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Among the great and fertile scientific conceptions which have either originated or become firmly established during the nineteenth century, the theory of evolution, if not the greatest of them all, will certainly take its place in the front rank. As a partial explanation (for no complete explanation is possible to finite intelligence) of the phenomena of nature, it illuminates every department of science, from the study of the most remote cosmic phenomena accessible to us to that of the minutest organisms revealed by the most powerful microscopes; while upon the great problem of the mode of origin of the various forms of life—long considered insoluble—it throws so clear a light that to many biologists it seems to afford as complete a solution, in principle, as we can expect to reach. THE NATURE AND LIMITS OF EVOLUTION
So many of the objections which are still made to the theory of evolution, and especially to that branch of it which deals with living organisms, rest upon a misconception of what it professes to explain, and even of what any theory can possibly explain, that a few words on its nature and limits seem to be necessary. Evolution, as a general principle, implies that all things in the universe, as we see them, have arisen from other things which preceded them by a process of modification, under the action of those all-pervading but mysterious agencies known to us as “natural forces,” or, more generally, “the laws of nature.” More particularly the term evolution implies that the process is an “unrolling,” or “unfolding,” derived probably from the way in which leaves and flowers are usually rolled up or crumpled up in the bud and grow into their perfect form by unrolling or unfolding. Insects in the pupa and vertebrates in the embryo exhibit a somewhat similar condition of folding, and the word is therefore very applicable to an extensive range of phenomena; but it must not be taken as universally applicable, since in the material world there are other modes of orderly change under natural laws to which the terms development or evolution are equally applicable. The “continuity” of physical phenomena, as illustrated by the late Sir William Grove in 1866, has the same general meaning, but evolution implies more than mere continuity or succession—something like growth or definite change from form to form under the action of unchangeable laws.


[Assistant] The “continuity” of physical phenomena, as illustrated by the late Sir William Grove in 1866, has the same general meaning, but evolution implies more than mere continuity or succession—something like growth or definite change from form to form under the action of unchangeable laws. The point to be especially noted here is, that evolution, even if it is essentially a true and complete theory of the universe, can only explain the existing conditions of nature by showing that it has been derived from some pre-existing condition through the action of known forces and laws. It may also show the high probability of a similar derivation from a still earlier condition; but the further back we go the more uncertain must be our conclusions, while we can never make any real approach to the absolute beginnings of things. Herbert Spencer, and many other thinkers before him, have shown that if we try to realize the absolute nature of the simplest phenomena, we are inevitably landed either in a contradiction or in some unthinkable proposition. Thus, suppose we ask, Is matter infinitely divisible, or is it not? If we say it is, we cannot think it out, since all infinity, however it may be stated in words, is really unthinkable.


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If we say it is, we cannot think it out, since all infinity, however it may be stated in words, is really unthinkable. If we say there is a limit—the ultimate atom—then, as all size is comparative, we can imagine a being to whom this atom seems as large as an apple or even a house does to us; and we then find it quite unthinkable that this mass of matter should be in its nature absolutely indivisible even by an infinite force. It follows that all explanations of phenomena can only be partial explanations. They can inform us of the last change or the last series of changes which brought about the actual conditions now existing, and they can often enable us to predict future changes to a limited extent; but both the infinite past and the remote future are alike beyond our powers. Yet the explanations that the theory of evolution gives us are none the less real and none the less important, especially when we compare its teachings with the wild guesses or the total ignorance of the thinkers of earlier ages. THE RISE AND PROGRESS OF THE IDEA OF EVOLUTION
If we trace, however briefly, the gradual development of knowledge and speculation on this subject, we shall perhaps appreciate more fully the advance we have really made during the present century.


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THE RISE AND PROGRESS OF THE IDEA OF EVOLUTION
If we trace, however briefly, the gradual development of knowledge and speculation on this subject, we shall perhaps appreciate more fully the advance we have really made during the present century. The first speculations on the nature and source of the phenomena of the universe, of which we have any knowledge, are those of the early Greek philosophers, such as Thales, Anaximander, Anaxagoras, and Empedocles; but as the more important of their teachings are embodied, with some approach to system and with much acuteness of reasoning, in the great poem of the Latin author Lucretius, “On the Nature of Things,” it will be sufficient to give a sketch of his main conclusions, making use of the excellent prose translation by Mr. H. A. J. Munro, of Trinity College, Cambridge.


[Assistant] Munro, of Trinity College, Cambridge. Lucretius had a very clear idea of the indestructibility of matter. He argues that things cannot have come out of nothing, and he says: “A thing never returns to nothing, but all things, after disruption, go back into the first bodies of matter.” He then argues that, as the actual processes of growth, decay, and other natural changes are imperceptible to us, therefore “Nature works by unseen bodies.” He justly claims great importance for the demonstration of the fact that in all matter whatever, however solid and hard it may be, there are vacancies, or, as he expresses it, “Mixed up in all things there is void or empty space.” He thus anticipated the modern doctrine that the molecules of matter do not come into actual contact. He then defines atoms thus: “First bodies are solid and without void”; and as nothing can be produced from nothing, he concludes that these first bodies (atoms or molecules) must be everlasting, and that they supply matter for the reproduction of all things. He then goes on to prove that these “first beginnings are of solid singleness, not formed of parts, but strong in everlasting singleness.” He further proves that these “first beginnings” (atoms) cannot be infinitely small, and also that the universe cannot be limited—that it is infinite. He thus anticipated the main ideas as to atoms and the universe which have been held by most materialistic thinkers down to our own times.


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He thus anticipated the main ideas as to atoms and the universe which have been held by most materialistic thinkers down to our own times. Lucretius was an absolute materialist, for though he did not deny the existence of the gods he refused them any share in the construction of the universe, which, he again and again urges, arose by chance, after infinite time, by the random motions and collisions and entanglements of the infinity of atoms. He assumes some forces analogous to gravitation and the molecular motions of gases in the following passage: “For the first beginnings of things move first of themselves; next these bodies which form a small aggregate and come nearest, so to say, to the powers of the first beginnings are impelled and set in movement by the unseen strokes of these first bodies, and they next in turn stir up other bodies which are a little larger.”
He also anticipated Galileo as to the equal speed of all falling bodies when not checked by the air in the following precise statement: “For whenever bodies fall through water and thin air they must quicken their descents in proportion to their weights, because the body of water and subtle nature of air cannot retard everything to an equal degree; on the other hand, empty void cannot offer resistance to anything in any direction at any time, but must continually give way; and for this reason all things must be moved and borne along with equal velocity, though of unequal weights, through the unresisting void.”
This is a wonderfully accurate general statement of the equal rate of motion of all kinds of matter under the same forces; and when we consider that there is no indication of any experimental basis for this conclusion, and that nothing equivalent to our sciences of physics or chemistry existed, we are amazed at the general correctness of many of his views, derived solely by a process of reasoning from the most obvious phenomena of nature. He argues that, given infinite matter and space and inherent motion, “things must go on and be completed,” and his general conclusion is thus expressed: “If you will apprehend and keep in mind these things, nature, free at once and rid of her haughty lords, is seen to do all things spontaneously of herself without the meddling of the gods.”
It is when he attempts to deal with the origin of living organisms that the absence of all knowledge of chemistry, physiology, and histology renders his task impossible and leads him into what seem to us the wildest absurdities. He has an elaborate but very unconvincing argument that sensation can arise out of atoms which have no sensation; and, taking the appearance of worms, etc., in the earth and in putrid matter as a proof that they are still actually produced _de novo_ in it, he argues that at some remote epoch the now worn-out earth was more fertile, and produced in like manner all kinds of animals. The first human infants he supposes to have been formed at some very remote time in the manner following: “For much heat and moisture would then abound in the fields; and therefore wherever a suitable spot offered wombs would grow, attached to the earth by roots; and when the warmth of the infants, flying the wet and craving the air, had opened these in the fulness of time, nature would turn to that spot the pores of the earth and constrain it to yield from its opened veins a liquid most like to milk.


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The first human infants he supposes to have been formed at some very remote time in the manner following: “For much heat and moisture would then abound in the fields; and therefore wherever a suitable spot offered wombs would grow, attached to the earth by roots; and when the warmth of the infants, flying the wet and craving the air, had opened these in the fulness of time, nature would turn to that spot the pores of the earth and constrain it to yield from its opened veins a liquid most like to milk. To the children the earth would furnish food, the heat raiment, the grass a bed rich in abundance of soft down.... Wherefore, again and again I say, the earth, with good title, has gotten and keeps the name of mother, since she of herself gave birth to mankind, and at a time nearly fixed shed forth every beast that ranges wildly over the great mountains, and at the same time the fowls of the air with all their varied shapes.”
The fact that this mode of origin commended itself to one of the brightest intellects of the first century B. C., enlightened by the best thought of the Grecian philosophers, may enable us the better to appreciate the immense advance made by modern evolutionists. THE FIRST REAL STEPS TOWARDS EVOLUTION
We have now a great blank of fifteen centuries—the dark ages of human progress—after which the era of observation and experiment began, and for the first time men really set themselves to study nature, thus laying the foundation for all the great theoretical advances of our time. As leading to the next great step in theories of evolution, we must note the life-long observations by Tycho Brahe of the apparent motions of the planets; the grand discovery of Kepler that all these apparently erratic motions were due to their revolution round the sun in elliptic orbits, with a fixed relation between their distance from the sun and their periods of revolution; and Newton’s epoch-making theory of universal gravitation by which all these facts and many others since discovered were harmonized and explained.


[Assistant] As leading to the next great step in theories of evolution, we must note the life-long observations by Tycho Brahe of the apparent motions of the planets; the grand discovery of Kepler that all these apparently erratic motions were due to their revolution round the sun in elliptic orbits, with a fixed relation between their distance from the sun and their periods of revolution; and Newton’s epoch-making theory of universal gravitation by which all these facts and many others since discovered were harmonized and explained. But all this implied no law of development, and it was long thought that the solar system was fixed and unchangeable—that some altogether unknown or miraculous agency must have set it going, and that it had in itself no principle of change or decay, but might continue as it now is to all eternity. It was at the very end of the eighteenth century that Laplace announced his “Nebular Hypothesis,” the first attempt ever made to explain the origin of the solar system under the influence of the known laws of motion, gravitation, and heat, acting upon an altogether different antecedent condition of things—a true process of evolution. Laplace supposed that the whole matter of the solar system was once in a condition of vapor, and that it formed an enormous nebulous mass many times larger than the then known dimensions of the planetary sphere. He showed how, under the influence of gravitation, this nebula would condense, and that such irregularities of motion and density as would be sure to exist would lead to rotation of the mass. Under the law of gravitation this would lead to outer rings being left behind by the contraction of the central mass, which rings would at a later period become drawn together at some point of initial greater density and thus form planets.


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Under the law of gravitation this would lead to outer rings being left behind by the contraction of the central mass, which rings would at a later period become drawn together at some point of initial greater density and thus form planets. The whole process is admitted to be mathematically demonstrable, given the initial conditions; but recent extensions of our knowledge of the interplanetary and interstellar spaces have shown that the supposed void is really full of invisible solid matter, ranging from the bulk of the smaller planets down to the finest dust, and it is very difficult to imagine any possible causes which would keep all the solid matter of the system in a state of vapor, when subject, on the confines of the mass, to the cold of interstellar space. The antecedent condition of our system is now thought to have been either wholly or partially meteoritic, but in either case we have a genuine theory of its evolution which has now been so extended as to include the appearance of comets and meteors, of nebulæ, and star clusters, of temporary, periodic, and colored stars, and many other phenomena of the stellar universe. It is no objection to these grand theories to urge that they do not explain the origin of the matter of the universe, either what it is or how it came to be where we now find it. We can only take one step at a time, and even if in these greater problems any further advance should be as yet denied us, it is still a great thing to have been able to take even one secure step into the vast and mysterious depths of the interstellar spaces. EVOLUTION OF THE EARTH’S CRUST
Although Pythagoras (500 B.


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EVOLUTION OF THE EARTH’S CRUST
Although Pythagoras (500 B. C.) believed that sea and land must often have changed places, and a few other observers at different epochs came to the same conclusion, yet, till quite recent times, the earth was generally supposed to have been always very much as it is now; people spoke of “the eternal hills”; and the great mountain ranges, the mighty ravines and precipices, as well as the deep seas and oceans, were believed to be the direct work of the Creator. It was only in the latter half of the eighteenth century that a few observers began to see the importance of studying the nature of the earth’s crust, so far as it could be reached in ravines, quarries, and mines; and one of the most earnest of these students, Dr. Hutton, of Edinburgh, after more than thirty years of travel and study, published his great work, _The Theory of the Earth_, which must be considered to be the starting-point of modern geology. He maintained that it was only by observing causes now in action that we can explain the phenomena presented by the stratified and igneous rocks; he showed that the former must have been laid down by water, and that the larger part of them, containing as they do marine shells and other fossils, must have been deposited on the sea-bottom. He showed how rain and rivers, frost and snow, wind and heat disintegrated the hardest rocks and would in time excavate the deepest valleys; while earthquakes, however small an elevation any one of them might produce, would in time raise the sea-bottom sufficiently high to form, when denuded, mountain ranges, plains, and valleys like those we now see everywhere upon the earth’s surface.


[Assistant] He showed how rain and rivers, frost and snow, wind and heat disintegrated the hardest rocks and would in time excavate the deepest valleys; while earthquakes, however small an elevation any one of them might produce, would in time raise the sea-bottom sufficiently high to form, when denuded, mountain ranges, plains, and valleys like those we now see everywhere upon the earth’s surface. He also showed that the most ancient stratified rocks, those that lie at the very base of the series, presented every indication of having been formed in exactly the same way as the most recent ones. Hence he stated a conclusion which excited a storm of opposition, in these words: “In the economy of the world I can find no traces of a beginning, no prospect of an end.” This was thought to imply a denial of creation, and was quite sufficient at that period to prevent the work of any man of science from being judged impartially. But although Playfair and a few others upheld Hutton’s views, they were too novel to receive much support by his contemporaries, and this was especially the case as regards the slow and continuous action of existing causes being sufficient to account for all the known phenomena presented by the crust of the earth. Hence the belief in catastrophes and cataclysms—in great convulsions tearing mountains asunder, and vast floods sweeping over whole continents—continued to prevail, till finally banished by the genius and perseverance of one man, Sir Charles Lyell. His _Principles of Geology_ was first published in 1830, and successive editions, revised and often greatly extended, continued to appear till the author’s death, forty-five years later.


[Assistant] His _Principles of Geology_ was first published in 1830, and successive editions, revised and often greatly extended, continued to appear till the author’s death, forty-five years later. As this work affords a fine example of the application of the principles of evolution to the later phases of the earth’s history, and as it not only revolutionized scientific opinion in its own domain, but prepared the way for the acceptance of the still more novel and startling application of the same principles to the entire organic world, it will be necessary to show what opinions prevailed at the time it first appeared in order that we may understand how great was the change it effected. In the earlier years of the nineteenth century the standard geological work, both in Great Britain and on the Continent, was Cuvier’s _Essay on the Theory of the Earth_. In 1827 a fifth edition of the English translation appeared, and there was a German translation so late as 1830—sufficient proofs of its wide popularity. Yet this work abounds in statements which are positively ludicrous to any one conversant with modern geology. It never appeals to known causes, but again and again assumes forces to be at work for which no evidence is adduced and which are totally at variance with what we see in the world to-day.


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It never appeals to known causes, but again and again assumes forces to be at work for which no evidence is adduced and which are totally at variance with what we see in the world to-day. A few examples justifying these statements must be here given. Cuvier shows that he was acquainted with the theory of modern causes, but he altogether rejects it, saying that “the march of nature is changed, and none of the agents she now employs would have been sufficient for the production of her ancient works.” He adduces “the primitive mountains” whose “sharp and bristling ridges and peaks are indications of the violent manner in which they have been elevated.” He allows that atmospheric agencies may form sea-cliffs, alluvial deposits, and taluses of loose matter at the foot of the precipices, but he adds: “These are but limited effects to which vegetation in general puts a stop, and which, besides, presuppose the existence of mountains, valleys, and plains—in short, all the inequalities of the globe—and which, therefore, cannot have given rise to those inequalities.” He contrasts the calm and peaceful aspect of the surface of the earth with the appearances discovered when we examine its interior. Here, in the raised beds of shells, the fractured rocks, the inclined or even vertical stratification, he finds abundant proofs “that the surface of the globe has been broken up by revolutions and catastrophes.”
He also refers to the numerous large blocks of the primitive rocks scattered over the surface of secondary formations, and separated by deep valleys or even by arms of the sea from the peaks or ridges from which they must have been derived, as further proofs of catastrophes; for, it is argued, they must have been either ejected by volcanic eruptions or carried by waters, which, in either case, “must have exceeded in violence anything we can imagine at the present day,” and he therefore concludes that “it is in vain we search among the powers which now act upon the surface of the earth for causes sufficient to produce the revolutions and catastrophes, the traces of which are exhibited in its crust.” He is quite confident that all these changes go on rapidly, periods of catastrophe alternating with periods of repose. The present surface of the earth he holds to be quite recent, and he maintains “that, if anything in geology be established, it is that the surface of our globe has undergone a great and sudden revolution, the date of which cannot be referred to a much earlier period than five or six thousand years ago; that this revolution overwhelmed and caused to disappear the countries which were previously inhabited by man, and the species of animals now best known; that, on the other hand, it laid dry the bottom of the last sea, and formed of it the countries which are at the present day inhabited.” And he further declares that “this event has been sudden, instantaneous, without any gradation; and what is so clearly demonstrated with respect to this last catastrophe is not less so with reference to those which preceded it.”
The method followed by Lyell was the very reverse of that of Cuvier. Instead of assuming hastily that modern causes were totally inadequate, and appealing constantly to purely imaginary and often inconceivable catastrophes, Lyell investigated these causes with painstaking accuracy, applying the tests of survey and time measurement, so as in many cases to prove that, given moderately long periods of time—not a few thousands only, but hundreds of thousands of years—they were fully adequate to explain the phenomena.


[Assistant] Instead of assuming hastily that modern causes were totally inadequate, and appealing constantly to purely imaginary and often inconceivable catastrophes, Lyell investigated these causes with painstaking accuracy, applying the tests of survey and time measurement, so as in many cases to prove that, given moderately long periods of time—not a few thousands only, but hundreds of thousands of years—they were fully adequate to explain the phenomena. He also showed that the imaginary causes of Cuvier would not explain the facts, for that everywhere in the crust of the earth we found conclusive proofs of very slow continuous changes exactly analogous to what now occur, never of great convulsions, except quite locally, as we have them now. He showed that modern volcanoes had poured out vast masses of melted rock during a single eruption, covering areas as extensive as those which any ancient volcano could be proved to have ejected in an equally short period; that strata were now in process of formation comparable in extent and thickness with any ancient strata; that organic remains are being preserved in them just as in the older rocks; that the land is almost everywhere rising or sinking as of old; that valleys are being excavated and plateaus or mountains upheaved; that earthquake shocks are producing faults beneath the surface; that vegetation is still preparing future coal beds; that limestones, clays, sandstones, metamorphic and igneous rocks are all still being formed; and that, given time, and the intermittent or continuous action of the causes we can now trace in operation, and all the varied features of the earth’s surface, as well as all the contortions and fractures which we discover in its crust, and every other phenomenon supposed to necessitate catastrophes and cataclysms will be again produced. In the massive volumes of the later editions of the _Principles of Geology_ all these points are discussed and illustrated with such a wealth of facts and such cogent yet cautious reasoning as have carried conviction to all modern students. It affords us perhaps the very best proof yet given of evolution in one department of the universe—that of the surface and the crust of the earth we inhabit. Not only have all the chief modifications during an almost unimaginable period of time been clearly depicted, but they have in almost every case been shown to be the inevitable results of real and comparatively well-known causes, such as we now see at work around us.


[Assistant] Not only have all the chief modifications during an almost unimaginable period of time been clearly depicted, but they have in almost every case been shown to be the inevitable results of real and comparatively well-known causes, such as we now see at work around us. The grand generalizations of Lyell have been strengthened since his death by more complete investigations of certain phenomena and their causes than were possible in his day; while the only objections to them seem to be founded, to some extent, upon a misconception. He has been termed a “Uniformitarian,” and it is alleged that it is unphilosophical to take the limited range of causes we now see in action, as a measure of those which have acted during all past geological time. But neither Lyell nor his followers make any such assumption. They merely say, we do not find any proof of greater or more violent causes in action in past times, and we do find many indications that the great natural forces then in action—seas and rivers, sun and cloud, rain and hail, frost and snow, as well as the very texture and constituents of the older rocks, and the mode in which the organisms of each age are preserved in them, must have been in their general nature and magnitude very much as they are now. Other objections, such as that the internal forces were greater when the earth was hotter, and that tidal effects must have been more powerful when the moon was nearer the earth, are altogether beside the question until we can obtain more definite measures of past time than we now possess in reference to both geological and cosmical phenomena.


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Other objections, such as that the internal forces were greater when the earth was hotter, and that tidal effects must have been more powerful when the moon was nearer the earth, are altogether beside the question until we can obtain more definite measures of past time than we now possess in reference to both geological and cosmical phenomena. It may well be that the physical changes above referred to have been so slow that they would have produced no perceptibly increased effect at the epoch of the early stratified rocks. Lyell’s doctrine is simply that of real against imaginary causes, and he only denies catastrophes and more violent agencies in early times, because there is no clear evidence of their actual existence, and also because known causes are quite competent to explain all geological phenomena. It must be remembered, too, that uniformitarians have never limited the natural forces of past geological periods to the precise limits of which we have had experience during the historical period. What they maintain is, that forces of the same _nature_ and of the same _order of magnitude_ are adequate to have brought about the evolution of the crust of the earth as we now find it. ORGANIC EVOLUTION, ITS LAWS AND CAUSES
We now come to that branch of the subject which is the most important and distinctive of our age, and which, in popular estimation, alone constitutes evolution—the mode of origin of the innumerable species of animal and plant life which now exist or have ever existed upon the earth.


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ORGANIC EVOLUTION, ITS LAWS AND CAUSES
We now come to that branch of the subject which is the most important and distinctive of our age, and which, in popular estimation, alone constitutes evolution—the mode of origin of the innumerable species of animal and plant life which now exist or have ever existed upon the earth. The origin of the different forms of life has till quite recent times been looked upon as an almost insoluble problem, although a few advanced thinkers, even in the eighteenth century, perceived that it was probably the result of some natural process of modification or evolution; but no force or law had been set forth and established in any way adequate to produce it until the publication of Darwin’s _Origin of Species_, in 1859. In the later editions of that work, Darwin has given a historical sketch of the progress of opinion on the subject. I shall, therefore, now only notice a few great writers which he has not referred to. We have seen what an impossible and even ludicrous explanation had to be given by Lucretius; and from his day down to the middle of the eighteenth century no advance had been made. Either the problem was not referred to at all, or the theological doctrine of a special creation was held to be the only possible one.


[Assistant] Either the problem was not referred to at all, or the theological doctrine of a special creation was held to be the only possible one. But in the middle of the eighteenth century the great French naturalist, Buffon, published his very important work, _Histoire Naturelle_, in fifteen volumes (1749–1767), in which, besides describing the characters and habits of all the animals then known, he introduced much philosophical and speculative thought, which would probably have been carried much further had he not felt obliged to conform to the religious prejudices of the age. We are indebted to Mr. Samuel Butler for having brought together all the important passages of Buffon’s voluminous and now little-read works bearing upon the question of evolution, and it is from his volume that I quote. Buffon lays stress on the great resemblance of all mammalia in internal structure, showing that the most unlike creatures may be really alike structurally. He says: “The horse, for example—what can at first sight seem more unlike mankind?


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He says: “The horse, for example—what can at first sight seem more unlike mankind? Yet when we compare man and horse, point by point and detail by detail, our wonder is excited rather by the resemblances than by the differences between them.” He then shows that all the parts of the skeleton agree, and that it is only in proportions, the increase of some bones and the suppression of others, that they differ, adding: “If we regard the matter thus, not only the ass and the horse, but even man himself, the apes, etc., might be regarded as forming members of one and the same family.” Then, after a few more illustrations, he remarks: “If we once admit that there are families of plants and animals, so that the ass may be of the family of the horse, and that the one may only differ from the other by degeneration from a common ancestor, we might be driven to admit that the ape is of the family of man, that he is but a degenerate man, and that he and man have had a common ancestor.... If it were once shown that we had right grounds for establishing these families, if the point were once gained that among plants and animals there have been even a single species which had been produced in the course of direct descent from another species, then there is no further limit to be set to the power of nature, and we should not be wrong in supposing that with sufficient time she could have evolved all other organized forms from one primordial type.”
This indicates clearly enough his own opinion, but to save himself from the ecclesiastical authorities he at once adds this saving clause: “But no! It is certain, from revelation, that all animals have alike been favored with the grace of an act of direct creation, and that the first pair of every species issued full formed from the hands of the Creator.”
Such examples of disarming religious prejudice are frequent, but he continually recurs to statements as to mutability which neutralize them. Here, for example, is a broad claim for nature as opposed to creation. He has been showing how variable are many animals, and how changes of food, climate, and general surroundings influence both their forms and their habits; and then he exclaims:
“What cannot nature effect with such means at her disposal?


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He has been showing how variable are many animals, and how changes of food, climate, and general surroundings influence both their forms and their habits; and then he exclaims:
“What cannot nature effect with such means at her disposal? She can do all except either create matter or destroy it. These two extremes of power the Deity has reserved for Himself only; creation and destruction are the action of His omnipotence. To alter and undo, to develop and renew—these are powers which He has handed over to the charge of nature.”
Here we have a claim for the power of nature in the modification of species which fully comes up to the requirements of the most advanced evolutionist. It is remarkable, too, how clearly he perceived the great factors so important for the evolution of organisms, rapid multiplication, great variability, and the struggle for existence. Thus he remarks: “It may be said that the movement of nature turns upon two immovable pivots—one, the illimitable fecundity which she has given to all species; the other, the innumerable difficulties which reduce the results of that fecundity and leave throughout time nearly the same quantity of individuals in every species.” Here the term “difficulties” corresponds to the “positive checks” of Malthus, and to the “struggle for existence” of Darwin; and he again and again refers to variability—as when he says: “Hence, when by some chance, common enough with nature, a variation or special feature makes its appearance, man has tried to perpetuate it by uniting together the individuals in which it has appeared.”
As Buffon thus clearly understood artificial selection, thoroughly appreciated the rapid increase of all organisms, and equally well saw that their inordinate increase was wholly neutralized through such destructive agencies as hunger, disease, and enemies, and as, at the same time, he had such unbounded faith in the power of nature to modify animal and vegetable forms, we feel assured that this great writer and original thinker only needed freedom to pursue this train of thought a little further and he would certainly have anticipated Darwin’s great discovery of natural selection by a whole century.


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Thus he remarks: “It may be said that the movement of nature turns upon two immovable pivots—one, the illimitable fecundity which she has given to all species; the other, the innumerable difficulties which reduce the results of that fecundity and leave throughout time nearly the same quantity of individuals in every species.” Here the term “difficulties” corresponds to the “positive checks” of Malthus, and to the “struggle for existence” of Darwin; and he again and again refers to variability—as when he says: “Hence, when by some chance, common enough with nature, a variation or special feature makes its appearance, man has tried to perpetuate it by uniting together the individuals in which it has appeared.”
As Buffon thus clearly understood artificial selection, thoroughly appreciated the rapid increase of all organisms, and equally well saw that their inordinate increase was wholly neutralized through such destructive agencies as hunger, disease, and enemies, and as, at the same time, he had such unbounded faith in the power of nature to modify animal and vegetable forms, we feel assured that this great writer and original thinker only needed freedom to pursue this train of thought a little further and he would certainly have anticipated Darwin’s great discovery of natural selection by a whole century. Even as it is we must class him as one of the great pioneers of organic evolution. The next distinct step towards a theory of organic evolution was made by the poet Goethe at the very end of the eighteenth century, in his views of the metamorphosis of plants. He pointed out the successive modifications of the leaf which produced all the other essential parts of the higher plants—the simple cotyledons or seed leaves became modified into the variously formed leaves of the fully grown plants; these again were successively modified into the calyx, corolla, stamens, and ovary of the flower. He supposed this to be due to the increased refinement of the sap under the influence of light and air, and to indicate the steps by which the various parts of the flower had been developed. It was, therefore, a theory of evolution; but it was very unsatisfactory, inasmuch as it in no way accounted for the wonderful variety of the floral organs, or indicated any purpose served by the most prominent and conspicuous part of the flower, the highly colored and often strangely formed corolla.


[Assistant] It was, therefore, a theory of evolution; but it was very unsatisfactory, inasmuch as it in no way accounted for the wonderful variety of the floral organs, or indicated any purpose served by the most prominent and conspicuous part of the flower, the highly colored and often strangely formed corolla. It was also erroneous in supposing that the corolla was a modified calyx, whereas it is now known to be a modification of the stamens. Next came the great work of Lamarck in the first decade of the nineteenth century, in which he proposed a general system of evolution of the whole animal world. Hence he may be termed the first systematic evolutionist. His system has been rather fully described by Lyell, who, in his _Principles of Geology_, devotes a whole chapter to a summary of his doctrines; while Mr. Butler gives copious quotations in three chapters of his _Evolution Old and New_; and any one who is not acquainted with the original work of Lamarck should read these two authors in order to understand how wide was his knowledge, how ingenious his explanations, and in how many important points he anticipated the views both of Lyell and Darwin.


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Butler gives copious quotations in three chapters of his _Evolution Old and New_; and any one who is not acquainted with the original work of Lamarck should read these two authors in order to understand how wide was his knowledge, how ingenious his explanations, and in how many important points he anticipated the views both of Lyell and Darwin. But he was half a century in advance of his age, and his only alleged causes of modification—changed conditions, use and disuse, habit and effort—were wholly insufficient to account for the vast range of the phenomena presented by the innumerable minute adaptations of living organisms to their conditions of life. He even imputed all the modifications of domestic animals to the changed conditions of food and habits to which they have been subjected by man, making no reference to the use of selection by breeders, in this respect falling short of his great predecessor, Buffon. The general laws which Lamarck deduces from his elaborate study of nature are these:
“Firstly. That in every animal which has not passed its limit of development, the more frequent and sustained employment of any organ develops and aggrandizes it, giving it a power proportionate to the duration of its employment, while the same organ, in default of constant use, becomes insensibly weakened and deteriorated, decreasing imperceptibly in power until it finally disappears. “Secondly.


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“Secondly. That these gains or losses of organic development, due to use or disuse, are transmitted to offspring, provided they have been common to both sexes, or to the animals from which the offspring have descended.”
The whole force of this argument depends upon the second clause—the inheritance of those individual modifications due to use and disuse. But no direct evidence of this has ever been found, while there is a good deal of evidence showing that it does not occur. Again, there are many structures which cannot have been produced by use, such, for example, as the feathers of the peacock’s train, the poison in the serpent’s fangs, the hard shells of nuts, the prickly covering of many fruits, the varied armor of the turtle, porcupine, crocodile, and many others. For these reasons Lamarck’s views gained few converts; and although some of his arguments have been upheld in recent years, the fatal objections to his general principle as a means of explaining the evolution of organic forms has never been overcome. Between the periods of Lamarck and Darwin many advances were made which clearly pointed to a general law of evolution in nature.


[Assistant] Between the periods of Lamarck and Darwin many advances were made which clearly pointed to a general law of evolution in nature. Such were Sir William Grove’s lectures on the “Correlation of the Physical Forces,” in 1842; Helmholtz on the “Conservation of Energy,” in 1847; and Herbert Spencer’s essay on “The Development Hypothesis,” in 1852. This latter work was a complete and almost unanswerable argument for a natural process of continuous evolution of the whole visible universe, including organic nature, man, and social phenomena. It is further extended in the later editions of the author’s _First Principles_, which, as a coherent exposition of philosophy, co-ordinating and explaining all human knowledge of the universe into one great system of evolution everywhere conforming to the same general principles, must be held to be one of the greatest intellectual achievements of the nineteenth century. It left, however, the exact method of evolution of organisms untouched, and thus failed to account for those complex adaptations and appearances of design in the various species of animals and plants which have always been the stronghold of those who advocated special creation. This difficulty was met by Darwin’s theory of _The Origin of Species by Means of Natural Selection_, published in 1859, and the series of works that succeeded it; and to a brief sketch of this theory the remainder of our space must be devoted.


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This difficulty was met by Darwin’s theory of _The Origin of Species by Means of Natural Selection_, published in 1859, and the series of works that succeeded it; and to a brief sketch of this theory the remainder of our space must be devoted. THE THEORY OF “NATURAL SELECTION”
Although, as we have seen, a succession of great writers and thinkers had for more than half a century shown the necessity for some process of evolution as the only rational or intelligible mode of origin of existing species of animals and plants, as well as of the whole physical universe, yet these views were by no means generally accepted by the educated classes, while few bodies of students were less influenced by them than zoologists and botanists, generally known as naturalists. Now, Darwin wrote especially for these classes, and no one knew better than he did their great prejudice on this matter. Not only had such men as Sir Charles Lyell and Sir John Herschel expressed themselves strongly against all theories of the transmutation of species, but the universal contempt and indignation of naturalists as well as theologians against _The Vestiges of Creation_, published anonymously a few years earlier, and giving a most temperate and even religious exposition of the general arguments for the universality of evolution, showed what any one might expect who advocated and attempted to demonstrate a similar theory. This accounts for Darwin writing to Sir Joseph Hooker, in 1844, of his being “almost convinced that species are not (it is like confessing a murder) immutable,” and again, in 1845, to the Rev. L.


[Assistant] L. Blomefield, that he now saw the way in which new varieties become exquisitely adapted to the external conditions of life and to other surrounding beings, and he adds: “I am a bold man to lay myself open to being thought a complete fool, and a most deliberate one.” It is only by a consideration of the frame of mind of even advanced thinkers at the time Darwin was preparing his work, and remembering how small was the effect which had been produced by Buffon, Goethe, Lamarck, the author of _Vestiges of Creation_, and the earlier writings of Herbert Spencer, that we can adequately realize the marvellous work that he accomplished. Let us now briefly consider the essential nature of this new theory, which in a few brief years became the established belief of the great majority of the students of nature, and which also gave a new interest in nature to the whole thinking world. The theory of natural selection is founded upon a few groups of thoroughly ascertained and universally admitted facts, with the direct and necessary results of those facts. The first group of facts consists of the great powers of increase of all organisms and the circumstance that, notwithstanding this great yearly increase, the actual population of each species remains stationary, there being no permanent increase. Now, these two facts were recognized by Buffon, but though, of course, known to all subsequent writers, were fully appreciated or thought out to their logical results by none of them.


[Assistant] Now, these two facts were recognized by Buffon, but though, of course, known to all subsequent writers, were fully appreciated or thought out to their logical results by none of them. Lamarck, so far as I can ascertain, took no notice of them whatever. Darwin has given illustrations of these facts in Chapter IV. of the _Origin of Species_, and I have added others in the second chapter of my _Darwinism_. That the population of each species remains stationary, with, of course, considerable fluctuations, is both a matter of observation and of reasoning. The powers of increase of all creatures are so great that if there is in any country room and food for a larger number of any species they will be produced in a year or two.


[Assistant] The powers of increase of all creatures are so great that if there is in any country room and food for a larger number of any species they will be produced in a year or two. It is impossible, therefore, to believe that, in a state of nature, where all kinds of animals and plants have lived together as they best could for thousands of years, there can be any important difference in their numbers from year to year or from century to century. Now, it is as a consequence of these two indisputable facts that the struggle for existence necessarily results. For if every year each pair of animals or each plant produces only ten young animals or plants, and this is very far below the average, and if the adult life of these is taken at ten years, again below the average of the higher plants and animals, then, unless some of the parents die, the whole of the offspring must die off every year; or, in other words, only as many young can survive as are necessary to replace the old ones that die. Hence the deaths must always (on the average and in the long run) equal the births. This terrible yearly destruction is an absolutely certain fact, as well as an inevitable result of the two preceding facts, and it is said to be due to the struggle for existence.


[Assistant] This terrible yearly destruction is an absolutely certain fact, as well as an inevitable result of the two preceding facts, and it is said to be due to the struggle for existence. This struggle is manifold in its nature. Individuals of the same species struggle together for food, for light, for moisture; they struggle also against other species having the same wants; they struggle against every kind of enemy, from parasitic worms and insects up to carnivorous animals; and there is a continual struggle with the forces of nature—frosts, rains, droughts, floods, and tempests. These varied causes of destruction may be seen constantly at work by any one who looks for them. They act from the moment of birth, being more especially destructive to the young; and, as only one in ten or fifty or a thousand (according to the rate of increase of the particular species) can possibly come to the full breeding age, we feel compelled to ask ourselves: What determines the nine or the forty-nine or the nine hundred and ninety-nine, as the case may be, which die, and the one which survives? Darwin calls this process of extermination one of “natural selection”—that is, by this process nature weeds out the weak, the unhealthy, the unadapted, the imperfect in any way.


[Assistant] Darwin calls this process of extermination one of “natural selection”—that is, by this process nature weeds out the weak, the unhealthy, the unadapted, the imperfect in any way. Of course, what may be called chance or accident produces many deaths of individuals otherwise well fitted to live, but if we think of the process going on day by day and year by year till only one in a hundred of those born in a given area are left alive, it is impossible to suppose that the _one_ which has passed through all the dangers and risks which have been fatal to, say, his ninety-nine relations was not, in all the faculties and qualities essential to the continuance of the race, decidedly better organized than the bulk of those which succumbed. Herbert Spencer calls the process the “survival of the fittest,” and though the term may not be strictly accurate in the case of any one species in any one year, yet when we consider that the struggle is going on _every_ year, during the whole duration of each species, we cannot doubt that, on the whole, and in the long run, those which survive are among the fittest. The struggle is so severe, so incessant, that the smallest defect in any sense organ, any physical weakness, any imperfection in constitution, will almost certainly, at one time or another, be fatal. This continual weeding out of the less fit, in every generation, and with exceptional severity in recurring adverse seasons, will produce two distinct effects, which require to be clearly distinguished. The first is the preservation of each species in the highest state of adaptation to the conditions of its existence; and, therefore, so long as these conditions remained unchanged, the effect of natural selection is to keep each well-adapted species also unchanged.


[Assistant] The first is the preservation of each species in the highest state of adaptation to the conditions of its existence; and, therefore, so long as these conditions remained unchanged, the effect of natural selection is to keep each well-adapted species also unchanged. The second effect is produced whenever the conditions vary, when, taking advantage of the variations continually occurring in all well-adapted and therefore populous species, the same process will slowly but surely bring about complete adaptation to the new conditions. And here another fact—the normal variability of all populous or dominant species, which is seldom realized except by those who have largely and minutely compared the individuals of many species in a state of nature—comes into play. There are some writers who admit all the preceding facts and reasoning, so far as the action of natural selection in weeding out the unfit and thus keeping every species in the highest state of efficiency is concerned, but who deny that it can modify them in such a way as to adapt them to new conditions, because they allege that “the right variations will not always occur at the right time.” This seems a strong and real objection to many of their readers, but to those who have studied the variability of species in nature, it is a mere verbal difficulty dependent on ignorance of the actual facts. A brief statement of the facts must therefore be given. Of late years, and chiefly since Darwin’s works were written, the variability of animals and plants in a state of nature has been carefully studied, by actual comparison and measurement of scores, hundreds, and even thousands of individuals of many common, that is, abundant and widely distributed species; and it is found that in almost every case they vary greatly, and, what is still more important, that every organ and every appendage varies independently and to a large amount.


[Assistant] Of late years, and chiefly since Darwin’s works were written, the variability of animals and plants in a state of nature has been carefully studied, by actual comparison and measurement of scores, hundreds, and even thousands of individuals of many common, that is, abundant and widely distributed species; and it is found that in almost every case they vary greatly, and, what is still more important, that every organ and every appendage varies independently and to a large amount. Some of the best known of these facts of variation are adduced in my _Darwinism_, and are illustrated by numerous diagrams, and much more extensive series have since been examined, always with the same general result. By large variability is meant a variation of from ten to twenty-five per cent. on each side of the mean size, this amount of variation occurring in at least five or ten per cent. of the whole number of individuals, and in every organ or part as yet examined, external or internal. Now, as the weeding-out process is so severe, only from one in ten to one in a hundred of those born surviving to produce young, the above proportion of variations affords ample scope for the selection of any variation needed in order to modify the species so as to bring it into harmony with new or changing conditions.


[Assistant] Now, as the weeding-out process is so severe, only from one in ten to one in a hundred of those born surviving to produce young, the above proportion of variations affords ample scope for the selection of any variation needed in order to modify the species so as to bring it into harmony with new or changing conditions. And this will be the more easy and certain if we consider how slowly land-surfaces and climates undergo permanent changes; and these are certainly the kind of changes that initiate and compel alterations, first, perhaps, in the distribution, and afterwards in the structure and habits of species. It follows, therefore, as an absolutely necessary conclusion from the facts, if natural selection can and does keep each continually varying species in close adaptation to an unchanging environment, that it preserves the fixity of its mean or average condition, and almost every objector admits this. Then, given a slowly changing environment, the same power must inevitably bring about whatever corresponding change is needed for the well-being and permanent survival of the various species which are subjected to those changed conditions. I shall not add here a further consideration of the objections and difficulties alleged by critics of the theory. All of these have, I believe, been fully answered either by Darwin or myself, many of the most recent having been discussed in review articles.


[Assistant] All of these have, I believe, been fully answered either by Darwin or myself, many of the most recent having been discussed in review articles. Suffice it to say here that this theory of natural selection—meaning the elimination of the least fit, and therefore the ultimate “survival of the fittest”—has furnished a rational and precise explanation of the means of adaptation of all existing organisms to their conditions, and therefore of their transformation from the series of distinct but allied species which occupied the earth at some preceding epoch. In this sense it has actually demonstrated the “origin of species,” and, by carrying back this process step by step into earlier and earlier geological times, we are able mentally to follow out the evolution of all forms of life from one or a few primordial forms. Natural selection has thus supplied that motive power of change and adaptation that was wanting in all earlier attempts at explanation, and this has led to its very general acceptance both by naturalists and by the great majority of thinkers and men of science. The brief sketch now given of the progress of human thought on the questions of the fact and the mode of the evolution of the material universe indicates how great has been the progress during the nineteenth as compared with all preceding centuries. Although the philosophical writers of classical times obtained a few glimpses of the action of law in nature regulating its successive changes, nothing satisfactory could be effected till the actual facts had been better ascertained by the whole body of workers who, during the last five centuries, have penetrated ever more and more deeply into nature’s mysteries and laws.


[Assistant] Although the philosophical writers of classical times obtained a few glimpses of the action of law in nature regulating its successive changes, nothing satisfactory could be effected till the actual facts had been better ascertained by the whole body of workers who, during the last five centuries, have penetrated ever more and more deeply into nature’s mysteries and laws. By their labors we became possessed of such a body of carefully observed facts that, towards the end of the eighteenth century, such thinkers as Laplace and Hutton were enabled to give us the first rudiments of theories of evolution as applied to the solar system and the earth’s crust, both of which have been greatly developed and rendered more secure during the century just passed away. In like manner Buffon and Goethe may be said to have started the idea of organic evolution, more systematically treated a little later by Lamarck, but still without any discovery of laws adequate to produce the results we see everywhere in nature. The subject then languished, till, after twenty years of observation and research, Charles Darwin produced a work which at once satisfied many thinkers that the long-desired clew had been discovered. Its acceptance by almost the whole scientific world soon followed: it threw new light on almost every branch of research, and it will probably take its place, in the opinion of future generations, as the crowning achievement of the nineteenth century. ALFRED RUSSEL WALLACE.


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ALFRED RUSSEL WALLACE. CHEMISTRY

The progress of the science of chemistry forms one phase of the progress of human thought. While at first mankind was contented to observe certain phenomena, and to utilize them for industrial purposes, if they were found suitable, “philosophers,” as the thinking portion of our race loved to call themselves, have always attempted to assign some explanation for observed facts, and to group them into similars and dissimilars. It was for long imagined, following the doctrines of the Greeks and of their predecessors, that all matter consisted of four elements or principles, names which survive to this day in popular language. These were “fire,” “air,” “water,” and “earth.” It was not until the seventeenth century that Boyle in his _Sceptical Chymist_ (1661) laid the foundations of the modern science, by pointing out that it was impossible to explain the existence of the fairly numerous chemical substances known in his day, or the changes which they can be made to undergo, by means of the ancient Greek hypotheses regarding the constitution of matter. He laid down the definition of the modern meaning of the word “element”; he declined to accept the current view that the properties of matter could be modified by its assimilating the qualities of fire, air, earth, or water, and he defined an element as the _constituent_ of a compound body.


[Assistant] He laid down the definition of the modern meaning of the word “element”; he declined to accept the current view that the properties of matter could be modified by its assimilating the qualities of fire, air, earth, or water, and he defined an element as the _constituent_ of a compound body. The first problem, then, to be solved, was to determine which of the numerous forms of matter were to be regarded as elementary, and which are compound, or composed of two or more elements in a state of combination; and to produce such compounds by causing the appropriate elements to unite with each other. One of the first objects to excite curiosity and interest was the air which surrounds us, and in which we live and move and have our being. It was, however, endowed with a semi-spiritual and scarcely corporeal nature in the ideas of our ancestors, for it does not affect the senses of sight, smell, or taste, and though it can be felt, yet it eludes our grasp. The word “gas,” moreover, was not invented until Van Helmont devised it to designate various kinds of “airs” which he had observed. The important part which gases play in the constitution of many chemical compounds was accordingly overlooked; and, indeed, it appeared to be almost as striking a feat of necromancy to produce a quantity of a gas of great volume from a small pinch of solid powder as for a “Jinn” of enormous stature but of delicate texture to issue from a brass pot, as related in the _Arabian Nights Entertainments_.


[Assistant] The important part which gases play in the constitution of many chemical compounds was accordingly overlooked; and, indeed, it appeared to be almost as striking a feat of necromancy to produce a quantity of a gas of great volume from a small pinch of solid powder as for a “Jinn” of enormous stature but of delicate texture to issue from a brass pot, as related in the _Arabian Nights Entertainments_. Gradually, however, it came to be recognized, not merely that gases have corporeal existence, but that they even possess weight. This, though foreshadowed by Torricelli, Jean Rey, and others, was first clearly proved by Black, professor of chemistry in Edinburgh, in 1752, through his masterly researches, as carbonic acid. The ignorance of the material nature of gases and of their weight lies at the bottom of the “Phlogistic Theory,” a theory devised by Stahl about the year 1690, to account for the phenomena of combustion and respiration and the recovery or “reduction” of metals from their “earths” by heating with charcoal or allied bodies. According to this inverted theory, a substance capable of burning was imagined to contain more or less phlogiston, a principle which it parted with on burning, leaving an earth deprived of phlogiston, or “dephlogisticated,” behind if a metal. This earth, when heated with substances rich in phlogiston, such as coal, wood, flour, and similar bodies, recovered the phlogiston, which it had lost on burning, and, with the added phlogiston, its metallic character.


[Assistant] This earth, when heated with substances rich in phlogiston, such as coal, wood, flour, and similar bodies, recovered the phlogiston, which it had lost on burning, and, with the added phlogiston, its metallic character. Other substances, such as phosphorus and sulphur, gave solids or acid liquids, to which phlogiston was not so easy to add; but even they could be rephlogisticated. On this hypothesis, it was the earths, and such acid liquids as sulphuric or phosphoric acids, which were the elements; the metals and sulphur and phosphorus were their compounds with phlogiston. The discovery of oxygen by Priestley and by Scheele in 1774, and the explanation of its functions by Lavoisier during the following ten years, gave their true meaning to these phenomena. It was then recognized that combustion was union with oxygen; that an “earth” or “calx” was to be regarded as the compound of a metal with oxygen; that when a metal becomes tarnished, and converted into such an earthy powder, it is being oxidized; that this oxide, on ignition with charcoal or carbon, or with compounds such as coal, flour, or wood, of which carbon is a constituent, gives up its oxygen to the carbon, forming an oxide of carbon, carbonic oxide on the one hand, or carbonic “acid” on the other, while the metal is reproduced in its “reguline” or metallic condition, and that the true elements are metals, carbon, sulphur, phosphorus, and similar bodies, and not the products of their oxidation. The discovery that air is in the main a mixture of nitrogen, an inert gas, and oxygen, an active one, together with a small proportion of carbonic “acid” (or, as it is now termed, anhydride)—a discovery perfected by Rutherford, Black, and Cavendish—and that water is a compound with oxygen of hydrogen, previously known as inflammable air, by Cavendish and by Watt, finally overthrew the theory of phlogiston; but at the beginning of this century it still lingered on, and was defended by Priestley until his death in 1804.


[Assistant] The discovery that air is in the main a mixture of nitrogen, an inert gas, and oxygen, an active one, together with a small proportion of carbonic “acid” (or, as it is now termed, anhydride)—a discovery perfected by Rutherford, Black, and Cavendish—and that water is a compound with oxygen of hydrogen, previously known as inflammable air, by Cavendish and by Watt, finally overthrew the theory of phlogiston; but at the beginning of this century it still lingered on, and was defended by Priestley until his death in 1804. Such, in brief, was the condition of chemical thought in the year 1800. Scheele had died in 1786, at the early age of forty-four; Lavoisier was one of the victims of the French Revolution, having been guillotined in 1794; Cavendish had ceased to work at chemical problems, and was devoting his extraordinary abilities to physical problems of the highest importance, while living the life of an eccentric recluse, and Priestley, driven by religious persecution from England to the more tolerant shores of America, was enjoying a peaceful old age, enlivened by occasional incursions into the region of sectarian controversy. The first striking discovery of our century was that of the compound nature of the alkalies and of the alkaline earths. This discovery was made by Humphry Davy. Born in Cornwall in 1778, he began the study of chemistry, self-taught, in 1796; and in 1799 he became director of the “Pneumatic Institution,” an undertaking founded by Dr.


[Assistant] Born in Cornwall in 1778, he began the study of chemistry, self-taught, in 1796; and in 1799 he became director of the “Pneumatic Institution,” an undertaking founded by Dr. Beddoes, at Bristol, for the purpose of experiments on the curative effects of gases in general. Here he at once made his mark by the discovery of the remarkable properties of “laughing gas,” or nitrous oxide. At the same time he constructed a galvanic battery, and began to perform experiments with it in attempting to decompose chemical compounds by its means. In 1801 Davy was appointed professor of chemistry at the Royal Institution, a society or club which had been founded a few years previously by Benjamin Thompson, Count Rumford, for the purpose of instructing and amusing its members with recent discoveries in chemistry and natural philosophy. In 1807 Davy applied his galvanic battery to the decomposition of damp caustic potash and soda, using platinum poles.


[Assistant] In 1807 Davy applied his galvanic battery to the decomposition of damp caustic potash and soda, using platinum poles. He was rewarded by seeing globules of metal, resembling mercury in appearance, at the negative pole; and he subsequently proved that these globules, when burned, reproduced the alkali from which they had been derived. They also combined with “oxymuriatic acid,” as chlorine (discovered by Scheele) was then termed, forming ordinary salt, if sodium be employed, and the analogous salt, “muriate of potash,” if the allied metal, potassium, were subjected to combustion. By using mercury as the negative pole, and passing a current through a strong solution of the chloride of calcium, strontium, or barium, Davy succeeded in procuring mixtures with mercury or “amalgams” of their metals, to which he gave the names calcium, strontium, and barium. Distillation removed most of the mercury, and the metal was left behind in a state of comparative purity. The alkali metals, potassium and sodium, were found to attack glass, liberating “the basis of the silex,” to which the name silicon has since been given.


[Assistant] The alkali metals, potassium and sodium, were found to attack glass, liberating “the basis of the silex,” to which the name silicon has since been given. Thus nearly the last of the “earths” had been decomposed. It was proved that not merely were the “calces” of iron, copper, lead, and other well-known metals compounds of the respective metals with oxygen, but Davy showed that lime, and its allies, strontia and baryta, and even silica or flint, were to be regarded as oxides of elements of metallic appearance. To complete our review of this part of the subject, suffice it to say that aluminum, a metal now produced on an industrial scale, was prepared for the first time in 1827 by Wöhler, professor of chemistry at Göttingen, by the action of potassium on its chloride, and alumina, the earthy basis of clay, was shown to be the oxide of the metal aluminum. Indeed, the preparation of this metal in quantity is now carried out at Schoffhausen-on-the-Rhine and at the Falls of Foyers, in Scotland, by electrolysis of the oxide dissolved in melted cryolite, a mineral consisting of the fluorides of sodium and aluminum, by a method differing only in scale from that by means of which Davy isolated sodium and potassium in 1806. To Davy, too, belongs the merit of having dethroned oxygen from its central position among the elements.


[Assistant] To Davy, too, belongs the merit of having dethroned oxygen from its central position among the elements. Lavoisier gave to this important gas the name “oxygen,” because he imagined it to be the constituent of all acids. He renamed the common compounds of oxygen in such a manner that the term oxygen was not even represented in the name—only inferred. Thus a “nitrate” is a compound of an oxide of nitrogen and an oxide of a metal; a “sulphate,” of the oxide of a metal with one of the oxides of sulphur, and so on. Davy, by discovering the elementary nature of chlorine, showed, first, that it is not an oxide of hydrochloric acid (or muriatic acid as it was then called); and, second, that the latter acid is the compound of the element chlorine with hydrogen. This he did by passing chlorine over white-hot carbon—a substance eminently suited to deprive oxy-compounds of their oxygen—and proving that no oxide of carbon is thereby produced; by acting on certain chlorides, such as those of tin or phosphorus with ammonia, and showing that no oxide of tin or phosphorus is formed; and, lastly, by decomposing “muriatic acid gas” (gaseous hydrogen chloride) with sodium, and showing that the only product besides common salt is hydrogen.


[Assistant] This he did by passing chlorine over white-hot carbon—a substance eminently suited to deprive oxy-compounds of their oxygen—and proving that no oxide of carbon is thereby produced; by acting on certain chlorides, such as those of tin or phosphorus with ammonia, and showing that no oxide of tin or phosphorus is formed; and, lastly, by decomposing “muriatic acid gas” (gaseous hydrogen chloride) with sodium, and showing that the only product besides common salt is hydrogen. Instead, therefore, of the former theory that a chloride was a compound of the unknown basis of oxymuriatic acid with oxygen and the oxide of a metal, he introduced the simpler and correct view that a chloride is merely a compound of the element chlorine with a metal. In 1813 he established the similar nature of fluorine, pointing out that on the analogy of the chlorides it was a fair deduction that the fluorides are compounds of an undiscovered element, fluorine, with metals; and that hydrofluoric acid is the true analogue of hydrochloric acid. The truth of this forecast has been established of recent years by Henri Moissan, who isolated gaseous fluorine by subjecting a mixture of hydrofluoric acid and hydrogen potassium fluoride contained in a platinum U tube to the action of a powerful electric current. He has recently found that the tube may be equally well constructed of copper; and this may soon lead to the industrial application of the process. The difficulty of isolating fluorine is due to its extraordinary chemical energy; for there are few substances, elementary or compound, which resist the action of this pale yellow, suffocating gas.


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The difficulty of isolating fluorine is due to its extraordinary chemical energy; for there are few substances, elementary or compound, which resist the action of this pale yellow, suffocating gas. In 1811 iodine, separated by Courtois from the ashes of sea-plants, was shown by Davy to be an element analogous to chlorine. Gay-Lussac subsequently investigated it and prepared many of its compounds; and in 1826 the last of these elements, bromine, was discovered in the mother-liquor of sea-salt by Balard. The elements of this group have been termed “halogens,” or “salt producers.”
While Davy was pouring his researches into the astonished ears of the scientific and dilettante world, John Dalton, a Manchester school-master, conceived a theory that has proved of the utmost service to the science of chemistry, and which bids fair to outlast our day. It had been noticed by Wenzel, by Richter, by Wollaston, and by Cavendish, towards the end of the last century, that the same compounds contain the same constituents in the same proportions, or, as the phrase runs, “possess constant composition.” Wollaston, indeed, had gone one step farther, and had shown that when the vegetable acid, oxalic acid, is combined with potash, it forms two compounds, in one of which the acid is contained in twice as great an amount relatively to the potash as in the other. The names _monoxalate_ and _binoxalate_ of potash were applied to these compounds, to indicate the respective proportions of the ingredients.


[Assistant]
The names _monoxalate_ and _binoxalate_ of potash were applied to these compounds, to indicate the respective proportions of the ingredients. Dalton conceived the happy idea that by applying the ancient Greek conception of atoms to such facts the relative weights of the atoms could be determined. Illustrating his views with the two compounds of carbon with hydrogen, marsh gas and olefiant gas, and with the two acids of carbon, carbonic oxide, carbonic “acid,” he regarded the former as a compound of one atom of carbon and one of hydrogen, and the second as a compound of one atom of carbon and two of hydrogen, and similarly for the two oxides of carbon. Knowing the relative weights in which these elements enter into combination, we can deduce the relative weights of the atoms. Placing the relative weight of an atom of hydrogen equal to unity, we have:
            Marsh     Olefiant  |           Carbonic  Carbonic              Gas        Gas     |            Oxide      Acid   Carbon      6          6      |  Carbon      5          6   Hydrogen    1          2      |  Oxygen      8         16
Thus the first compound, marsh gas, was regarded by Dalton as composed of an atom of carbon in union with an atom of hydrogen; or, to reproduce his symbols, as ⊜◉; while the second, olefiant gas, on this hypothesis, was a compound of two atoms of hydrogen with one of carbon, or ◉◍◉. Similarly the symbols ◍○, and ○◍○ were given to the two compounds of carbon with oxygen.


[Assistant] Similarly the symbols ◍○, and ○◍○ were given to the two compounds of carbon with oxygen. So water was assigned the symbol ◉○, for Dalton imagined it to be a compound of one atom of hydrogen with one of oxygen. Compounds containing only two atoms were termed by him “binary”; those containing three, “ternary”; four, “quaternary,” and so on. The weight of an atom of oxygen was eight times that of an atom of hydrogen; while that of an atom of carbon was six times as great as the unit. By assigning symbols to the elements, consisting of the initial letters of their names, or of the first two letters, formulas were developed, indicating the composition of the compound, the atomic weights of the elements being assured. Thus, NaO signified a compound of an atom of sodium (natrium), weighing twenty-three times as much as a similar atom of hydrogen, with an atom of oxygen, possessing eight times the weight of an atom of hydrogen.


[Assistant] Thus, NaO signified a compound of an atom of sodium (natrium), weighing twenty-three times as much as a similar atom of hydrogen, with an atom of oxygen, possessing eight times the weight of an atom of hydrogen. Therefore, thirty-one pounds of soda should consist of twenty-three pounds of sodium in combination with eight pounds of oxygen, for, according to Dalton, each smallest particle of soda contains an atom of each element, and the proportion is not changed, however many particles be considered. It has been pointed out by Judge Stallo, of Philadelphia, in his _Concepts of Physics_, that such a hypothesis as that of Dalton is no explanation; that a fact of nature, as, for example, the fact of simple and multiple proportions, is not _explained_ by being minified. Allowing the general truth of this statement, it is, nevertheless, undoubted that chemistry owes much to Dalton’s hypothesis—a lucky guess at first, it represents one of the fundamental truths of nature, although its form must be somewhat modified from that in which Dalton conceived it. Dalton’s work was first expounded by Thomas Thomson, professor at Glasgow, in his _System of Chemistry_, published in 1805; and subsequently in Dalton’s own _New System of Chemical Philosophy_, the three volumes of which were published in 1808, in 1810, and in 1827. The determination of these “Constants of Nature” was at once followed out by many chemists, Thomson among the first.


[Assistant] The determination of these “Constants of Nature” was at once followed out by many chemists, Thomson among the first. But chief among the chemists who have pursued this branch of work was Jacob Berzelius, a Swede, who devoted his long life (1779–1848) to the manufacture of compounds, and to the determination of their composition, or, as it is still termed, the determination of the “atomic weights”—more correctly, “equivalents”—of the elements of which they are composed. It is to him that we owe most of our analytical methods, for, prior to his time, there were few, if any, accurate analyses. Although Lavoisier had devised a method for the analysis of compounds of carbon, viz., by burning the organic compounds in an atmosphere of oxygen contained in a bell-jar over mercury, and measuring the volume of carbon dioxide produced, as well as that of the residual oxygen, Berzelius achieved the same results more accurately and more expeditiously by heating the substance, mixed with chlorate of potassium and sodium chloride, and then estimating the hydrogen as well as the carbon; this process was afterwards perfected by Liebig. Berzelius, however, was able to show that compounds of carbon, like those of other elements, were instances of combination in constant and in multiple proportions. In 1815 two papers were published in the _Annals of Philosophy_ by Dr.


[Assistant] In 1815 two papers were published in the _Annals of Philosophy_ by Dr. Prout, which have had much influence on the progress of chemistry. They dealt with the figures which were being obtained by Thomson, Berzelius, and others, at that time supposed to represent the “atomic weights” of the elements. Prout’s hypothesis, based on only a few numbers, was that the atomic weights of all elements were multiples of that of hydrogen, taken as unity. There was much dispute regarding this assertion at the time, but as it was contradicted by Berzelius’s numbers, the balance of opinion was against it. But about the year 1840 Dumas discovered an error in the number (12.12) given by Berzelius as the atomic weight of carbon; and with his collaborator, Stas, undertook the redetermination of the atomic weights of the commoner elements—for example, carbon, oxygen, chlorine, and calcium.


[Assistant] But about the year 1840 Dumas discovered an error in the number (12.12) given by Berzelius as the atomic weight of carbon; and with his collaborator, Stas, undertook the redetermination of the atomic weights of the commoner elements—for example, carbon, oxygen, chlorine, and calcium. This line of research was subsequently pursued alone by Stas, whose name will always be remembered for the precision and accuracy of his experiments. At first Dumas and Stas inclined to the view that Prout’s hypothesis was a just one, but it was completely disproved by Stas’s subsequent work, as well as by that of numerous other observers. It is, nevertheless, curious that a much larger proportion of the atomic weights approximate to whole numbers than would be foretold by the doctrine of chances, and perhaps the last has not been heard of Prout’s hypothesis, although in its original crude form it is no longer worthy of credence. One of the most noteworthy of the discoveries of the century was made by Gay-Lussac (1778–1850) in the year 1808. In conjunction with Alexander von Humboldt, Gay-Lussac had rediscovered about three years before what had previously been established by Cavendish—namely, that, as nearly as possible, two volumes of hydrogen combine with one volume of oxygen to form water, the gases having been measured at the same temperature and pressure.


[Assistant] In conjunction with Alexander von Humboldt, Gay-Lussac had rediscovered about three years before what had previously been established by Cavendish—namely, that, as nearly as possible, two volumes of hydrogen combine with one volume of oxygen to form water, the gases having been measured at the same temperature and pressure. Humboldt suggested to Gay-Lussac that it would be well to investigate whether similar simple relations exist between the volumes of other gaseous substances when they combine with each other. This turned out to be the case; it appeared that almost exactly two volumes of carbonic oxide unite with one volume of oxygen to form carbon dioxide; that equal volumes of chlorine and hydrogen unite to form hydrochloric acid gas; that two volumes of ammonia gas consist of three volumes of hydrogen in union with one volume of nitrogen, and so on. From such facts, Gay-Lussac was led to make the statement that: The weights of equal volumes of both simple and compound gases, and therefore their densities, are proportional to their empirically found combining weights, or to rational multiples of the latter. Gay-Lussac recognized this discovery of his to be a support for the atomic theory; but it did not accord with many of the then received atomic weights. The assumption that equal volumes of gases contain equal numbers of particles, or, as they were termed by him, _molécules intégrantes_, was made in 1811 by Avogadro, professor of physics at Turin (1776–1856).


[Assistant] The assumption that equal volumes of gases contain equal numbers of particles, or, as they were termed by him, _molécules intégrantes_, was made in 1811 by Avogadro, professor of physics at Turin (1776–1856). This theory, which has proved of the utmost importance to the sciences both of physics and of chemistry, had no doubt occurred to Gay-Lussac, and had been rejected by him for the following reasons: A certain volume of hydrogen, say one cubic inch, may be supposed to contain an equal number of particles (atoms) as an equal volume of chlorine. Now these two gases unite in equal volumes. The deduction appears so far quite legitimate that one atom of hydrogen has combined with one atom of chlorine. But the resulting gas occupies two cubic inches, and must therefore contain the same number of particles of hydrogen chloride, the compound of the two elements, as one cubic inch originally contained of hydrogen, or of chlorine. Thus we have two cubic inches containing, of uncombined gases, twice as many particles as is contained in that volume, after combination.


[Assistant] Thus we have two cubic inches containing, of uncombined gases, twice as many particles as is contained in that volume, after combination. Avogadro’s hypothesis solved the difficulty. By premising two different orders of particles, now termed _atoms_ and _molecules_, the solution was plain. According to him, each particle, or molecule, of hydrogen is a complex, and contains two atoms; the same is the case with chlorine. When these gases combine, or rather _react_, to form hydrogen chloride, the phenomenon is one of a change of partners; the molecule, the double atom, of hydrogen splits; the same is the case with the molecule of chlorine; and each liberated atom of hydrogen unites with a liberated atom of chlorine, forming a compound, hydrogen chloride, which equally consists of a molecule, or double atom. Thus two cubic inches of hydrogen chloride consist of a definite number of molecules, equal in number to those contained in a cubic inch of hydrogen, plus those contained in a cubic inch of chlorine.


[Assistant] Thus two cubic inches of hydrogen chloride consist of a definite number of molecules, equal in number to those contained in a cubic inch of hydrogen, plus those contained in a cubic inch of chlorine. The case is precisely similar, if other compounds of gases be considered. Berzelius was at first inclined to adopt this theory, and indeed went so far as to change many of his atomic weights to make them fit it. But later he somewhat withdrew from his position, for it appeared to him that it was hazardous to extend to liquids and solids a theory which could be held only of gases. Avogadro’s suggestion, therefore, rested in abeyance until the publication, in 1858, by Cannizzaro, now professor of chemistry in Rome, of an essay in which all the arguments in favor of the hypothesis were collected and stated in a masterly manner. It will be advisable to revert to this hypothesis at a later point, and to consider other guides for the determination of atomic weights.


[Assistant] It will be advisable to revert to this hypothesis at a later point, and to consider other guides for the determination of atomic weights. In 1819, Dulong (1785–1838), director of the Ecole Polytechnique at Paris, and Petit (1791–1820), professor of physics there, made the discovery that equal amounts of heat are required to raise equally the temperature of solid and liquid elements, provided quantities are taken proportional to their atomic weights. Thus, to raise the temperature of 56 grammes of iron through one degree requires approximately the same amount of heat as is required to raise through one degree 32 grammes of sulphur, 63.5 grammes of copper, and so on; these numbers representing the atomic weights of the elements named. In other words, _equal numbers of atoms have equal capacity for heat_. The number of heat units, or calories (one calory is the amount of heat required to raise the temperature of 1 gramme of water through 1° C.), which is necessary to raise the atomic weight expressed in grammes of any solid or liquid element through 1° C. is approximately 6.2; it varies between 5.7 and 6.6 in actual part.


[Assistant] is approximately 6.2; it varies between 5.7 and 6.6 in actual part. This affords a means of determining the true value of the atomic weight of an element, as the following example will show: The analysis of the only compound of zinc and chlorine shows that it contains 47.49 per cent. of zinc and 52.16 per cent. of chlorine. Now one grain of hydrogen combines with 35.5 grains of chlorine to form 36.5 grains of hydrogen chloride; and, as already remarked, one volume of hydrogen and one volume of chlorine combine, forming two volumes of hydrogen chloride. Applying Avogadro’s hypothesis, one molecule of hydrogen and one molecule of chlorine react to yield two molecules of hydrogen chloride; and as each molecule is supposed to consist in this case of two atoms, hydrogen chloride consists of one atom of each of its constituent elements.


[Assistant] Applying Avogadro’s hypothesis, one molecule of hydrogen and one molecule of chlorine react to yield two molecules of hydrogen chloride; and as each molecule is supposed to consist in this case of two atoms, hydrogen chloride consists of one atom of each of its constituent elements. The amount of that element, therefore, which combines with 35.5 grains of chlorine may give the numerical value of the atomic weight of the element, if the compound contains one atom of each element; in that case the formula of the above compound would be zinc, and the atomic weight of zinc, 32.7; but if the formula is ZuCl3, the atomic weight of zinc would be 32.7 × 2; if ZuCl3, 32.7 × 3, and so on. The specific heat of metallic zinc enables this question to be solved. For it has been found, experimentally, to be about 0.095; and 6.2 ÷ 0.095 = 65.2, a close approximation to 32.7 × 2 = 65.4. The conclusion is therefore drawn that zinc chloride is composed of one atom of zinc in combination with two atoms of chlorine, that the atomic weight of zinc is 65.4, and that the molecular weight of zinc chloride is 65.4 + (35.5 × 2) = 136.4. Inasmuch as the relative weight of a molecule of hydrogen is 2 (that of an atom being 1), zinc chloride in the gaseous state should be 136.4 ÷ 2 = 68.2 times that of hydrogen, measured at the same temperature and pressure.


[Assistant] Inasmuch as the relative weight of a molecule of hydrogen is 2 (that of an atom being 1), zinc chloride in the gaseous state should be 136.4 ÷ 2 = 68.2 times that of hydrogen, measured at the same temperature and pressure. This has been found, experimentally, to be the case. The methods of determining the vapor densities, or relative weights of vapors, are three in number; the first method, due to Dumas (1827), consists in vaporizing the substance in question in a bulb of glass or of porcelain, at a known temperature, closing the bulb while still hot, and weighing it after it is cold. Knowing the capacity of the bulb, the weight of hydrogen necessary to fill it at the desired temperature can be calculated, and the density of the vapor thus arrived at. A second method was devised by Gay-Lussac and perfected by A. W.


[Assistant] W. Hofmann (1868); and a third, preferable for its simplicity and ease of execution, is due to Victor Meyer (1881). In 1858, as already remarked, Cannizzaro showed the connection between these known facts, and for the first time attention was called to the true atomic weights, which were, up to that time, confused with equivalents, or weights of elements required to replace one unit weight of hydrogen. These were generally regarded as atomic weights by Dalton and his contemporaries. Some exceptions had been observed to the law of Dulong and Petit, viz., beryllium, or glucinium, an element occurring in emeralds; boron, of which borax is a compound; silicon, the component of quartz and flint, and carbon. It was found by Weber that at high temperatures the specific heats of these elements are higher, and the atomic heats approximate to the number of 6.2; but this behavior is not peculiar to these elements, for it appears that the specific heat of all elements increases with rise of temperature.


[Assistant]
It was found by Weber that at high temperatures the specific heats of these elements are higher, and the atomic heats approximate to the number of 6.2; but this behavior is not peculiar to these elements, for it appears that the specific heat of all elements increases with rise of temperature. A certain number of exceptions have also been noticed to the law of Gay-Lussac, which may be formulated: the molecular weight of a compound in a gaseous state is twice its density referred to hydrogen. Thus equal volumes of ammonia and hydrogen chloride unite to form ammonium chloride. It was to be expected that the density should be half the molecular weight, thus:
    NH3 +  HCl  =  NH4Cl; and 53.5 ÷ 2 = 26.75 = density. (14+3) (1+35.5)  53.5
But the density actually found is only half that number, viz., 13.37; and for long this and similar cases were supposed to be exceptions to the law of Gay-Lussac, viz., that equal volumes of gases at the same pressure expand equally for equal rise of temperature. In other instances the gradual decrease in density with rise of temperature can be followed, as with chloral hydrate, the products of which are chloral and water.


[Assistant] In other instances the gradual decrease in density with rise of temperature can be followed, as with chloral hydrate, the products of which are chloral and water. It was recognized by St. Claire Deville (1857) that the decrease in density of such mixtures of gases was due, not to their being exceptions to Avogadro’s law, but to the gradual decomposition of the compound body with rise of temperature. To this gradual decomposition he gave the name _dissociation_. This conception has proved of the utmost importance to the science, as will be seen in the sequel. To take the above instance of ammonium chloride, its abnormal density is due to its dissociation into ammonia and hydrogen chloride; and the gas which is obtained on raising its temperature consists, not of gaseous ammonium chloride, but of a mixture of ammonia and hydrogen chloride, which, as is easily seen, occupy, when separate, twice the volume that would be occupied by the gaseous compound.


[Assistant] To take the above instance of ammonium chloride, its abnormal density is due to its dissociation into ammonia and hydrogen chloride; and the gas which is obtained on raising its temperature consists, not of gaseous ammonium chloride, but of a mixture of ammonia and hydrogen chloride, which, as is easily seen, occupy, when separate, twice the volume that would be occupied by the gaseous compound. Of recent years it has been shown by Brereton Baker that, if perfectly free from moisture, ammonium chloride gasifies as such, and that its density in the state of vapor is, in fact, 26.75. The molecular complexity of gases has thus gradually become comprehended, and the truth of Avogadro’s law has gained acceptance. And as a means of picturing the behavior of gaseous molecules, the “Kinetic Theory of Gases” has been devised by Joule, Clausius, Maxwell, Thomson (Lord Kelvin), and others. On the assumption that the pressure of a gas on the walls of the vessel which contains it is due to the continued impacts of its molecules, and that the temperature of a gas is represented by the product of the mass of the molecules, or the square of their velocity, it has been possible to offer a mechanical explanation of Boyle’s law, that at constant temperature the volume of a gas diminishes in proportion as the pressure increases; of Gay-Lussac’s law, that all gases expand equally for equal rise of temperature, provided pressure is kept constant; the condition being that equal volumes of gases contain equal numbers of molecules. A striking support is lent to this chain of reasoning by the facts discovered by Thomas Graham (1805–1869), professor at University College, London, and subsequently master of the Royal Mint.


[Assistant] A striking support is lent to this chain of reasoning by the facts discovered by Thomas Graham (1805–1869), professor at University College, London, and subsequently master of the Royal Mint. Graham discovered that the rate of diffusion of gases into each other is inversely as the square roots of their densities. For instance, the density of hydrogen being taken as unity, that of oxygen is sixteen times as great; if a vessel containing hydrogen be made to communicate with one containing oxygen, the hydrogen will pass into the oxygen and mix with it; and, conversely, the oxygen will pass into the hydrogen vessel. This is due to the intrinsic motion of the molecule of each gas. And Graham found, experimentally, that for each volume of oxygen which enters the hydrogen vessel four volumes of hydrogen will enter the oxygen vessel. Now, 4 = √16; and as these masses are relatively 1 and 16, and their temperatures are equal, the square of their velocities are respectively 1 and 16.