting theoretical relations of the hydrates will hereafter be discussed. "When the hydrogen of an acid is only in part re- placed, or is replaced by more than one metallic ele- ment, the constitution of the resulting salt may still be indicated by the name, as in the following examples : Hydro-disodic phosphate H,Na 2 P04 Potassio-aluminic sulphate K 3 A1 2 (S0 4 ) 4 174 CHEMICAL REACTIONS. In like manner the relative proportions of the several ingredients of a salt may be indicated, as in — Tetrahydro-calcic diphosphate H 4 Ca(P04)2 Disodic tetraborate (borax) Na2B 4 07 But, as is evident, names like the last two are prac- tically useless, and, when we attempt to extend the nomenclature to organic compounds, we are led into still greater absurdities; so that, although by giving arbitrary names to various groups of atoms called com- pound radicals we have been able, to a limited extent, to adapt the nomenclature to this class of substances, yet we have been compelled in many cases to resort to trivial names like those used before the adoption of the nomenclature. The names oil of vitriol, corrosive sub- limate, calomel, saltpetre, borax, cream-of-tartar, etc., of the last century, have their counterparts in aldehyde, glycol, phenol, urea, morphine, naphthaline, and many other familiar names of our modern science. Of course, such names are subject to no rules, and, although they have been usually selected with care, and indicate by their etymology important relations or qualities, they must be associated separately with the substances they designate. LECTURE VIII. CHEMICAL CHANGES CLASSIFIED. Among chemical reactions we may distinguish three classes : 1. These in which the molecules are broken up into atoms ; 2. Those in which atoms are united to form molecules ; and, 3. Those in which the atoms of one molecule change places with those of another. Reactions of the first kind are called analysis, those of. the second synthesis, and those of the third metathesis —terms derived from the Greek, and signifying re- spectively to tear apart, to bind together, and to inter- change. This classification is one of great theoretical impor- tance. But it must be further stated that a simple ana- lytical or synthetical reaction, as here defined, is sel- dom if ever realized in Nature. Almost every chemi- cal process is attended both with the breaking up of molecules into atoms and the regrouping of these atoms to form new molecules, that is, it involves both analysis and synthesis; and this is true even in the many cases where the products or factors of the chemi- cal reaction are elementary substances ; for, when the molecules of the elementary substances consist of two or more atoms, the breaking apart or coalescing of these atoms, although they are atoms of the same ele- 176 CHEMICAL CHANGES CLASSIFIED. ment, constitutes analysis or synthesis, as here defined. Thus, when, in the burning of hydrogen gas, this ele- mentary substance unites with the oxygen of the air to form water, the molecules of oxygen must be divided into atoms before the synthesis of the water molecule is possible ; and so, on the other hand, when water is decomposed, the resulting atoms of oxygen unite by twos to form molecules of oxygen gas ; and this pair- ing is, according to our definition, a process of syn- thesis. The chemical reactions, which express these changes, illustrate very clearly the point here made : Burning of Hydrogen Gas. Decomposition of Water. 2H-H + 0=0 = 2H 2 0. 2EU0 = 2H-H + 0=0. Hydrogen Oxygen Water. Water. Oxygen Hydrogen Gas. Gas. Gas. Gas. The first states that from one molecule of oxygen are formed two molecules of water, and this, of course, ne- cessitates a division of the oxygen molecules; while the second states that from two molecules of water only one molecule of oxygen gas results, a process which involves the union of the two oxygen atoms, previously separated in the two molecules of water. Indeed, a purely analytical or a purely synthetical reaction would only be possible theoretically in those cases where ele- mentary substances were involved, whose molecules con- sist of a single atom, that is, where the molecule and the atom are identical, and we can recall no well-de- fined reactions of this kind. But, although we should be obliged to seek among the unfamiliar facts of chemistry for examples of pure analysis or pure synthesis, yet processes, in which one or the other is the predominant feature, and which il- lustrate the special characteristics of each, are close at hand. Some of these I now propose to bring before you, beginning with the analytical processes, and I PREPARATION OP OXYGEN GAS. 177 shall select such examples as incidentally illustrate im- portant principles, or interesting facts, of the science. Afterward, we will pass to the metathetical reac- tions, which are not only very common, but constantly occur undisturbed by other modes of chemical change ; and the study of this very important class of phenom- ena will show us some of the latest phases which our chemical philosophy has assumed. Of the analytical reactions I will select for our first illustration the process by which oxygen gas is usually made. The common source of oxygen is a white salt, now well known under the name of chlorate of potash, but which, in the nomenclature of our modern chem- istry, is called potassic chlorate. I presume, if we should inquire into the cause of the present notoriety of this chemical preparation, we should find that it owed its reputation to the chlorate of potassa troches, and there is no doubt that, when judiciously used, this salt has a very soothing effect on an irritated throat. But, after all, the great mass of the potassic chlorate manufactured is used for fireworks or for mak- ing oxygen gas, and it is to the last use we now pro- pose to apply it. For this purpose, we have only to heat the salt to a low, red heat in an appropriate ves- sel. "We use here a copper flask, and connect the exit- tube with the now familiar pneumatic trough. While my assistant is preparing the oxygen gas, I will explain to you the process. Although potassic chlorate is a non-volatile solid, and we have no direct means of weighing its molecules, yet, from the purely chemical evidence we possess, there is no doubt whatever about its molecular consti- tution. It is expressed by the symbol KC10 3 , and, in the process before us, the potassic chlorate simply 11 8 CHEMICAL CHANGES CLASSIFIED. breaks up into another salt called potassic chloride and oxygen gas, K010 3 = KC1 + O s , Potassic Chlorate. Potassic Chloride. Oxygen Atoms. that is, each molecule of the salt gives a molecule of potassic chloride and three atoms of oxygen. Notice that I say three atoms ; for this is a point to which I must call your attention. We are not dealing here with an example of pure analysis, although that feature of the reaction pre- dominates over every other. Oxygen gas is the product formed; and, as I have several times said, we know that the molecules of oxygen consist of two atoms. Hence, the three atoms which the heat drives off must pair, and, from three atoms, we can only make one molecule. What, then, is to become of the third atom, which seems to be left out in the cold \ You must have already answered this question; for you remember that our symbols only express the change in one of the many millions of molecules which are breaking up at the same instant ; so there can be no want of a mate for our solitary atom. In- deed, two molecules of chlorate will give us just the number of atoms we want to make three molecules of oxygen gas. Hence, we should express the change more accurately by doubling the symbols : 2KC10 3 = 2KC1 + 30=0. Potassic Chlorate. Potassic Chloride. Oxygen Gas. Let me next remind you that these symbols express exact quantitative relations ; and, as some of my young friends may desire to know how to calculate the amount of chlorate they ought to use in order to make a given - volume, say, ten litres of oxygen, I will, even at the risk of a little recapitulation, go through the calcula- PRECAUTIONS. 179 tion : A molecule of KC10 3 weighs 39.1+35.5+48 = 122.6 m.c, and two molecules will weigh 245.2 m.c. These yield 2KC1, weighing 2 (39.1 + 35.5) = 149.2 m.c, and 30-0, weighing 96 m.c. "We must next find the weight of 'ten litres of oxygen gas. To find the weight of one litre we multiply the specific gravity of the gas, or half molecular weight, by j^. Now, ^xlG = 1.44 gramme. Hence, ten litres weigh 14.4 grammes. But, if 96 m.c. of gas are made from 245.2 m.c. of salt, then 14.4 grammes would be obtained from a quantity easily found from the proportion : 96 : 245.2 = 14.4 : x = 36.78 grammes. I think, after this, we will assume that these quan- titative relations are all right, and let them take care of themselves. Returning to the experiment, before I show that the products are those which I have de- scribed, let me give just a word of caution to any of my young friends present, who may like to repeat it. We find that it is best to mix our chlorate with a heavy black powder, known in commerce as black ox- ide of manganese. What the effect of the powder is we do not know, for it is wholly unchanged in the process. But, in some way or other, it eases off the decomposition, which is otherwise apt to be vio- lent. In buying the black oxide of manganese you must take care that it has not been adulterated with coal-dust — for a mixture of coal-dust and chlorate ex- plodes with dangerous violence when heated, and seri- ous accidents have resulted from the cupidity which led to such adulteration. Let me, moreover, say in general that, although I highly approve of chemical experi- ments, as a recreation for boys, they ought always to be made under proper oversight, and according to exact 180 CHEMICAL CHANGES CLASSIFIED. directions, and I would warmly recommend, as a trust- worthy companion for all beginners, the abridgment of "Eliot and Storer's Manual of Chemistry," recently edited by Prof. Nichols, of the Institute of Technol- ogy- But how shall I show you that this gas we have obtained is oxygen ? I know of no better way than to test it with one of our watch-spring matches. ... In no other gas will iron burn like this. So much for the oxygen. Let us next turn to the other product, that I called potassic chloride. This is left in the retort, forming a solid residue, but, as it would take a long time to bring what we have just made into a presentable condition, we must be content to see some of the product of a former process, which I have in this bottle. At a distance, you cannot distinguish the white salt from the potassic chlorate with which we started, but, if you compared the two carefully, yon would see that there was a very great difference between them. I can only show you that the crystals of the two salts have wholly different forms. For this purpose I have crystallized them on separate glass plates, and I will now project a magnified image of the crystals on the screen. There you see them beautifully exhibited on the two illuminated disks side by side. The square figures on the left-hand disk (Fig. 23) are the projections of the cubes of potassic chloride, which differ utterly in form from the rhombic plates of potassic chlorate that appear on the right (Fig. 24). The second example of an analytical process which I have to show you is also familiar to many of my audience, and cannot fail to be interesting to the rest ; for it is the process by which nitrous oxide is prepared, PREPARATION OF NITROUS OXIDE. 181 the gas now so much used by the dentists as an anaes- thetic. It was formerly called laughing-gas, but the peculiar intoxication it causes, when inhaled under cer- tain conditions, has been almost forgotten in its present Fro. 28.— Crystals of Potassic Chloride. Fio. 24. — Crystals of Potassic Chlorate. beneficent application in minor surgery. Nitrous oxide is made from a well-known white salt, prepared from one of the secondary products of the gas-works, and called nitrate of ammonia, or ammonic nitrate. When this salt is gently heated in a glass flask, its molecules split up into those of nitrous oxide and water. Again, let us make use of the time required for the experiment to explain the process. The molecules of ammonic nitrate have the constitution NjB^Os, and the change may be represented thus : N 2 H,0 3 = Ammonic Nitrate. 2H a O Water. + N 2 0. Nitrous Oxide. The experiment has been arranged so as to show both of the products (Fig. 25). The water condenses in this test-tube, while the gas passes forward, and is collected over a pneumatic trough. But what evidence can I give you that these are, in fact, the products ? As re- gards the water, you would readily recognize the fa- 182 CHEMICAL CHANGES CLASSIFIED. miliar liquid, which has collected in the tube, could you examine and taste it. Eut, as I cannot offer you this evidence, I will seek for another. Most of you must be familiar with the remarkable action of the alkaline metals on water. You see how this lump of potassium inflames the moment it touches the liquid. Fig. 25. — Preparation of Nitrous Oxide and Water, from Amnionic Nitrate. Let us now see whether it will act in a similar way on the liquid which has condensed in our tube. . . . There can be no doubt that we are dealing with water. Next for the gas. Nitrous oxide has the remarkable quality, not only of producing anaesthesia, but also of sustain- ing the combustion of ordinary combustibles with great brilliancy — like oxygen gas. But there is a marked difference between nitrous oxide and oxygen, which an experiment will serve to illustrate, and this, at the same time, will show us that the gas we have obtained in our experiment is really nitrous oxide. Taking a lump of sulphur, I will, in the first place, ignite it, and when it is only burning at a few points I will immerse it in a jar of oxygen. As you see, it at once burns up with great brilliancy. Taking now a sim- EXPLOSION OF IODIDE OF NITROGEN. 183 ilar lump of sulphur, and waiting until you all admit that it is ignited more fully than before, I will plunge it into this jar of gas we have just prepared, and which we assume to be nitrous oxide. ... It at once goes out, and the reason is obvious. There is an abundance of oxygen in the nitrous oxide — relatively, more than twice as much as in the air ; but, in the molecules of N 2 0, the oxygen atoms are bound to the atoms of ni- trogen by a certain force, which the sulphur at this temperature is unable to overcome. Let me, however, heat the sulphur to a still higher temperature, until the whole surface is burning, and you see that it burns as brilliantly in the compound as it does in the element- ary gas. The last example of an analytical reaction, which we shall have time to examine, is furnished by a re- markable compound of iodine and nitrogen, called iodide of nitrogen. Iodine is an elementary substance, resembling chlorine, which is extracted from kelp, that common broad-leafed sea-weed abounding on our coast. It is a very volatile solid, and gives a violet-colored va- por, whence its name from the Greek wordttofij;?. When heated gently with aqua ammonia, the iodine takes from the ammonia a portion of nitrogen, and forms with it a very explosive compound whose molecule has the constitution NI 3 . "We have prepared a small quan- tity of the substance, and the black powder is now rest- ing on this anvil, wrapped in filtering-paper. The slightest friction is sufficient to determine the break- ing up of these very unstable molecules, and the de- composition of the compound into iodine and nitro- gen. A mere touch with a hammer is followed by a loud report, when you notice a cloud of violet vapor, which indicates that the iodine has been set free : 184 CHEMICAL CHANGES CLASSIFIED. 2OT 3 = NW + 3I-I. Iodide of Nitrogen. Nitrogen Gas. IoQine-Vapor. In this case, as in previous examples, the atoms, when liberated, unite in pairs to form molecules of nitrogen gas on the one side, and molecules of iodine-vapor on the other ; and, since a single molecule does not yield an even number of atoms of either kind, we double the symbols. There is cne characteristic of analytical reactions which must be carefully noticed. The parting of atoms (and it must be remembered that by an analytical reac- tion we merely mean this phase of a chemical process) is attended by the absorption of heat ; although — as in the last experiment — the effect is often masked by other causes. But this truth can only be made evident by comparing the results of careful measurements, which cannot be made rapidly, and whose discussion, even, would be out of place at this time. I must, therefore, content myself with stating the fact as one of the defi- nite results of science, and pass on to some examples of synthesis — reactions of the opposite class. One of the most striking illustrations of the direct union of two molecules, to form a third, is furnished by the action of ammonia gas on hydrochloric-acid gas. "Without entering into any details in regard to the pro- cesses by which these two aeriform substances are pre- pared, let it be sufficient to say that, in the glass flask on the right-hand side of this apparatus (Fig. 26), are the materials for making hydrochloric acid, and in the similar flask on the left those for making ammonia. The exit-tubes from these flasks deliver the two gases into this large glass bell, where they meet, and the chemical reaction takes place. The reaction is very simple, and one in regard to which we have no doubt, DIRECT UNION OP TWO GASES. 185 for the molecules of both of the factors have been weighed and analyzed. It is expressed thus : NH 3 Ammonia Gas. + HCl Hydrochloric-Acid Gas. NH,01. Amnionic Chloride. Fig. 26. — Combination of Ammonia and Hydrochloric-Acid Gases. As you see, the atoms of a molecule of ammonia unite with those of a molecule of hydrochloric acid to form a single molecule of amnionic chloride, and, although the reaction may imply the breaking up, to a certain extent, of the molecules of the two factors, yet the subsequent synthesis is the chief feature. Ammonic chloride is a solid, and the sudden production, from two invisible gases, of the white particles of this salt, which fill the bell with a dense cloud, is a very strik- ing phenomenon. The second example of synthesis I have chosen is equally striking. Here, also, the factors of the reaction are both gases. The lower jar (Fig. 27) contains a gas called nitric oxide, like nitrous oxide, a compound of oxygen and nitrogen, but containing a relatively larger proportion of oxygen. Its molecule has the constitution NO. 186 CHEMICAL CHANGES CLASSIFIED. The upper jar contains oxygen, and, on removing the thin glass which now separates the two gases, you no- tice an instantaneous change. A deep-red vapor soon fills the glass. This red prod- uct is still another compound of nitrogen and oxygen, called nitric peroxide, whose symbol is N0 2 , and the reaction is simply this : 2NO + 0=0 = 2N0 2 . Nitric Oxide. Nitric Peroxide. Here a molecule of nitric oxide takes only an atom of oxygen, and, since each mole- cule of oxygen gas consists of two atoms, it will supply the need of two molecules of NO. Since the two factors and the single prod- uct of this process are all gases, the reaction before us is well adapted to illustrate another S&ygen f act m regard to our symbols, of which I GaSi have not as yet directly spoken. If, in writing reactions, care is taken that each term shall always represent one or more perfect molecules, so far as their constitution is known — then the symbols will always indicate, not only the relative weights, but also the relative volumes of the several factors and products when in the state of gas. That this must be the case, you will see when you remember that equal volumes of all gases under the same conditions have the same number of molecules, and hence that all gas-molecules have the same volume. The symbol of one molecule rep- resents what we will call a unit volume, and the number of these unit volumes concerned in any reaction is the same as the number of molecules. We can read the reaction before us thus : Two volumes of nitric-oxide and one volume of oxygen gas yield two volumes of nitric peroxide. Fig. 27.— Com- bination of TINSEL BUKNT IN CHLORINE GAS. 187 Three volumes, therefore, become two. If this is the case, there must be a partial vacuum in the jar, and, on opening the stop-cock, you hear the whistle which the current of air produces as it rushes in to es- tablish an equilibrium. We come now to still another example of a syn- thetical reaction, and, to illustrate this, the apparatus before you has been prepared (Fig. 26). The metallic leaf in the upper of the two glass jars is made of brass, which consists of the two metals, zinc and copper. In the lower jar we have chlorine gas. The air has been exhausted from the upper jar by a pump, and, on opening the stop- cock, the chlorine gas will rush in from the lower jar to take its place. Chemical union at once results, and notice the ap- pearance of flame, which is an indication that great heat is produced by this chemical change. The change here is very simple. The atoms of chlorine unite directly with fig. 2b.— union of . , , „ . j j? Chlorine with the atoms both of zinc and 01 copper, Tinsel, forming two compounds, which we call respectively zincic chloride, and cupric chloride. One reaction will serve for both metals, as the two are sim- ilar, differing only in the symbols of the metals. Take copper — Cu + Ol-CI = CuCl 2 . Copper. Chlorine Gas. Cupric Chloride. As in analytical reactions heat is absorbed, so in synthetical reactions heat is evolved. You were all witnesses of the fact that heat was evolved in this last reaction, and it is equally true that heat was devel- oped in each of the two previous experiments. In the combination of ammonia with hydrochloric acid (Fig. 188 CHEMICAL CHANGES CLASSIFIED. 26), this fact was made evident by the thermometer we placed in the bell for the purpose, and in the combina- tion of nitric oxide with oxygen by the initial expan- sion which attended the first union of the two gases, and which would have lifted off the upper bell (Fig. 27), had I not firmly held it in its place. As, however, the product rapidly cooled to the temperature of the air, the initial expansion was soon followed by the con- densation to which I called your attention. Now, the principle illustrated by these three experi- ments is universally true, and the point is so impor- tant that I will make still another experiment in order to illustrate this feature of synthetical reactions still further. In this glass I have placed a small piece of phosphorus, and now I will drop upon it a few crys- tals of iodine. Direct combination between the phos- phorus and iodine at once takes place, and the heat developed by this union is sufficient to inflame the un- combined phosphorus which I have intentionally added in excess. The principle here illustrated is one of the greatest importance in the theory of chemistry, and this - class of phenomena has been the object of extended inves- tigation. Not only has it been shown that the prin- ciple here stated is in general true, but also that the amount of heat liberated by the union of the same atoms to form the same molecules is always constant, and this amount has, in very many cases, been meas- ured. It has further been proved by actual experiment that, when, by any cause, the atoms thus joined are sep- arated, exactly the same amount of heat is absorbed. In chemical processes, where, as a general rule, there are both analysis and synthesis, the thermal relations depend primarily on the extent to which these two ef- PHOSPHORUS BURNT IN AIR. 189 f ects neutralize each other ; but changes in the state of aggregation, and other physical causes, constantly inter- vene to modify the result. There is one class of chemical processes in which the thermal effects are so great, so striking, and so im- portant, as to subordinate all other phenomena. I re- fer to the common processes of combustion, on which we depend for all our artificial light and heat. To these processes I shall next ask your attention, for, al- though they are only further illustrations of the princi- ple just stated, yet, they play such an important part in Nature, and have been so often the battle-ground be- tween rival chemical theories, that they demand our separate attention. I will open the subject by burning in the air a piece of phosphorus. Before this intelligent audience it is surely unneces- sary to dwell on the elementary facts connected with the class of phenomena of which this is the type. It will only be necessary for me to call to your recollec- tion the main points, and then to pass to the few feat- ures which I desire especially to illustrate. In regard to the main points, no experiment could be more in- structive than this. This large glass jar is filled with the same atmospheric air in which we live. Of this atmospheric air one-fifth of the whole material consists of molecules of oxygen gas in a perfectly free and un- combined condition ; for, although they are mixed with molecules of nitrogen gas, in the proportion of four to one, and, although the presence of this great mass of inert material greatly mitigates the violence of our or- dinary processes of burning, it does not, in any other re- spect, alter the chemical relations of the oxygen gas to combustible substances. These combustibles are, for the most part, compounds of a few elements — carbon, 190 CHEMICAL CHANGES CLASSIFIED. hydrogen, sulphur, and phosphorus — including the ele- mentary substances themselves, and our common com- bustibles are almost exclusively compounds of hydrogen and carbon only. Their peculiar relations to the atmos- phere depend solely on the fact that the atoms of these bodies attract oxygen atoms with exceeding energy, and it is only necessary to excite a little molecular ac- tivity in order to determine chemical union between the two. This union is a simple synthetical reaction, and, like all processes of that class, it is attended with the liberation of heat. The chief feature which dis- tinguishes the processes of burning from other synthet- ical reactions is the circumstance that the heat gen- erated during the combination is sufficient to produce ignition — in other words, to raise the temperature of the materials present to that point at which they be- come luminous, and the brilliant phenomena which thus result tend to divert the attention from the sim- ple chemical change, of which they are merely the out- ward manifestation. In the case of our ordinary com- bustibles, the real nature of the process is still further obscured by the additional circumstance that the prod- ucts of the burning — carbonic dioxide and aqueous va- por — are invisible gases, which, by mixing with the atmosphere, so completely escape rude observation that their existence even was not suspected until about a century ago, when carbonic dioxide was first discovered by Dr. Black. Although these aeriform products neces- sarily contain the whole material, both of the combus- tible and of the oxygen with which the combustible has combined, there is a seeming annihilation of the com- bustible, which completely deceived the earlier chem- ists. In the case before us, however, the product of the combustion is a solid, and it is this circumstance POINT OF IGNITION. 191 which makes the experiment so instructive. Almost every step of the process can be here seen. Tou no- ticed that we lighted the phosphorus in order to start the combustion — for this combustible, like every other, must be heated to a certain definite temperature before it bursts into flame. This temperature is usually called the point of ignition, and differs greatly for different combustibles. While phosphorus inflames below the temperature of boiling water, coal and similar combus- tibles require a full red heat. If, as our modern theory assumes, increased temperature merely means an in- creased velocity of molecular motion, the explanation of these facts would seem to be that a certain intensity of molecular activity is necessary in order to bring the molecules of oxygen sufficiently near to those of the combustible to enable the atoms to unite, and that the point of ignition is simply the temperature at which the requisite molecular momentum is attained. But the process once started continues of itself, for it is a characteristic of those substances we call combustible that, as soon as a part of the body is inflamed, the heat developed by the chemical union is sufficient to mainr tain the temperature of the adjacent mass at the igni- tion-point. Passing next to the chemical process itself, nothing could be simpler than the change which is taking place in the experiment before us. It is an example of di- rect synthesis. This white powder which you see falling in such abundant flakes is the solid smoke of this fire. It is formed by the union of the phosphorus and oxygen — two atoms of phosphorus uniting with five of oxygen to form a molecule of this solid, which we call phosphoric oxide, and whose symbol we may write thus, P 3 5 . 192 CHEMICAL CHANGES CLASSIFIED. But, neither the conditions of the burning nor the chemical change itself, although so beautifully illus- trated here, are nearly so prominent facts as the mani- festation of light and heat, which attends the process ; and these brilliant phenomena wholly engrossed the attention of the world until comparatively recently, and indeed they still point out what is really the most im- portant circumstance connected with this class of phe- nomena. The union of combustible bodies with oxy- gen is attended with the development of an immense amount of energy, which takes the form of light or heat, as the case may be. Moreover, it is also true that the amount of energy thus developed depends solely on the amount of combustible burnt, and not at all on the circumstance that the burning is rapid or slow. Thus, in the case before us, the amount of heat devel- oped by the burning of an ounce of phosphorus is a perfectly definite quantity, and would not be increased if the combustion were made vastly more intense. So it is with other combustibles. The table before you gives the amount of energy developed by the burning of one pound of several of the more common combus- Galorific Power from One Pound of Each Combustible. English Units of Heat. Foot-pounds. Hydrogen Marsh-gas Olefiant gas. . . . Wood-charcoal Alcohol Sulphur 62,032 23,513 21,344 14,544 12,931 4,070 47,888,400 18,152,350 16,477,880 11,228,000 9,982,890 3,141,886 tibles, estimated, in the first place, in our common units of heat, and, in the second place, in foot-pounds. But, although the amount of energy is thus constant, de- PHOSPHORUS BURNT IN OXYGEN GAS. 193 pending solely on the amount of the combustible burnt, the brilliancy of the effect may differ immensely. A striking illustration of this fact I can readily show you. For this purpose I will now repeat the last experi- ment, with only this difference, that, instead of burning the phosphorus in air, I will burn the same amount as before in a globe filled with pure oxygen. We shall, of course, expect a more violent action, because, there being here no nitrogen-molecules, there are five times as many molecules of oxygen in the same space. Hence, there are five times as many molecules of oxygen in con- tact with the phosphorus at once, and five will combine with the phosphorus in the same time that one did be- fore. But, with this exception, all the other conditions of the two experiments are identical. We have the same combustible, and the same amount of it burnt. We have, therefore, the same amount of energy devel- oped, and yet how different the effect ! Phosphorus burns brightly even in air, but here we have vastly greater brilliancy, and the intensity of the light is blinding. What is the cause of the difference ? One obvious explanation will occur to all : The energy in this last experiment has been concentrated. Although only the same amount of heat is produced in the two cases, yet, in the last, it is liberated in one fifth of the time, and the effect is proportionally more intense. The inten- sity of the effect is shown simply in two circumstances : first, a higher temperature ; and, secondly, a more brill- iant light. Of these, the first is fully accounted for in the explanation just suggested ; for, if five times as much heat is liberated in a given time, it must neces- sarily raise the temperature of surrounding bodies to a much higher degree. I need not go beyond your famil- 194 CHEMICAL CHANGES CLASSIFIED. iar experience to establish this principle, although tem- perature is a complex effect, depending, not only on the amount of heat liberated, but also on the nature of the material to be heated, and on conditions which deter- mine the rapidity with which the heat is dissipated. But the matter of the light is not so obvious. Why should more rapid burning be attended with more brill- iant light ? It is so in the present case ; but is it al- ways so ? We can best answer this question by a few experiments, which will teach us what are the condi- tions under which energy takes the form of light ; but these experiments we must reserve until the next lect- ure. LECTURE IX. THE THEOET OF COMBUSTION. As our last hour closed, we were studying the phe- nomena of combustion. I had already illustrated the fact that, so far as the chemical change was concerned, these processes were examples of simple synthesis, con- sisting in the union of the combustible atoms with the oxygen atoms of the air, and that the sole circumstance which distinguished these processes from other synthet- ical reactions was the amount of energy developed. There were three points to which I directed your at- tention in connection with this subject : 1. The con- dition of molecular activity, measured by the tempera- ture or point of ignition, which the process requires. 2. The chemical change itself, always very simple. 3. The amount of energy developed, and the form of its manifestation. This last point is the phase of these phenomena which absorbs the attention of be- holders, and the one which we have chiefly to study. I stated in the last lecture that the amount of energy de- veloped depended solely on the nature and amount of the combustible burnt, but I also showed that both the intensity and the mode of manifestation of this energy varied very greatly with the circumstances of the ex- periment. The intensity of the action we traced at 196 THE THEORY OF COMBUSTION. once to the rapidity of the combustion, but the condi- tions which determine whether the energy developed shall take the form of heat or light we have still to in- vestigate, and no combustible is so well adapted as hydrogen gas to teach us what we seek to know. Here, then, we have a burning jet of hydrogen. It is not best for me to describe, in this connection, either the process" or the apparatus by which this elementary substance is made, and a constant supply maintained at the burner, as I wish now to ask your attention ex- clusively to the phenomena attending the burning of the gas ; and let me point out to you, in the first place, that hydrogen burns with a very well-marked flame. The flame is so slightly luminous that I am afraid it cannot be seen at the end of the hall, but I can make it visible by puffing into it a little charcoal-powder. Now, all gases burn with a flame, and flame is sim- ply a mass of gas burning on its exterior surface. As the gas issues from the orifice of the burner, the cur- rent pushes aside the air, and a mass of gas rises from the jet. If' the gas is lighted — that is, raised to the point of ignition — this mass begins to combine with the oxygen atoms of the air at the surface of contact, and the size of the flame depends on the rapidity with which the gas is consumed as compared with the rapid- ity with which it is supplied. By regulating the sup- ply with a cock, as every one knows, I can enlarge or diminish the size at will. The conical form of a quiet flame results from the circumstance that the gas, as it rises, is consumed, and thus the burning mass, which may have a considerable diameter near the orifice of the jet, rapidly shrinks to a point as it burns in ascending. But we must not spend too much time with these HYDROGEN GAS BUENT UST AIR. 197 details, lest we should lose sight of the chemical phi- losophy, which it is the main object of this course to illustrate. The chemical change here is even more simple than in the experiment with phosphorus, and consists solely in a direct union of the hydrogen atoms of the gas with the oxygen atoms of the air. Indeed, in another connection, we studied the reaction at an early stage in this course of lectures ; when, in order to illustrate the characteristic feature of chemical combi- nation, we exploded a mixture of hydrogen and oxygen gases. The reaction obtained under those conditions was identical with that here. We had not then learned to express the chemical change with symbols ; but now I may venture to write the reaction on the black-board : 2H-H + 0=0 = 2H 2 0. Hydrogen Gas. Oxygen Gas. Steam. It would be very easy to show you that, as the sym- bols indicate, from two volumes of hydrogen, and one of oxygen, two volumes of steam are formed ; but the experiment requires a great deal of time, and the re- sult could not readily be made visible to this audience. I must content myself with proving that water is really produced by the hydrogen flame. The apparatus we use looks complicated, but is, in fact, very simple (Fig. 29). By means of an aspira- tor the products of combustion are sucked through a long glass tube, which is kept cool by a current of wa- ter in a jacket outside. The flame bums under the open and flaring mouth of the tube, and the liquid, which condenses, drops into a bottle at the other end. You must not expect that any considerable amount of water can be produced in this way. In the union of the two gases to liquid water, a condensation of 1,800 times takes place, so that, in order to obtain a 19 8 THE THEORY OP COMBUSTION. quart of liquid water, we must burn 1,200 quarts of hydrogen gas, and take from the air 600 quarts of pure oxygen; and this, on the scale of our experiment, would be a very slow process. We have here obtained barely an ounce of liquid, although the jet has been burning for more than an hour. In order to show that the product is really water, I will apply the same test I used in a former experiment. We will pour the liquid into a shallow dish, and drop upon it a bit of potassium. . . . The hydrogen - flame, which at once bursts forth, gives the evidence we seek. Fio. 29.— The Synthesis of Water. Such, .then, being the nature of the chemical pro- cess before us, let me pass on to that feature of this flame which is at once the most conspicuous and the most important phase of the phenomenon, namely, the development of energy. Here, again, we have become acquainted with the important facts bearing on this question. In a previous lecture I told you that, in the burning of a pound of hydrogen, sufficient energy was developed to raise a weight of 47,888,400 pounds to the height of one foot, and these figures are included, COMPOUND BLOW-PIPE. 199 among other data of the same kind, in the diagram still before you. {See page 186.) I also endeavored to impress on your minds the magnitude of this energy by showing that, with a hydrogen-flame, a temperature can be obtained at which steel burns like tinder. In that experiment, however, the energy was intensified to a far greater degree than in the flame we have here ; for, although this flame is very hot, it is wholly inade- quate to produce the effects you before witnessed. The intensity was then gained just as in our experiment with phosphorus, by burning the hydrogen in pure oxygen, instead of air ; and you remember the apparatus, called the compound blow-pipe, by which this result was ob- tained. The flame of the blow-pipe emits a pale-blue light, but is so slightly luminous that it can hardly be seen at any distance in this large hall, and yet, as we know, it is, intensely hot. You have seen how steel defla- grates before it, and I will now show you its effect on several other metals (copper, zinc, silver, and lead). You notice that they all burn freely, and that each im- parts to the flame a characteristic color, and, I may add, in passing, that spectrum analysis, which has achieved such great results during the last few years, is based on these chromatic phenomena. But the experiments you have just seen, although so brilliant and instructive, have not yet given us much help toward the solution of the problem we proposed to investigate, viz., the conditions under which the en- ergy of combustion is manifested in the form of light. They have, however, helped us thus far : they have shown that the light cannot depend upon the rapidity of the combustion or the temperature of the flame alone, for here we have intense energy and a very high tern- 200 THE THEORY OF COMBUSTION. perature without light. Moreover, they have presented us with a phenomenon, which differs from that we wit- nessed at the close of the last lecture, in the very point we are investigating : phosphorus burns in oxygen with a most brilliant light ; hydrogen burns in oxygen with scarcely any light. Now, it is evident that the cause of the light must be some circumstance of the first experiment, which does not exist in this, and, by comparing the two to- gether, we may hope to reach a definite result. At first sight, this comparison reveals only resemblances. Both processes consist in the union of combustible material with oxygen. In the one case it is the atoms of phos- phorus, and in the other the atoms of hydrogen, which combine with the atoms of the oxygen gas. Otherwise the chemical change is the same in both cases, and we cannot therefore refer the light to any difference in the process. Again, in both processes a very large amount of energy is developed, but, so far as there is any differ- ence, that difference is in favor of the hydrogen, which gives the least light. So, also, in both processes, a very high temperature is attained ; but a simple calcu- lation will show that the temperature of the hydrogen- name is higher than that of the phosphoAs-flame, and so the light cannot be an effect solely of temperature. Can it be that the difference is due to the circumstance that the combustible in one case is a solid, and in the other a gas ? Here, at least, is a difference, which gives us a starting-point in our investigation. But we shall not pursue the investigation far before we find that this difference is wholly illusory. It will appear that phosphorus is a very volatile solid, and that it is wholly converted into vapor before burning ; so that, in fact, we are dealing in both cases with burning gag. ON WHAT DOES LUMINOUS POWER DEPEND? 201 In looking round for other differences we shall recognize that there is a marked difference in the products of the two processes. The product in one case is phosphoric oxide, and in the other case water. Water is volatile, and is evolved in the state of vapor. Phosphoric oxide is a highly-fixed solid, and condenses in those snow-like flakes which you saw falling in the jar at the last lecture. May it not be that the circum- stance that the product in the one case is a solid, and in the other a gas, is the cause of the difference in the light? In the phosphorus flame there are solid parti- cles of phosphoric oxide, while in the hydrogen-flame there are no solid particles whatever. Can this be the cause of the difference? Here, at least, is another starting-point for our investigation. An obvious mode of discovering whether there is any value in this suggestion is to introduce non-vola- tile solid matter into the blow-pipe flame, and observe whether the light of the flame is affected thereby. The temperature of the flame is so high that there are but few solids which are sufficiently fixed for our experi- ment. One, however, which is admirably adapted for our purpose, is at hand, and that is lime. In order, then, to answer the question that has been raised, let us introduce into the flame a bit of lime, or, what amounts to the same thing, allow the flame to play against a cylinder of this material. (In an instant the hall is most brilliantly illuminated.) The question is answered, and there is no plainer answer than that given by a well-considered experiment. And here let me ask your attention to the method we have followed, because it illustrates, in the most striking manner, the method of science. When we wish to discover the cause of an effect observed in any 202 THE THEORY OF COMBUSTION. phenomenon, we begin by varying the conditions of the phenomenon until at last we find that the effect varies, or perhaps even disappears. That is, we try a series of experiments, varying the conditions at each trial, until at last we succeed in eliminating the effect. This having been done, we next compare the condi- tions under which the effect appears and those under which it does not. Those conditions common to both experiments are at once eliminated, while those which are different in the two are carefully considered, and experiments are devised to test their influence on the effect until at last the cause is made evident. Thus we sought to find the cause of the light generally pro- duced by combustion. We began by burning different combustibles until we found one which gave out little or no light. We next compared the burning of phos- phorus in oxygen, which gave a very intense light, with the burning of hydrogen, which gave little or none. We found that the only important difference between the two cases was the circumstance that the phosphorus-flame contained particles of solid matter, while the hydrogen-flame contained none, and in order to test the effect of the difference, which the compari- son suggested, we placed solid matter in the hydrogen- flame, when the cause of the light became evident. This method of comparing phenomena as a means of discovering the cause of effects which are prominent in one, although common to both, is frequently called differentiation, and it is one of the most valuable methods of science. If I have succeeded in giving you some idea of the method, the time we have de- voted to these experiments has been well spent. You will grant, I think, that we have now established the following points in regard to the theory of combus- POINTS ESTABLISHED. 203 tion : 1. That the process requires. a certain degree of molecular activity, measured roughly by what we call the point of ignition. 2. That the chemical change consists simply in the union of the combustible with the oxygen of the air. 3. That these processes differ from other examples of synthesis chiefly in the circum- stance that the union of the oxygen atoms with those of our ordinary combustibles is attended with an extraordinary development of energy. 4. That the amount of this energy is constant for the same com- bustible, and is in each case exactly proportional to the amount of fuel burnt. 5. That the intensity of the effect depends on the rapidity of the combustion, the energy usually manifesting itself as heat, but tak- ing also the form of light when non-volatile solid parti- cles are present. 1 Were we to limit our regards solely to the theory of combustion, there would be no necessity of pursu- ing the subject further; but additional experiments may be of value by helping you to associate these principles with your previous experience. To this end I propose to ask your attention to the burning of one of the most familiar combustibles, viz., carbon in the form of charcoal, and, in order to hasten the process, we will burn the charcoal in oxygen gas instead of air. Placing, then, a few lumps of charcoal, previously ignit- ed, in a deflagrating spoon, I will introduce them into this large jar of oxygen gas. ... As you see, the char- coal burns more brilliantly than in air. But even in the pure gas the burning is by no means very rapid, and the reason is obvious. Since carbon, in all its 1 In order to give a complete view of the subject, it would be necessary to show further that liquids, and even vapors, under certain conditions, may become brilliant sources of light. 204 THE THEORY OF COMBUSTION. forms, is non-volatile, the molecules of the charcoal cannot leave the solid lumps. They do not, therefore, go half-way to meet the oxygen-molecules, but simply receive those which are driven against the surface of the coals. Hence the process depends on the activity of the oxygen-molecules alone, and, since the number of these molecules which can reach the combustible in a given time is limited by the extent of its surface, it is evident that with these lumps of coal we cannot expect very rapid burning even in pure oxygen-. If, however, our theory is correct, we should greatly in- crease the rapidity by breaking up the lumps, and thus increasing the surface of contact with the gas. Let us see if the result answers our expectations. Taking, then, some finely - pulverized charcoal, already ignited (by heating the mass in an iron dish over a spirit-lamp), I will sift the red-hot powder from an iron spoon into another large jar filled with oxy- gen. . . . Nothing we have yet seen has exceeded the splendor of the chemical action which now results. This dazzling light is radiated by the glowing particles of charcoal, which, after they have become incandes- cent, retain their solid condition until the last atom of carbon is consumed, giving us another illustration of the influence of this circumstance on the light : and let me again call your attention to the great fixity of carbon which the experiment also illustrates, and you will at once recognize the importance of this quality of the elementary substance in localizing our fires, as well as limiting their intensity, and will see that the use of coal as fuel wholly depends upon it. Turn next to the chemical change itself. This, as in the other similar processes we have studied, is an example of simple synthesis, consisting in the union CHARCOAL BURNT IN OXYGEN GAS. 205 of the carbon atoms with oxygen. As to the nature of the product formed, a single experiment will give you all the information you desire. After removing the deflagrating spoon with the residue of the charcoal lumps from the first of the two jars, I will ask you to notice the fact that the atmosphere within remains as transparent as before. The eye can detect no evidence of change, yet all the charcoal that has disappeared has been taken up by this atmosphere, and, could we readily weigh the mass of gas, I could show you that the weight had been in- creased by the exact weight of the coal absorbed. In- deed, the density has been so greatly enhanced that I can pour the gas from one vessel to another very much as I would water. Let me pour some of it from the jar into a tall glass half filled already with lime water. ... It looks like child's-play; but the transfer has been made, and now, on shaking the gas and lime- water together, the liquid becomes milky. You at once recognize the product : chalk has been formed in the lime-water, and the gas left after the burning ceased in the jar must be the same carbonic dioxide we have previously studied. We made the analysis of this aeriform substance in a previous lect- ure, and we have now made the synthesis. See how simply we express the reaction : C + 0=0 = C0 2 . Coal. Oxygen Gas. Carbonic Dioxide. A fact is indicated by this reaction, which we must not overlook. The volume of the carbonic dioxide (C0 2 ) obtained is exactly equal to the volume of the oxygen gas (0=0) employed. In this experiment we used a jarful of oxygen and we obtained a jarful of carbonic dioxide. The material of the burnt charcoal 206 THE THEORY OF COMBUSTION. is taken up into the gas atom by atom, actually ab- sorbed by it as a sponge absorbs water. Every mole- cule of oxygen which strikes against the charcoal flies off with an atom of carbon, forming with it the mole- cule of carbonic dioxide which, of course, occupies the same space as the previous molecule of oxygen gas. Hence it is that the vast amount of carbon which is being constantly absorbed by the atmosphere, as it passes through our grates and furnaces, does not alter its volume. Would that I might impress this re- markable fact on your imagination ! Consider how much coal is being burnt every day in a city like this — hundreds and hundreds of tons ! Conceive of what a mass it would make, more than filling this large hall from floor to ceiling, and yet in our city alone this enormous black mass is in twenty-four hours absorbed by the transparent air, picked up and carried away bodily, atom by atom, by the oxygen -molecules. Turn now to the energy developed in this process. Our diagram indicates that the amount of energy de- veloped by the burning of a pound of coal is very much less than that obtained with a pound of hydro- gen. But then it must be remembered how attenuated hydrogen gas is ; if, instead of comparing equal weights, we compare equal volumes, we shall find that the differ- ence is vastly in favor of carbon. Most of the combustible materials, however, which we use as fuel, consist of both hydrogen and carbon ; but the phenomena we have studied in the burning of the elementary substances reappear with these familiar combustibles, and, in regard to them, there are only a few special points to be noticed. On many of these substances, such as naphtha, paraffine, stearine, wax, oil, and the like, the effect of the heat is to generate illu- THE FAMILY GAS-FACTORY. 207 minating gas, which is the source of most of our arti- ficial light. In our cities and large towns the gas is made for us by a special process, but it must be remem- bered that every lamp and candle is a small gas-fac- tory. Flame is always burning gas, and the gas which we burn in our lamps and candles is very similar to that supplied by the Boston Gas Company : the only difference is that the gas, instead of being made from bituminous coal, is made from petroleum or wax, and, instead of being made at the "North End" and dis- tributed through pipes to distant burners, is burnt as fast as it is made. The heat generated by the burning gas is so great that it volatilizes the oil or wax fast enough to supply the flame, and then the mechanism of the wick comes into play to keep the parts of these natural gas machines in perfect running order. In* deed, a common candle, simple as it appears to be, is a most wonderful apparatus, and I should be glad to occupy the whole hour in explaining the adaptation of its parts ; but I have only time for a few illustrations, which show that in these luminous flames, as in the other cases of combustion we have studied, the light comes from incandescent solid particles. Of the two constituents of the combustible gas which forms the flame, hydrogen is the most combusti- ble, and under ordinary conditions is the first to burn, setting free, for a moment, the accompanying carbon in the form of a fine soot which fills the light-giving cone. This dust is at once intensely heated, and each glowing particle becomes a centre of radiation, .throwing out its luminous pulsations in every direction. The sparks last, however, but an instant, for the next moment the charcoal is itself consumed by the fierce oxygen, now aroused to full activity, and only a transparent gas rises 208 THE THEORY OF COMBUSTION. from the flame. But the same process continues ; other particles succeed, which become ignited in their turn, and hence, although the sparks are evanescent, the light is continuous. I might illustrate this theory by the familiar fact that soot is at once emitted from all these luminous flames, whenever the draft becomes so far interrupted that it does not supply sufficient oxygen to burn completely the carbon particles ; but a still more striking illustration is furnished by the simple contrivance we employ in the laboratory for preventing the deposition of this soot on the heating surfaces of our chemical vessels. We use for this purpose a gas-burner invented by Prof. Eunsen, of Heidelberg, and known by his name, in which air is mixed with the hydrocarbon gas before it is burnt. But this air, while it prevents the formation of soot, at the same time destroys the illuminating power of the flame. The molecules ot the hydrocarbon gas being now in near proximity to the molecules of oxygen re- quired for complete combustion, the difference of af- finity of oxygen for the carbon and hydrogen atoms does not come into play. There is enough oxygen for all, and the result is that no carbon-particles are set free in the flame. We have no soot, and therefore no light. In this Bunsen lamp the size of the apertures, by which the air enters at the base of the burner, may be regulated by a valve, and you notice that on closing this valve the flame at once becomes luminous. Open it again so that the gas shall mix with air before burn- ing, and the energy no longer takes the form of light. See, nevertheless, how brightly the flame ignites this coil of platinum wire, showing that there is no want of energy, only it now appears wholly as heat. WHAT BECOMES OF THE CANDLE. 209 The flame of a wood or soft-coal fire is also a gas- flame. The first effect of heat on these bodies is to generate illuminating gas, and to this circumstance, as in the case of the candle, the flame is due,. but after a while all the hydrogen is driven off, and we have then, in the glowing embers, the flameless combustion of carbon. The chemical change which takes place in the burn- ing of hydrocarbon fuels is in no way affected by the circumstance that the hydrogen and carbon are in chemical union. All the hydrogen-atoms burn to water, and all the carbon-atoms to carbonic dioxide, and these products can be detected in the smoke of every flame.; indeed, with a few unimportant excep- tions, they are the sole products of the combustion. Take, for example, this candle-flame. On holding over it a cold bell-glass the glass soon becomes be- dewed, and, before long, drops of water begin to trickle down the sides ; and now, on inverting the bell, and shaking up in it some lime-water, the milky appear- ance, which the clear solution immediately assumes, indicates the presence of carbonic dioxide. Of course, all the material of the candle passes into these colorless and insensible aeriform products which mingle with the atmosphere, and this absorption of combustible material into the atmosphere, this melting of firm, solid masses of coal and wood into thin air, has such an appearance of annihilation that it requires all the power of the reason, aided by experiment, to cor- rect the false impression of the senses. Yet nothing is easier than to show that the smoke, colorless and insensible as it is, weighs more than the material burnt, and, although the experiment must be familiar to many of my audience, I will repeat it, because it 210 THE THEOET OF COMBUSTION. Fib. 30. may aid some to clearer views of this all-important subject. Let mc call your attention, then, to this candle which, in a candlestick of peculiar construction, is hanging equipoised from one end of the beam of this balance (Fig. 30). You know that both aqueous vapor and car- bonic dioxide are eagerly absorbed by caustic soda, and this apparatus is so ar- ranged that the smoke of the candle is sucked through two glass tubes filled with this absorbent material. You no- tice that my balance is in equilibrium, and I will now light the candle under its tin chimney. The products of the com- bustion rise to the top of the chimney, which is closed excepting two small apertures, through which the smoke is sucked into the glass tubes contain- ing the caustic soda. Now you must picture to your- selves the molecules of oxygen of our atmosphere rushing in on this candle-flame from every side, each one seizing its atom of carbon, or its four atoms of hydrogen, as the case may be. You must, then, follow the molecules of carbonic dioxide and water thus formed, as they are caught up by the current of air — which our aspirator draws through the apparatus — and hurried into the glass tubes, where they are seized upon and held fast by the caustic soda. All the smoke of the candle being thus retained, it is evident that, if the process is as I have described it, we should expect that the apparatus would increase in weight as the candle burns, while, on the other hand, were any part of the material lost, there would be a corresponding diminution in weight. And we not only find that the weight increases, as the bal THE PRODUCTS HARMLESS. 211 ance shows, but that the increase is exactly equal to the amount of oxygen consumed. Not only none of the material of the candle escapes from the apparatus, but a portion of the oxygen of the air is also retained, and that causes the increase of weight. In connection with this experiment, I must not fail to call your attention to the circumstance that the prod- ucts of this combustion are as harmless as they are im- perceptible to the senses. Remember that thousands of tons of carbonic dioxide and aqueous vapor are dis- charged into the air of this city in a single day. Remem- ber, also, what a howl of remonstrance goes up if, from some manufactory, a few pounds of similar but noisome products escape, and you cannot fail to recognize the importance of this fact in the economy of Nature. Add to this what you already know, that the smoke of our fires and the exhalations of our lungs is the food of the plant — that the whole vegetable world is con- stantly absorbing carbonic dioxide, and giving back the oxygen to the atmosphere while storing up the regen- erated carbon in its tissues, and you will be still further impressed by the wonderful revelations we are study- ing. Nor must we, in this connection, fail to notice again the enormous amount of energy which the burning of our common forms of fuel liberates. The table is still before you which shows how great is the amount of energy which can be obtained by the burning of a sin- gle pound either of hydrogen gas or of charcoal, and the relations of these elementary substances in this re- spect are not in the least altered by their association in common wood or coal. In round numbers, it may be said that a cubic foot of cannel coal contains sufficient energy, if wholly utilized, to raise a weight of 3,269 212 THE THEORY OF COMBUSTION. tons one hundred feet, or 732,000,000 pounds one foot. I said, if wholly utilized, for, although we are able to make use of the whole energy in the form of heat, we have not yet succeeded in applying more than about one-twentieth of it to mechanical work. Eut still the energy exists stored up for use in every foot of wood or coal, and is ready to be set free when the fuel is burnt. When standing before a grand conflagration, wit- nessing the display of mighty energies there in action, and seeing the elements rushing into combination with a force which no human energy can withstand, does it seem as if any power could undo that work of destruc- tion, and rebuild those beams and rafters which are melting into air ? Yet, in a few years thay will he re- built. This mighty force will be overcome ; not, how- ever, as we might expect, amid the convulsions of Na- ture or the clashing of the elements, but silently in a delicate leaf waving in the sunshine. As I have al- ready explained, the sun's rays are the Ithuriel wand, which exerts the mighty power, and under the direction of that unerring Architect, whom all true science rec- ognizes, the woody structure will be rebuilt, and fresh energy stored away to be used or wasted in some future conflagration. My friends, this is no theory, but sober, well-estab- lished fact. How the energy comes and how it is stored away, we attempt to explain by our theories. Let these pass. They may be true, they may be mere fancies ; but, that the energy comes, that it is stored away, and that it does reappear, are as much facts as any phe- nomena which the sun's rays illuminate. I know of no facts in the whole realm of Nature more wonderful than these, and I return to them in the annual round of my instruction with increasing wonder and admira- THE EARTH A CINDER. 213 tion, amazed at the apparent inefficiency of the means, and the stupendous magnitude of the result. In an- other course of lectures in this place I endeavored to show what weighty evidence these-facts give in support of the argument that all the details have been arranged by an intelligent Designer.' The plan of this course does not give me time to do more than allude to this point, and I only refer to it here to ask for the argu- ment your own careful consideration. There is still another point, in connection with this subject, to which also I can only barely allude. The crust of our globe consists almost wholly of burnt ma- terial. Our granite, sandstone, and limestone rocks, are the cinders of the great primeval fire, and the at- mosphere of oxygen the residue left after the general conflagration — left because there was nothing more to burn. "Whatever of combustible material, wood, coal, or metal, now exists on the surface of the earth, has been recovered from the wreck of the first conflagration by the action of the sun's rays. One-half of all known material consists of oxygen, and, ou the surface of the globe, combination with oxygen is the only state of rest. In the process of vegetable growth, the sun's rays have the power of freeing from this combination hydrogen and carbon atoms, and from these are formed the numberless substances of which both the vegetable and animal organisms consist. From the material of these organisms we make charcoal, and Nature makes her coal-beds, and supplies her petroleum-wells. More- over, with these same materials, man has been able to separate the useful metals from their ores, and, by the aid 1 " Religion and Chemistry ; or, Proofs of God's Plan in the Atmos- phere and its Elements," ten lectures by Josiah P. Cooke, Jr , published by Charles Seribner. New York, 1864. 214 THE THEORY OF COMBUSTION. of various chemical processes, to isolate the other ele- mentary substances from their native compounds ; but the efficiency of all these processes depends on em- ploying the energy which the sun's rays impart to the carbon and hydrogen atoms to do work. A careful analysis of the conditions will show that it is just as truly the sun's energy which parts the iron from its combination in the ore, as it is solar power which parts the carbon from the carbonic dioxide in the leaf. "We have here, however, but a single example of a general truth. All terrestrial energy comes from the sun, and every manifestation of power on the earth can be traced directly back to his energizing and life-giving rays. The force with which oxygen tends to unite with the other elements may be regarded as a spring, which the sun's rays have the power to bend. In bending this spring they do a certain amount of work, and, when, in the process of combustion, the spring flies back, the energy reappears. Moreover, the instability of all organized forms is but a phase of the same action, and the various processes of decay, with the accompa- nying phenomenon of death, are simply the recoiling of the same bent spring. Amid all these varied phe- nomena, the one element which reappears in all, and frequently wholly engrosses our attention, is energy; and, if I have succeeded in fixing your attention on this point, my great object in this lecture has been gained. In the early part of this course, I stated that all modern chemistry rests on the great truth that Matter is inde- structible, and is measured bt weight. This evening we have seen glimpses of another great central truth, which, although more recently discovered, is not less far-reaching or important, namely, Energy is inde- structible, AND IS MEASURED BY WORK. Add to these THE THREE PHASES OF NATURE. 215 two a third, namely — Intelligence is indestructible, and is measured bt adaptation — and you have, as it seems to me, the three great manifestations of Na- ture : Matter, Energy, and Intelligence. These great truths explain and supplement each other. Give to each its due weight in your philosophy, and you will avoid the extremes of idealism on the one side, and of materialism on the other. Note. — The doctrine that energy is indestructible is known in physics as the conservation of energy, and it would give greater prominence to the corresponding doctrine — that matter is indestructible — if it were called the conservation of mass. The recognition of this last truth by Lavoisier has already been indicated, and its important influence on the history of chemical philosophy has been discussed ; but the student is not likely to appreciate its full significance unless he dwells upon it, and a compre- hensive view of the subject will probably be best gained by bringing the chemical changes, in which the, truth can only be verified by careful in- vestigation, into comparison with those familiar mechanical processes in which it is perfectly obvious. That there is no annihilation of material in the conversion of water into oxygen and hydrogen gases is a phase of the same truth, which we accept as self-evident in the common mechanical operations of the arts. Just as gold coins contain the metal from which they were struck, so the oxygen and hydrogen gases contain the material of the water from which they were made, and, if the student fully grasps the truth which this statement involves he will, in the first place, perceive that mass may be an attribute of matter underlying those accidents in which substances differ ; and, in the second place, he will see that the as- sumption of immutable atoms is an obvious explanation of the conserva- tion of mass in Nature. 10 LECTUKE X. GUNPOWDER AND NTTEO GLTCEEINE. Theee is one further point in connection with the theory of combustion to which I wish to call your at- tention, at the outset of my lecture this evening. In the only cases of burning we have studied, the combus- tible unites with the oxygen of the atmosphere. It is possible, however, to have combustion without atmos- pheric air, the combustible obtaining the required oxygen from some associated substance. There are several substances in which a large amount of oxygen is so loosely combined, or, in other words, in which the oxygen-atoms are held in combination by such a fee- ble force, that they will furnish oxygen to the combus- tible as readily as the atmosphere, and in a vastly more concentrated form. Two of these substances are well known, nitre (potassic nitrate) and chlorate of potash (potassic chlorate). One ounce of this last salt— the quantity in this small crucible — contains enough oxygen to fill a large jar (1.7 gallon), and by simply heating the salt we should obtain that amount of oxygen gas. We have provided also one-third of an ounce of pul- verized sugar, and we will now mix the two powders thoroughly together. Consider the conditions in this SUGAR BURNT BY POTASSIC CHLORATE. 21V mixture : The sugar is a combustible substance, and every particle of this combustible is in contact with, or, I should rather say, in close proximity to, grains of chlorate of potassa, which contain sufficient oxygen to burn the whole. All is now quiescent, because both materials, being in the solid condition, their molecules are, as it were, imprisoned, and a certain degree of mo- lecular activity is required to produce chemical change. This molecular activity we can readily excite by heat, but a more convenient, although less intelligible way, is to touch the mixture with a drop of sulphuric acid. Here we have not merely a pretty firework, but an experiment which illustrates a very important phase of the phenomena of combustion, and one of immense practical value. I have chosen this particular example because you are familiar with both of the materials employed. You have seen that sugar contains a large amount of combustible carbon. You also know that potassic chlorate contains a large volume of oxygen, which can readily be driven off by heat ; for you have seen me make oxygen from this very salt. You can, therefore, fully appreciate the conditions we had in our crucible at the beginning of the experiment, namely, a combustible with the oxygen required to burn it in close proximity. You will be prepared, then, to understand — 1. That the burning we have just witnessed does not dif- fer from ordinary burning, except in the single point I have mentioned ; that the combustible derives its oxygen from potassic chlorate, instead of from the air ; and, 2. that it is possible to inclose in a confined space, as a gun-barrel or a bomb, all the conditions of combustion. In a word this experiment illustrates the simple theory of gunpowder. What, then, is gunpowder? Essentially a mixture 218 GUNPOWDER. of two substances — saltpetre and charcoal, with merely a small amount of sulphur added to facilitate the kin- dling of the charcoal. In the manufacture of this explosive agent, as is well known, the materials are first reduced to a very fine powder, and then inti- mately mixed together. Afterward, by great pressure, the mass is compacted to a firm, hard cake, which is subsequently broken up into grains of different sizes, adapted to various uses. Here we have some samples of these grains, varying from the size of a walnut to that of a millet-seed. These black grains, although they appear so homogeneous, are, in fact, a very inti- mate mixture of a combustible material (charcoal and a little sulphur) with a substance rich in oxygen (salt- petre), and, when we ignite the powder, the charcoal burns at the expense of the oxygen of the saltpetre. Two parallel experiments will make the whole matter clear. In this jar Ave have about one gallon (100 grains) of pure oxygen, enough to combine with 37£ grains of charcoal. This quantity of charcoal we will place in a copper spoon, and, having ignited the coal, we will plunge it into the jar of oxygen. We have at once a brilliant combustion, and a repetition of the experi- ment which you witnessed at the last lecture. We then learned that the process consists in the union of the oxygen with the carbon, and that each molecule of oxygen gas actually picks up an atom of carbon to form a molecule of carbonic dioxide. There are, therefore, just as many molecules in - the jar at the close of the ex- periment as at the first, only they now consist of three atoms, instead of two ; 0=0 has become 0=C=0. In the second jar is a cup containing a small quan- tity of gunpowder, and so arranged that the powder WHEN BURNT, RESOLVED INTO GAS. 219 can be exploded by a voltaic battery. As the oxygen- atoms required for the burning are lying in the cup side by side with the charcoal, we do not need the air in our experiment. Accordingly, we have connected the jar with an air-pump, so that we can exhaust the air. . . . The gauge of the pump now indicates that the greater part of the air has been removed. Notice further that, when we readmit a little air, the mercury column falls, and thus, as you see, this gauge will tell us when any gas enters the jar. . . . Having again completed the exhaustion, let us fire the powder. . . . The powder has disappeared ; but the gauge indicates that a large volume of gas has been formed. A simple test will now show that the aeriform prod- ucts in the two last experiments are identical. Here are two glasses, each filled with lime-water. To one we will add some of the gas from the first jar, pouring it in upon the lime-water, and to the other we will add some of the gas from the gunpowder, by pouring as before. On shaking the gas and liquid together, we obtain in both cases the familiar milky turbidness which indicates the presence of carbonic dioxide. It is true that the carbonic dioxide from the gunpowder is not quite so pure as that found in the other jar, but this is an unessential matter. Having seen that gunpowder, burnt in a vacuum, is quietly resolved into gas, we will next take an equal amount of powder and inclose it in a pasteboard case, which we call a cartridge, using the same arrangement for firing the powder as before. We make the connec- tion, and off it goes ! . . . There can be no occasion, I think, to seek far for the cause of the explosion. The chemical process must have been identical with that in our jar ; but, while in the jar there was room for all the 220 GUNPOWDER. gas-molecules formed in the burning, the small volume of the cartridge could not hold them, and they burst out, tearing away the paper walls in their course, The gas evolved would occupy, at the ordinary pressure of the air, about three hundred times the volume of the pow- der used, and, if confined in the space previously filled with the powder, would exert a pressure equal to about 300 x 14 = 4,200 lbs., or two tons, on a square-inch. The pressure obtained is really far greater than this, on ac- count of the heat developed by the combustion. More- over, as the powder burns rapidly, this pressure is sud- denly applied, and has all the effect of an immensely heavy blow, which no strength of materials is sufficient to withstand. Of course, any chamber in which the powder is confined gives way at the weakest point. In the chamber of a gun the ball usually yields before the breech, and is hurled with violence from the mouth of the piece ; but fearful accidents not unfrequently occur when, for any reason, the ball has been too tightly wedged, or when the metal of the breech is too weak. You all know that a large amount of gas condensed into a small chamber must exert great pressure, and therefore you will undoubtedly regard the explanation I have given of the force exerted by gunpowder as satisfactory and sufficient. But, although this is the usual way of presenting the phenomena, I am anxi- ous that you should view them in the light of our modern molecular theory, which gives to the imagi- nation a far more vivid picture of the manner in which the power acts. Begin with the black grains as they lie in the cham- ber of the gun behind the ball. You must remember that all the ingredients of the powder are in a solid condition, and picture to your imagination the mole- PROJECTILE FORCE EXPLAINED. 221 cules as held in their places by those forces which I attempted to make evident to you in a former lecture, incapable of any motion except a slight oscillation about the centres of force. The gun is now fired, and the powder burns. We need consider but two of the immediate consequences : first, there is a large volume of gas formed ; and, secondly, there is a very great amount of energy developed. Picture to yourselves, now, an immense number of gas -molecules suddenly set free in the chamber of the gun, and animated with all the velocity which great energy is capable of im- parting. See these molecules rushing against the ball with their whole might, and, when at last it starts, im- parting to the projectile their moving power, until it acquires the fearful velocity with which it rushes from the mouth of the gun. -The molecules impart their motion to the ball, just as one billiard-ball imparts mo- tion to another. The effect is due to the accumulation of small impulses ; for, although the power imparted by a single molecule may be as nothing, the accumu- lated effect of millions on millions of these impulses becomes immense. Within a few years our community have become familiar with the name and terrible effects of a new ex- plosive agent, called nitro-glycerine, and I feel sure that you will be glad to be made acquainted with the re- markable qualities and relations of this truly wonderful substance. Every one knows that clear, oily, and sweet- tasting liquid called glycerine, and probably most of you have eaten it for honey. But it has a great many valuable uses, which may reconcile you to its abuse for adulterating honey, and it is obtained in large quanti- ties as a secondary product of the manufacture of soap 222 NITRO-GLYCERINE. and candles from our common fats. Now, nitro-glycer- ine bears the same relation to glycerine that saltpetre bears to caustic potash. Common saltpetre, which is the oxygenated ingredient of gunpowder, is called in chemistry potassic nitrate, and, although the com- mercial supply comes wholly from natural sources, it can easily be made by the action of nitric acid on caustic potash. My assistant will pour some nitric acid into a solution of caustic potash, and you will soon see crystals of saltpetre appear, shooting out from the sides of the dish, whose image we have projected on the screen. In a similar way we can prepare nitro-glyce- rine by pouring glycerine in a fine stream into very strong nitric acid, rendered more active by being mixed with sulphuric acid — oil of vitriol. We could easily make the experiment, but you could see nothing. There is no apparent change, and it is a remarkable fact that, when pure, nitro-glycerine re- sembles, externally, very closely glycerine itself, and, like it, is a colorless, oily fluid — the reddish-yellow color of the commercial article being due to impurities. As soon as the chemical change is ended, the nitro-glycer- ine must be very carefully washed with water, until all adhering acid has been removed. The material thus obtained has most singular qualities, and not the least unexpected of these is its stability under ordinary con- ditions. After the terrible accidents that have hap- pened, it would, perhaps, be rash to say that it did not readily explode ; but I can assure you that it is not an easy matter to explode pure nitro-glycerine. It is not nearly so explosive as gunpowder, and I am told that the flame of an ordinary match can be quenched in it without danger, although I confess that I should be un- willing to try the experiment. Still, there can be no EXPERIMENT AT THE TORPEDO STATION. 223 doubt that, under ordinary circumstances, a small flame will not ignite it. My knowledge of the matter is de- rived from Professor Hill, of the Torpedo Station at Newport, who has studied very carefully the preparation and application of the material. He is of opinion that most of the accidents which have given to nitro-glycer- ine such an unfortunate notoriety have been caused by the use of an impure article, and that proper care in its preparation would greatly lessen the danger attending its use. JSTitro-glycerine is usually exploded, not by the direct application of heat, but by a sudden and vio- lent concussion, which is obtained by firing in contact with it a fuse of 6ome fulminating powder. The ef- fects of this explosion are as peculiar as the method by which it is obtained, and I can best illustrate the sub- ject by describing an experiment with nitro-glyeerine which I witnessed myself at the Torpedo Station a few months since. It is so inconvenient to handle liquid nitro-glycerine that it is now usual to mix it with some inert and im- palpable powder, and the names dualine and dynamite have been given to different mixtures of this kind ; but in both of these the powder merely acts as a sponge. In the experiment referred to, a canister holding less than a pound of dynamite, and only a few ounces of nitro-glycerine, was placed on the top of a large bowl- der-rock, weighing two or three tons. In order that you may fully appreciate the conditions, I repeat that this tin case was simply laid on the top of the bowlder, and not confined in any way. The nitro-glycerine was then exploded by an appropriate fuse fired from a dis- tance by electricity. The report was not louder than from a heavy gun, but the rock on which the canister lay was broken into a thousand fragments. 224 NITRO-GLYCERINE. This experiment strikingly illustrates the peculiar action of nitro - glycerine. In using gunpowder for blasting it is necessary to confine it, by what is called tamping, in the hole prepared for it in the rock. Not so with nitro-glycerine. This, though it may be put up in small tin cartridges for convenience, is placed in the drill-holes without tamping of any kind. Some- times the liquid itself has been poured into the hole, and then a little water poured on the top is the only means used to confine it. As an agent for blasting, nitro-glycerine is so vastly superior to gunpowder that it must be regarded as one of the most valuable dis- coveries of our age. Already it is enabling men to open tracks for their iron roads through mountain- barriers which, a few years ago, it would have been thought impracticable to pierce, and, although its intro- duction has been attended with such terrible accidents, those best acquainted with the material believe that, with proper care in its manufacture, and proper precau- tions in its use, it can be made as safe as or even safer than gunpowder, and the Government can do no bet- ter service toward developing the resources of the coun- try than by carrying forward the experiments it has instituted at the Torpedo Station at Newport, until all the conditions required for the safe manufacture and use of this valuable agent are known, and, when this result is reached, imposing on the manufacturers, deal- ers, and carriers, such restrictions as the public safety requires. Of course, we cannot expect, thus, to prevent all accidents. Great power in the hands of ignorant'* or careless men implies great danger. Sleepless vigi- lance is the condition under which we wield all the great powers of modern civilization, and we cannot RENDING POWER. 225 expect that the power of nitro-glycerine will be aiiy ex- ception to the general rule. 1 But, while nitro-glycerine has such great rending power, it lias no value whatever as a projectile agent. Exploded in the chamber of a gun, it would burst the breech before it started the ball. Indeed, there is a great popular misapprehension in regard to the limit of the projectile power of gunpowder, and inventors are constantly looking for more powerful projectile agents as the means of obtaining increased effects. But a study of the mechanical conditions of projec- tion will show not only that gunpowder is most admi- rably adapted to this use, but also that its capabilities far exceed the strength of any known material, and the student will soon be convinced that what is wanted is not stronger powder, but stronger guns. I do not mean- to say that we cannot conceive of a better pow- der than that now in use, but merely that its short- coming is not want of strength. Having described the properties of nitro-glycerine, the question at once arises, " Can these singular proper- tics be explained ? " In order to answer this question I shall next ask your attention to the theory of its ac- tion, and I think you will find that our modern chem- istry is able to give a very intelligible account of the phenomena we have described. I will begin by saying that the chemical action in the explosion of nitro-gly- cerine is very similar to that in the burning of gun- powder. In both cases we have the same two results : 1. The production of a large volume of gas; 2. The 1 The recent improvements in the manufacture of gun cotton, and the discovery that, even when too wet to burn, it can be exploded by con- cussion if the fuse is sufficiently powerful, promise to furnish an explo- sive agent nearly equal to nitro-glycerine in strength, and free from all ordinary risks. 226 NITRO-GLYCERINE. liberation of a large amount of energy which gives to the confined gas-molecules an immense moving power. Moreover, essentially the same aeriform products are formed in the two cases, and in both the process con- sists, for the most part, in the union of carbon and hydrogen atoms with oxygen. But, while in the gun- powder the carbon and oxygen atoms are ia different molecules, although lying side by side in the same grains, in the nitro glycerine they are in different parts of the same molecule. And here comes our first glimpse of the most recondite chemical principle the science has yet attained, one which I have been aiming to reach throughout this whole course of lectures, and one which it will be my object in the three remaining lectures clearly to set before you. I can, as yet, only state the principle as a theorem to be proved ; but, if I can succeed in making this difficult subject clear, I feel confident that you will regard the proof as satisfactory. The principle is this : Every molecule has a definite structure. It not only consists of a definite kind and a definite number of atoms, but these atoms are arranged or grouped together in a definite order, and it is the great object of modern chemistry to discover what that grouping is. Almost all the great chemists of the world are, at this moment, engaged in investigating this very prob- lem, and, what is more, they have succeeded, in many cases, in solving it. and we have reached as much cer- tainty in regard to the grouping of the atoms in the molecules of a very large number of substances, as we have in regard to any phenomena so wholly super-sen- sible. For example, we feel well assured that we know how the atoms are grouped in the molecule of nitro- glycerine, and the diagram before you represents in MOLECULAR STRUCTURE. 227 H O H-0-O-N=O O H H H O H-6-0-NC9 N-O-O-6-C-O-N i u « ill « H-C-O-N-O H O H O H O 0=N=0 Order of Atoms in the Molecule Same Order, but different Form of of Nitro-glycerine. Symbol. our rude way the result we have reached. The let- ters signify single atoms, and the lines between the letters merely show how the atoms are severally united. Begin with the three atoms of carbon, which are united together, say, by a certain force, which the lines denote. To these are directly united five atoms of hydrogen, and then to each of the carbon-atoms is also bound the atomic group — 0-N(q, the four atoms of the group having a definite arrangement among themselves. There is no virtue in the mere form of the arrangement of the letters on the diagram. It is perfectly possible that the atoms may be arranged so as to form regular geometrical figures, such as some theorists have amused themselves in constructing ; but we do not pretend to have any accurate knowledge on this point. All we affirm is, that the atoms are united, one with another, in the order I have indicated, and the second diagram, in which the several atoms are united as before, although the form of the arrangement is different, means, to the chemist, precisely the same thing as the first. Now, as I said, I present to you this diagram of the constitution of a molecule of nitro-glycerine simply as a theorem to be proved. As it hangs before you, I have no doubt that it will shake your faith in the credi- bility of the scientific investigators who bring forward 228 NITRO-GLYCERINE. this as the sober conclusion at which they have ar- rived. Indeed, when I first saw these attempts to represent the grouping of atoms, they appeared to me to be the vagaries of a diseased scientific imagination ; for, remember, this molecule, whose structure is here portrayed, cannot be larger than the •rF.Tnnr.ToT °f an inch. Bat, as the evidence pressed upon me, I re- luctantly examined it. Finding that it could not be gainsaid, I was forced to accept the conclusion, and soon I found myself busy at the same work. Now, I only ask you to accept this diagram as a theorem to be proved, and, assuming it for the time to represent, although very rudely, a real truth, see how fully it ex- plains the properties of nitro-glycerine. Indeed, the facts already before us furnish the strongest evidence possible of the general truth of the principle I have asked you to assume ; for, if you accept the principles I have previously endeavored to establish, and once ad- mit that there are such things as molecules and atoms, the properties of nitro-glycerine will force you to admit that its molecules have a definite structure. See how the case stands. Nitro-glycerine has been analyzed, and, unless the principles of our modern chemistry are all wrong, its molecules have the composition indicated by the sym- bol CgHsNA,. Note that there are already in the mole- cule nine atoms of oxygen, more than enough to satisfy all the atoms, both of carbon and of hydrogen. When carbon burns, C 3 only takes 6 , H 5 only 2 ,, and why is not the afiinity of these atoms for oxygen satisfied al- ready ? The only answer that can be suggested is, be- cause the oxygen-atoms, although parts of the same molecule, are not in combination with the carbon or hydrogen atoms in those molecules ; and what is this HOW IT EXPLODES. 229 but an admission that the molecules have a definite structure by which these atoms are kept apart i In the next place, admitting that the structure is that represented above, you see how the atoms are kept apart. Three of the oxygen - atoms form the links, as it were, between the carbon and nitrogen atoms, and the rest of the oxygen-atoms are united with the nitrogen-atoms, and not with those of either carbon or hydrogen. Now, when the substance ex- plodes, what takes place is simply this : The oxygen- atoms at one end of the molecule rush for the atoms of carbon and hydrogen at the other end, and the molecule is broken up, as our next diagram indicates ; only, as there are not enough atoms to form even mole- II H-6-0-N-0 H-O-H 0=0=0 h-c-o-n(q h-o-h o=c=o H=sr H-0-0-N=0 H-O-H 0=0=0 KeN I !1 Water. Carbonic Nitrogen H O Dioxide. Gas. Nitro-glycerine. cules, we must consider that one atom of hydrogen and one of nitrogen are borrowed from the fragments of a neighboring molecule, broken up at the same time. You see, therefore, that the chemical action is very nearly the same as in the burning of gunpowder, the difference being that, while in the powder the car- bon and oxygen atoms belong to different molecules, in nitro-glycerine they belong to the same molecule. In both cases the carbon burns, but in the nitro-glycer- ine the combustion is within the molecule. This differ- ence, however, which the theory indicates, is one of great importance, and shows itself in the effects of the explosion. 230 NITRO-GJiYCERINE. In gunpowder the grains of charcoal and nitre, although very small, have a sensible magnitude, and consist each of many thousand if not of many million molecules. The chemical union of the oxygen of the nitre with the carbon-atoms of the charcoal can take place only on the surface of charcoal-grains ; the first layer of molecules must be consumed before the second can be reached, and so on. Hence the process, although very rapid, must take a sensible time. In the nitro- glycerine, on the other hand, the two sets of atoms, so far from being in different grains, are in one and the same molecule, and the internal combustion is essen- tially instantaneous. Now, this element of time will explain a great part of the difference in the effect of the two explosions, but a part is also due to the fact that nitro glycerine yields fully nine hundred times its volume of gas, while with gunpowder the volume is only about three hundred times that of the solid grains. There is a further difference in favor of the nitro-gly- cerine in the amount of energy liberated, but this we will leave out of account, although it is worthy of notice that energy may be developed by internal mo- lecular combustion as well as in the ordinary processes of burning. The conditions, then, are these : With gunpowder we have a volume of gas, which would normally occupy a space three hundred times as great as the grains used, liberated rapidly, but still in a perceptible inter- val. With nitro-glycerine a volume of gas, nine hun- dred times that of the liquid used, is set free, all but instantaneously. Now, in order to appreciate the difference of effect which would follow this difference of condition, you must remember that all our experi- ments are made in air, and that this air presses with an EXPLANATION OF THE EFFECTS. 231 enormous weight on every surface. If a volume of gas is suddenly liberated, it must lift this whole weight, which, therefore, acts as so much tamping material. This weight, moreover, cannot be lifted without the expenditure of a large amount of work. Let us make a rough estimate of the amount in the case of nitro- glycerine. We will assume that in the experiment at Newport the quantity exploded yielded a cubic yard of gas. Had the air given way, instead of the rock, the liberation of this volume of gas must have lifted the pressure on one square yard (about nine tons) one yard high, an amount of work which, using these large units, we will call nine yard-tons or about 60,000 foot-pounds. Moreover, this work must have been done during the excessively brief duration of the explo- sion, and, it being less work to split the rock, it was the rock that yielded, and not the atmosphere. Com- pare, now, the case of gunpowder. The same weight of powder would yield only about one-third of the volume of gas, and would, therefore, raise the same weight to only one-third of the height ; doing, therefore, but one- third of the amount of work, say 20,000 foot-pounds. Moreover, the duration of the explosion being at least one hundred times longer than before, the work to be done in lifting the atmosphere during the same ex- ceedingly short interval would be only ^ of 20,000 foot-pounds, or 200 foot-pounds, and, under these cir- cumstances, you can conceive that it might be easier to lift the air than to break the rock. If there are some who have not followed me through this simple calculation, they may, perhaps, be able to reach clear views upon the subject by looking at the phenomena in a somewhat different way. It can readi- ly be seen that the sudden development of this large 232 NITEO-GLTCEKINE. volume of gas, ■which becomes at once a part of the at- mosphere, would be equivalent to a blow by the atmos- phere against the rock ; or, what would be a more ac- curate representation of the phenomenon, since the air is the larger mass, and acts as the anvil, a blow by the rock against the air. It may seem very singular that our atmosphere can act as an anvil, against which a rock can be split, and yet it is so, and, if the blow has velocity enough, the atmosphere presents as effective a resistance as would a granite ledge. The following consideration will, I think, convince you that this is the case : I have here a light wooden surface, say, one yard square ; the pressure of the air against the surface is equal, as I just stated, to about nine tons ; but the air presses equally on both sides, and the molecules have such great mobility that, when we move the sur- face slowly, they readily give way, and we encounter but little resistance. If, however, we push it rapidly forward, the resistance greatly increases, for the air- molecules must have time to change their position, and we encounter them in their passage. If, now, we in- crease the velocity of the motion to the highest speed ever attained by a locomotive — say, one and one-fifth mile per minute — we should encounter still more par- ticles, and find a resistance which no human muscle could overcome. Increase that velocity ten times, to twelve miles a minute, the velocity of sound, and the air would oppose such a resistance that our wooden board would be shivered into splinters. Multiply again the velocity ten times, and not even a plate of boiler- iron could withstand the resistance. Multiply the ve- locity once more by ten, and we should reach the ve- locity of the earth in its orbit, about 1,200 miles a minute, and, to a body moving with this velocity, the HISTORY OF THE THEORY OF COMBUSTION. 233 comparatively dense air at the surface of the earth would present an almost impenetrable barrier, against which the firmest rocks might be broken to fragments. Indeed, this effect has been several times seen, when meteoric masses, moving with these planetary velocities, penetrate our atmosphere. The explosions which have been witnessed are simply the effect of the concussion against the aeriform anvil at a point where the atmos- phere is far less dense than it is here. So, in the case of the nitro-glycerine, the rock strikes the atmosphere with such a velocity that it has the effect of a solid mass, and the rock is shivered by the blow. In concluding my illustrations of the theory of com- bustion, a few words in regard to its history will not be out of place. We owe this theory to the great French chemist Lavoisier, who was murdered by the French communists during the reign of terror which accompanied the first French Revolution. The theory came almost perfect from his hands, and caused a revo- lution in the science of chemistry. Some would even date the beginning of scientific chemistry at this epoch. In this connection, there is a recent incident which' amusingly illustrates, not only the importance of the- ory, but also the not unfrequent contrast between the theorizing and investigating mind — the scientific poet and the scientific philosopher. About three years since, Professor Wurtz, of Paris, to whom modern chemistry owes as much as to any individual man, an Alsatian by birth, a German by descent and in many of his traits of mind, but a genuine Frenchman in sympathy and spirit, published a work on the history of modern chemical the- ories, which opens with this amusing and characteristic French panegyric : " Chemistry is a French science. It was founded by Lavoisier, of immortal memory ; '" 234 LAVOISIER AND SCHEELE. and the author goes on to make good his claim that a large part of the great generalizations in the science have been made by Frenchmen. This unmeasured as- sumption, coming, as it did, on the eve of the Franco- Prussian War, was the occasion of no little bitterness, and was answered in very much the same spirit in which it was uttered. The old controversies were re- vived, and the old arguments repeated, proving, -what was undoubtedly true, that Lavoisier did not add a new fact of prime importance to chemistry. But he did add one of the grandest generalizations. Lavoisier was not a chemical investigator in our modern sense, but he had, to a very high degree, that quality of mind which the Frenchmen call clarte, and the great good fortune — if you please so to style the opportunities of genius — to advance his theory of combustion at the time the discoveries of Priestley, Cavendish, Black, and Scheele, had prepared the world to receive it. His contemporary, Scheele, a poor apothecary in an out-of- the-way village of Sweden, had done more than any one else to supply the facts which made the theory credible ; but, not only did he not see clearly the bear- ing of his facts, but he had not the vantage-ground which would have enabled him to impress his ideas on his age ; and, although, with his extremely restricted means, he added more knowledge to the stock of chemical science in a single year than did Lavoisier in his lifetime, yet it was Lavoisier, and not Scheele, who made the great generalization which revolutionized chemistry. It is unnecessary to add that this Franco-German controversy was as irrational as it was useless. Both Lavoisier and Scheele filled well the place to which they were called, and did faithfully the work which Provi- BECHER AND STAHL. 235 dence assigned them in the development of chemical science, and it is sheer presumption in any man to say that one was more important or more honorable than the other. It is true that chemistry, as a science of exact quan- titative relations, begins with the introduction of the balance into the science, and that Lavoisier was one of the first to recognize the importance of this instrument for investigating chemical problems. But, from the beginning of the seventeenth century, chemistry as a science of qualitative relations was actively studied at all the great centres of learning in Europe, and was illustrated by some of the most learned men of the age. For over a century previous to the time of Lavoisier, who died in 1794, the doctrines of the science centred around a theory of combustion which is known in his- tory as the phlogiston theory. This theory was first ad^ vanced in 1682, by Becher, a German chemist then liv- ing in England, and was worked out into a complete system some years later by Stahl. According to this the- ory, the principle of fire is everywhere diffused through- out Nature, but enters into the composition of different bodies to a very unequal extent. Combustible sub- stances are bodies very rich in phlogiston, and burning consists in the escape of phlogiston into the atmosphere. I have already referred to this theory, and shown that it was in variance with the great principle of the law of gravitation, that quantity of matter is proportional to weight. Still, as I said before, this principle of Newton made its way into chemistry very slowly, and the theory of Stahl was in complete accordance with the philosophy of Aristotle, which, at the time, held an entire supremacy over the intellectual world. And was the theory wholly false ? I believe not ; and I am 236 PHLOGISTON AND ENERGY. persuaded that every theory, which gains among think- ing men such universal acceptance as did this theory of Stahl, has its element of truth. The men of the seventeenth century were not less acute thinkers than ourselves, and we must be careful not to judge of their ideas from our stand-point. The authoss of the theory never attached to phlogiston the idea of weight which we necessarily associate with all matter. It was to them a principle, an undefined essence, and not matter in the sense we understand it. Vague and indefinite idea, no doubt, like many of the metaphysical ideas of the time, but not absurd. And that it was not absurd a single consideration will show. Translate the word phlogiston energy, and in Stahl's work on chemistry and physics, of 1731, put energy where he wrote phlo- giston, and you will find there the germs of our great modern doctrine of conservation of energy — one of the noblest products of human thought. It was not a mere fanciful speculation which ruled the scientific thought of Europe for a century and a half. It was a really grand generalization ; but the generalization was given to the world clothed in such a material garb that it has required two centuries to unwrap the truth. Still, the sparkle of the gem was there, and men fol- lowed it until it led them into a clearer day. It is a great error to suppose that the theory of Lavoisier su- perseded that of Stahl. It merely added to it. Stahl clearly saw that the chief characteristic of burning was the development of energy, and, although he called energy phlogiston, and did not comprehend its real essence, he recognized that it was a fundamental prin- ciple of Nature. He did not understand the chemical change which takes place in the process, and this La- voisier discovered. But, both Lavoisier and his fol- A NEW ASPECT OF AN OLD THEORY. 237 lowers, to a great extent, ignored the more important phenomenon in magnifying the less, and it is only within a few years that the true relations of the two have been understood. All honor to these great pio- neers of science, and let their experience teach us that, in science, as in religion, we see as through a glass darkly, and that we must not attach too much impor- tance to the forms of thought, which, like all things human, are subject to limitations, and liable to change. LEOTUEE XI. QtrANTTVALENCE AND METATHESIS ALKALIES AND ACIDS. Before studying metathesis, the third, as you will remember, of the three classes into which we divided chemical reactions, I must ask your attention, at the be- ginning of my lecture this evening, to a most important general principle, to which a study of the results of analysis and synthesis has led, and which will greatly help to elucidate the metathetical processes we have yet to investigate. HOI H 2 H 3 1T H 4 NaCl Hg01 2 SbOIs COl, PCle CrFe H 3 HgO NOC1 C0 2 COCl 2 OOH„ POOU CrOs Circa, The diagram on the curtain before us illustrates the truth we have to present. The story, indeed, is here told in our chemical hieroglyphics, but let us try to de- cipher them. In attacking our work, let us not fail to remember that these symbols really exhibit the con- stitution of the molecules of the definite substances THE BASIS OF FACT. 239 they represent. The symbol H 2 0, for example, shows that a molecule of water consists of two atoms of hydrogen and one of oxygen. Remember that this symbol is not the expression of a mere hypothesis, but represents the results of actual experiment. In a former lecture we have dwelt at length on the evi- dence on which it is based. We cannot continually retrace our steps ; but be sure that you recall this evi- dence, so that we may plant the ladder, on which we shall attempt to climb higher, on firm ground. ISTow, what is true of the symbol of water, is true of all the symbols on this diagram. There is not one of them in regard to which there is a shade of doubt. Our atoms may be mere fancies, I admit, but, like the mag- nitudes we call waves of light, the magnitudes we have measured and called atoms must be magnitudes of something, however greatly our conceptions in regard to that something may change. Our whole atomic theory may pass, the words molecule and atom may be forgotten ; but it will never cease to be true that the magnitude which we now call a molecule of water con- sists of two of the magnitudes which, in the year 1872, were called atoms of hydrogen, and of one of the mag- nitudes which were called, at the same period, atoms of oxygen. Look, now, at the first line of symbols, and see in what a remarkable relation the atoms there repre- sented stand to each other. In a molecule of hydro- chloric-acid gas (HC1), one atom of chlorine is united to one atom of hydrogen. In the molecule of water (H 2 0) one atom of oxygen is united to two of hydrogen. In the molecule of ammonia gas (NH g ) one atom of nitro- gen is united to three atoms of hydrogen, and in the molecule of marsh gas (CH 4 ) the atom of carbon is 240 QUANTIVALENCE ANT) METATHESIS. united to four atoms of hydrogen. It would appear, then, that the atoms of chlorine, oxygen, nitrogen, and carbon, have different powers of combination, uniting respectively with one, two, three, and four atoms of hydrogen. In order to assure yourselves that this rela- tion is not an illusion, depending on the collocation of selected symbols, but results from a definite quality of the several atoms, examine the symbols of the second line, and you will see that, in a similar way, the atoms of sodium (Na), mercury (Hg), antimony (Sb), carbon (C), and phosphorus (P), unite respectively with one, two, three, four, and five atoms of chlorine. Moreover, on comparing the two lines, notice that the atom of chlo- rine, which combines with one atom of hydrogen, com- bines also with one atom of sodium. Again notice that the atom of carbon, which combines with four atoms of hydrogen, combines also with four atoms of chlorine. Further, observe on the third line that the atom of mercury, which combines with two atoms of chlorine, combines with only one of oxygen ; and that the atom of carbon, which combines with either four atoms of chlorine or four atoms of hydrogen, combines with two atoms of oxygen ; and compare with these facts those first noticed, that the atom of oxygen com- bines with two atoms of hydrogen, and the atom of chlorine with but one. Eelations so far reaching and so intricate as these cannot be accidental ; and when you are told that the examples here given have been selected, on account of their simplicity, from a countless number of instances in which similar relations have been observed, you will not be satisfied until you find some explanation of the cause of these facts. The explanation which our modern chemistry gives ATOMIC BONDS. 241 is this : It is assumed that each of the elementary atoms has a certain definite number of bonds, and that by these alone it can be united to other atoms. If you wish to clothe this abstract idea in a material conception, picture these bonds as so many hooks, or, what is probably nearer the truth, regard them as poles like those of a magnet. If we have grasped this idea, let us turn back to our dia- gram and we shall find that the relations we had but dimly seen have become clear and intelligible. The hydrogen, sodium and chlorine atoms have only one bond or pole, and hence, in combining with each other, they can only unite in pairs. The oxygen-atom, has two bonds or poles, and can combine, therefore, with two hydrogen-atoms, one at each pole. The mercury-atom has also two bonds, and takes, in a similar manner, two atoms of chlorine ; but it can only combine with a sin- gle atom of oxygen, for the two poles of one just satisfy the two poles of the other. Again, the atom of car- bon has four bonds, which may be satisfied by either four atoms of hydrogen, or four atoms of chlorine, or two atoms of oxygen, or one atom of oxygen and two of chlorine, or, lastly, one atom of oxygen and two of hydrogen. Further, the atom of phosphorus has five bonds, and holds five atoms of chlorine, or three atoms of chlorine and one of oxygen. Finally, the chromium atom binds six atoms of fluorine, or three of oxygen, or two of oxygen and two of chlorine. This quality of the atoms, which we endeavor to represent to our minds by the conception of hooks, bonds, or poles, we call, in our modern chemistry, quantivalence, and we use the Latin terms univalent, bivalent, trivalent, quadrivalent, quin- quivalent, sexivalent, etc., to designate the atoms which have one, two, three, four, five, six, etc., hooks, bonds, or poles, respectively. 242 QUANTIVALENCE AND METATHESIS. H- -0- -N- -0- i Cl- -S- i -P- i -Si- i F- -Ca- -Sb- i -Sn- i K- -Mg- i -As- i -Ti- i Na- -Hg- i -B- i -Pt- i Ag- -Zn- i -Au- i -Zr- i In the above diagram we have classified a few only of the more important elementary atoms according to their quantivalence, and the diagram also shows how, by a slight addition to our symbolical notation, we can indicate the number of bonds in each case. In writing symbols of molecules, a dash between two letters indi- cates the union of two bonds, and one bond or pole on each atom is then said to be closed. Two dashes indi- cate that two bonds on each atom are closed — and so with a larger number. The next diagram is in part a repetition of that on page 232, with the exception that the bonds are indicated. H-H H-Cl Hg=0 C1-N=0 0-C = You notice that this idea of quantivalence suggests, or, rather, as I should say, implies the idea that the molecules have a definite structure. Thus in the mole- H H H-O-H H-N-H H-C-H i H CI 01 Cl-Hg-Cl Cl-Sb-Cl Cl-C-Cl i CI QUANTTVALENCE IMPLIES STRUCTURE. 243 cule CH 4 we conceive that the carbon-atom is united at four distinct points with the four hydrogen-atoms. There is not an indiscriminate grouping of the five atoms, hut a definite arrangement with the carbon - atom at the centre of the system. So, also, in CC1 4 , which has the same structure, as CUt, determined, as before, by the quadrivalence of the nucleus. Passing next to C0 2 we find an equally definite structure, the four bonds of the same nucleus being satisfied by two bivalent atoms of oxygen ; and intermediate in struct- ure, between the two molecules last mentioned, we have the molecule of phosgene gas, COCl 2 , and the molecule of formic aldehyde, COH 2 . The symbols of these molecules indicate an obvious limitation to this idea of structure, which must not be overlooked, and which cannot too early be called to your notice. All that we, as yet, feel justified in infer- ring from the phenomena we have described, are simply the facts that in the molecule CC1 4 , for example, the four chlorine-atoms are united to the carbon-nucleus by four different bonds, and that in the molecule C0 2 the two oxygen-atoms are united to the same nucleus, each by two bonds. Further than this we assert noth- ing. It may hereafter appear that the different bonds of the carbon-atom have different values ; or, perhaps, have a fixed position, and that there are distinctions of right and left, top and bottom, or the like ; but, until we are acquainted with phenomena which require assump- tions of this sort, we may group our symbols around the nucleus of the molecule as we find most convenient, provided only we satisfy the condition of quantivalence. Thus it is unimportant whether we write CI Ol-Hg-Cl, or Hg(°'; Ol-O-Ol, or 0-0 O 01. 244 QUANTIVALENCE AND METATHESIS. The quantivalence of the atoms, moreover, is by no means an invariable quality ; but this circumstance does not in the least obscure the general principle we have been discussing : because, in the first place, any change in the quantivalence of an atom is accompanied with a change in all its chemical relations ; and, in the second place, the change is circumscribed by definite limits, which are easily defined. This point will be best illustrated by a few examples. When in a previous lecture, as an example of a synthetical process, we united ammonia gas with hydro- chloric acid, there was a change in the quantivalence of the nitrogen-atom, from three to five, as will be seen on comparing the symbol of the first factor with the sole product of the reaction : H H H-N H )N-C1 i H x i H H Ammonia Gas. Amnionic Chloride. Now, from ammonia gas can be derived a large class of compounds, in all of which nitrogen is trivalent ; and, in like manner, from amnionic chloride can be derived another class of compounds, in which nitrogen is quin- quivalent ; but, although they all contain the same atom as a nucleus, the two classes differ from each other as widely as if they were composed of different elements. A similar fact is true of phosphorus, which forms two well-marked chlorides : ^ CI CI i CI i Cl-P )P-C1 1 CI i CI w CI Phosphorous Chloride. Phosphoric Chloride. One of the most striking instances of the variation of quantivalence is to be found in the atom of man- VARIATIONS OF QUANTIVALENCE. 245 ganese. This elementary substance forms no less than four compounds with fluorine, whose molecules have probably the constitution represented by the symbols given below : F F F i i i F-Mn-F F-Mn-F F-Mn-Mn-F i i i F F F . F F F-Mn-F F F In the first, the manganese-atom is bivalent ; in the second and third it is quadrivalent; and in the last, sexivalent. The third molecule, it will be noticed, contains two quadrivalent atoms of manganese, united by a single bond, and the two together form a complex nucleus, which is sexivalent. Here, as in the previous examples, it is true that there is a distinct class of com- pounds corresponding to each of the four conditions of the nucleus, and that the difference between the chem- ical relations of the bivalent and those of the sexiva- lent atom of manganese is almost as great as that be- tween the atom of zinc and the atom of sulphur. The compounds of iron furnish a more familiar ex- ample of the effect produced by a variation of quantiva- lence, than either of those which have been adduced. There are two classes of these compounds, which are distinguished in chemistry as the ferrous and fche fer- ric compounds. The first class consists of molecules, of which the nucleus is a bivalent atom of iron, while the molecules of the second class are grouped around a nucleus, consisting of two quadrivalent atoms united as explained above. Thus the symbols of ferrous and ferric chloride are : 246 QUANTIVALENCE AND METATHESIS. 01 CI I I Fe01 2 or Cl-Fe-Cl, and Fe a Cl 8 or Cl-Fe-Fe-01. CI CI. Now, I have before me four glasses, which, contain solutions in water of FeCls, Fe 2 Cl«, CuCl 3 NiCl s , Ferrous Chloride, Ferric Chloride, Cupric Chloride, and Nickel Chloride ; and I will add to each glass a portion of a solution of a yellow salt, which is well known in commerce, under the name of yellow prussiate of potash, and in chemis- try as potassic ferrocyanide. Notice, in the first place, what a different effect the reagent produces on the last two solutions. From the solution of cupric chloride, we obtain a red precipitate, and, from the solution of nickel chloride, a white precipitate. Next, we will add the same reagent to the solutions of the two com- pounds of iron, and, as you see, the difference of effect produced is even greater than before. Moreover, if, going behind the outward manifestations, you study the constitution of the products formed, you will find that the variations of color correspond to more funda- mental differences in the case of the two conditions of iron than in that of the two separate elements, cop- per and nickel. The result, then, at which we arrive, is this, that, although a fixed quantivalence is not an invariable of quality of every atom, it is at least an in- variable quality of each condition of every given atom, and thait, in every marked class of compounds of any elementary substance, the atoms of that element always have the same quantivalence. Lastly, as to the limits to which this variation of quantivalence may extend. There are several of the chemical elements, and these among the most impor- LAW OF THE VARIATION. 247 tant and most widely distributed, whose quantivalence appears to be invariable. This is especially true of hydrogen, it is likewise true of the alkaline metals, lith- ium, sodium, potassium, caesium, and rubidium, and it is also true of silver, all elements whose atoms are univa- lent. It is further true of the trivalent element boron. Again, oxygen is always bivalent, and so are also* the metallic radicals of the alkaline earths, calcium, barium, strontium, and magnesium, and so are, moreover, the well-known metallic elements, lead, zinc, and cadmiumi Lastly, aluminum, titanium, silicon, and carbon, are al- ways quadrivalent, although, in the single instance of the molecule, CO, the carbon-atom appears to be bivalent. But, in addition to the fact that the variations in quantivalence are confined to a limited number of the elementary atoms, these variations appear to follow a remarkable law, which is thought to point to an ex- planation of their cause. As is shown in this diagram, the successive degrees of quantivalence in gold and phosphorus follow the order of the odd number : AuCl An01 3 PCI, P01 6 while those of manganese follow the order of the even numbers : MnF, MnF 4 MnF s Now, what is true of these atoms is, in general, true of the atoms of all those elements which have several degrees of quantivalence : at each successive step the quantivalence increases by two bonds, and never by a single bond. The explanation of the fact is thought to be that the bonds of any atom, when not in use to hold other atoms, are satisfied by each other, and that, so far as these unused bonds are concerned, the atom is in 248 QUANTIVALENCE AND METATHESIS. the condition of a horseshoe magnet, with its north pole directed toward and neutralized by its south pole. Thus it is assumed that, in both of the two compounds of car- bon and oxygen, the carbon atom is quadrivalent, the only difference being that, while in CO.,, all four bonds are employed to hold the two atoms of oxygen, in CO only two are so used, the other two neutralizing each other thus : (M>0 cC=0. Of course, then, if the unused bonds are in all cases neutralized in this way, it must be that the quantiva- lence of an atom will fall off from the highest degree of which it it susceptible, by two bonds at each step ; so that, if the highest degree is odd, all must be odd, and, if the highest is even, all must be even, as in the illustrations given above. Atoms with odd degrees of quantivalence have been called perissads, and those with even degrees have been called artiads, and the classifi- cation appears to be a fundamental one ; but there are important exceptions to the general principle, which have never yet been reconciled with the theory. The doctrine of quantivalence, which we have en- deavored to illustrate in this lecture, is one of the dis- tinctive features in which the new chemistry differs from the old, and the recognition of the fact that a defi- nite quantivalence is an inherent quality of each ele- mentary atom was one of the chief causes of the revo- lution in the science which has recently taken place. In the old chemistry, the question of how the element- ary substances were united in a compound was hardly raised, much less answered ; but now the manner in which the atoms are grouped together in the molecule has become an all-important question. Every mole- cule is a unit in which all the atoms are joined to- ATOMIC CLAMPS. 249 gether by their several bonds, and it becomes an object of investigation to determine the exact manner in which the molecular structure is built up. Moreover, it ap- pears that the qualities and chemical relations of a com- pound are determined fully as much by the structure of its molecules as by the nature of the atoms of which the molecules consist. For example, it was formerly supposed that the qualities of au alkali or an acid were simply the characteristics of the compounds of certain elements with oxygen, but it now appears that they are the result of a definite molecular structure, and are only slightly modified by the characteristics of the in- dividual atoms which may chance to be the nucleus of the molecule. We are thus fairly brought face to face with the question of molecular structure that is to occupy our attention during the remainder of this course of lect- ures. In regard to this question, there are a few pre- liminary points which need barely be mentioned, as they can easily be apprehended, and require, therefore, no extended illustration. It is evident that with univa- lent atoms solely we can only form molecules con- sisting of two atoms, like Na-Cl, or H-Br. "When we introduce bivalent atoms the structure becomes more complexes in H-O-H or K-O-Cl. With several biva- lent atoms we can form molecules in which the atoms seem to be strung together in a chain, sometimes of great extent, as — n- ^7n a 7? _H ' or H -0-Pb-0-Pb-0-Pb-0-H. Calcic Hydrate. Triplnmbic Hydrate. And, with atoms of higher quantivalence, we obtain groups of very great complexity, of which the multiva- lent atom ' is the nucleus, and serves to bind together 1 The atom with a high degree of quantivalence. 250 QUANTIVALENCE AND METATHESIS. the parts of the molecule. The molecule of calcic sul- phate, for example, is supposed to have the complex con- • Sulphate of Lime. Sulphate of Itod. Nitrate of Zinc. As expounded and illustrated by Berzelius, the dualistic theory had the charm of great simplicity, and was greatly strengthened by the electro-chemical facts which he brought forward in its support. The division of the elementary substances into electro-positive and electro-negative elements corresponded very closely to 1 To avoid confusion, all our symbols stand for the new atomic weights, and this must be remembered in comparing these formulae with those in the old books. 294 ELECTRO-CHEMICAL THEORY. the distinction between metals and metalloids. Bases were compounds of electro-positive elements with oxy- gen ; and acids, on the other hand, the oxides of electro- negative elements. Again, among these binary com- pounds the basic oxides were electro-positive, and the acid oxides electro - negative. Moreover, the wider apart in their electrical relations, the stronger was seen to be the tendency of both the elements and of their oxides to combine, and, just as the metals united to metalloids, so bases united with acids. Thus was formed the class of ternary compounds, called, as above, salts. 1 Among these, also, could be distinguished a similar op- position of relations, although less marked, to that be- tween bases and acids, and, from the union of two salts, resulted the class of quaternary compounds, or double salts. In this way the theory advanced from element- ary substances to the most complex compounds through the successive gradations of binaries, ternaries, and qua- ternaries ; the elements or compounds only combining with substances of the same order, two and two togeth- er, like two magnetic poles, or two electrified bodies. This dualistic theory was certainly a most admira- ble system, and served the purposes of a rapidly-grow- 1 The word salt was used in chemistry very early to describe any saline substance resembling externally common salt; but, under the dualistic system, the term came to be applied to that class of compounds which were supposed to be formed by the union of basic and acid oxides, as described above. Absurdly enough, however, common salt was thus ruled out of the very class of compounds of which it had previously been regarded as the type, and Berzelius, in his electro-chemical classification, made a distinct family of those substances which resemble common salt in their chemical composition, and called it the haloids. But this name — bodies resembling salt — only rendered the anomaly the more glaring, and it was always a blemish on the dualistic system. In the modern chemistry, the word salt, although still used as a. descriptive name, has no technical meaning. WHEREIN THE DUALISTIC THEORY FAILED. 295 ing science for more than half a century. We now feel assured that the old theory undervalued essential circumstances, and misinterpreted important facts. We maintain that hydrogen is an essential, not an accident- al constituent of all acids and all alkalies, and that, when the alkali is neutralized by the acid, the reaction consists in the replacement of this hydrogen, and not in the direct union of two oxides. Nevertheless, given the old facts, the old theory was logical and consistent, and it is no longer tenable, not because the old facts have changed, but simply because a whole new order of facts has been discovered by which the old facts must be interpreted. During the last twenty-five years there has been discovered a great mass of truths, connected chiefly with the compounds of carbon, in what was for- merly called the domain of organic chemistry, and this is to-day the most prominent and attractive portion of our science. Moreover, the law of Avogadro and the doctrine of quantivalence are two new principles which our modern science has added to the old chemistry, and these principles have supplanted the dualistic theory. Let us not, however, undervalue the old theory. It was an important stage in the progress of science, and a noble product of human thought. Theories are means, not ends ; but they are the appointed means by which man may raise himself above the low level of merely sensuous knowledge to heights where his intel- lectual eye ranges over a boundless prospect which it is the special privilege of the student to behold. What though his vision be not always clear, and his imagination fill the twilight with deceptive shapes which vanish as the light of knowledge dawns ; yet, to have enjoyed the intellectual elevation, is reward enough for all his devotion and all his toil. LECTUEE XIII. ISOMERISM, AND THE SYNTHESIS OF ORGANIC COMPOUNDS. Having, in the previous lectures of this course, made you familiar with the conception that the molecules of every substance have a definite atomic structure, which is a legitimate object of scientific investigation, I en- deavored in my last lecture to illustrate, by numerous examples, the mode now generally employed in chem- istry of exhibiting this structure by means of what are called structural formulae, and, during the whole course of these lectures, it has been a chief object to develop the fundamental principles on which these formulae are based, in order that, having reached this stage, you might be able to see for yourselves that they were legitimately deduced from the facts of observation. I have freely ad- mitted that they were the expression of theoretical con- ceptions which we could not for a moment believe were realized in Nature in the concrete forms, which our dia- grams embody. But I have claimed that they were at present our only mode of representing to the mind a large and important class of facts, and were to be val- ued as the first glimpses of some great, general truth, toward which they direct our investigation. Theories are the only lights with which we can penetrate the ORGANIC COMPOUNDS. 297 obscurity of the unknown, and they are to be valued just so far as they illuminate our path. This ability to lead investigation is the only true test of any theory, and it will be my object in this my last lecture to show that the modern chemical theory of molecular struct- ure has a claim to be regarded as one of the most val- uable aids to discovery which science has ever received. The illustrations of molecular structure thus far studied have been mostly taken from those classes of compounds long known in chemistry under the names of acids, bases, and salts, and they were selected be- cause it was with such substances that the old theory had almost exclusively to deal, and they were therefore the best adapted to illustrate the differences between the new and the old chemistry. But, as I have already said, the strongest evidence in favor of the new theory is to be obtained from a class of substances about which the old chemistry knew almost absolutely noth- ing, and whose number has been enormously increased during the past twenty-five years. Indeed, the modern theory is so completely the outgrowth of new discov- eries that, given alone the old facts, the question be- tween the old and the new theories would be at least of doubtful issue, even if the new could ever have been conceived. The class of substances to which I refer are the compounds of the elementary substance car- bon. The number of known compounds of this one element is far greater than that of all the other elements besides, and these compounds exhibit a great 'diversity in their molecular structure, which is often highly complex. As a rule they consist of a very few chemical elements (besides carbon, only hy- drogen, oxygen, and nitrogen), but the number of atoms united in a single molecule may be very large, 298 ISOMERISM. sometimes even exceeding one hundred. Carbon is peculiarly the element of the organic world, for, leav- ing out of view the great mass of water which liv- ing beings always contain, organized material consists almost exclusively of carbonaceous compounds. Hence these substances, with the exception of a few of the simplest, were formerly called organic compounds, and in works on chemistry they are usually studied to- gether under the head of organic chemistry. It was formerly supposed that the great complexity of these substances was sustained by what was called the vital principle; but, although the cause which determines the growth of organized beings is still a perfect mys- tery, we now know that the materials of which they consist are subject to the same laws as mineral mat- ter, and the complexity may be traced to the pe- culiar qualities of carbon. In like manner the notion that these so-called organic substances .owed their ori- gin to some mysterious energy, which overruled the ordinary laws of chemical action, for a long time pre- cluded from the mind of the chemist even the idea that they could be formed in the laboratory by purely chemical processes ; so that, although the analysis of >these compounds was easily effected, the synthesis was thought impossible. But within a few years we have succeeded in preparing artificially a very large number of what were formerly supposed to be exclusively organic products ; and not only this, but the processes we have discovered are of such general application that we now feel we have the same command over the syn- thesis of organic, as of mineral substances. The chem- ist has never succeeded in forming a single organic cell, and the whole process of its growth and development is entirely beyond the range of his knowledge ; but he BUTYRIC ACID AND ACETIC ETHER. 299 has every reason to expect that, in the no distant future, he will he able to prepare, in his laboratory, both the material of which that cell is fashioned, and the various products with which it becomes filled during life. The number of elements which enter into the com- position of organic compounds being so restricted, it- is evident that the immense variety of qualities which they present -cannot be referred solely to the influence of the simple radicals which they contain. 1 Moreover, there appears among these organie substances a most remarkable phenomenon, which, although not unknown in the mineral kingdom, is peculiarly characteristic of these complex compounds of carbon. "We are ac- quainted with a large number of cases of two or more wholly different substances having exactly the same composition and the same vapor density. Here, for example, are two such substances : The first, butyric acid, is an oily liquid with whose smell we are only too familiar, since, when formed in rancid butter, it imparts to this article of our food its peculiarly offensive odor. But, though, as the odor shows, it must slowly volatilize at the ordinary tem- perature, it does not boil lower than 156° C, and does not easily inflame. Further, as its name denotes, it has the qualities of an acid, reddening litmus-paper, and causing an effervescence with alkaline carbonates. Utterly different from this offensive acid is the sec- ond substance, which wc call acetic ether, a very lim- pid liquid, with a pleasant, fruity smell, highly volatile, boiling at 74° and inflaming with the greatest ease. Notice, also, that it does not in the least affect the colors of these sensitive vegetable dyes. Yet, butyric acid and acetic ether have exactly the 1 Compare pages 243 and 262. 300 ISOMERISM. same composition, and the same vapor density.' The results both of actual chemical analysis and of the determination of vapor density are given in this dia- gram, and the figures obtained in the two cases do not differ more than we should expect the results of different analyses of the same substances to differ ; for it must be remembered that, in such experimental work, we can only attain a certain degree of accuracy, and that we may disregard all variations which are within the limit of probable error : Analyses of Isomeric Compounds. By Grunzweig. By Liebig. Butyric Acid — Acetic Ether — Carbon 54.51 Carbon 54.47 Hydrogen 9.26 Hydrogen 9.67 Oxygen 36.23 Oxygen 35.86 100.00 100.00 By Cahours. By Boullay and4)umas. Sp. Gr 44.3 Sp. Gr 44.1 Moleo. weight 88.0 Molec. weight 88.0 If, now, from these experimental results, we come to calculate the symbols of the two substances, accord- ing to the method I have so fully described, we shall obtain in both cases precisely the same formula, C 4 H 8 2 , and it must, therefore, be that the molecules of these two substances contain the same number of atoms of the same three elements, carbon, hydrogen, and oxygen. Here, then, we come face to face with a most remarkable fact. For, to affirm no more than can be absolutely demonstrated, this pleasant odor of apples and this dis- gusting smell of rancid butter come from substances consisting of the same elements united in the same proportions. What, then, can be the cause of the dif- ference ? We cannot allow such a fundamental fact as THE QUESTION STATED, 301 this to pass unchallenged. It is evident that there is an all-important condition which has escaped our ele- mentary analysis. The circumstances demand investi- gation, and it would be a disgrace to our science not to attempt to answer the question. Can you wonder, then, that, for the past ten years, a great part of the intellectual force of the chemists of the world has been applied to the problem, and in this course of lectures I have been endeavoring to present to you the result they have reached. The answer they have obtained is, that the difference of qualities depends on molecu- lar structure, and that the same atoms arranged in a different order may form molecules of different sub- stances having wholly different qualities. But they have gained more than this general result. These isomeric compounds, as we call them, when acted on by chemical agents, break up in very different ways, and, by studying the resulting reactions, we are frequently able to infer that certain groups of atoms (or compound radicals) are present in the compounds, because we know that they exist in the products which these compounds respectively yield ; our knowledge of the structure of these very radicals probably depending on yet other reactions, by which they again may be re- solved into still simpler groups. Thus, for example, if we act on acetic ether with potassic hydrate, we obtain two products, potassic ace- tate and common alcohol. Now, we know that alcohol has the symbol C 2 H 5 -0-H and contains the radical C 2 H 5 , which we call ethyl. Further, we know that potassic acetate has the symbol K-0-(C 2 H 3 0) and con- tains the radical C 2 H 3 0, which we call acetyl. Hence we infer that the ether contains both of these groups, and that its symbol must be C 2 H 5 -0-C 2 H 3 0. The reac- 302 ISOMERISM. tion obtained with potassic hydrate is, then, seen to consist in a simple metathesis between K and C 2 H 5 . C 2 H 6 -0-C 2 H 3 Acetic Ether. K-O-H Potassic Hydrate. I = K-OCH.O Potassic Acetate. c 2 h 6 -o-h Alcohol. Passing next to the radical ethyl C 2 H 5 , we can show that it may be formed in a compound which contains the radical CH 3 , called methyl, by substituting for one of the hydrogen -atoms of this radical another group of the atoms CH 3 , thus : H H-O-X 1 B Tirflt Methyl Compound. H H-6-Y 1 H Second Methyl Compound. H H H-C-C-X 1 1 H H Ethyl Compound. H-Y Hydrogen Compound. In this assumed reaction the terminal hydrogen- atom of the first methyl compound changes place with the methyl radical of the second, thus producing the compounds in the second column. Such a reaction can actually be produced with a variety of substances, and these symbols may be supposed to stand for any of the substances between which the reaction is possible. We use X and Y, instead of writing the symbols of definite compounds, in order to confine the attention to the change which takes place in the radical alone. In reactions of this kind we form the radical ethyl in such a way as to leave no doubt whatever in regard to its structure, and in a precisely similar way we have STRUCTURE OF ACETIC ETHER. 303 worked out the structure of acetyl. We represent the structure in the two cases thus : H H OH H-O-C- -C-O-H i i i H H H Ethyl. Acetyl. Hence we conclude that the structure of a molecule of acetic ether should be represented as follows : H H OH H-6-6-0-0-0-H i > i H II H Acetic Ether. Moreover, since we are led to the same result, whether we study the reactions by which the ether may be pre- pared or those by which it may be decomposed, we feel great confidence in our result. If, now, we act on butyric acid, the isomer of acetic ether, with potassic hydrate, the same reagent as before, we obtain wholly different products. They are potas- sic butyrate and water; and here the knowledge of acids, bases, and salts, which we obtained at the last lecture, comes in to help us interpret the reaction. It must be' simply as follows : n-0-C 4 H,0 ) ( K-0-C,H,0 Butyric Acid. f I Potassic Butyrate. K-O-H f 1 H-O-H Potassic Hydrate. J \ Water. Evidently, then, butyric acid, instead of containing the two radicals C 2 IT 5 and C 2 H 3 0, like acetic ether, contains the more complex radical C 4 H 7 0, and the simple radi- cal H. But, although the last reaction shows that butyric acid contains the radical C 4 H 7 0, it gives us no infor- 304 ISOMERISM. mation in regard to the grouping of the atoms in the radical. Of course, we have sought to discover what the structure is, and the result of the investigation is most remarkable, for it appears that there are two dif- ferent radicals having the same composition and corre- sponding to two distinct varieties of butyric acid, which differ in their odor, their boiling-point, and other quali- ties, and, further, various reactions show that the atoms of the radicals are arranged in the two acids as the fol- lowing formulae indicate : H O H H H O H-6-H H-O-O-O-C-O-H H-O-6 O-H iii i H H H H-O-H Normal Butyric Acid. I (Prepared from batter or by fermentation.) H Isobutyric Acid. (A product of synthesis.) There are, therefore, at least three substances having the composition C 4 H 8 2 . Now, by studying in a similar way the whole scheme of carbon compounds, and connecting by reactions the more complex with the simpler, it has been found pos- sible, in a very large number of instances, to deter- mine the manner in which the atoms are grouped in the respective molecules, and thus to show what the variations of structure are which determine the differ- ence of qualities in these isomeric bodies. Moreover, having discovered how the atoms are grouped, it has been found possible, in many cases, to reproduce the com- pounds ; and, more than this, chemists have frequently been led to the discovery of wholly new bodies, isomeric with old compounds, by studying the possible variations of the structural symbol. This last fact has such an im- portant bearing on our subject, tending greatly to sub- HOMOLOGOUS COMPOUNDS. 305 stantiate the general truth of our theory of molecular structure, that a few illustrations will be interesting. One of these we have already seen, for the isomeric modification of butyric acid, we have just been dis- cussing, was foreseen by theory before it was discov- ered, and it is, therefore, an example in point, but there are many other cases of the kind which are equally remarkable. Butyric acid is the fourth body in that series of volatile acids before mentioned (page 277), of which formic and acetic acids are the first and second mem- bers. It was then said that the molecules of these acids increase in weight by successive additions of CH 2 as we descend in the series, and it has been shown since H H i i (page 296), that the radical ethyl, -0-0 -H, may be H H H derived from methyl, - C - H, by replacing the terminal H H by another methyl group. It is obvious that this H H H H H ii a iii process repeated on -C-O-H would give -O-O-O- H, H ii H H H and that the result of successive replacements of the same kind would be a series of hydrocarbon radicals differing from each other by CH 2 like the volatile acids mentioned above. Furthermore, it is equally obvious that, theoretically at least, the same process might be applied to any compound containing a hydrocarbon radical ; and you will not be surprised, therefore, to learn that there are many series of carbon compounds, between whose members we find this same common S06 ISOMERISM. difference. Bodies so related are said to be the homo- logues of each other ; and of these homologous series, so called, no one has been more carefully studied than that of the volatile acids, of which nineteen members are known. Now, it is obvious that, as the hydrocarbon radical in the series of volatile acids increases in complexity, the possibilities of varying the atomic grouping in- crease also. Next to butyric acid, C 4 H 8 2 , comes va- leric acid, C 5 H 10 O 2 , and, while we had only two butyric acids, we can have four valeric acids, whose molecular structure is indicated by the following symbols : O H H H H H-O-O-C-C-C-C-H i i i i H H H H Normal Valeric Acid. H O H H-C-H H-O-O-0 O-H H H-C-H i H First Isovaleric Acid. H O H-O-H H H-O-C C O-H H-6-H H i H Second Isovaleric Acid. H O H-C-H H H n i ii H-O-C C C-C-H i i i H H H Third Isovaleric Acid. Of these possible modifications of valeric acids, pointed out by theory, the first three have already been identified in the investigations to which the theory led, and the discovery of the fourth is pvobably only a ques- tion of time. Examples similar, to this are already nu- merous and are rapidly multiplying, but I have only time to cite one other instance. CYANIC ETHER. 307 A compound called cyanic ether has long been known, and its symbol was always assumed to be — • (C a H 8 )-0-C=N, after the analogy of the other ethers ; that is, it was assumed to contain the compound radicals, ethyl, C 2 H 5 , and cyanogen, CJST, united through an atom of oxygen. But, as is obvious, we may, without changing the radi- cal ethyl, group the other atoms thus : (C.H 6 )-N=C=0, and, on searching for- this substance, an isomer of the supposed cyanic ether was actually obtained, and called cyanetholine. Very singularly, however, further inves- tigation proved that the new compound was the real cyanic ether, and that the old one had the constitution represented by the last symbol. Evidently, then, we are not infallible ; but the very mistake has been in- structive ; for, in detecting and correcting the error, we have the more clearly shown that our methods are trustworthy. I hope I have been able to give some general no- tions of the manner in which we have obtained our knowledge of the grouping of the atoms in the com- pounds of carbon. More than this cannot be expected in a popular lecture ; for, so interwoven is the web of evidence on which the conclusions are based, that, to enter into full details in regard to any one of the more complex compounds, would be wearisome, and the work is much better suited for the study than the lecture room. Indeed, I fear that I have already imposed too great a burden on your patience ; but, if you have fol- lowed me thus far, you will be interested in some of the results which we have reached, and which you arc now prepared to understand. I must necessarily pre- 308 SYNTHESIS OF ORGANIC COMPOUNDS. sent these results as they have been formulated by our theory of atomic bonds ; for, without the aid of these foramlse, we cannot either think or talk clearly about the subject. The one characteristic of carbon on which the great complexity and variety of its compounds depend is, the power which its atoms possess of combining among themselves to an almost indefinite extent. As a rule, chemical combination takes place readily only between dissimilar atoms. It is true that we have met with many examples of the union of similar atoms, as in the molecules of several of the elementary gases, like — H-H Cl-Cl = E"=¥ Hydrogen Gas. Chlorine Gas. Oxygen Gas. Nitrogen Gas. So, also, in the compounds — 01 CI O 01-Fe-Fe-Cl and Al-Al 11 s \ CI 01 Ferric Chloride. Aluminic Oxide. and likewise in O / \ Cl-Hg-Hg-Cl and Cu-Cu Mercurous Chloride. Cuprous Oxide — two atoms are united by a single bond, forming a bi- nary group, which is the radical of the metallic com- pound. But, in all these cases, the power of combina*- tion is very limited, admitting the grouping together of only a very few atoms at the most, and generally of only two. The carbon-atoms, however, not only unite with each other in large numbers, but form groups of great stability, which, in organic compounds, take the place of the elementary radicals of the mineral king- dom. Let us begin, then, by constructing these radi- cals: CARBON RADICALS. 309 D/°VO °^'° II o O II « I o \o^-\ NO' o ^O I « O II ■« I o 'Ox.' o*. ,o. I »o .© \Q /0 v ND/°^0 / \ O o I © « O - || ■* O II i>. 0-*ll O. <° ^O- II » O II S ' \J O ' x O ' ©^ '°\ y v /D\ /0( \ I ,O v ,2^ «i-o- ■* ii o « i ,o °° o; ' i s o ; i s i , . -o- ' )o v " )o( N >°\^ ^ )°)o(°< i I -O- I -O- I -O- I -o- I -o- I -o- I i -o- i -o- I -o- <*_0- • I oo-o- o i «_0- S I I -O- I -O- I -D- I -O- I -o- I I -o- I -Q- I -o- I I -D- 310 SYNTHESIS OF ORGANIC COMPOUNDS. The carbon -atoms being quadrivalent, they may- unite with each other either by one, two, three, or four bonds, and the larger the number of bonds which are thus closed, the less will evidently be the com- bining power of the resulting radical. Hence may arise radicals like those represented in the diagram on the previous page. It is evident that this table might be extended indefinitely, but the number of terms given is sufficient to illustrate the simple rela- tion between the several radicals thus formed. Each group of carbon-atoms can have a maximum quantiva- lence of 2n + 2 (the letter n denoting the number of carbon-atoms in the group), and from this maximum the quantivalence may fall off by two bonds at a time until it is reduced to zero. Thus we have for the six- atom group a maximum of 14 ; but the same group may also have a quantivalence of 12, 10, 8, 6, 4, or 2. The symbols, however, given in the table do not by No. 1. 2. s. i i l. -C- -C- i i i i i iii iii _0-0-0-0-0- -0-0-0- -0-0-0- I I I I I III III -0- -0- I I No. 2. i. a. i i i i i i i i -0-0=0-0- 0=0-0-0- I I III 8. 4. I \ / -0- 5. II \ / \ / I 0=0 i i.O- ii s \ / \ -0-0 i -0- , u n _ I any means exhaust the possibilities of combination with the given number of carbon-atoms; for further CARBON RADICALS. 311 variations may be obtained by changing the relative position of the atoms while retaining the same quan- tivalence. Thus, the radical (C 5 ) xu may be constructed in the several ways shown in diagram No. 1, and, al- though the several radicals thus obtained contain the same number of atoms, and have the same quantiva- lence, they are fundamentally different. The differ- ence consists, not in the mere grouping of the letters on the page, which is purely arbitrary, but in the fact that, while in 1 no carbon-atom is united with more than two others, in 2, one of the atoms is united with three others, and, in 3, with four. As the num- ber of atoms in the group increases, the number of possible variations must necessarily become very great- ly augmented. Moreover, when some of the atoms are united by double bonds, a variation may be obtained by shifting the position of this double bond as well as by varying the position of the atoms with respect to each other. This is illustrated by diagram No. 2, which shows the possible forms of the group (C 4 ) TiU . It is unnecessary, however, to multiply illustrations; for it is evident that a great multitude of radicals may be obtained with even a very limited number of car- bon-atoms, and to attempt to exhaust the possibilities would be an endless task. Some of my audience, how- ever, may be interested to study the subject further, and I would, therefore, set them as a problem to find the number of possible combinations which can be made with a group of six carbon-atoms, having a quan- tivalence of twelve. Such investigations are not with- out their profit ; for, although many of the possibilities may not be realized in Nature, yet the practice will give a clear idea of what is meant by an essentially dif- ferent structure. It may hereafter appear that changes 312 SYNTHESIS OP ORGANIC COMPOUNDS. of position corresponding to the upper and lower, or the left and right hand sides of onr diagram, constitute really essential variations of structure ; but, although there are some facts looking in this direction, we do not as yet admit that any such differences are of im- portance, and we regard any two groups as the same when, by any change that does not alter the relative order of the atoms, or the number of bonds by which they are united, the two can be made to coincide thus : 1 1 \ / / -0- -c- 0- -0 -C-C-C-isthesameas-C-C- iii ii i 0-. 1 ' ,and o 0- the sami -C- -c- 1 1 C = \ \ / \ / \ \/ 0-C C-0 0-0 as 0, but not the same as C or /\ / \ y\ s\ \ /^ C-C = C-0 /\ /\ / \ / /\ The radicals thus formed may be regarded as the skel- etons of the organic compounds. These carbon -atoms, locked together like so many vertebrae, form the frame- work to which the other elementary atoms are fastened, and it is thus that the complex molecular structures, of which organized beings consist, are rendered possi- ble ; moreover, when we remember that, while the ele- mentary substance carbon is a fixed solid, the three elementary substances, oxygen, hydrogen, and nitro- gen, with which it is usually associated, are permanent gases, this analogy of the carbon-nucleus to the skele- ton of the vertebrate animal becomes still more strik- ing. Having thus shown how the skeletons may be formed, let us next see how these dry bones may be clothed. In order to illustrate this point, I will sim- PETROLEUMS. 313 ply take two of the numberless carbon-radicals, which are theoretically possible, and show how from them a set of familiar organic products can be derived. Let the two be the radicals represented in this diagram : i -c s o^ II I ill _c 0- -0-0-0- \ y ill , o I To such carbon-skeletons a large number of different elementary atoms and compound radicals can be attached by various chemical processes ; but the number of those usually met with in organic compounds is very limited, and only the following will be considered in this con- nection, namely : H-, -0- H-0-, |)N-, °)N- Hydrogen. Oxygen. Hydroxyl. Amidogen. Nitryl. Indeed, by doubling this number, we could obtain the materials for constructing nearly the whole scheme of modern organic chemistry. Beginning, then, with the nucleus - C - - -, let us, in the first place, satisfy all the open bonds with hydror gen-atoms. The result is — H H H i i i H-C-C-0-H i i i II H H Propylic Hydride. a combustible gas, which is found mixed with numer- ous other compounds of the same class in our petrole- um-wells. Propyl hydride is the third in a series of homologous compounds, of which no less than nine have been identified in our Pennsylvania petroleums. 314 SYNTHESIS OF ORGANIC COMPOUNDS. Methylic hydride CH, Gas. Ethylic hydride C2H0 Propylic hydride CaH 8 Butylic hydride C4H10 32 Amylic hydride CeHu 86 Hexylic hydride CaHi 4 142 Heptylic hydride O7H10 194 Octylie hydride.... C 8 H 18 247° Nonylic hydride C»H 2 o 303 The diagram, above, gives their names and boiling- points. Our common kerosene is chiefly a mixture of hexylic and heptylic hydride, and the light naphthas a mixture of amylic and hexylic hydrides. Notice here, again, the common difference, CH 2 , between the symbols of any two consecutive members of this series of hydrocarbons. If, next, we substitute an atom of oxygen for two of the hydrogen-atoms which, in propylic hydride, are united to either of the terminal atoms of the carbon- nucleus, we obtain a compound called propylic alde- hyde. This is a member of another series of homo- logues, parallel to the last, and of which nearly as many members are known. The aldehydes, as these bodies are all called, have very striking and characteristic qual- ities ; and these qualities may be, to a great extent, traced to their peculiar molecular structure. If we only make so small a change as to transfer the oxygen- atom from the terminal to one of the central atoms of the carbon-nucleus, we obtain a class of compounds which, though isomeric with the aldehydes, have wholly different qualities, and are called ketones. The ketone isomeric with propylic aldehyde is called acetone : H H O H O H I " « 1 11 1 H-C-C-C-H H-C-C-C-H II 1 1 H H H H Propylic Aldehyde. Acetone. ALCOHOLS. 315 Going back again to the hydrocarbon, C 3 H 8 , and replacing either of the terminal hydrogen-atoms by the radical hydroxyl (-0-H), we obtain one of a very important class of compounds, called alcohols. H H H H H H H-C-C-C-H H-C-C-C-O-H iii iii H H H H H H Propylic Hydride gives Propylic AlcohoL Propylic alcohol is the third member of still another series of homologous compounds, of which our common alcohol is the second member. Normal Alcohols. Methylio alcohol (wood-spirit) C H a -O-H Ethylic alcohol (common alcohol) C 2 H 6 -O-H Propylic alcohol C 3 H 7 -O-H Butylic alcohol C, H 9 -O-H Amylic alcohol (fusel-oil) 6 Hn-O-H Hexylic alcohol 8 Hu-O-H Heptylic alcohol O7 H15-O-H Octylic alcohol C 8 Hn-O-H The structure of the alcohol may obviously be varied, like that of the aldehyde, by transferring the hydroxyl from the terminal to one of the central atoms of the carbon-nucleus ; but we thus, as before, obtain a wholly new set of substances, which, although resem- bling the normal alcohols in many respects, differ from them in important particulars. There is, for example, an isopropylic alcohol, which is isomeric with the nor- mal propylic alcohol, and, like it, resembles externally common alcohol. But the pseudo-alcohol, as we call it, boils at 85° Cent., while the normal alcohol boils at 97°, and, when acted 'on by chemical agents, yields wholly different products : 316 SYNTHESIS OP ORGANIC COMPOUNDS. H H H H H H III ' ' L -rr H-C-C-C-O-H H-C-C-C-H ill i i i H H H H O H Propylic Alcohol. I H Isopropylic Alcohol. Continuing, now, this process of clothing the car- bon-skeleton, let us, in the next place, substitute for two of the hydrogen-atoms of the normal alcohol an atom of oxygen, selecting for replacement the two hy- drogen-atoms which are connected with that terminal carbon-atom to which the hydroxy 1 is united : H H H H H H-C-C-C-O-H H-C-C-C-O-H 1 1 I H H H Propylic Alcohol gives H H Propionic Acid. Now, propionic acid is the third member of that ho- mologous series of volatile acids of which a partial list has already been given (page 277), and of two of whose members the possible variations of structure have al- ready been discussed (pages 298 and 300). Again, we may substitute in propionic acid a second oxygen- atom for two of the remaining atoms of hydro- gen, and we thus obtain a liquid body called pyruvic acid, a perfectly definite substance, although one with which I can give you no familiar associations : H H H O O H-C-C-C-O-H H-C-C-O-O-H i i i H H H Propionic Acid. Pyruvic Acid. The acids and alcohols we have thus far formed around our three-atom carbon-nucleus have been all monatomic. The* atomicity of a compound, you re- member, is determined by the number of atoms of hy- drogen which are easily replaced by metathesis, and GLYCOL AND GLYCERINE. 317 only those atoms of hydrogen can be so replaced which are united to the carbon-nucleus through an atom of oxygen. Hence, with one hydroxyl group we can only produce monatomic compounds. Use two hydroxyl groups, and we can form around the same skeleton a number of diatomic compounds. The following are a few examples. After what has been said, the symbols require no detailed description ; but it must be remem- bered that the grouping is no play of fancy, and that a good reason can be given for the position of every letter : H H H H H H H-O-O-O-C-O-H H-O-C-C-C-H iii iii (Ni Hor H H H H O H bt yet discovered.) I ormal Propyl Glycol. H Propyl Glycol. O H H O H H H-O-C-C-C-O-H H-O-O-C-C-H ii ii H H OH Normal or Paralactic Acid. I H Common Lactic Acid. Attach to the nucleus three hydroxyl groups, and there result triatomic compounds, among which is a very fa- miliar substance : H H H H-O-C-O-C-O-H i i i H O H i H Glycerine. O H H OHO H-O-C-C-C-O-H H-O-C-C-C-O-H i i i OH O i i H H Glyceric Acid. Tartronic Acid. 318 SYNTHESIS OF ORGANIC COMPOUNDS. Lastly, replace the three terminal hydrogen-atoms of glycerine by nitryl (N0 2 ), and we meet again an old acquaintance : ^ o H H H O jr-o-c-c-c-o-N H iii ii O H O H O Kitro-glycerine. I think that this last symbol will not now appear to you so strange as when I first called your attention to it a few lectures back. It is true that I have not act- ually proved that this grouping of the letters repre- sents the structure of the nitro-glycerine molecule, but I have led you to a point where you are prepared to accept it as a definite result of investigation, and can feel assured that the proofs await your examination in the due course of your study. You can now understand more clearly than before how it is that, by the struct- ure of the molecule, the oxygen-atoms are kept apart from the atoms of carbon and hydrogen for which the fire-element has such a strong affinity, and how these atoms rush into more stable combinations when the delicate balance of forces, on which the structure de- pends, is disturbed. You have now seen what a number of distinct com- pounds can be obtained by attaching to one of the very simplest of the carbon-nuclei atoms of hydrogen and oxygen alone. Almost every commutation we could make with these few atoms is actually realized in a defi- nite substance. Of course, with the names of many of these bodies you have no association. You must accept the assurance that they stand for definite substances, and that our symbols represent the results of care- ful investigation, and, knowing this, you can gain some COAL-TAE PRODUCTS. 319 conception of the knowledge we have acquired of the structure of this class of compounds ; and, when you add to this that, in many of these cases, the theory has gone before discovery, and, by suggesting possible com- mutations of the atoms, has prefigured compounds which were subsequently obtained, you must admit that, rude and unreal as our representations of molecular struct- ure may be, they have a positive value, both as means of classifying facts and as aids to new discoveries. Lastly, let us turn our attention to the second of the two carbon-skeletons, whose dry bones we proposed to clothe with the features of definite compounds. The group of bodies whose molecules contain, as we assume, this nucleus (Fig. 32), has been very fully investigated by Professor Kekule, of Bonn, and to him we owe the theory of their structure which our diagram repre- sents. It may appear superfluous for me to repeat that, in such diagrams, the only es- sential points are the relative order of the atoms and the number of the bonds ; but the hexagonal shape in which we find it convenient to represent on our page the structure of this nucleus suggests the idea of defi- nite form so forcibly, that additional caution may be needed to avoid misconstruction. The bodies with which we are now to deal are, for the most part, products either already existing in coal- tar, or which may be obtained from it by various chem- ical processes. Among them are those gorgeous ani- line dyes which, within a comparatively few years, have added so much to the elegances of common life. From a very large number of compounds, I can only select a few examples. Still, I shall not restrict the selection to compounds whose molecules contain only six carbon- \ / 0- # \ \ / c- ■ c / \ Fig. 82. 320 SYNTHESIS OF ORGANIC COMPOUNDS. atoms, but I shall endeavor to show that molecules of extreme complexity can be built up either by the addi- tion of hydrocarbon radicals to the nucleus represented in Fig. 32, or by the coaleascing of two or more of these nuclei into one. As I have not time to enter into details, the symbols must, to a great extent, be allowed to speak for themselves. Coal-tar is a mixture of a very large number of sub- stances whose boiling-points vary from 80° Cent, upward. When the tar is distilled, and the distillate rectified, the more volatile product obtained is chiefly a mixture of two hydrocarbons — benzol and toluol. This mix- ture, the commercial benzol, is used in large quantities for the preparation of the aniline dyes : H H i i H 0-0 i ^ % -H H-C-C C-H i \ / H = i i H H Toluol. When benzol and toluol are treated with strong ni- tric acid the products are : H H H H >i ii 0-0 O H C-0 O H-0 C-ST H-C-0 C-N C = C / O h O-c' O II II H H H H Nitrobenzol or Nitrotoluol. When nitrobenzol and nitrotoluol are acted on by nascent hydrogen (in the arts a mixture of iron-filings and acetic acid is used), we obtain : ll H 1 0- 1 -0 Ji- -C \ = = 1 H H Benzol. ANILINE CO LORS. H H H H 6-6 h s \ 1 H-0 0-N \ / i 0=0 H H H-6- i H 6-6 h -6 0-N \ / i = H 1 1 H H Aniline, or 1 1 H H Toluidine. 321 When the mixture of aniline and toluidine, obtained in the arts from commercial benzol, is treated with various oxidizing agents, we obtain salts of H H \ / C-C ^ -v H-C 0-H \ / c = c 14 / ^ 14 H "^N N' n H \ / v / H 0-0 C-C H H-6-o" "b-N-o' 0-6- H i \ / i \ / i H 0=0 H 0=0 H / \ / \ H H H H Kosaniline. Eosaniline is a base like ammonia. As I have before stated, when the molecule NH 3 unites with acids to form salts, the quantivalence of the nitrogen-atom ap- pears to be increased by two bonds which bind the atoms of the acid molecules (see page 238). So, when rosaniline combines with acids, the atoms of the acid molecule join to one or the other of the nitrogen-atoms in the complex molecule of this base. Moreover, as there are three of these nitrogen-atoms in the molecule of rosaniline, it can bind either one, two, or three mole- cules of acid ; for example, it can unite either with HC1, with 2HC1, or with 3HC1. Thus, there may be 322 SYNTHESIS OF ORGANIC COMPOUNDS. formed three classes of salts, and those which contain the smallest amount of acid are used in the arts as coloring agents. These salts, when crystallized, have a very brilliant beetle-like lustre, and yield beautiful rose-red solutions. They possess, moreover, a most wonderful coloring power. Taking only a few crystals (one grain in weight) of the hydrochlorate of rosaniline, called fuchsine in com- merce, and, first rubbing them up in a mortar with some alcohol, I will pour the concentrated solution into a large glass jar, holding two gallons of water, and you see that this very small quantity of dye shows a brilliant red color even when diffused through the large body of liquid. By combining the base with dif- ferent acids we obtain only slight variations of tint, but very marked alterations of color can be produced in another way. By recurring to the symbol of rosaniline, it will be seen that there are three hydrogen - atoms directly united to the three atoms of nitrogen which the radical contains. Now, it is possible to replace either one, two, or all three of these hydrogen-atoms by various hy- drocarbon radicals ; such as — -CH 3 -0,H. -C 6 H S ; Methyl, Ethyl, or Phenyl. and we thus obtain other bases whose salts are violet or blue — the blue tint increasing with the degree of replacement. I have in these five jars solutions of some of these salts, the aniline violets and blues of commerce, and they will illustrate to you the °Ta- dations of color we can obtain by the replacements I have described. Among the less volatile products of the distil- NAPHTHALINE. 323 lation of coal-tar is the compound called phenol or carbolic acid, which is so much used as an antisep- tic agent. Here is its symbol and also the symbol of another compound which has recently acquired great theoretical importance, but which, although closely al- lied to phenol, is derived from a wholly different source : H H H H ii ii C-0 C-C R-6 C-O-H H-0 C-H \ / \ / 0-0 0=0 II II H H 0-0 Phenol. Quinone. One of the least volatile products obtained in the distillation of coal-tar is a hydrocarbon called naphtha- line, whose molecule appears to be formed by the coa- lescing of two molecules of benzol. This body yields a very large number of derivatives having the same general structure, some of which have such a deep color that they can be used as dyes : H H i i H-0* ^G C-H i * n i H-C C 0-H c o I I H H Naphthaline. Associated with naphthaline in coal-tar is a still less volatile hydrocarbon, called anthracene, which may be regarded as formed by the coalescing of three mole- cules of benzol : 324 SYNTHESIS OF ORGANIC COMPOUNDS. H H i i O C H • \ / \ / H-0 C C H i ii i / H-0 0-0 ^c' ^O^ \)-H i \ / H 0-0 i i H H Anthracene. Lastly, from anthracene has been derived the fol- lowing product : * H ^O H \ i C H ? \ / \ / 0-0 C H ii i i / 0-0 0-0 \ / \ s % C-H / \ / O = / I I H H H Anthraquinonic Acid (Alizarine). This brings us to one of the latest and most note- worthy results of our science. Alizarine is the color- ing principle of the madder-root, which has long been the chief dyestuff used in printing calicoes. But, al- though the mordanted cloth extracts from a decoction of the root the coloring material in a condition of great purity, yet it has been found exceedingly difficult to isolate the alizarine. For this reason, although the subject had been most carefully investigated, there was for many years a question in regard to the exact com- 1 For further details see " Principles of Chemical Philosophy," by Josiah P. Cooke, Jr., published by John Allyn, Boston, third edition, 1874. ALIZARINE. 325 position of the substance. A short time since, Graebe, a German chemist, in investigating a class of com- pounds called the quinones, 1 determined incidentally the molecular structure of a body closely resembling alizarine, which had been discovered several years be- fore. This body was derived from naphthaline, and, like many similar derivatives, was reduced back to naph- thaline when heated with zinc-dust. This circumstance led the chemist to heat also madder alizarine with zinc- dust, when, to his surprise, he obtained anthracene. Of course, the inference was at once drawn that ali- zarine must have the same relation to anthracene that the allied coloring-matter bore to naphthaline, and, more than this, it was also inferred that the same chem- ical processes which produced the coloring-matter from naphthaline, when applied to anthracene, would yield alizarine. The result fully answered these expectations, and now alizarine is manufactured on a large scale from the anthracene obtained from coal-tar. Here are two pieces of cloth, one printed with mad- der and the other with artificial alizarine, and the most expert calico-printer could not distinguish between them. This certainly is a most remarkable achievement. A highly-complex organic product has been actually con- structed by following out the indications of its molecu- lar structure, which the study of its reaction, and those of allied compounds, had furnished. It is a result that all can appreciate, and which the world will accept as the most trustworthy credential that the molecular 1 The name quinone is applied to a class of bodies whose molecules contain two atoms of oxygen united to a carbon-nucleus in the peculiar way shown in the symbol of the typical compound of the class, given above (page 317).. 326 CONCLUSION. theory of chemistry could offer. The circumstance that this substance is the important madder-dye, and that the new process has a great commercial value, of course, really adds nothing to the force of the evidence in favor of the theory. To the scientific mind the evidence of any one of hundreds of substances which have been constructed in a similar way, but of which the world at large has never heard, is equally conclu- sive. Still, we have great reason to rejoice that this is one of the few instances where purely theoretical study has been unexpectedly crowned with great practical re- sults. Let us accept the gift with gratitude, and pay due honor to those through whose exertions it has been received. Let us remember, however, that it came as a free gift, and that the result was achieved by men who, with single-hearted zeal, worked solely to extend knowledge. Forget not, then, to encourage those who are devoting their lives to the same noble service, and have the manly courage to sow the seed whose harvest they can never hope to reap. /Honor those who seek Knowledge for her own sake, and remember they are the great heroes of the world, who work in faith, and leave the result with God 1 / INDEX. The numbers of this index refer to pages. Attention is called to the lists of experiments, graphic symbols, reactions, and tables given under these several headings. Acetic acid, 281, 283. Acetic ether, 302, 303 ; isometric with butyric acid, 299. Acetone, 314. Acetyl, 303. Acids, 249, 255, 262, 267, 292. Acids and alkalies, 256, 258, 265 ; differences, 271, 273. Aggregation, states of, 14. Alchemy, 106. Alcohols, 137, 315. Aldehydes, 314. Alizarine, 325. Alkali, 249, 253, 255 [see also Acids and Alkalies). Alum, potassic, 250 ; amnionic, 290. Aluminic oxide, 308. Aluminum, action on potassic hy- drate, 265. Amidogen, 313. Ammonia gas, 185, 244. Amnionic chloride, 244; nitrate, 181. Ampere's law, 13. Analysis, 96, 122, 175; of acetic ether, 300; of alcohol, 137; of butyric acid, 300 ; of nitric acid, 259; of water, 124, 138-, of salt and sugar, 124. Andalusite, 290. Anhydride, 288. Aniline, 321. Anthracene, 324. Anticipations in science, 11. Aristotle, 98, 235. Arithmetic, chemical, 150, 179. Artiads and perissads, 248. Atomic bonds, 241 ; clamps, 250 ; theory, 102. Atomicity of hydrates, 284. Atoms, 36, 135; specific heat of, 133 ; polarity of, 273 ; weight of, 117, 124, 130. Avogadro's law, 13, 37, 63. Barometer, 39. Bases, 292. Basic, definition of the term, 266. Beauxite, 290. Becker and Stahl, 235. Benzol, 320. Berzelius, 270, 292. Binary compounds, nomenclature of, 170. Bonds, atomic, 241. Boric acid, 286. Boyle's law, 41. Bunsen's lamp, 208. Burning (see Combustion). Butyric acid, 283, 299, 304. Calcic hydrate, 249, 285 ; oxalate, 286 ; oxide (see Lime) ; sulphate, 250. Calcium, 160. Candle, 207, 209. Carbolic acid, 321. Carbon, 155, 158, 308 ; atomic 328 INDEX. weight, 127; radicals, 309, 310; quantivalence, 248. Carbonic dioxide, 143, 163, 190, 205 ; dioxide action on lime-water, 161 ; dioxide decomposed by plants, 156 ; dioxide decomposed by so- dium, 153. Carbonic oxide, 279. Chalk, decomposed by acids, 167; decomposed by heat, 166 ; forma- tion, 162 ; solution, 164. Changes, chemical and physical, 95. Charcoal, burning of, 203, 218. Charles's law, 48. Chemical changes, 95, 175; com- pounds, 96, 99, 107. Chlorine, atomic weight, 125 ; gas burns tinsel, 187. Chrysoberyl, 290. Chrysolite, 289. Coal, burning of, 205 ; energy stored in, 206. Combustibles, 189. Combustion, 189-237; of charcoal, 203, 204, 218 ; of hydrogen, 100, 196, 199 ; of phosphorus, 189, 193;. of slow-match in oxygen, 91 ; of sulphur in nitrous oxide, 183 ; of sulphur in oxygen gas, 182; of tinsel in chlorine gas, 187 ; of watch-spring in oxygen, 91 ; history of theory, 233. Compound blow-pipe, 199. Compound radicals, 268. Compounds (see Chemical Com- pounds) ; not mixtures, 107. Corundum, 290. Cream-of-tartar, 146. Crith, 67, 70. Crystallization of ice, 55 ; of sal- ammoniac, 53 ; of urea, 54. Crystals, effects on polarized light, 57-62. Cuprous oxide, 308. Cyanic ether and cyanetholine, 307. Dalton's atomic theory, 108. Definite proportions, law of, 107. Density, 68. Density of vapors, 76, 260. Design, in Natfire, 213. Diaspore, 290. Diatomic hydrates, 285. Differentiation, a method of inves- tigation, 201. Dihydro-sodic phosphate, 286. Dipotassic oxalate, 285. Disodic sulphate, 284. Divisibility of matter, 35. Dualistic theory, 270, 271. Dumas's method for vapor density, 78. Electrical polarity, 275. Electrolysis, 270. Elementary substances, 109-113 ; ta- ble of, 1 12 ; nomenclature of, 169. Energy from burning, 190-206 ; from the sun, 214 ; indestructible, 214 ; required to decompose water, 99. Ether of space, 22. Ethyl, 303. Expansion by heat, of gases, 19 ; of liquids, 18. Experiments : aluminum and potas- sic hydrate, 266 ; ammonia and hy- drochloric-acid gas, 185 ; bands on soap-film, 31 ; burning charcoal, 203 ; burning charcoal powder, 204 ; burning hydrogen gas, 90 ; burning iron, 110; burning phos- phorus in air, 189 ; burning phos- phorus in oxygen, 1 93 ; burning watch-spring, 91 ; calcining chalk, 166 ; chalk and acid, 167 ; chlo- rine gas and tinsel, 1 87 ; coloring power of aniline dyes, 322 ; com- pound blow-pipe, 100 ; crystalliza- tion of sal-ammoniac, 53 ; crystal- lization of urea, 54 ; decomposition of sugar, 87; decomposition of water, 89, 92 ; density of vapors, 77, 80 ; expansion of liquids by heat, 18 ; explosion of iodide of nitrogen, 183 ; explosion of hy- drogen and oxygen; 100 ; forma- tion of vapors, 17; globular form of liquids, 52 ; gunpowder burnt in vacuo, 219 ; gunpowder burnt in air, 2-19 ; ice-flowers, 55 ; iodine • and phosphorus, 188 ; iron and hydrochloric acid, 263 ; iron and sulphur, 164 ; lime-water and car- bonic dioxide, 161 ; magnetic curves, 61 ; Mariotte's law, 41 ; nitric oxide and oxygen gas, 185 ; INDEX. 329 with polarized light, 57-62 ; Pha- raoh's serpent, 108 ; potassic hy- drate and nitric acid, 258 ; potas- sium and water, 257 ; preparation of nitrous oxide, 181 ; preparation of oxygen gas, 177; products of combustion weigh more than the candle, 210 ; slaking of lime, 161 ; sodie carbonate and eream-of-tar- tar, 146 ; sodic carbonate and mu- riatic acid, 141 ; sodic silicate and muriatic acid, 289 ; sodium and carbonic dioxide, 153 ; sodium and water, 250 ; sulphur burnt in nitrous oxide, 183 ; sulphuric acid and zinc, 263 ; sulphuric acid and zinc oxide, 264 ; synthesis of formic acid, 279 ; variations of quantivalence, 246 ; weight of carbonic dioxide, 142. Feldspar, 291. Ferric chloride, 308. Filtering, 161. Flame, 196 ; how colored, 199, 253, 257 ; light of, 207 ; of wood and coal, 209. Formic acid, 279, 283. French system of weights and meas- ures, 67. Fuel, constituents of, 206; energy of, 211; products harmless, 211. Garnet, 291. Gas, cause of its tension, 43 ; charac- teristics of a, 38. Gas illuminating, 207. Gas-volumes, how represented, 186. Gay-Lussac's law, 65. Gibbsite, 290. Glass not absolutely homogeneous, 21 ; size of molecules, 28. Glyceric acid, 317. Glycerine, 221, 317. Gold, variations of quantivalence, 247. Graebe, synthesis of alizarine, 325. Gramme, 67. Graphic symbol, 251 ; acetone, 314.; acetic ether, 303 ; acetyl, 303 ; alizarine, 324; aluminic oxide, 308 ; amidogen, 313 ; ammonia alum, 290 ; ammonia gas, 244 ; amnionic chloride, 244 ; ammoni- um, 269 ; andalusite, 290 ; aniline, 321 ; anthracene, 324 ; beauxite, 290 ; benzol, 320 ; butyric acid, 304 ; calcic hydrate, 249 ; calcic sulphate, 250 ; carbon radicals, 309-313 ; corundum, 290 ; chryso- beryl, 290; chrysolite, 289; cu- prous oxide, 308; cyanogen, 269; diaspore, 290; ethyl, 269, 303; feldspar, 291 ; ferric chloride, 308 ; formic acid, 280 ; fluorides of manganese, 245 ; garnet, 291 ; gibbsite, 290 ; glyceric acid, 317 ; glycerine, 317 ; hydrochloric acid, 278; hydroxyl, 313; hypochlo- rous acid, 278 ; chlorides of iron, 246; lactic acid, 317; methyl, 269 ; mercurous chloride, 308 ; naphthaline, 323 ; nitric acid, 274 ; nitro- benzol, 320; nitro - toluol, 320; nitro-glycerine, 318 ; nitryl, 313; phenol, 323; phosphorous chloride, 244 ; triplumbic hydrate, 249 ; potassic aluminic sulphate, 250 ; potassic hydrate, 273 ; pro- pionic acid, 316; propylic alde- hyde, 314 ; propylic glycol, 317 ; propylic hydride, 313 ; pyruvic acid, 316; quinone, 323; rosani- line, 321 ; silicic hydrates, 288 ; tartronic acid, 317 ; toluidine, 321 ; toluol, 320 ; valeric acid, 306 ; wollastonite, 289. Gunpowder, 217; energy exerted, 220 ; products of combustion, 219. Hare's compound blow-pipe, 100. Heat, nature of, 44-49 ; developed bv burning, 192 ; whenever atoms unite, 188. Hexatomic hydrates, 290. Hofmann's method for vapor den- sity, 80. Homologues, 306; aeries of, 283, 314, 315. Hydrates, nomenclature of, 173 ; alkaline and acid, 266 ; atomicity of, 284; definition of, 266, 284; instability of, when complex, 287 ; yield water when heated, 287. Hydrides of methyl ethyl, propyl, etc., 314. Hydrochloric acid, 278 ; action on 330 INDEX. iron nails, 263 ; action on sodic carbonate, 141 ; combines with ammonia, 185 ; neutralizes alka- lies, 256. Hydrodisodic phosphate, 286. Hydrogen, atomic weight of, 128. Hydrogen gas, 91 ; burning of, 196- 201 ; preparation of, 263 ; synthe- sis of water, 197. Hydropotassic oxalate, 283. Hydrosodic sulphate, 283. Hydroxyl, 284, 313. Hypochlorous acid, 278. Ice, crystalline structure, 55. Ignition, point of, 191. Imponderables, 98. Intelligence in Nature, 213, 215. Iodide of nitrogen, 183. Iodine, 183, 188. Iron chlorides, 246. Iron, variations of quantivalence, 245. Isomerism, 299. Isopropylic alcohol, 315. Kekule benzol theory, 319. Kerosene, 314. Ketones, 314. Lactic acid, 317. Lamp, a gas-factory, 207. Lavoisier, 233, 291. Law of Ampere, 13; Avogadro, 13, 50; Boyle, 41; Charles, 48-50; definite proportions, 107 ; Gay- Lussac, 65 ; Mariotte, 41 ; mul- tiple proportions, 115; Newton, 97. Liebig, 268. Light, when manifested, 193, 201 ; dimensions of waves, 24 ; disper- sion of, 27 ; polarized, 56 ; wave theory, 22. Lime, action on water, 161.; compo- sition of, 160. Lime-kiln, 167. Limestones, how formed, 165. Lime-water, 161. Liquids, characteristics, 50; globu- lar form, 52. Litmus-paper, 166. Luminous flames, 207. Madder-dve, 325. Magnesie" hydrate, 284; sulphate, 286. Magnetic curves, 61 ; polarity, 273. Manganese fluorides, 245; varia- tions of quantivalence, 245, 247. Mariotte's law, 41. Matter, relations to space, 20 ; inde- structible, 144. Maxwell, " theory of heat," 49 ; " on molecules," 36. Measures and weights, French sys- tem, 67. Mercuroua chloride, 308. Metathesis, 175. Metathetical reactions, 251. Metre, 67. Mieroerith, 73, 120. Mixture, distinguished from a chem- ical compound, 107. Molecular structure, 249. Molecules, 13, 35, 37, 42, 136. Moleeules, chemical definition, 85, 86 ; physical definition, 84 ; dis- tinguished from atoms, 118 ; how divided, 86-89, 93 ; of elementary substances, 119-126, 176 ; their integrity depends on what, 250; size of, 28, 34 ; structure of, 226, 242, 248, 251, 277; weight of, 66, 71, 83. Monatomic hydrates, 284. Multiple proportions, law of, 115. Multivalent, 249. Naphthaline, 323. Naphthas, 314. Nature, her three manifestations, 215. Newton, Sir Isaac, 97. Nitrate of zinc, 293. Nitric acid, 258, 274, 278 ; symbol determined, 258. Nitro-benzol and nitro-toluol, 320. Nitrogen, compounds with oxygen, 116; influence on combustion, 189 ; variations of quantivalence, 244. Nitro glycerine, 221, 318 ; experi- ment at Newport, 223 ; molecular structure, 227, 819 ; theory of its action, 225-233. Nitrous oxide, 181, 182. INDEX 331 Nitryl, 313. Nobat's bauds, 25. Nomenclature, principles of, 169. Ores, smelted by solar energy, 214. Organic compounds, 297. Oxalic acid, 285. Oxides, nomenclature of, 170 ; acid and basic, 292. Oxygen, atomic weight of, 125; chemical centre of Nature, 292 ; relations to dualistie theory, 292. Oxygen gas, 91 ; relations to com- bustibles, 189-237 ; preparation of, 178. Perissads and artiads, 248. Phenol, 322. Phlogiston theory, 98, 235. Phosgene gas, 279. Phosphoric acid, 280 ; chloride, 244 ; oxide, 191. Phosphorous chloride, 244. Phosphorus, combustion of, 189, 193 ; variation of quantivalence, 244, 247. Physical changes, definition, 95. Plants decompose carbonic dioxide, 156. Pneumatic trough, 167. Polarity of atoms, 273. Polarized light, 56-62. Potassic chlorate, crystals of, 181 ; used for making oxygen gas, 177, 178 ; burning sugar, 216. Potassic chloride, crystals of, 181. Potassic hydrate, 257, 273 ; acted on by aluminum, 266. Potassic nitrate (saltpetre), 218, 222, 288. Potassium, and water, 257. Projectile agents, 225. Propionic acid, 283, 316. Proportional numbers, 115; old sys- tem, 140. Propylic alcohol, 315 ; aldehyde, 314; glycol, 317; hydride, 313. Pseudo-alcohols, 315. Pyruvic acid, 316. Quantitative analysis, 123. Quantivalence, 238-251 ; distinctive feature of the new chemistry, 248 ; how far fixed, 246 ; variations of, 244-248. Quinone, 323, 325. Radicals, simple and compound, 268 ; consisting of carbon-atoms, 309 ; metals and metalloids, 270 ; electro-positive and electro-nega- tive, 270 ; serial relations, 271. Reactions, analytical, 177 ; syntheti- cal, 184; metathetical, 251; de- scribe results of experiments, 165 ; expressed by symbols, 144 ; indicate structure, 251, 301 ; nu- merical values calculated, 150, 179 ; acetic ether and potassic hy- drate, 302 ; ammonia and hydro- chloric acid, 185 ; ammonic ni- trate, when heated, 181 ; butyric acid and potassic hydrate, 303 ; carbonic dioxide and sodium, 153, 158 ; carbonic dioxide and sun- light, 158; carbonic oxide and chlorine gas, 279 ; carbonic oxide and oxygen gas, 279 ; chalk, when calcined, 165 ; chalk and hydro- chloric acid, 167 ; coal and oxy- gen, 205 ; metallic copper and chlorine gas, 187 ; electrolysis of water, 176 ; burning of hydrogen gas, 176 ; hydrogen and oxygen, 197; hydrochloric acid and iron, 263 ; iodide of nitrogen, when ex- ploded, 184 ; lime and water, 161 ; lime-water and carbonic dioxide, 162 ; magnesium and water, 284 ; nitric oxide and oxygen gas, 186 ; potassic chloride, when heated, 178 ; potassic hydrate and alumi- num, 266 ; potassic hydrate and nitric acid, 261 ; potassium and water, 257 ; sodic carbonate and cream-of-tartar, 147; sodic car- bonate and hydrochloric acid, 144, 147, 149, 150; sodic hydrate and hydrochloric acid, 256 ; sodi- um and water, 254; sulphuric acid and zinc, 263 ; sulphuric acid and zinc oxide, 264. " Religion and Chemistry '' — refer- ence, 213. Rochelle-salts formed in bread, 148. 332 INDEX. Socks, cinders of a primeval fire, ,213. R6saniline, 321. Rules of chemical arithmetic, 151. Sal-ammoniac, crystallization of, S3. Salts, nomenclature of, 172; defini- tion, 294. Scbeele,-234. Science, its method illustrated, 202. Series of homologues, 283, 306, 314, 315; of volatile acids, 283. Silicic hydrates, 286-288. Slaking of lime, 161. Snow-flakes, 56. Soap-bubbles, 29. Soap-film, effect on light, 30 ; thick- ness of, 32. Soda, caustic, 253. Soda-water, 163. Sodic carbonate and muriatic acid, 141 ; hydrate, 254. Sodium, action on water, 252 ; va- por colors flame, 253. Solids, characteristics of, 53 ; struct- ure illustrated, 53-62. Specific gravity distinguished from density, 68 ; of liquids and solids, 69; of gases and vapors, 71, 77, 80. Specific heat of elementary sub- stances, 131. Spectroscopic analysis, basis of, 199, 253, 257. Spectrum, solar, 25. Stahl, 235. Structure of molecules, 226, 242; determines qualities, 299 ; shown by reactions, 301 (see Molecular Structure). Substances defined by their mole- cules, 86; elementary, 109. Sugar burnt by potassic chlorate, 216 ; decomposed by heat, 87 ; decomposed by sulphuric acid, 87. Sulphate of lime, 293. Sulphuric acid, action on zinc, 263 ; action on zinc oxide, 264 ; graphic symbol, 285. Sun the source of energy, 214. Symbols, chemical, 141-150, 186; how determined, 136, 254, 258. Synthesis, 96, 175 ; of alizarine, 324 ; of organic compounds, 298. Synthetical reactions, 184-187. Table of alcohols, 315 ; of atomic weight of carbon, 127 ; of atomic weight of chlorine, 125 ; of atomic weight of hydrogen, 128 ; of atom- ic weight of oxygen, 125 ; of calo- rific power of combustibles, 192 ; of compounds of manganese and fluorine, 116; of compounds of nitrogen and oxygen, 116 ; of di- mensions of light-waves, 24; of elementary substances, 112 ; of hydrides of methyl, ethyl, etc., 314 ; law of multiple proportions, 117 ; quantivalence of atoms, 238, 242 ; specific heat of elementary substances, 132 ; thickness of soap-film, 32. Tartronic acid, 317. Temperature, 44-49 ; absolute scale, 46 ; centigrade scale, 45 ; Fahren- heit scale, 45. Test-papers, 166. Tetratomic hydrates, 286. Thermometer, 46. Thomson, Sir William, size of mol- ecules, 35. Toluidine, 321. Toluol, 320. Triatomic hydrates, 286. Triplumbic hydrate, 249. Trisodic pho'sphate, 286. Turmeric-paper, 166. Urea, crystallization of, 54. Valeric acid, normal, 283 ; isomeric modifications, 306. Vapors, condition of, 15 ; interpene- tration of, 17 ; specific gravity of, 76-83. Volatile acids, series of, 283. Water decomposed by electricity, 89, 92; decomposed by sodium, 252 ; decomposed by potassium, 257 ; hardness of, 164 ; influences chemical changes, 145 ; synthesis of, 197. INDEX. 333 Waves ~o( light, 24. Weight, important relations chemistry, 97 ; of molecules, • 71-83. Weights of atoms, 117. Weights and measures (French sys- tem), 67. Wollastonite, 289. » Zinc sulphate, 263. mw m u ... ■ >m i m }* .mm * \ y *m*mm**mm THE <$H^ SCIENTIFIC SERIES n ii i i nm i in-nri i iinl «T m f ii'iii - t a r i t i i f nni ii tii inn 'in-