THE PROGRESS
OF THE
CENTURY

BY ALFRED RUSSEL WALLACE; PROF. WILLIAM RAMSAY; PROF. WILLIAM MATTHEW FLINDERS-PETRIE; SIR JOSEPH NORMAN LOCKYER; EDWARD CAIRD; WILLIAM OSLER; W. W. KEEN; PROF. ELIHU THOMSON; PRESIDENT THOMAS CORWIN MENDENHALL; SIR CHARLES WENTWORTH DILKE; CAPTAIN ALFRED T. MAHAN; ANDREW LANG; THOMAS C. CLARKE; CARDINAL JAMES GIBBONS; REV. ALEXANDER V. G. ALLEN; PROF. RICHARD J. H. GOTTHEIL; PROF. GOLDWIN SMITH

NEW YORK AND LONDON
HARPER & BROTHERS PUBLISHERS
1901

Copyright, 1901, by Harper & Brothers.
Copyright, 1901, by The Sun Printing and Publishing Association.
All rights reserved.

CONTENTS

EVOLUTION

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.

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.

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.

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.

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.

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. 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.

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. 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. 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. 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. 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. 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. 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.

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. 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.

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. 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? 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? 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. 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. 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. 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. 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. 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.

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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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.

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. 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. 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. 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. 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. 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. 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.

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. 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. 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. 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. 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:

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. 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. 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. 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. 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. 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. 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). 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. 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. 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.

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. 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. 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. 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. 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.

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.

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. 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. 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.

The question of the molecular complexity of gases being thus disposed of, it remains to be considered what are the relative complexity of liquid molecules. The answer is indicated by a study of the capillary phenomena of liquids, one method of measuring which is the height of their ascent in narrow or capillary tubes. We shall not enter here into detail as to the method and arguments necessary; suffice it to say that the Hungarian physicist Eötvös was the first to indicate the direction of research, and that Ramsay and Shields succeeded in proving that the complexity of the molecules of most liquids is not greater than that of the gases which they form on being vaporized; and also that certain liquids, e.g., water, the alcohols, and other liquids, are more or less “associated,” i.e., their molecules occur in couplices of two, three, four, or more, and as the temperature is raised the complexity of molecular structure diminishes.

As regards the molecular complexity of solids, nothing definite is known, and, moreover, there appears to be no method capable of revealing it.

While the researches of which a short account has now been given have led to knowledge regarding the nature of molecules, the structure of the molecule has excited interest since the early years of the century, and its investigation has led to important results. The fact of the decomposition of acidified water by an electric current, discovered by Nicholson and Carlisle, and of salts into “bases” and “acids” by Berzelius and Hisinger in 1803, led to the belief that a close connection exists between electric energy, or, as it was then termed, “electric force,” and the affinity which holds the constituents of chemical compounds in combination. In 1807 Davy propounded the theory that all compounds consist of two portions, one electro-positive and the other electro-negative. This idea was the result of experiments on the behavior of substances, such, for example, as copper and sulphur—if portions of these elements be insulated and then brought into contact they become oppositely electrified. The degree of electrification is intensified by rise of temperature until, when combination ensues, the electrification vanishes. Combination, therefore, according to Davy, is concurrent with the equalization of potentials. In 1812 Berzelius brought forward an electro-chemical theory which for the following twenty years was generally accepted. His primary assumption was that the atoms of elements, or, in certain cases, groups of atoms, are themselves electrified; that each atom, or group of atoms, possesses two poles, one positive, the other negative; that the electrification of one of these poles predominates over that of the other, so that the atom or group is itself, as a whole, electro-positive, or electro-negative; that combination ensued between such oppositely electrified bodies by the neutralization, partial or complete, of their electric charges; and, lastly, that the polarity of an element or group could be determined by noting whether the element or group separated at the positive or at the negative pole of the galvanic battery, or electrolysis. For Berzelius, oxygen was the most electro-negative and potassium the most electro-positive of the elements, the bridge between the “non-metals” and the “metals” being hydrogen, which, with nitrogen, forms a basic, or electro-positive, group, while with chlorine, etc., it forms electro-negative groups. The fact that an electric current splits compounds in solution into two portions led Berzelius to devise his “dualistic” system, which involved the assumption that all compounds consist of two portions, one electro-positive, the other electro-negative. Thus sulphate of magnesium and potassium was to be regarded as composed of electro-positive potassium sulphate in combination with electro-negative magnesium sulphate; the former in its turn consisted of electro-negative sulphur trioxide (SO₃) in combination with electro-positive oxide of potassium (K₂O); while each of these proximate constituents of potassium sulphate were themselves composed of the electro-negative oxygen in combination with electro-positive sulphur, or potassium. On contrasting sulphur with potassium, however, the former was considered more electro-negative than the latter; so that the group SO₃ as a whole was electro-negative, while K₂O was electro-positive. The symbols given above, which are still in universal use, were also devised by Berzelius for the purpose of illustrating and emphasizing his views. These views, however, met with little acceptance at the time in England.

Lavoisier’s idea, that oxygen was the necessary constituent of all acids, began about this time to lose ground. For Davy had proved the elementary nature of chlorine; and hydrochloric acid, one of the strongest, was thus seen to contain no oxygen, and Davy expressed the view, founded on his observation, that iodic “acid,” I₂O₅, was devoid of acid properties until dissolved in water, and that the essential constituent of all acids was hydrogen, not oxygen. The bearing of this theory on the dualistic theory is, that while, e.g., sulphuric acid was regarded by Berzelius as SO₃, containing no hydrogen, and was supposed to be separated as such at the positive pole of a battery, Davy’s suggestion led to the opposite conclusion that the formula of sulphuric acid is H₂SO₄, and that by the current it is resolved into H₂ and SO₄. Faraday’s electrolytic law, that when a current is passed through electrolytes in solution the elements are liberated in quantities proportional to their equivalents, led to the abandonment of the dualistic theory. For when a current is passed in succession through acidified water, fused lead chloride, and a solution of potassium sulphate, the quantities of hydrogen and oxygen from the water, of lead and chlorine from the lead chloride, and the potassium of the sulphate are in accordance with Faraday’s law. But in addition to the potassium there is liberated at the same pole an equivalent of hydrogen. Now, if Berzelius’s theory be true, the products should be SO₃ and K₂O, but if the opposite view be correct, then K₂ is liberated first and by its subsequent action on water it yields potash and its equivalent of hydrogen. This was pointed out first by Daniell, professor at King’s College, London, and it was regarded as a powerful argument against Berzelius’s system. In 1833, too, Graham investigated the phosphoric acids, and prepared the salts of three, to which he gave the names, ortho-, pyro-, and meta- phosphoric acids. To understand the bearing of this on the doctrine of dualism it must be remembered that P₂O₅, pentoxide of phosphorus, was at that date named phosphoric acid. When dissolved in water it reacts with bases, forming salts—the phosphates. But the quantity of water necessary was not then considered essential; Graham, however, showed that there exist three series of salts—one set derived from P₂O₅,3H₂O, one from P₂O₅,2H₂O, and a third from P₂O₅,H₂O. His way of stating the fact was that water could play the part of a base; for example, the ordinary phosphate of commerce possessed, according to him, the formula P₂O₅,2Na₂O,H₂O, two-thirds of the “water of constitution” being replaced by oxide of sodium. Liebig, then professor at Giessen (1803–1873), founded on these and on similar observations of his own the doctrine of poly-basic acids—acids in which one, two, three, or more atoms of hydrogen were replaceable by metals. Thus, instead of writing, as Graham did, P₂O₅,2Na₂O,H₂O, he wrote, PO₄Na₂H; and for orthophosphoric acid PO₄H₃. The group of atoms (PO₄), therefore, existed throughout the whole series of orthophosphates, and could exist in combination with hydrogen, with hydrogen and metals, or with metals alone. Similarly the group (P₂O₇) was characteristic of pyrophosphates and (PO₃) of metaphosphates, for P₂O₅,2H₂O=(P₂O₇)H₄; and P₂O₅,H₂O=2(PO₃)H.

The first clear ideas of the structure of the molecule were, however, gained from the study of the compounds of carbon. It was difficult to apply the dualistic theory to them. For few of them are electrolytes, and therefore their products of electrolysis, being non-existent, could not be classified. Nevertheless, Gay-Lussac regarded alcohol, C₂H₆O, as a compound of C₂H₄, ethylene, and H₂O, water; and oxalic acid (anhydrous), C₂O₃, as one of CO₂ with CO. The discovery of “isomeric compounds,” i.e., of compounds which possess the same ultimate formula and yet differ entirely in their properties, forced upon chemists the necessity of attending to the structure of the molecule; for only by such a supposition could the difference between two isomeric bodies be explained. In 1823 Liebig discovered that silver fulminate and silver cyanate both possessed the empirical formula AgCNO; in 1825 this was followed by the discovery by Faraday that oil gas contains a hydrocarbon identical in composition with ethylene, C₂H₄, yet differing from it in properties; and in 1829 Wöhler, professor in Göttingen (1800–1882), discovered that urea, a constituent of urine, could be produced by heating ammonium cyanate, NH₄CNO, a substance of the same formula. It therefore became clear that the identity of a compound must depend on some other cause than its ultimate composition.

In 1833 Liebig and Wöhler took an important step in elucidating this question by their investigations on benzoic acid and acid obtainable by distilling a resin named gum benzoin. They showed that this acid, C₇H₆O₂, could be conceived as consisting of the group C₇H₅O, to which they gave the name “benzoyl,” in combination with OH; that benzoic aldehyde, C₇H₆O, might be regarded as its compound with hydrogen; that it also formed compounds with chlorine, and bromine, and sulphur, and replaced hydrogen in ammonia (C₇H₆O,NH₂). They termed this group, benzoyl, a “compound element” or a “radical.” This research was followed by one by Robert Bunsen, professor at Heidelberg, born in 1811, and recently (1899) dead, which bore reference to cacodyl, a compound of arsenic, carbon and hydrogen, in which the idea of a radical was confirmed and amplified.

The idea of a radical having thus become established, Jean Baptiste Andrée Dumas, professor in Paris (1800–1884), propounded the theory of “substitution,” i.e., that an element such as chlorine or oxygen (which, be it noticed, is electro-negative on Berzelius’s scale) could replace hydrogen in carbon compounds, atom for atom, the resulting compound belonging to the same “type” as the one from which it was derived. And Laurent, warden of the mint at Paris (1807–1853), and Gerhardt, professor at Montpelier and at Strasburg (1816–1856), emphasized the fact that one element, be it what it may, can replace another without fundamentally altering its chemical character, and also that an atom of hydrogen can be replaced by a group of atoms or radical, behaving for the occasion like the atom of an element. It is to Laurent and Gerhardt that we owe the definition of an atom—the smallest quantity of an element which can be present in a compound; an equivalent—that weight of an element which combines with or replaces one part by weight of hydrogen; and a molecule—the smallest quantity which can exist in a free state, whether of an element or a compound. They recognized, too, that a molecule of hydrogen, chlorine, etc., consists of two atoms.

In 1849 Wurtz, professor in Paris (1817–1884), and Hofmann, then professor in the College of Chemistry in London, afterwards at Berlin (1818–1892), discovered a series of compounds allied to ammonia, NH₃, in which one or more atoms of hydrogen were replaced by a group or radical, such as methyl (CH₃), ethyl (C₂H₅), or phenyl (C₆H₅). Wurtz referred such compounds to the ammonia “type.” They all resemble ammonia in their physical properties—smell, taste, etc.—as well as in their power of uniting with acids to form salts resembling ammonium chloride (NH₄Cl), and other ammonium compounds. Shortly afterwards Williamson, professor at University College, London, added the “water type,” in consequence of his researches on “mixed ethers”—bodies in which the hydrogen of water might be regarded as replaced by organic radicals. Thus we have the series:

H. O. H.; CH₃. O. H.; CH₃. O. CH₃; and NH₃; NH₂; H₃; NH(CH₃)₂; and N(CH₃)₃; the first representing compounds following the water type, the latter the ammonia type. This suggestion had been previously made by Laurent, in 1846. But Williamson extended his views to inorganic compounds; thus, sulphuric acid was represented as constructed on the double water type—HO. SO₂. OH, being derived from H. O. (H. H) O. H, the two hydrogen atoms enclosed in brackets being replaced by the radical SO₂. To these types Gerhardt added the hydrogen and hydrogen chloride types, H.H. and H.Cl; and, later, Kekulé, professor in Bonn (1829), added the marsh gas type C(H)₄. The next important step was taken by Frankland, professor in the Royal School of Mines, London; his work, however, had been anticipated by Cunn Brown, professor at Edinburgh University, in a pamphlet even yet little known. It was to attribute to elements one or more powers of combination. To these he gave the name “valency,” and the capacity of possessing valency was called “quantivalence.” Thus hydrogen was taken as a “monad,” or monovalent. Chlorine, because it unites with hydrogen atom to atom, is also a monad. Oxygen, having the power to combine with two atoms of hydrogen, was termed a dyad, or divalent; nitrogen a triad, or trivalent; carbon a tetrad, or tetravalent, and so on. This is evident from inspection of the formulas of their compounds with hydrogen, thus:

H H H
/ \ /
H——Cl; H——O——H; H——N ; C
\ / \
H H H

Instances of penta, hexa, and even hepta-valency are not wanting.

This was the key to unlock the structure of chemical compounds; and Frankland’s views, just stated, are still held by chemists. The determination of the constitution of compounds, chiefly those of carbon, occupied the attention of chemists, almost exclusively, until 1880. The plan of action is much the same as that of a mechanician who wishes to imitate a complicated mechanism. He must first dissect it into groups of mechanical contrivances; these are next constructed; and they are finally built together into the complete machine. In certain cases the atoms of carbon are arranged in “chains,” as, for example, in pentyl alcohol:

H₃C——C——C——C——C——O——H
H₂ H₂ H₂ H₂

each atom being tetrad, and its “affinities,” or powers of combination, saturated either with hydrogen or with those of neighboring atoms of carbon; in others they are in the form of a “ring,” as in benzene, the formula of which was first suggested by Kekulé, viz.:

H H
C——C
/ \
HC CH;
\ /
C==C
H H

or in both, as in ethyl benzene,

H H
C——C
/ \ H H
HC C——C——CH.
\ / H H
C==C
H H

One or more atoms of nitrogen, or of oxygen, may form part of the circle, as in pyridine:

H H H H
C——C C C
/ \ /
N CH and furfurane, O == ,
\ / \
C==C C C
H H H H

and so on. By means of conceptions such as these many interesting compounds have been built up out of the elements which they contain; e.g., urea and uric acid, constituents of urine; theobromine and caffeine, the essential principles of cocoa and tea; alizarine and indigo, valuable dyestuffs; and several of the alkaloids, bitter principles contained in plants, of great medicinal value.

They have led, too, to the discovery of many brilliant colors, now almost universally employed, to the exclusion of those less brilliant, because less pure, derived from plants, and in one or two cases from animals; the manufacture of gun-cotton, dynamite, and similar high explosives; and to the development of the candle industry; the sugar manufacture; to improvement in tanning, in brewing, and in the preparation of gas and oils for illuminating purposes. In short, it may be said that the industrial progress of the latter half of the century has been due to the theoretical views of which a short sketch has just been given.

Such formulas, however, can evidently not represent the true constitution of matter, inasmuch as the atoms are imagined to lie on a plane, whereas it is evident that they must occupy space of three dimensions and possess the attributes of solidity. The conception which led to the formulation of such views was due first to Pasteur, in his later years director of the institute known by his name at Paris, and more directly to LeBel and Van’t Hoff, now professor at Berlin, independently of each other. In 1848 Pasteur discovered that it was possible to separate the two varieties of tartaric acid from each other; and that that one which rotated the plane of polarized light to the right gave crystals with an extra face, unsymmetrically disposed with regard to the other faces of the crystal. The variety, the solution of which in water was capable of producing left-handed rotation, also possessed a similar face, but so placed that its reflection in a mirror reproduced the right-handed variety. Pasteur also showed that a mixture of these acids gave crystals not characterized by an unsymmetrically placed face; and also that the solution was without action on polarized light. These observations remained unexplained, until LeBel and Van’t Hoff, in 1874, simultaneously and independently devised a theory which has, up till now, stood the test of research. It is briefly this: Imagine two regular tetrahedra, or three-sided pyramids, standing each on its triangular base. An idea can best be got by a model, easily made by laying on a table three lucifer matches so as to form an equilateral triangle, and erecting a tripod with three other matches, so that each leg of the tripod stands on one corner of the triangle. At the centre of such a tetrahedron, an atom of carbon is supposed to be placed. Marsh gas, CH₄, is supposed to have such a structure, each corner, or solid angle of the structure (of which there are four), being occupied by an atom of hydrogen. This represents the solid or stereochemical formula of methane or marsh gas. Now, suppose one of the atoms of hydrogen in each of these structures to be replaced by chlorine, the group (OH), or any other monovalent element or group. It is evident that if not exactly similar (owing to the replacement not having been made at similar corners in each), the two structures can be made similar by turning one of them round, until the position of the substituting atom or group (which we will term X) coincides in position with X in the stationary one. If two such replacements be made, say, with X and Y in each, coincidence can again be made to take place; but the same is not the case if X, Y, and Z replace three atoms of hydrogen in the structure; for there is one way of replacement which is the optical image of the other, and represents the other’s reflection in a mirror.

(Tetrahedron XYZ) and (Tetrahedron XZY)

Now, it is found that when the four corners of such a structure are occupied by four separate atoms or groups, or when (as the expression goes) the body contains an “asymmetrical carbon atom,” if the substance or one of its derivations can be obtained in a crystalline form, the crystals are also asymmetric, i.e., arc develops a face which is the mirror-reflection of a similar face developed on the other variety; and if a beam of polarized light be passed through the solution of the substance, its plane is rotated to the left if one variety be used, and, if the other, to the right. This hypothesis of LeBel’s and Van’t Hoff’s has had an enormous influence on the progress of organic chemistry. By its means Fischer, now professor at Berlin, has explained the reason of the existence of the enormous number of bodies analogous to grape and cane sugar, and has prepared many new varieties; and it appears likely that the terpenes, a class of bodies allied to turpentine, and comprising most of the substances to which the odor of flowers is due, may thereby find their explanation. It may be mentioned in passing that Pasteur, having found that ordinary mould destroyed one variety of tartaric acid rather than the other in a mixture of the two, and made use of this observation in order to prepare the unattached variety in a state of purity, was led to study the action of organisms more or less resembling mould; and that this has led to the development of the science of bacteriology, which has had an enormous influence on our views regarding fermentation in general, and guides the work of our physicians, our surgeons (witness Lister’s antiseptic treatment), our sanitary engineers in their estimate of the purity of drinking-water and of the disposal of sewage, of our manufacturers of beer and spirits, of wine-growers, and more recently of farmers. All these processes depend upon the action of organisms in producing chemical changes, whether in the tissues of the body, causing or curing disease, or in the production of flavored alcohol from sugar, or in the manufacture of butter and cheese, or in preparing the land for the reception of crops. We also owe to the genius of Van’t Hoff the most important advance of recent times in the region of physical chemistry. It has been observed by Raoult, professor at Grenoble, that the freezing-point of a solvent as a general rule is lowered to the same extent if there be dissolved in it quantities of substances proportional to their molecular weights. Thus, supposing 1.80 grams of grape-sugar be dissolved in 100 grams of water and the solution cooled below 0° with constant stirring, ice separates suddenly in thin spicules, and the temperature rises to −0.185°. If 3.42 grams of cane-sugar be similarly dissolved in 100 grams of water, the freezing-point of the solution is again −0.185°. Now, 1.80 and 3.42 are respectively the hundredth part of the molecular weights of grape-sugar (C₆H₁₂O₆) and cane-sugar (C₁₂H₂₂O₁₁). Similarly, Raoult found that quantities proportional to molecular weights dissolved in a solvent depress the vapor pressure of that solvent equally, or, what comes to the same thing, raise its boiling-point by an equal number of degrees. But ordinary salts, such as sodium chloride, potassium nitrate, etc., dissolved in water, give too great a depression of the freezing-point and too high a boiling-point. Next, it has been observed by botanists, Devries, Pfeffer, and others, who had examined the ascent of sap in plants, that if a vessel of unglazed porcelain, so treated as to cause a film of cupric ferrocyanide (a slimy red compound) to deposit in the pores of its walls, be filled with a weak (about 1 per cent.) solution of sugar or similar substance, and plunged in a vessel of pure water, water entered through the pores. By attaching a monometer to the porous vessel the pressure exerted by the entering water could be measured. Such pressure was termed “osmotic pressure,” referring to the “osmosis” or passage through the walls of the vessel. Such prepared walls are permeable freely to water, but not to sugar or similar bodies. Van’t Hoff pointed out that the total pressure registered is proportional to the amount of substance in solution, and that it is proportional to the absolute temperature, and he showed, besides, that the pressure exerted by the sugar molecules is the same as that which would be exerted at the same temperature were an equal number of molecules of hydrogen to occupy the same volume as the sugar solution. This may be expressed by stating that when in dilute solution sugar molecules behave as if they were present in the gaseous state. Here again, however, it was noticed that salts tended to give a higher pressure; it was difficult to construct a semi-permeable diaphragm, however, which would resist the passage of salt molecules, while allowing those of water to pass freely. Lastly, Arrhenius, of Stockholm, had shown that the conductivity of salt solutions for electricity may be explained on the assumption that when a salt, such as KNO₃ is dissolved in water, it dissociates into portions similar in number and kind to those it would yield if electrolyzed (and if no secondary reactions were to take place). Such portions (K and NO₃, for example) had been named ions by Faraday. The conductivity of such solutions becomes greater, per unit of dissolved salt, the weaker the solution, until finally a limit is reached, after which further dilution no longer increases conductivity. Now Van’t Hoff united all these isolated observations and showed their bearing on each other. Stated shortly, the hypothesis is as follows: When a substance is dissolved in a large quantity of a solvent, its molecules are separated from each other to a distance comparable with that which obtains in gases. They are, therefore, capable of independent action; and when placed in a vessel the walls of which are permeable to the solvent, but not to the dissolved substance (“semi-permeable membrane”), the imprisoned molecules of the latter exert pressure on the interior surface of these walls as if they were gaseous. Van’t Hoff showed the intimate connection between this phenomenon and the depression of freezing-point and the use of vapor pressure already alluded to. He pointed out further that the exceptions to this behavior, noticed in the case of dissolved salts, are due to their “electric dissociation,” or “ionization,” as it is now termed; and that in a sufficiently dilute solution of potassium nitrate, for example, the osmotic pressure, and the correlated depression of freezing-point and rise of boiling-point, are practically equal to what would be produced were the salt to be split into its ions, K and NO₃. These views were vigorously advocated by Ostwald, professor at Leipzig, in his Zeitschrift für physikalische Chemie, and he and his pupils have done much to gather together facts in confirmation of this theory, and in extending its scope.

It must be understood that the ions K and NO₃ are not, strictly speaking, atoms; they are charged atoms; the K retains a +, and the NO₃ a − charge. On immersing into the solution the poles of a battery, one charged + and the other −, the + K atoms are attracted to the − pole, and are there discharged; as soon as they lose their charge they are free to act on the water, when they liberate their equivalent of hydrogen. Similarly, the − NO₃ groups are discharged at the + pole, and abstract hydrogen from the water, liberating an equivalent quantity of oxygen. Thus the phenomenon of electrolysis, so long a mysterious process, finds a simple explanation. The course of ordinary chemical reactions is also readily realized when viewed in the light of this theory. Take, for example, the ordinary equation:

AgNO₃.Aq + NaCl.Ag = AgCl + NaNO₃.Aq;

i.e., solutions of silver nitrate and sodium chloride give a precipitate of silver chloride, leaving sodium nitrate in solution. By the new views, such an equation must be written:

+ − + − + −
Ag.Aq + NO₃.Aq + Na.Aq + Cl.Aq = AgCl + Na.Aq + NO₃.Aq.

The compound, silver chloride, being insoluble in water, is formed by the union of the ions Ag and Cl, and their consequent discharge, forming an electrically neutral compound; while the sodium ions, charged positively together with the NO₃ ions, negatively charged, remain in solution.

One more application of the principle may be given. Many observers—Andrews, Favre, and Silbermann, but especially Julius Thomsen, of Copenhagen, and M. Berthelot, of Paris—have devoted much labor and time to the measurement of the heat evolved during chemical reactions. Now, while very different amounts of heat are evolved when chlorine, bromine, or iodine combine respectively with sodium or potassium, the number of heat units evolved on neutralizing sodium or potassium hydroxide with hydrochloric, hydrobromic, hydriodic, or nitric acids is always about 13,500. How can this fact be explained? It finds its explanation as follows: These acids and bases are ionized in solution as shown in the equation:

+ − + − + −
H.Aq + Cl.Aq. + Na.Aq + OH.Aq = H.OH + Na.Aq + Cl.Aq.

Water is the only compound formed; and it is produced by the union of the hydrogen-ion originally belonging to the acid, and the OH or hydroxyl-ion originally belonging to the base. No further change has occurred; hence the uniform evolution of heat by the interaction of equivalent quantities of these acids and bases.

It now remains to give a short account of the greatest generalization which has as yet been made in chemistry. It has been termed the “Periodic Arrangement of the Elements.”

In 1864 Newlands, of London, and Lothar Meyer, late of Tübingen, found that by arranging the elements in the order of their atomic weights certain regularities were to be observed between each element, and in general the eighth in succession from it, in the order of their numerical value. Such similar elements formed groups or quantities; while the elements separating them belong to a period, hence the name “periodic arrangement.” Commencing with lithium, a light, lustrous metal found in silicate in certain minerals, we have the following series:

and so on. It is unnecessary to point out in detail the resemblances between the elements which stand in the vertical columns; but it may be stated that the resemblance extends also to the formulas and properties of their compounds. Thus the chlorides of lithium and sodium are each white soluble salts, of the formulas LiCl and NaCl; oxides of magnesium and of beryllium are both insoluble white earthy powders, MgO and BeO (GeO), and so on. Newlands, in his preliminary sketch, termed this order the “Law of Octaves,” and predicted the existence of certain undiscovered elements which should occupy unfilled positions in the table. Mendeléef, professor at St. Petersburg, in 1869 amplified and extended these relations; and he and Meyer pointed out that the volume occupied by equal numbers of atoms of such elements underwent a periodic variation when the elements are classified as above. The prediction of undiscovered elements was made by Mendeléef in a more assured manner; and in several cases they have been realized. Thus what Mendeléef called “ekaboron” has since been discovered by Lecoq de Boisbandron and named, patriotically, “gallium”; Mendeléef’s “eka-silicon” is now known as “germanium,” discovered by Winkler; and “eka-aluminum” is now Cléve’s “scandium.” Moreover, the atomic weights of cæsium, beryllium, molybdenium, and mercury have been altered so that they fit the periodic table; and further research has justified the alteration.

The valency of these elements increases from right to left, as will be seen by inspection of the following series:

The elements of no valency are of recent discovery. In 1894 Lord Rayleigh had determined the density of the nitrogen of the atmosphere, having separated from it the oxygen and carbon dioxide which is mixed with nitrogen in air. He found it to be of somewhat higher density than that obtainable from ammonia and other compounds of nitrogen. In conjunction with Ramsay he investigated atmospheric nitrogen; it was absorbed either by a method devised by Cavendish, or by making it combine with magnesium at a red heat. They found that the unabsorbable residue possessed an unknown spectrum, and that its density was nearly 20. To this new gas they gave the name “argon,” or inactive, seeing that all attempts to cause it to enter into combination had failed. In 1895 Ramsay, searching for possible combinations of argon in minerals, experimented with one which had been previously examined by Hillebrand, of Baltimore, and obtained from it helium, a gas of density 2, possessing a spectrum which had been previously discovered in 1868 in the chromosphere of the sun, by Jannsen, of Paris, and named helium by Frankland and Lockyer. Subsequent liquefaction of crude argon by means of liquid air, prepared by a process invented simultaneously by Linde and Hampson, gave a residue which was named by its discoverers, Ramsay and Travers, “neon.” Liquid argon has yielded two other gases also, “krypon” and “xenon.” These elements form a separate group in the Periodic Table, commencing with helium, with atomic weight, 4; neon, 20; argon, 40; krypon, 82; and xenon, 128. They all agree in being mono-atomic, i.e., their molecules consist of single atoms; and they have no tendency to form compounds, i.e., they possess no valency.

In this sketch of the progress of chemistry during the century which has just passed, attention has been paid chiefly to the progress of thought. Allusions must, however, be made to the applications of chemistry to industrial purposes. The development of the soda industry, the preparation of carbonate of soda and caustic from common salt—initiated in France by LeBlanc (1742–1806)—has been developed by Tennant, in Scotland, and Muspeath and Gossage, and by Hargreaves, Weldon, and Maetea, in England; this process has at present a serious rival in the ammonia-soda process, developed by Solway, in Belgium, and by Brunner and Mond, in England. The main action of sulphuric acid, so long associated with the alkali process, has made enormous strides during the present century, but is still, in the main, the original process of causing sulphur dioxide in presence of water to absorb the oxygen of the air through nitric oxide. But the saving of the oxides of nitrogen through the invention of a sulphuric acid power by Gay-Lussac, known by his name, and the re-utilization of these oxides in the “Glover” power, invented by John Glover, of Newcastle, have greatly lessened the cost of the acid. Concentration of the acid in iron vessels is now common, the cost of platinum or of fragile glass vessels being thereby saved. The desulphurization of iron and the removal of silicon, carbon, and phosphorus by Bessemer’s process, modified by Thomas and Gilchrist through the introduction of a “basic magnesia lining” for the convertors, has made it possible to obtain pure iron and steel from ores previously regarded as of little value.

The use of artificial manures, prepared by mixing refuse animal matters with tetra-hydrogen, calcium phosphate, and nitrate of soda, or sulphate of ammonia, first introduced by Liebig, has created a revolution in agricultural methods and in the weight of crops obtainable from a given area of soil. The influence of manures on crops has been fully studied by Lawes and Gilbert for more than fifty years in their experimental farm at Rothampstead. The most remarkable advances which have been made, however, are due to cheap electric current. The electrolysis of alumina, dissolved in fused cryolite to obtain aluminum, an operation carried out at Schaffhausen-on-the-Rhine, and at the Falls of Foyers, in Scotland; the electro-deposition of pure copper for electric wires and cables, electro-silvering, gilding, and nickelling, all these are instances where decomposition of a compound by the electric current has led to important industrial results. At present soda and chlorine are being manufactured by the electrolysis of salt solution contained in rocking trays, one of the electrodes being mercury, by the Castner-Kellner process. This manufacture is being carried on at Niagara, as well as in England. But electricity as a heating agent finds ever-extending application. Louis Moisson, professor at Paris, led the way by utilizing the enormous heat of the ore in his electric furnace, thereby, among other interesting reactions, manufacturing diamonds, small, it is true, though none the less real. The use of electricity as a heating agent has received new applications. Phosphorus is now made by distilling a mixture of phosphates of lime and alumina with coke; a new polishing agent has been found in “carborundum,” a compound of carbon and silicon, produced by heating in an electric furnace a mixture of sand and coke; and cyanide of potassium, almost indispensable for the extraction of gold from ores poor in gold, is now manufactured by heating a mixture of carbon and carbonate of barium in an electric furnace in a current of carbon monoxide. These are but some of the instances in which electricity has been adopted as an agent in effecting chemical changes; and it may be confidently predicted that the earlier years of the twentieth century will witness a great development in this direction. It may be pointed out that the later developments of industrial chemistry owe their success entirely to the growth of chemical theory; and it is obvious that that nation which possesses the most competent chemists, theoretical and practical, is destined to succeed in the competition with other nations for commercial supremacy and all its concomitant advantages.

William Ramsay.

ARCHÆOLOGY

To write of the progress of archæology in this century is scarcely possible, as the idea of the subject was unknown a hundred years ago; it is, therefore, the whole history of its opening and development that we have to deal with. The conception of the history of man being preserved to us in material facts, and not only in written words, was quite disregarded until the growth of geology had taught men to read nature for themselves, instead of trusting to the interpretations formed by their ancestors. Even down to the present the academic view is that classical archæology is more important than other branches, because it serves to illustrate classical literature. Looked at as archæology, it is, on the contrary, the least important branch, because we already know so much more of the classical ages than we do of others.

It is only within the present generation that it has been realized that wherever man has lived he has left the traces of his action, and that a systematic and observant study of those remains will interpret to us what his life was, what his abilities and tastes were, and the extent and nature of his mind. Literature is but one branch of the archæology of the higher races; another—equally important for the understanding of man—is art; these two give the highest and most complex and characteristic view of the nature of a race. At the opposite end of the scale are the rudest stone weapons which remain as the sole traces of the savages who used them. These highest and lowest evidences of mind, and all that lies between them, are the domain of archæology.

We now purpose to review the growth of archæology in contact with geology, where it concerns man as the last of the links of life on the globe; and then to notice the archæology of each country in turn, as it leads on to the times of historical record, and so passes down to modern times.

A century ago the world of thought was divided between the old and new ideas very differently from what is now the case. Then there stood on one side the idea of a special creation of an individual man, at 4000 B. C.; the compression of all human history into a prehistoric age of about three thousand years, and a fairly logical solution of most of the difficulties of understanding in a comfortable teleology. On the other hand stood many who felt the inherent improbability of such solutions of the problem of life, and who were feeling their way to some more workable theory on the basis of Laplace, Lamarck, Erasmus Darwin, and others; vaguely mingling together questions of physics, geology, archæology, anthropology, and theology, each of which we now see must be treated on its own basis, and be decided on internal evidence, before we can venture to let it affect our judgment on other points.

The great new force which thrust itself in to divide and decide on these questions is the scientific study of man and his works. Strangely shaped flints had been noticed, but no one had any knowledge of their age. One such, when found with the bones of a mammoth, was attributed to the Roman age, because no person could have brought elephants into Britain except some Roman general. The argument was excellent and irrefutable until geology found plenty more remains of the mammoth and showed that it was here long before the Romans. It was less than half a century ago that our eyes began to open to the abundant remains of flint-using man. Then a single rude stone weapon was an unexplained curiosity; now an active collector will put together his tens of thousands of specimens, will know exactly where they were found, their relation of age and of purpose, and their bearing on the history of man.

Not only have worked flint implements been found in the river gravels of France and England, where they were first noticed in the middle of this century, but also in most parts of Europe, in Egypt on the high desert, in Somaliland, at the Cape of Good Hope, in India, America, and other countries; and the most striking feature is the exact similarity in form wherever they have been found. So precisely do the same types recur, so impossible would it be to say from its form whether a flint had been found in Europe, Asia, or Africa, that it appears as if the art of working had spread from some single centre over the rest of the world. This is especially the case with the river-gravel flints—the earlier class—usually called Paleolithic. Soon after the general division had been made between polished stone-work of the later or Neolithic times, found on the surface, and the rough chipped work of the earlier or Paleolithic times, found in geological deposits, a further sub-division was made by separating the Paleolithic age into that of the river gravels and that of the cave-dwellers. The latter has again been divided into three classes by French writers, named, from their localities, Mousterien, Solutrien, Magdalenien; and, though these classes may be much influenced by locality, they probably have some difference of age between them.

And now within the last few years a still earlier kind of workmanship has been recognized in flints found in England on the high hills in Kent. Though at first much disputed, the human origin of the forms is now generally acknowledged, and they show a far ruder ability than even the most massive of the Paleolithic forms. The position also of these flints, in river deposits lying on the highest hills some six hundred feet above the present rivers, shows that the whole of the valleys has been excavated since they were deposited, and implies a far greater age than any of the gravel beds of the Paleolithic ages.

We, therefore, have passed now at the beginning of this century to a far wider view of man’s history, and classify his earlier ages in Europe thus:

First—Eolithic: Rudest massive flints from deposits 600 feet up.

Second—Paleolithic: Massive flints from gravels 200 feet up and less (Achuleen).

Third—Paleolithic—Cave-dwellers: Flints like the preceding and flakes (Mousterien).

Fourth—Paleolithic—Cave-dwellers: Flints well worked and finely shaped (Solutrien).

Fifth—Paleolithic—Cave-dwellers: Abundant bone working and drawing (Magdalenien).

Sixth—Neolithic: Polished flint working, pastoral and agricultural man.

What time these periods cover nothing yet proves. The date of 4000 B. C. for man’s appearance, with which belief the nineteenth century started, has been pushed back by one discovery after another. Estimates of from 10,000 to 200,000 years have been given from various possible clews. In Egypt an exposure of 7000 years or more only gives a faint brown tint to flints lying side by side with Paleolithic flints that are black with age. I incline to think that 100,000 years B. C. for the rise of the second class, and 10,000 B. C. for the rise of the sixth class will be a moderate estimate.

Passing now from Paleolithic man of the latest geological times whose works lie under the deposit of ages, to Neolithic man of surface history whose polished stone tools lie on the ground, we find also how greatly views have changed. For ages past metal-using man has looked on the beautifully polished or chipped weapons of his forefathers as “thunderbolts,” possessing magic powers, and he often mounted the smaller ones to wear as charms. At the beginning of this century well-finished stone weapons were only preserved as curiosities which might belong to some remote age, but without any definite ideas about them. The recognition of long ages of earlier unpolished stone work has now put these more elaborate specimens to a comparatively late period, and yet they are probably older than the date to which our forefathers placed the creation of man.

The beginning of a more intelligent knowledge of such things was laid by the systematic excavations of the burial mounds scattered over the south of England, which was done in the early part of this century by Sir Richard Colt Hoare. A solid basis of facts was laid, which began to supersede the romances woven by Stukeley and others in the last century. Gradually more exact methods of search were introduced, and in the last thirty years Canon Greenwell has done much, and General Pitt Rivers has established a standard of accurate and complete work with perfect recording, which is the highest development of archæological study. These and other researches have opened up the life of Neolithic man to us, and we see that he was much as modern man, if compared with the earlier stage of man as a hunter. The Neolithic man made pottery, spun and wove linen, constructed enormous earthworks both for defence and for burial, and systematically made his tools of the best material he could obtain by combined labor in mining. The extensive flint-mines in chalk districts of England show long-continued labor; and the perfect form and splendid finish of many of the stone weapons show that skilled leisure could be devoted to them, and that æsthetic taste had been developed. The large camps prove that a thorough tribal organization prevailed, though probably confined to small clans.

About the middle of the century a new type of dwelling began to be explored—the lake dwelling; this system of building towns upon piles in lakes had the great advantage of protection from enemies and wild beasts, and a constant supply of food in the fish that could be hooked from the water below. Though such settlements were first found in the Swiss lakes, and explored there by Keller, they have since been found in France, Hungary, Italy, Holland, and the British Isles. The earlier settlements of this form belong to the Neolithic age, but only in central Europe. In these earliest lake dwellings weaving was known, and the cultivation of flax, grapes, and other fruit and corn; while the usual domestic animals were kept and cattle were yoked to the plough; pottery was abundant, and was often ornamented with geometric patterns. The type of man was round-headed. Following the Neolithic lake dwellings came those of the Bronze age, and as the bronze objects are similar to those found in other kinds of dwellings we shall notice them in the Bronze age in general. The type of man was longer-headed than in the earlier lake settlement. The domestication of animals shows an advance; the horse was common, and the dog, ox, pig, and sheep were greatly improved. Pottery was better made and elaborately decorated, often with strips of tin-foil.

The Bronze age marks a great step in man’s history. In many countries the use of copper, hardened by arsenic or oxide, was common for long before the alloy of copper and tin was used. In other countries, where the use of metals was imported, copper only appears as a native imitation of the imported bronze. Hence there is a true age of copper in lands where the use of metals has grown. It must by no means be supposed that copper excluded the use of flint; it was not until bronze became common that flint was disused. The existence of a Bronze age was first formulated, as distinct from a Stone age, about seventy years ago; and the existence of a Copper age has been much disputed in the last thirty years, but has only been proved clearly ten years ago, in Egypt.

In the eighteenth century the bronze weapons found in England were attributed to the Romans by some writers, though others, with more reason, argued that they were British. In the first year of the century began the comparative study of such weapons with reference to modern savage products. The development of the metal forms from stone prototypes was pointed out in 1816; the tracing out of the succession of the forms and the modes of use appeared in 1847. Further study cleared up the details, and within the last twenty years the full knowledge of the Bronze age in other countries has left no question as to the general facts of the sequence of its history. In each type of tool and weapon there appears first a very simple form imitated from the stone implements which were earlier used. Gradually the facilities given by the casting and toughness of the metal were used, and the forms were modified; ornamentation was added, and thin work in embossed patterns gave the stiffness and strength which had been attained before by massive forms. The general types are the axe—first a plain slip of metal, later developed with a socket; then the chisel, gouge, sickle, knife, dagger, sword, spear, and shield; personal objects, as pins, necklets, bracelets, ear-rings, buttons, buckles, and domestic caldrons and cups. Most of these forms were found together, all worn out and broken, in the great bronze-founder’s hoard at Bologna.

Lastly in the prehistory of Europe comes the Iron age, which so much belongs to the historical period that we can best consider it in noticing separate countries.

From the recent discoveries in Egypt we can gain some idea of the date of these periods. We ventured to assign about 10,000 B. C. for the rise of the Neolithic or polished-stone period (it may very possibly be earlier); the beginning of the use of copper may be placed about 5000 B. C.; the beginning of bronze was perhaps 3000 or 2000 B. C., as its free use in Egypt is not till 1600 B. C.; and the use of iron beginning about 1000 B. C., probably in Armenia, spreading thence through Europe until it reached Italy, perhaps 700 years B. C., and Britain about 400 B. C. Such is the briefest outline of the greater part of the history of man, massed together in one general term of “prehistoric,” before we reach the little fringe of history nearest to our own age. The whole of this knowledge results from the work of the century.

We now turn to the historical ages of each of the principal countries, to review what advance has been made even where a basis of written record has come down to us, equally accessible in all recent times.

EGYPT

At the beginning of the century Egypt was a land of untouched and inexplicable mystery; the hieroglyphics were wondered at, and puzzled over, without any idea of how they were to be read, whether as symbols or as letters. The history was entirely derived from the confused accounts of Greek authors, the lists remaining of Manetho’s history, written about 260 B. C., and the allusions in the Bible. The attempt to make everything fit to the ideas of the Greeks, and to make everything refer to the Biblical history, greatly retarded the understanding of the monuments, and is scarcely overcome yet. The first great step forward was when an inscription was found at Rosetta, in 1799, written in two methods, the monumental hieroglyphic and the popular demotic, along with a Greek version. By 1802 some groups of each writing had been translated. Young identified more signs, and Gell, by 1822, could successfully apportion three-quarters of the signs to the Greek words. The next step was to apply the modern Coptic language, descended from the ancient Egyptian, to the reading of the words. Gell had been doing so, but it needed a student of Coptic—Champollion—to carry this out thoroughly, as he did in 1821–32. Since then advance in reading has been only a matter of detail, not requiring any new principles.

The knowledge of the art began with the admiration for the debased work of Roman times, the principal interest at the beginning of the century. Then the excavations among the Rameside monuments at Thebes, about 1820–30, took attention back to the age of 1500–1000 B. C. The work of Lepsius, and later of Mariette, from 1840–80, opened men’s eyes to the splendid work of the early dynasties, about 4000–3000 B. C. And lastly the excavations of 1893–99 have fascinated scholars by a view of the rise of the civilization and the prehistoric period before 5000 B. C.

Throughout the greater part of the century the archæology of Egypt lay untouched; all attention was given to the language; and even Gardner Wilkinson’s fine view of the civilization (1837) depended largely on Greek authors, and had no perspective of history in tracing changes and development. It is only in the last ten or fifteen years that any exact knowledge has been acquired about the rise and progress of the various arts of life; this study now enables us to date the sculpture, metal work, pottery, and other art products as exactly as we can those of the Middle Ages.

The view that we now have of the rise and decay of this great civilization and its connection with other lands is more complete and far-reaching than that of any other country. In the early undated age, before the monarchy which began about 4800 B. C., a flourishing civilization was spread over upper Egypt. Towns were built of brick, as in later times; clothing was made of woven linen and of leather; pottery was most skilfully formed, without the potter’s wheel, hand-made, yet of exquisite regularity and beauty of outline, while the variety of form is perhaps greater than in any other land; stone vases were made entirely by hand, without a lathe, as perfect in form as the pottery, and of the hardest rocks, as diorite and granite; wood was carved for furniture; the art of colored glazing was common, and was even applied to glazing over large carvings in rock crystal; ornaments and beads were wrought of various stones and precious metals; ivory combs with carved figures adorned the hair; ivory spoons were used at the table; finely formed weapons and tools of copper served where strength was needful, while more useful were flint knives and lances which were wrought with a miraculous finish that has never been reached by any other people; and games were played with dainty pieces made of hard stone and of ivory. But all this tasteful skill of 6000–5000 B. C. had its negative side; in the artistic copying of nature the mechanical skill of these people carried them a very little way; their figures and heads of men and animals are strangely crude. And they had no system of writing, although marks were commonly used. They always buried the body doubled up, and often preserved the head and hands separately. Commerce was already active, and large rowing-galleys carried the wares of different countries around the Mediterranean. These people were the same as the modern Kabyle, of Algeria, and akin to the South European races, but with some negro admixture. Our whole knowledge of this age has only been gained within the last five years.

At about 5000 B. C. there poured into Egypt a very different people, probably from the Red Sea. Having far more artistic taste, a commoner use of metals, a system of writing already begun, and a more organized government, these fresh people started a new civilization in Egypt; adopting readily the art and skill of the earlier race, they formed by their union the peculiar culture known as Egyptian, a type which lasted for four thousand years. The same foundation of a type is seen in the bodily structure; the early historical people had wider heads and more slender noses than the prehistoric, but from 4000 B. C. down to Roman times the form shows no change.

From this union of two able races came one of the finest peoples ever seen, the Egyptians of the old kingdom, 4500—3500 B. C. Full of grand conceptions, active, able, highly mechanical, and yet splendid artists, they have left behind them the greatest masses of building, the most accurate workmanship and exquisite sculptures in the grand pyramids and tombs of their cemeteries. They perfected the art of organizing combined labor on the immense public works. In all these respects no later age or country has advanced beyond this early ability. The moral character and ideas are preserved to us in the writings of these people; and we there read of the ability, reserve, steadfastness, and kindliness which we see reflected in the lifelike portraiture of that age.

After a partial decay about 3000 B. C. this civilization blossomed out again nobly in the twelfth dynasty about 2600 B. C.; though the works of this age hardly reach the high level of the earlier times, yet they are finer than anything that followed them. At this period more contact with other countries is seen; both Syria and the Mediterranean were known, though imperfectly.

To this succeeded another decadence, sealed by the disaster of the foreign invasion of the Hyksos. But this was thrown off by the rise of a third age of brilliance—the eighteenth dynasty, 1500 B. C.—which, though inferior to early times in its highest work, yet shines by the widespread of art and luxury throughout the upper classes. Magnificence became fashionable, and the lower classes contented themselves with most barefaced imitations of costly wares. Foreign islands came closely in contact with Egypt. The ships of the Syrian coast and Cyprus continually traded to and fro, exchanging silver, copper, and precious stones for the gold of Egypt. Greece also traded its fine pottery of the Mycenæan age for the showy necklaces of gold and the rings and amulets with names of Pharaohs. Egypt then dominated the shores of the western Mediterranean, the plains of the Euphrates, and the fertile Soudan. But this power and wealth led to disaster. Like Rome, later on, she could not resist the temptation to live on plunder; heavy tribute of corn was exacted, large numbers were employed in unproductive labor, and national disaster was the natural consequence. Egypt never recovered the dominion or the splendor that were hers in this age. Of this period some slight notions are given us from literary remains in the Bible and Greek authors; but archæology is, so far, our only practical guide, as in the earlier ages. The great temples and monuments of the eighteenth-twentieth dynasties (1600–1100 B. C.) bear hundreds of historical inscriptions, the tombs are covered with scenes of private life, the burials and the ruins of towns furnish us with all the objects of daily use. This age is one of the fullest and richest in all history, and hardly any other is better known even in Greece or Italy. Yet all this has been brought to light in the century, and the knowledge of the foreign relations of Egypt is entirely the result of the last fifteen years.

The final thousand years of the civilization of Egypt is checkered with many changes; sometimes independent, as in the ages of Shishak of Necho, and of the Ptolemies; at other times a prey to Ethiopians, Persians, Greeks, or Romans. Its arts and crafts show a constant decay, and there was but little left to resist the influence of Greek taste and design, which ran a debased course in the country. There was, however, a spread of manufactures and of cheap luxuries into lower and lower classes; and the wealth of the country accumulated under the beneficent rule of the earlier Ptolemies (300–200 B. C.).

The principal discoveries about these later ages have been in the papyri, which have been largely found during the last twenty years. The details of the government and life of the country in the Ptolemaic (305–30 B. C.) and Roman (30 B. C.–640 A. D.) periods have been cleared up; and many prizes of classical literature have also been recovered. The archæology of the Middle Ages in Egypt has also been studied. Many of the Arabic buildings have been recently cleaned and put in good condition, and the splendid collection of manuscripts in Cairo has opened a view of the beautiful art of the thirteenth-fifteenth centuries so closely akin to what was done in Europe at the same time.

Egypt is, then, before all other lands, the country of archæology. A continuous history of seven thousand years, with abundant remains of every period to illustrate it, and a rich prehistoric age before that, give completeness to the study and the fullest value to archæological research.

MESOPOTAMIA

The valley of the Euphrates might well rival that of the Nile if it were scientifically explored, but unhappily all the excavation has been done solely with a view to inscription and sculpture, and no proper record has been made, nor have any towns been examined, the only work being in palaces and temples.

The earliest study on the ground was by Rich (1818–20), who gathered some few sculptures and formed an idea of Assyrian art. The French Consul, Botta, excavated Khorsabad (founded 700 B. C.) in 1834–35, and Layard excavated Nimrud in 1845–47; these were both Assyrian sites. The older Babylonian civilization was touched at Erech by Loftus, in 1849–52; and this age has attracted the most important excavations made since, at Tello by Sarzec (1876–81), and at Nippur by Peters and Haynes, of Philadelphia, during the last few years.

The cuneiform characters were absolutely unexplained until Grotefend, in 1800, resolved several of them by taking inscriptions which he presumed might contain names of Persian kings and comparing them alongside of the known names; thus—without a single fixed point to start from—he tried a series of hypotheses until he found one which fitted the facts. Bournouf (in 1836) and Lassen (1836–44) rectified and completed the alphabet. But the cuneiform signs were used to write many diverse languages, as the Roman alphabet is used at present; and the short Persian alphabet was only a fraction of the great syllabary of six hundred signs used for Assyrian. Rawlinson had independently made out the Persian alphabet, using the Zend and Sanskrit for the language. He next, from the trilingual Behistun inscription in Persian, Assyrian, and Vannic, resolved the long Assyrian syllabary, using Hebrew for the language. Since then other more obscure languages written in cuneiform have been worked with more or less success; the most important is the Turanian language, used by the earlier inhabitants of Babylonia before the Semitic invasion; this is recorded by many syllabaries and dictionaries, and translations compiled by the literary Semitic kings.

The general view of the civilization which has been obtained by these labors of the century shows it to have been more important to the world than any other. Cuneiform was the literary script of the world for at least six thousand years, the only medium of writing from the Mediterranean to the Indian Ocean. The Babylonian culture was almost certainly the source of the oldest present civilization—that of China. And the arts were developed probably even earlier than in Egypt. The first inhabitants were called Sumirian (or river folk) in distinction from the Accadian (or highland) people, who came from Elam down into the Euphrates valley, bringing with them the use of writing. Their earliest writing was of figure symbols (like the Egyptian and Hittite); but as in the valley clay tablets were the only material for writing, the figures became gradually transformed into groups of straight lines and spots impressed on the clay; hence the signs were formalized into what we call cuneiform. The Semitic invaders were using cuneiform characters by about 3000 B. C.

The early civilization was intensely religious, the main buildings being the temples, which were placed on enormous piles of brick-work. The sculpture was at a high level in the time of Naram-Sinn, about 3750 B. C.; and yet below his ruins at Nippur there are no less than thirty-five feet depth of earlier ruins, which must extend back to 6000 or 7000 B. C. In early times stone implements were used alongside of copper and bronze, as we find in Egypt 4000 B. C. Pottery was well made, and also reliefs in terra-cotta. Personal ornaments of engraved gems and gold-work were common.

The main landmarks in the later time of this civilization are the Elamite invasion of Kudur-nan-khundi (2280 B. C.) which upset the Semitic rulers, and the Assyrian invasion of Tiglath-Adar (1270 B. C.), after which interest centres in the Assyrian kingdom and its development of the Mesopotamian culture which it borrowed. The main buildings of the Assyrian kings were their enormous palaces, the mass of which was of unbaked bricks, faced with alabaster slabs; such were the works of Assurnazir-pal (Nimrud, 880 B. C.), Sargon (Khorsabad, 710 B. C.), Sennacherib and Assurbani-pal (Kouyunjik, 700 B. C.). The later, Assyrian, form of the civilization was to the earlier Chaldean much what Rome was to Greece, a rather clumsy borrower, who laboriously preserved the literature and art. Some of the Assyrian sculpture of animals is, however, perhaps unsurpassed for vivid action. The systematic libraries, containing copies of all the older literature for general study, were most creditable, though the Assyrian himself composed nothing better than chronicles. Nearly all that we possess of Babylonian religion, and much of the history, is in the copies scrupulously made from the ancient tablets by the Assyrian scribes, who noted every defect in the original with critical fidelity.

The Mesopotamian civilization has left its mark on the modern world. Its religion greatly influenced Hebrew, and thence Christian, thought, the psalms, for instance, being a Babylonian form of piety. Its science fixed the signs of the zodiac, the months of the year, the days of the week, and the division of the circle in degrees, all of which are now universal. And its art, carried by the Phœnicians, was copied by the Greeks and Etruscans, and thus passed on into modern design.

SYRIA

The knowledge of Palestine was but slight, and of northern Syria nothing to speak of, a century ago. Travellers with some scientific ability, such as Robinson (1838 and 1852), De Saulcy (1853), and Van de Velde (1854), greatly extended our view and led up to the splendid survey by the Palestine Exploration Fund (1866 and on), which exhausted the surface study of the land. The more archæological work of excavation was begun at Jerusalem (1867–70), and resumed (1892–99) at Lachish, Jerusalem, etc. The topographical results are all important, and leave nothing to be done until excavation can be freely applied; and the small amount of digging yet done has fixed the varieties of pottery back to 2000 B. C. and given some early architecture. But the ruins of Syria, and indeed of Turkey in general, are practically yet untouched. The discovery (1868) of the inscription of Mesha, King of Moab (896 B. C.), opened a new prospect of research which cannot yet be entered upon. In the north of Syria nothing has been done except the German work at Singerli, from which came an Aramean inscription of about 740 B. C. And in the south a large number of early inscriptions of the Arabian dynasties, reaching back some centuries B. C., have been copied; but there, also, excavation is impossible.

The main new light from Syria has been on the Hittite power. Burckhardt, in 1812, had noticed a new kind of hieroglyph at Hamath. After several ineffective copies, Wright made casts of the stones in 1872. Several other such inscriptions have been found, and from these and the Egyptian and Assyrian references to the Hittites we now realize that they were a northern people, with a great capital on the Euphrates, at Karkhemish, and ruling over nearly all Syria and Asia Minor. Little has yet been fixed about the writing; a few signs are read and some have passed into the Cypriote alphabet. A striking proof of the spread of Babylonian culture is seen in the tablets found in Egypt at Tel-el-Amarna in 1887, which show that all the correspondence between Egypt and Syria in the fifteenth century B. C. was carried on in cuneiform. These hundreds of letters give a vivid picture of life in Syria at that early date.

GREECE

The revival of interest in Greek civilization was at first purely literary, and remained so during two or three centuries. But during the last century various travellers and residents abroad made collections which awoke an interest in the art; and though most of these collectors were content with merely showy sculpture, greatly restored and falsified for the market, yet some—such as Hamilton—took a real archæological interest in the unearthing and collecting of ancient art. The condition of study at the end of the eighteenth century was that many private men of wealth had bought large quantities of sculpture which was but little understood, and looked on more from a decorative than a scientific point of view, while there were the beginnings of a serious appreciation of it which had been just laid down by Winckelmann.

The nineteenth century opened with a grand work of publishing the principal treasures of classical art in England, which was finally issued in 1809 by Payne, Knight, and Townley; this marks the highest point of the dilettante collecting spirit, which was soon eclipsed by truer knowledge. Hitherto the best sculpture had hardly been known but at second hand through Roman copies; a closer acquaintance began with the travels of Dodwell, Gell, and Leake, all in the first decade of the century. The free opening of the British Museum, in 1805, and the accumulation there of all the best collections within the first quarter of the century, also served to educate a public taste. The first struggle of scientific and artistic knowledge against the dilettante spirit was over the Elgin marbles; by 1816 they were accepted as the masterpieces which all later criticism has proved them to be. The Æginetan and Phigaleian sculptures, brought to Munich and London, helped also to show the nobility of early Greek art; so that the last two generations have had a canon of taste to rely upon, the value of which cannot be overestimated.

Following on this noble foundation, other collectors worked in Greece and Asia Minor, and the British Museum profited by the labors of Burgon, Fellows, and Woodhouse between 1840 and 1860. The diplomatically supported work of Newton on the Mausoleum (1857–58), and Wood at Ephesus (1863–75), filled out our knowledge of the middle period of Greek art (350 B. C.). Comparatively little has been done since then by England, but the activity of the Germans at Olympia has given us the only original masterpiece that is known—the Hermes of Praxiteles (350 B. C.), and their work at Pergamon revealed the great altar belonging to the later age (180 B. C.). The excavations at Athens (in 1886) have produced the impressive statues dedicated to Athene about 520 B. C., which reveal the noble rise of Attic sculpture. But attention during the last quarter-century has been largely fixed upon the earlier ages. The discoveries of Schliemann at Hissarlik (Troy, 1870–82), Mycenæ (1876), Orchomenos (1880–81), and Tiryns (1884), opened a new world of thought and research. Though at first bitterly attacked, it is now agreed that these discoveries show us the civilization of Greece between 2000 and 1000 B. C. Lastly, during ten years past Egypt has provided the solid chronology for prehistoric Greece by discoveries of trade between the two countries.

We can now very briefly estimate the present position of our knowledge as gained during the century. Setting aside the early foreign pottery found in Egypt, which belongs probably to Greece or Italy at 5000 and 3000 B. C., we first touch a civilized city in the lowest town of Troy, where metal was scarcely yet in use, which is certainly before 2000 and probably about 3000 B. C. in date. Succeeding that is the finely built second Troy, rich in gold vases and ornaments, which—though mistaken by Schliemann for the Homeric Troy—must yet be long before that, probably before 2000 B. C. After the burning of that come three other rebuildings before we reach the town of the age of Mycenæ, about 1500 B. C. Of this, which was in Greece the climax of the prehistoric civilization, there are the splendid treasures found at Mycenæ, the magnificent domed tombs, the abundance of fine jewelry and metal-work, of beautiful pottery and glazed ornament. To this age belong the great palaces of Mycenæ, Tiryns, Athens, and other hill fortresses, of which hardly more than the plans can now be traced. And it is this civilization which traded eagerly with Egypt, exchanging the valued manufactures of each country. This period was at its full bloom from 1500–1200 B. C., and began to decay by 1100 B. C., this dating being given by the contact with Egypt.

This natural decadence of art in Greece was hastened by the invasion of the barbarous Dorians about 1000 B. C. Art, however, was by no means extinguished, but only repressed by the troubles of the age; and Athens, which was not conquered by the Dorians, was the main centre of the revival of the arts. Other examples of such a history are familiar in Egypt (after the Hyksos invasion) and in Italy (after the Lombards), where earlier abilities revive and bloom afresh when vigorous invaders become united to an artistic stock. After the centuries of warfare a quieter age allowed the growth of fine arts again in the seventh century B. C., largely influenced by Egyptian and Assyrian work at second hand, through the Greek settlements in Cyprus and Egypt. By 600 B. C. definite types of sculpture were started, and a course was begun which only ended in the fall of classical civilization. The century before the Persian invasion, in 480 B. C., was one of rapid development; and in sculpture and vase-painting we see that this century carried forward the arts to technical perfection and the highest power of expression. Immediately after the Persian wars came the supreme works of Pheidias and Myron, most familiar in the Parthenon and the Discobolus; and in vase-painting comes the reversal from vases drawn in black on a red ground to the blocking out of the ground in black, leaving the figure in red, thus giving far greater scope to the filling in of finely drawn detail. The civilization of Athens was also at its height in this age, under Pericles, and the minor arts received their most refined and perfect treatment. After this comes nothing but ripening to decay. It must always be remembered that we have but very few examples of original work of the great artists. Nearly all the actual marbles preserved are copies made in later times, which show little of the delicacy of the original; and the few original marbles that remain are mostly of unknown subjects by unknown men. The great work in Greek archæology during the last fifty years has been comparing the records of ancient art (in Pliny, Pausanias, etc.) with the remaining sculptures, critically assigning the various types of statues to their celebrated originals, and thus forming some idea of the real history of Greek art.

From these studies, full of detail and controversy, we may briefly sum up the characteristics of the principal artists and their imitators. At about 440 B. C. Pheidias showed in the Parthenon the highest expression of divine and mythic forms, in a simple and heroic style which was never equalled. Half a century later Polykleitos followed a more human expression, using motives (as in the Doryphoros), but yet portraying an abstract humanity. By 330 B. C. Praxiteles brought the expression of moods to his works, graceful, animated, and with a full ripeness, as in the Hermes of Olympia, or the Faun. Skopas, slightly later, marked his work by his great vigor and strong personality. This was the second turning-point, when ripeness passed into decay; and in Lysippos there is mere vivid naturalism and an impressionist manner without much soul or thought, as in his Apoxyomenos, about 330 B. C. After this mere triviality and genre subjects are usual, portraiture is a common aim, and dignity was vainly striven for in colossal size. The glorification of showing dead and vanquished enemies is seen in the Dying Gaul and figures of slain foes at Pergamon. Later on, about 180 B. C., we see the violent, complicated, and straining action of the figures around the great altar of Pergamon, which also appears in the groups of the Laocoon and Farnese Bull. In the Græco-Roman age a conscious artificiality took the place of life and expression, as we see in the Apollo Belvidere, the Venus di Medici, and the Farnese Hercules. Art was saved in the first century A. D. by the devotion of portraiture, which gave a sense of reality and conviction which is entirely absent in the imaginative works. Lastly, a painstaking study and admiration of earlier works led, under the wealthy patronage of Hadrian (130 A. D.), to an eclectic revival which was wholly artificial, and passed away within a generation. We have fixed on sculpture as the most complete expression of Greek art; in other directions there is neither enough material nor enough research to give us a connected view. Not a single town, hardly a single house, in Greece has been excavated; there is no consecutive knowledge of the ordinary products and objects of life; and there is very little recorded of the discoveries of the tombs. The artistic interest of the sculpture and architecture has starved other branches of archæology, and for Greece more remains to be done than for some less celebrated lands.

ITALY

The interest in Italy at the beginning of the nineteenth century was mainly for the sake of its second-hand version of Greek art, and for the architecture and painting of the Renaissance. On the contrary, now the objects from Greece itself have far eclipsed the Italian copies, and the interest lies in the early Italian civilization and its purely Roman derivatives; while modern taste values the mediæval art of Italy far from the bastard products of the florid age which followed. The first detailed studies in Italy were those on Pompeii, especially by Gell (1817), which made that debased style very popular, and paved the way for appreciation of better work. The various isolated discoveries of Etruscan tombs were summed up in the admirable work of Dennis (1848), which presented a general view of that civilization which has not been superseded. The earlier Italic culture has been examined in many places where accidental discoveries have revealed it during the latter half of the nineteenth century, and especially in the systematic work of Zannoni, at Bologna (1870–75), and of Orsi, lately, in Sicily. The history of the city of Rome has been almost rewritten in the last thirty years owing to the great changes of the new government; these have been largely worked by Lanciani, and recorded by him and Middleton. The view of Italian history at present begins in the Stone age, which has been well studied, and has links with the later periods, as in the general use of black pottery. The earliest metal objects are very simple blades of daggers, found in graves, mingled with flint arrow-heads and knives. The admirable Italian plan of preserving whole burials undisturbed in museums enables us to see these graves complete in the Kircherian Museum. A special branch of the early Bronze age life was the system of lake dwellings (natural or artificially water girt), which abound in the northern Italian lakes and over the plain of Lombardy. These towns (“terra mare”) are arranged on a rectangular plan, and form the earliest stage of many of the present cities. The full development of the Bronze age civilization seems to have been later than in Greece, at about 800 B. C., to which belong the great discoveries of tombs, weapons, and tools at Bologna, and the cemetery of Falerii.

Upon all the native Italic civilization came an entirely different influence from the immigrant Etruscan. Traditionally coming from Asia Minor, he brought art and religion which had no relation to the Italic. The earliest Etruscan paintings are strongly northern in style, influenced by north European feeling (Veii). But soon the Etruscan borrowed largely from other races, from the Greek mainly, but also from Assyria and Egypt. Thus the fascinating problem in Italy is to distinguish the various sources of Italic, Etruscan, Græco-Etruscan, Oriental-Etruscan, and pure Greek, which are found in all degrees of combination before Roman times, and which can still be traced through the Roman age. The characteristics of Etruscan taste are: (1) The extraneous objects and figures, such as rows of pendants to a metal vase, monstrous heads standing out from a bowl, and statuettes placed for handles; (2) in forms of vases and furniture, the combination of many different parts and curves which never form a whole design; (3) and in sculpture the large round head and staring eyes. In general, an air of clumsy adaptation by a race deficient in originality. The glory of the Etruscan was his engineering, which he handed as a legacy to Rome. Strange to say, although thousands of Etruscan inscriptions are known, and many words are translated, yet the language is sealed to us, and none of the many attempts to read it has succeeded. The scientific study of Etruscan tombs has been well followed lately, as shown in the Florence Museum, where a separate room is devoted to each city.

In the south of Italy Greek art prevailed, and many of the finest works belong to this civilization. The Greek in Italy had rather different ideals to those of Greece; he started more from the level of Polykleitos and Praxiteles than from the severe age; his favorite type is that of youth and adolescence, never of maturity. The grace and feeling of such bronze statues as the Hermes and so-called Sappho of Herculaneum are peculiar to southern Italy. And when the Greek artist penetrated north and allied himself with the mechanical skill of the Etruscan, such splendid work was done as the Orator of Sanguineto.

Rome in the earlier centuries was an Italic town which came under Etruscan influence as Tuscany was conquered. But from the age of foreign conquest in the first century B. C., Greek art in a debased form ruled over all else, and ran into utter degradation in the third century A. D. It was this art that the power of Rome spread around the whole Mediterranean, from Palmyra to Britain, and is the parent of most modern decoration. But in the great reconstruction of the empire under Diocletian the debased Greek taste was mostly shaken off, and Rome went back to the old Italic-Etruscan style and motives. The statues have the round heads and staring eyes of old Etruria; the taste for quaint accessories, such as lions supporting objects, came back and passed into mediæval art, and the exaggerated, lengthy forms of men and animals reappeared.

Of the Christian period De Rossi’s work in the catacombs has given a firm base of facts for the third to the sixth century A. D., the actual tomb and body of Saint Cecilia being the most striking result. The later Roman and mediæval age in Italy is full of interest, but in that—as in the rest of mediæval Europe—research has been mainly on architecture and objects which are not the result of excavation.

INDIA

The Hindus have never been chronologists or historians, and their great Sanskrit literature tells practically nothing about the rise of Buddhism, the invasion of Alexander, or the spread of civilization in Indo-China. All before the Islamic conquest in the tenth century A. D. is in a mist of Puranic mythology. Here, then, more than in other countries, archæology has restored the history, and done so entirely within the nineteenth century.

The existence of Sanskrit literature was revealed to the West by Sir William Jones at the end of the last century, and this gave scope to Oriental scholars, while antiquities only interested the collector. But serious exploration was led by Prinsep, whose decipherment of the Asoka inscriptions in 1837, which ranks with the achievements of Champollion and Rawlinson, gave the key to a mass of inscriptions.

His assistant, Cunningham, excavated many sites and collected coins, being head of the Archæological Survey from 1861 to 1885. Fergusson was the historian of Indian architecture; Burgess has published the cave-temples in west and south India; Sewell in Madras and Führer in the northwest have excavated and explored, and a few native pundits have been educated to such research. The government, in financial difficulty, has withdrawn from the work, but the congress of Orientalists in 1897 resolved to establish an Indian exploration fund.

Inscriptions abound in India, on copper plate, stone pillars, and native rock. Those in Sanskrit, or modern vernaculars, are records of land grants or local dynasties. The oldest—in two different alphabets (of Semitic origin)—are the famous edicts of Asoka (third century B. C.), who has been called The Buddhist Constantine. He placed these monuments of his power and religion around his frontiers of northern India; but their meaning was forgotten until Prinsep’s decipherment. The Hindus seem to have a coinage of stamped silver plate before Alexander; but regular coinage begins in the Bactrian kingdoms (200 B. C.–200 A. D.), with Greek and native inscriptions. Since then the coinage is continuous, and invaluable for history. No stone building or sculpture is older than Alexander (327 B. C.), or certainly earlier than Asoka (264–233 B. C.). Greek influence is plain in the Punjab, but native style is seen in the cave-temples. The richest results have been from the mounds, some of which are ruins of forts or palaces, but the more important are the stupas, lofty domes erected two to one thousand years ago to enshrine Buddhist relics. These domes are surrounded with sculptured reliefs of scenes in the life of Buddha, and are often dated by inscriptions. From one lately opened the Buddha relic has been sent to the King of Siam, the only Buddhist king. Much has been done by the government in publishing and providing casts and photographs; but India yet needs a scientific archæologist to record details with the accuracy demanded by modern research.

AMERICA

Archæological work in the United States and in Central America was begun by Squier about the middle of the century, and the attention thus drawn to the subject has borne fruit in the more accurate and scientific explorations connected with the surveying and geological departments, and, above all, those of the Smithsonian Bureau of Ethnology. The names of Whitney, Wright, Cyrus Thomas, Holmes, Fowke, Mindeleff, and others, will be familiar to all American readers by their work of the last twenty years, and need no introducing here.

The earliest remains of man in America—or perhaps in the world—are those beneath the great lava beds of California; since those were deposited the rivers have cut their beds through two thousand to four thousand feet of lava rock, implying an erosion during tens, or perhaps hundreds, of thousands of years. But little can be assigned, however, with any certainty to a date before the Christian era, though mounds of refuse on both ocean shores may probably belong to an age before any human history.

The most important studies have been those on the highest civilization of the continent, that of Central America. The destroying Spaniards preserved but little of native record, except incidentally, and the first collector of Aztec manuscripts was Benaduci (1736), of whose treasures but an eighth survived his imprisonments and persecutions, one of the greatest disasters to history. The first great publication of manuscripts was the magnificent work of Lord Kingsborough (1830); and almost at the same time appeared Prescott’s history. Though the later researches have shown that the land was divided into many small kingdoms, rather than under one power, as Prescott supposed, yet his account of the calendar and chronology of the Aztecs has been verified and added to, and far more has been done in reading the manuscripts than he supposed possible. Aubin, after years of work in Mexico, brought to Europe manuscripts of an entirely new kind, showing a fully developed system of phonetic writing, which he has largely deciphered with success, having analyzed over one hundred syllabic values correctly.

One of the most complete studies has been that of the Mayan Quiché peoples, and especially of the Mayans of Yucatan. In 1864 Landa’s work on Yucatan (written 1566) was rediscovered, and the account of the calendar has sufficed to enable Goodman to discover the meaning of a very large number of signs (1897); these enable the numerical documents to be translated, and show that a period of as much as eight thousand years was dealt with by the Mayans, perhaps belonging to mythical ages. The alphabetic signs of Landa have proved useless so far, and Goodman even disbelieves in any record except that of numbers. Seler has shown the identical origin of the signs used by Aztecs and Mayans for the days and months. Little had been done to make known these remains until the recent explorations, casts, and publications of Maudsley, who has worked magnificently for seventeen years at Copan, Palenque, and Chichen-Itza; these, however, are but three of innumerable cities of Guatemala and Yucatan that need exploration.

In New Mexico the many ruins from the Colorado to the Rio Grande have been proved to resemble those of the modern Pueblo Indians, and to have none of the characteristics of Central American architecture; there are no sculptures, and the rock inscriptions are too primitive to be interpreted. Nothing points to an Aztec occupation, and probably the ancestors of the present people were the builders.

The innumerable earthworks of the Mississippi valley were formerly supposed to belong to some vanished race. And the view that they were connected with the Central American civilization is favored by the pyramid mound, which was hardly known otherwise, and by the excellence of the minor sculpture. But there are great differences between the two civilizations. The mound-builders were far inferior in metal-working, and their burial customs are peculiar. The use of materials from both east and west coasts shows an extensive commerce. The best summing up of the researches is that by Prof. Cyrus Thomas, after his extensive excavations. He concludes that the remains of the mound-builders show no great antiquity; that they were formed by tribes like the existing Indians; that the builders were of the same culture as were the Indians when discovered; that such mounds continued to be made and used for burial during the European period, and that the principal builders were the Cherokees.

It will be seen now how totally our view of man’s history has been changed by the study of archæology, and how fundamentally this science affects our ideas of the past and our expectations for the future of our race. The main outlines have been dimly seen; but in every country the greater part yet remains to be done, and in Turkey, Persia, and China most important civilizations are as yet quite untouched by exploration. The new century will no doubt see a harvest from these lands; and it is to be hoped that what yet remains in the safe keeping of the earth may be found by able men, who will preserve it for instruction and enable posterity to trace the fortunes of our species.

[India and America are here treated with the assistance of Mr. J. S. Cotton and Mr. D. MacIver.]

W. M. Flinders Petrie.

ASTRONOMY

In looking back over a century’s work in the oldest of the sciences, one is struck not only by the enormous advance that has been made in those branches of the science dealing with the motions of the heavenly bodies which were cultivated at least eight thousand years ago by early dwellers in the valleys of the Nile, Tigris, and Euphrates, but with the fact that during the century that has just passed away a perfectly new science of astronomy arose. By annexing physics and chemistry astronomers now study the motions of the particles of which all celestial bodies are composed; a new molecular astronomy has now been firmly established side by side with the old molar astronomy which formerly alone occupied the thoughts of star-gazers.

Along this new line our knowledge has advanced by leaps and bounds, and the results already obtained in expanding and perfecting man’s views of nature in all her beauty and immensity are second to none which have been garnered during the last hundred years.

THE POSITION AT THE BEGINNING OF THE CENTURY

It may be well before attempting to obtain a glimpse of recent progress that we should try to grasp the state of the science at the time when the nineteenth century was about to dawn, and this, perhaps, can be best accomplished by seeing what men were working at this period, at which the greatest activity was to be found in Germany; there was no permanent observatory in the southern hemisphere or in the United States.

First and foremost among the workers—he has, in fact, been described as “the greatest of modern astronomers”—was William Herschel, a German domiciled in England. In the year 1773 he hired a telescope, and with this small instrument he obtained his first glimpses of the rich fields of exploration open in the skies. From that time onward he had one fixed purpose in his mind, which was to obtain as intimate knowledge as possible of the construction of the heavens.

To do this, of course, great optical power was necessary, and such was his energy that, as large instruments were not to be obtained at any price, he set to work and made them himself.

Herschel presented the beginning of the nineteenth century not only with a definite idea of the constitution of the stellar system, based on a connected body of facts and deductions from facts, as gleaned through his telescopes, but observations without number in many fields. He discovered a new planet, Uranus, and several satellites of the planets; published catalogues of nebulæ; established the gravitational bond between many “double stars,” and carried on observations of the sun, then supposed to be a habitable globe. What Herschel did for observational astronomy and deductions therefrom, Laplace did for the furtherance of our knowledge concerning the exact motions of the bodies comprising the solar system. Newton had long before announced that gravitation was universal, and Laplace brought together investigations undertaken to determine the validity of this law. These were given to the world in his wonderful book on Celestial Mechanics, the first volumes of which appeared in 1799.

A survey of the work of these two great astronomers gives one an idea of what was going on in observational and mathematical astronomy at the beginning of the century.

The study was now destined to make rapid strides, as not only were new optical instruments—some designed for special purposes—introduced, new mathematical processes applied, fresh fields for research opened up, but the number of workers was considerably augmented by the increased means available; so much so, indeed, that the first astronomical periodical was founded by Von Zach in 1800 to facilitate intercommunications between the observers.

The first evening of the nineteenth century (January 1, 1801) augured well for progress. It had long been thought that all the members of the solar system had not as yet been discovered, and there was a very notable gap between the planets Mars and Jupiter, indicated by Bode’s law. Observers were organized to make a thorough search for the missing planet, portions of the sky being divided between them for minute examination. It fell to the Italian observer, Piazzi, to discover a small body which was moving in an orbit between these two planets on the date named. The century thus began with a sensation, and because the new body, which was named “Ceres,” was not of sufficient size to be accepted as the “missing planet,” the idea was suggested that perhaps it was a fragment of a larger planet that had been blown to pieces in the past.

An opportunity here arose for mathematical astronomy to come to the help of the observer, for Ceres soon was lost in the solar rays, and in order to rediscover it, after it had passed conjunction, an approximate knowledge of its path and future position was necessary.

With the then existing methods of computation of orbits it was imperative to have numerous measured positions to use as data for the calculation. The scanty data available in the case of Ceres were not sufficient for the application of the method. The occasion discovered a man, one of the greatest mathematicians of the nineteenth century, Karl Frederick Gauss, who, although only twenty-five years of age, undertook the solution of the problem by employing a system which he had devised, known as “the method of least squares,” which enabled him to obtain a most probable result from a given set of observations.

This, with a more general method of orbit computation, also elaborated by himself, was sufficient to enable him to calculate future positions of Ceres, and on the anniversary of the original discovery, Olbers, another great pioneer in orbit calculations, found the planet in very nearly the position assigned by Gauss. So great was the curiosity regarding the other portions of the planet, which was supposed to have been shattered, that numerous observers at once commenced to search after other fragments.

These were the actualities of 1801 and thereabouts; but the seed of much future work was sown. Kant and Laplace had already occupied themselves with theories as to the world formation, and spectrum analysis as applied to the heavenly bodies may be said to have been started by Wollaston’s observations of dark lines in the solar spectrum in 1802. Fraunhofer was then a boy at school. In the same year the first photographic prints were produced by Wedgewood and Davy.

OBSERVATORIES

It has been stated that at the beginning of the century there were no permanent observatories either in the southern hemisphere or in the United States. The end of the century finds us with two hundred observatories all told, of which fourteen are south of the equator and forty-seven in the United States, among which latter are the best-equipped and most active in the world.

The observatory of Parramatta was the first established (1821) in the southern hemisphere. This was followed by that at the Cape of Good Hope in 1829. Of the more modern southern observatories from which the best work has come we may mention Cordova, the seat of Gould’s important investigations, established in 1868, and Arequipa, a dependency of Harvard, whence the spectra of the southern stars have been secured, erected still more recently (1881).

I believe, but I do not know, that the large number of American observatories have radiated from Cincinnati, where, in consequence of eloquent appeals, both by voice and pen, from Mitchell, then professor of astronomy, an observatory was commenced in 1845. There can be no doubt that at the present moment, with the numerous well-equipped and active observatories, and the careful and thorough teaching established side by side with them, which enables numberless students to use the various instruments, the United States, in matters astronomical, fills the position occupied by Germany at the beginning of the century.

In Europe special observatories have been established at Meudon, Kensington, and Potsdam, so that new astrophysical inquiries may be undertaken without interfering with the prosecution or extension of the important meridional work carried on at Paris, Greenwich, and Berlin. A large proportion of the observations made by the Lick and Yerkes observatories in the United States has been astrophysical.

One of the special inquiries committed to the charge of the Solar Physics Observatory at Kensington at its establishment by the British government had relation to the possibility of running home meteorological changes on the earth, especially those followed by drought and famines in various parts of the empire, to the varying changes in the sun indicated by the ebb and flow of spots on its surface. With this end in view observations of the sun were commenced in India and the Mauritius to supplement those taken at Greenwich. At the same time other daily observations of sun spots by a different method were commenced at Kensington.

This kind of work was at first considered ideally useless; we shall see later on what has become of it.

IMPROVEMENTS IN TELESCOPES

The progress in astronomical science throughout the nineteenth century has naturally to a great extent depended upon the advances made both in the optics of the telescope and the way in which they are mounted, either with circles to record exact times and positions, or made to move so as to keep a star or other celestial objects in the field of view while under observation. The perfection of definition and the magnitude of the lenses employed in the modern instrument have been responsible for many important discoveries.

Ever since the telescope was invented—Galileo’s lens was smaller than those used in spectacles—men’s minds have been concentrated on producing instruments of larger and larger size to fathom the cosmos to its innermost depths.

At the beginning of the century we were, as we have seen already, in possession of reflectors of large dimensions; Herschel’s four-foot mirror, the instrument he was using in 1801, which had a focal length of forty feet, was capable of being employed with high magnifying powers; and it was the judicious use of these, on occasions when the finest of weather prevailed, that enabled him to enrich so extensively our knowledge of the stellar and planetary systems. For the ordinary work of astronomy, however, especially when circles are used, refractors are the more suitable instruments. This form suffers less from the vicissitudes of weather and temperature, and is, therefore, more suited where exact measurements are required.

Towards the end of the eighteenth century a Swiss artisan, Pierre Guinard, after many years of patient labor, succeeded in producing pure disks of flint glass as large as six inches in diameter. The modern refracting telescope thus became possible.

In 1804 there was started at Munich the famous optical and mechanical institute, which soon made its presence felt in the astronomical world. Reforms in instrument making were soon taken in hand, and under the leadership of the great German astronomer, Bessel, great strides were made in instruments of precision. Fraunhofer, who had been silently working away at the theory of lenses, and making various experiments in the manufacture of glass, was joined, in 1805, by Guinard. In 1809 Troughton invented a new method of graduating circles, according to Airy the greatest improvement ever achieved in the art of instrument making.

In 1824 Fraunhofer successfully completed and perfected an object-glass of 9.9 inches in diameter for the Dorpat Observatory. This objective might literally have been called a “giant,” for nothing approaching it in size had been previously made.

England, which was at one time the exclusive seat of the manufacture of refracting telescopes, was now completely outstripped by both Germany and France, and for this we had to thank “the short-sighted policy of the government, which had placed an exorbitant duty on the manufacture of flint glass.” In 1833 the Dorpat refractor was eclipsed by one of fifteen inches aperture made for the Pulkowa Observatory by Merz & Mähler, Fraunhofer’s successors, who about ten years later supplied a similar instrument to Harvard College. At that time Lord Rosse emulated with success the efforts of Herschel and rehabilitated the reflector by producing a metallic mirror of six-foot aperture and fifty-four-foot focal length which he mounted at Parsonstown. The speculum weighed no less than four tons. To mount this immense mass efficiently and safely was a work of no light nature, but he successfully accomplished it, and eventually both mirror and the telescope, which weighed now altogether fourteen tons, were so well counterpoised that they could be easily moved in a limited direction by means of a windlass worked by two men. The perfection of the “seeing” qualities of this instrument and its enormous light-grasping powers were particularly striking, and observational astronomy was considerably enriched by the discoveries made with it.

Speculum metal was not destined to stay; ten years later (1857) the genius of Léon Foucault introduced glass mirrors with a thin coating of silver deposited chemically, and these have now universally superseded the metallic ones.

The long supremacy of Germany in the matter of refractors was broken down ultimately by the famous English optician and engineer, Thomas Cooke, of York. His first considerable instrument, one of seven inches aperture, was finished in 1851; and in 1865, a year before his lamented death, he completed the first of our present giant refractors, one of twenty-five inches aperture, for Mr. Newall, of Gateshead. In consequence of the success of Cooke’s achievement other large refractors were soon undertaken.

Alvan Clarke, the famous optician of Cambridgeport, Massachusetts, at once commenced a twenty-six-inch for the Washington Observatory. The next was one of twenty-seven inches, made by Grubb for the Vienna Observatory. Object-glasses now grew inch by inch in size, depending on the increased dimensions of disks that could be satisfactorily cast. Gautier, of Paris, completed a twenty-nine-and-a-half-inch for the Nice Observatory, while Alvan Clarke made an objective of thirty inches for Pulkowa. In 1877 the latter successfully completed the mounting of an objective of thirty-six inches for the Lick Observatory, but this immense lens was only achieved after a great number of failures. Even this large object-glass was surpassed in size by the completion in 1892 of the forty-inch which he made for the Yerkes Observatory, and by that made by Gautier for the Paris Exhibition of 1900.

So much, then, for the largest refractors. In recent years, since the introduction of the silver on glass mirrors, with their stability of figure and brilliant surface, which can be easily renewed, reflectors of large apertures are again being produced. The first of these was one of thirty-six inches aperture made by Calver for Dr. Common, who demonstrated its fine qualities and his own skill by the beautiful photographs of the nebula of Orion he was enabled to secure with it. Dr. Common himself has since turned his attention to the making and silvering of large mirrors of this kind, and the largest he has actually completed and mounted equatorially is one with a diameter of five feet. Another of thirty-six inches aperture is in use at the Solar Physics Observatory at Kensington.

The progress of depositing silver on glass has led of late years to important developments in which plane mirrors are used. Foucault was the first to utilize such mirrors in his “siderostat,” in which such a mirror is made to move in front of a horizontal fixed telescope, which may be of any focal length, and no expensive dome or rising floor is required. The plane mirror of the siderostat in the Paris Exhibition telescope is six feet in diameter.

A variation of this instrument is the cœlostat more recently advocated by Lippmann. The Coudé equatorial mounting also depends upon the use of plane mirrors; with such a telescope the observer is at rest at a fixed eye-piece or camera in a room which may be kept at any temperature.

Now that in astronomical work eye observations are indispensably supplemented by the employment of photography, an important modification of the refracting telescope has become necessary; this was first suggested by Rutherfurd.

The ordinary achromatic object-glass consists, as a rule, of two lenses, one made of flint and the other of crown glass; but in this form the photographic rays are not brought to the same focus as the visual rays. This, however, can be achieved by employing three lenses instead of two, each of different kinds of glass. The most modern improvement in the telescope is due to Mr. Dennis Taylor, of Cooke & Sons, and to Dr. Schott and Professor Abbe, whose researches in the manufacture of old and new varieties of optical glass have rendered Mr. Taylor’s results feasible. By the Taylor lens outstanding color is abolished, all the rays being brought absolutely to the same focus; such lenses can therefore be used either for visual observations or for photography for spectroscopy.

SPECTROSCOPIC ASTRONOMY

The branch of physics which at the present day has assumed such mighty and far-reaching proportions in astronomical work is that dealing with spectrum analysis, which, although suggested as early as the time of Kepler, did not receive any impetus as regards its application to celestial bodies until the beginning of the present century at the hands of Wollaston and Fraunhofer. Then, however, it still lacked the chemical touch supplied afterwards by Kirchhoff and Bunsen. They showed us that the spectrum observed when the light from any heated body is passed through a prism is an index to the chemical composition of the light source; the constitution of a vapor when in a condition to absorb light can be determined by an extension of the same principle, first demonstrated by Stokes, Angström, and Balfour Stewart, when the century was about half completed.

The first celestial body towards which the spectroscope was turned was our central luminary, the sun.

Wollaston first discovered that its spectrum was crossed by a few dark lines; we learned next from Fraunhofer, who in 1814 worked with instruments of greater power, that the solar spectrum was crossed not only by a few dark lines, but by some hundreds. Not content with examining the light of the sun, Fraunhofer turned his instrument towards the stars, the light of which he also examined, so that he may be justly called the inventor of stellar spectrum analysis. It is not to the credit of modern science that from this time forward spectrum analysis did not become a recognized branch of scientific inquiry, but, as a matter of fact, Fraunhofer’s observations were buried in oblivion for nearly half a century. The importance of them was not recognized till the origin of the dark lines, both in sun and stars, had been explained by Stokes and others, as before stated. The lines in the solar spectrum were mapped with great diligence by Kirchhoff in 1861 and 1862, and later by Angström and Thalen, and this was done side by side with chemical work in the laboratory. The chemistry of the sun was thus to a great extent revealed; it was no longer a habitable globe, but one with its visible boundary at a fierce heat, surrounded by an atmosphere of metallic vapors, chief among them iron, also in a state of incandescence. To these metallic vapors Angström added hydrogen shortly afterwards.

Here, then, was established a firm link between the heavens and the earth; the first step to the problem of the chemistry of space had been taken.

It was only natural that as advances were made the instrumental equipment should keep pace with them. Spectroscopes were built on a larger scale; more prisms, which meant greater dispersion, were employed to render the measurements of the lines in spectra more accurate. The growth of our knowledge especially necessitated the making of maps of the lines in the solar spectrum, and in the spectra of the chemical elements which had been compared with it on a natural scale. This was done by Angström, who utilized for this purpose the diffraction grating invented by Fraunhofer, and defined the position of all lines in spectra by their “wave lengths,” in ten-millionths of a millimetre or “tenth-metres.”

In 1862 Rutherfurd extended Fraunhofer’s work on the stars by a first attempt at classification. Two years later Huggins and Miller produced maps of the spectra of some stars. Donati demonstrated that comets gave radiation spectra, and Huggins did the same for nebulæ.

By these observations comets and nebulæ were shown to be spectroscopically different from stars, which at that time were studied by their dark lines only.

Chiefly by the labors of Pickering, the energetic head of the Harvard Observatory, science has been enriched during the later years by observations of thousands of stellar spectra, the study of which has brought about the most marvellous advance in our knowledge.

These priceless data have enabled us now to classify the stars not only by their brightness, or their color, but by their chemistry.

Next to be chronicled is the application of the so-called Doppler-Fizeau principle, which teaches us that when a light source is approaching or receding from us the light waves are crushed together or drawn out, so that the wave length is changed. The amount of change gives us the velocity of approach or recess, so that the rate of movement of stars towards or from the earth, or the up-rush or down-rush of the solar vapors on the sun’s disk can be accurately determined. A further utilization of this principle is found when the stars are so close together that they appear as one if the plane of motion passes near the earth. A line common to the spectra of both stars will appear double twice in each revolution, when the motion to or from the earth, or, as it is termed, “in the line of sight,” is greatest. “Spectroscopic doubles,” as these stars are called, yield up many of their secrets which otherwise would elude us. Their time of revolution, the size of the orbit, and the combined mass can be determined.

To return from the stars to the sun.

By the device of throwing an image of the sun on the slit of the spectroscope the spectra of solar spots have been studied from 1866 onward, and a little later the brighter portions of the sun’s outer envelopes, revealed till then only during eclipses, were brought within our ken spectroscopically, so that they are now studied every day.

CELESTIAL PHOTOGRAPHY

Wedgewood and Davy, in 1802, made prints on paper by means of silver salts, but it was not until 1830 that Niepce and Daguerre founded photography, which Arago, in an address to the French Chamber, at once suggested might subsequently be used to record the positions of stars.

In 1839 we find Sir John Herschel carrying out a series of experiments so important for our correct knowledge of the sequence of steps in the early stages of photography that I have no hesitation in quoting from one of Herschel’s manuscripts relating to a deposit on a glass plate of “muriate” [chloride] of silver from a mixed solution of the nitrate with common salt. The manuscript states: “After forty-eight hours [the chloride] had formed a film firm enough to bear draining the water off very slowly by a siphon. Having dried it, I found that it was very little affected by light, and by washing it with nitrate of silver, weak, and drying it, it became highly sensitive. In this state I took a camera picture of the telescope on it.”

The original of the above-mentioned photograph, the first photograph ever taken on glass, is now in the science collection at the Victoria and Albert Museum, South Kensington.

In the early days of photography colored glasses were first used to investigate the action of different colors on the photographic plate. Sir John Herschel was among the first to propose that such investigations should be made direct with a spectrum, and he, like Dr. J. W. Draper, stated that he had found a new kind of light beyond the blue end of the spectrum, as the photographic plate showed a portion of the spectrum there which was not visible to the eye. Advance followed advance, and in 1842 Becquerel photographed the whole solar spectrum, in colors, with nearly all the lines registered by the hand and eye of Fraunhofer, not only the blue end, but the complete spectrum, from Draper’s “latent light,” as he called the ultra-violet rays, to the extreme red end.

The first photograph of a celestial object was one of the moon, secured by Dr. J. W. Draper in 1840; we had to wait till 1845, so far as I know, before a daguerreotype was taken of the sun; this was done by Foucault and Fizeau, while the first photograph of a star—Vega—was taken at Harvard in 1850. After the introduction of the wet-collodion process regular photography of the sun’s surface was commenced, at Sir John Herschel’s recommendation, at Kew in 1858, and the total solar eclipse of 1860 was made memorable by the photographs of De La Rue, who before that time had secured most admirable photographs of the moon, as also had Rutherfurd.

Photography now began to pay the debt she owed to spectrum analysis.

The first laboratory photograph of the spectra of the chemical elements was taken by Dr. W. A. Miller in 1862.

Rutherfurd was the first to secure a photograph of the solar spectrum with considerable dispersion by means of prisms.

In 1863 Mascart undertook a complete photographic investigation of the ultra-violet portion of the solar spectrum, a work of no mean magnitude. He, however, did not employ a train of prisms for producing the spectrum, but a diffraction grating, using the light reflected from the first surface. The first photograph of the spectrum of a star was secured by Henry Draper, the son of Dr. J. W. Draper, one of the pioneers in photography in 1872.

It was not till the introduction of dry plates in 1876 that the photography of the fainter celestial objects or of their spectra was possible, as a long exposure was naturally required. Stellar spectra were photographed by Huggins in 1879, and in the next year Draper photographed the nebula of Orion. As the dry plates became more rapid, and as longer exposures were employed, revelation followed revelation; the nebulæ as seen by the naked eye, and even some stars, were found by the Henrys, Roberts, Max Wolf, Barnard, and others, to be but the brighter kernels of large nebulous patches.

This new application of photography, depending upon long exposures (the longest one I know of has extended to forty hours), had an important reflex action on the mechanical parts of the telescope; it was not only necessary to keep the faintest star exactly on the same part of the plate during the whole of the exposure, but night after night the stellar image must be brought on to the same part of the plate so that the exposure might be continued.

A system of electric control of the going of the driving-clock of the telescope by means of a sidereal clock was introduced, the simplest one being designed by Russell, of Sydney; a most elaborate one by Grubb, of Dublin.

Another application of the method of long exposures has been the discovery of minor planets by the trails impressed by their motion among the stars on the photographic plates on which the images of both are impressed.

A complete spectroscopic survey of the stars by means of photography was commenced in 1886 at Harvard College, as a memorial to Draper, who died while he was laboring diligently and successfully in securing advances in astrophysical inquiries. To carry on this work at Harvard, Professor Pickering wisely reverted to the method first employed by Fraunhofer, and utilized by Respighi and another in 1871, of placing prisms in front of the object-glass.

In the photographing of stellar spectra by means of objective prisms, the driving-clock of the telescope must not go exactly at sidereal rate, but at certain speeds depending on the brightness and position of the star under examination.

This is necessary because the image of the spectrum of a star on the photograph is only a thin line in which it is impossible to see the spectral lines; the spectrum must be broadened, and this is accomplished by making the star image “trail” to a certain degree on the plate. This trailing is accomplished by means of the clock, the rate of which is made to vary. In this way the trail of a spectrum of a star on the photographic plate is always obtained of the same width, while the density of the image is made fairly constant by increasing the rate for bright stars and decreasing it for fainter ones. In this way spectra of the brighter stars rivalling in perfection and detail those obtained of the spectrum of the sun itself thirty years ago have been obtained. Such photographs have rendered a minute chemical classification of the stars possible.

One of the most interesting applications of photography to spectrum analysis during the latter part of the century has been the utilization by Messrs. Deslandres and Hale of a suggestion made by Janssen, that by employing photography images of the sun and its surroundings can be obtained in light on one wave length. In this way we can study the distribution of any one of the chemical constituents of the sun separately, and note its behavior, not only on the sun itself, but in the atmosphere which enfolds the disk.

It is strange that, in spite of the suggestions of Faye, and others after him, one of the great advantages of the employment of photography in astronomical work, namely, the abolition of “personal equation,” has so far been almost entirely neglected. What “personal equation” is can be perhaps illustrated by considering an observer who is observing the transit of a star over the wires in a transit instrument.