THE NINETEENTH CENTURY SERIES EDITOR : JUSTIN MCCARTHY. ASSOCIATE EDITORS : W. P. TRENT, LL.D. T. G. MARQUIS. CHARLES G. D. ROBERTS. REV. W. H. WITH ROW;, D.D. PROGRESS OF SCIENCE IN THE CENTURY BY J. ARTHUR THOMSON, M.A., Regius Professor of Natural History in the University of Aberdeen ; Author oj " The Study of Animal Life," " The Science of Life," " Outlines of Zoology" etc; Joint-Author of u The Evolution of Sex." THE LINSCOTT PUBLISHING COMPANY TORONTO AND PHILADELPHIA LONDON : 47 Paternoster Row VV. & R. CHAMBERS, LIMITED EDINBURGH : 339 High Street 1906 Entered, according to Act of Congress, in the Year One Thousand Nine Hundred and Three, by the Bradley-Garretson Co., Limited, in the Office of the Librarian of Congress, at Washington. Entered, according to Act of Parliament of Canada, in the Year One Thousand Nine Hundred and Three, by the Bradley-Garretson Co., Limited, in the Office of the Minister of Agriculture. All Rights Reserved. $ R A jf To discuss in a single volume the progress of sci- ence in the nineteenth century has been no easy task, and the author craves the reader's indulgence. It must be remembered that the book does not pretend to be a history of nineteenth century science; it is designed simply as an introduction to many histo- ries— some still unwritten. It is not a consecutive story of the marvellous progress of knowledge which the century witnessed ; it is simply a record of some of the great scientific events. Many famous names and many important discoveries have been left un- mentioned, for any attempt at exhaustiveness would have made a volume of this size a mere catalogue. On the other hand, there has been a serious attempt to discuss the great theme so as to give prominence to the salient steps of progress. To have attempted this in an easy-going mood would have been irreverent to the past and insulting to the serious reader ; therefore no apology is offered for the difficulty of some of the pages, nor does it seem necessary to apologise for the numerous quotations from expert authorities, — they help to give personal reality to some of the pages, and they were needed as acknowledgments of the author's indebtedness. J. A. T. USIVEBSITY OF ABERDEEN, SEPTESCBEB, 1902. Note. — The reader will understand that the absence of any reference to radium and its marvellous properties is due to the fact that the book was printed before the discovery had been made. In the same way it will be obvious why Sir Oliver Lodge and Sir William Kamsay are not duly entitled, and why some great men of science no longer with us. such as Gegenbaur, Spencer, and Zittel, are referred to in the present tense. CONTENTS. BOOK ONE. INTRODUCTORY. CHAPTER I. THE SCIENTIFIC MOOD. PACT The Meaning of Science. — A Contrast of Moods. — Charac- teristics of the Scientific Mood — (a) A Passion for Facts — (b) Cautiousness — (c) Clearness of Vision — (d) Sense of Inter-Relations. — The Aim of Science. — Scientific Method.. 1 CHAPTER II. THE UNITY OF SCIENCE. Classification of the Sciences. — The Correlation of Knowledge. — Need for Criticism of Scientific Work. — Unity of Life. — Unity of Science. — Unity of Nature 25 CHAPTER III. PROGRESSIVENESS OF SCIENCE. The First Condition of Scientific Progress. — The Fact of Pro- gress.— Its Necessity. — Scientific Conclusions of the First Magnitude. —Factors in Further Progress. — Justification of Science. — Science and Practical Utility 41 viii CONTENTS. BOOK TWO. MATTER AND ENERGY. CHAPTER IV. A CENTURY OF CHEMISTRY. PAOB Search for the Elements. — Theory of Combustion and the Conservation of Matter. — The Atomic Theory. — De- velopment of the Atomic Theory. — Development of Organic Chemistry. — The Periodic Law. — Co-operation of Chemistry and Physics. — The Circulation of Matter. — Chemical Affinity 70 CHAPTER V. THE PROGRESS OF PHYSICS. Introductory. — The Newtonian Foundation. — Conservation of Energy. — Heat as a Mode of Action. — Kinetic Theory of Gases. — Undulatory Theory of Light. — Theory of Electricity. — Theories of Matter. — Theory of the Ether .. 131 CHAPTER VI. ADVANCE OF ASTRONOMY. From Copernicus to Newton. — Applications of the Gravita- tion-Formula.—The Study of the Stars.— Extension and Intensifying of Observation. — Physical and Chemical Problems. — Spectrum Analysis. — The Evolution-Idea in Astronomy 179 CONTENTS. IX CHAPTER VIL GROWTH OF GEOLOGY. PAQE Cataclysmal, Uniformitarian, Evolutionary. — Foundation- Stones of Geology. — The Evolution-Idea in Geology. — Age of the Earth. — Reading the Rock-Record. — Prob- lems of Earth-Sculpture. — Recognition of Ice Ages. — The Hand of Life upon the Earth.— The Problem of Petrog- raphy.— Note on the Scientific Development of Geog- raphy.—An Illustration of Oceanography ..v 225 BOOK THREE. SCIENCE OF OKGANISMS : LIFE-LORE. CHAPTER VIII. THE DEEPENING OF PHYSIOLOGY. Historical Outline. — Physiology of the Living Organism as a Whole. — Study of the Functions of Organs. — Physiology of Tissues. — The Life of Cells. — As regards Protoplasm. — The Unsolved Secret of the Organism. 283 The Morphological Question and its Progressive Answers. — Foundations of Morphology. — The Appreciation of Fossils. — Minute Analysis 329 CHAPTER X. GEXEOLOGICAL. Geneology.— Development of the Individual. — Experimental Embryology.— Heredity and Inheritance 365 X CONTENTS. CHAPTER XL THE THEORY OF ORGANIC EVOLUTION. FAQI The General Idea of Organic Evolution. — History of the Evolution -Idea. — The Present Aspect of the Evolution Theory 424 BOOK FOUR PSYCHOLOGY, ANTHROPOLOGY, AND SOCIOLOGY. (MIND, MAN, AND SOCIETY.) CHAPTER XII. PROGRESS OF PSYCHOLOGY. Changes in Aims and Methods. — Correlation of Mind and Body. — Experimental Psychology. — Comparative Psycho- logy.— Development and Evolution of Mind. — Conclusion.. 442 CHAPTER XIII. ADVANCE OF ANTHROPOLOGY. Man's Place in Nature. — Antiquity of Man. — The Human Species. — Races of Mankind. — Evolution of Language. — Appreciation of Folk-Lore. — Factors in the Evolution of Man 473 CHAPTER XIV. SUGGESTIONS OF SOCIOLOGY. Scope of Sociology. — Historical Note. — Lines of Sociologi- cal Inquiry. — The Social Organism. — "Lieu, Travail, Famille."— Classification of the General Factors of Social Evolution 496 PROGRESS OF SCIENCE IN THE CENTURY. BOOK ONE. INTRODUCTORY. CHAPTEK I. THE SCIENTIFIC MOOD. THE MEANING OF SCIENCE. MANY attempts have been made to define -what we mean by " Science." " A higher development of common knowledge" (Spencer) ; " organised common sense " (Huxley) ; " classified and criticised knowl- edge " ; " the universal element in knowledge " ; " an understanding of facts " ; " our correlated ex- perience,"— are among the many suggestions. It will be noted that these definitions, though all some- what vague, suggest two ideas: (a) that science is not something by itself, apart from other knowledge, or confined to any particular order of facts; and (&) that it has none the less a distinctive feature, as expressed by some word like " organised " or " sys- tematised." The fact is that whenever we gather 2 PROGRESS OF SCIENCE IN THE CENTURY. facts and classify them, detect their inter-relations and formulate their sequences, there is science. The subject of enquiry may be man or beast, star or tree, a language or the atmosphere, institutions or fossils, the growth of ideas or the development of an egg — all come within the scope of scientific en- quiry whose far-off goal is an interpretation of the known world. The distinctive feature is in the method, — making sure of facts, observing their inter- relations, grouping them according to their like- nesses of sequence, and inventing descriptive for- mulae which sum them up. Facts are essential, but it is evident that they alone do not constitute a science; they must be correlated, interpreted, for- mulated. As Sir Lyon Play fair once put it,* " isolated facts may be viewed as the dust of science," — dust only, but dust is not to be despised, for, as he went on to say, " to it when the rays of light act upon its floating particles we owe the blue of the heavens and the glories of the sky." Though it may sound for a moment like a paradox, the scientific mood does not necessarily involve any particular knowledge of this or that science. Many business men, for instance, who are almost quite ignorant of chemistry or physics, botany or zoology, astronomy or geology, but who have carefully disciplined themselves in regard to some restricted series of facts involved in their daily work, have acquired the scientific mood in a high degree of development. The same may be said of many a one well disciplined in the " Humanities," though his title of " scholar " is often used as if it stood in antithesis to " man of science." * Pres. Address, Rep. Brit. Ass. for 1885, p. 18. THE SCIENTIFIC MOOD. 3 A CONTRAST OF MOODS. We receive in our inheritance what may be meta- phorically called a bundle of moods — of various shapes and sizes, like a bundle of sticks gathered in the forest. Among these moods, or predispositions to particular lines of activity, three stand out prom- inently— the scientific, the artistic, and the practical mood. Most of us have at least the rudiments of these, but in most cases one is dominant. It is part of the aim of education to adjust the propor- tions of our moods, and to foster a minute rudiment into realisation. First there is the mood of the dominantly practical man, who, though in part scien- tific and usually a man of feeling, is characteristi- cally concerned with the possibilities of action. The whole trend of his mind is towards doing, not towards knowing. He is seeking after social amelioration, not after descriptive formulae. There is obviously much to be said for the dom- inance of the practical mood. It seems likely that man's first relations to nature were predominantly practical, and it is certain that in old practical lore many of the sciences — such as astronomy, botany, physiology — had their roots, and that fresh vigour has often come to science by a tightening of its con- tact with the affairs of daily life. There is no doubt that the practical mood is as natural and necessary and dignified as any other. Without it science tends to become pedantic and art decadent. Yet when the practical mood becomes altogether dominant, when things get into the saddle and over-ride ideas and ideals and all good feeling, when the multiplication of loaves and fishes becomes the only problem in the world, we know the results to be vicious. The 4 PROGRESS OF SCIENCE IN THE CENTURY. vices of the hypertrophied practical mood are — be- littlement, baseness, brutality. We cannot but have a great respect for the dominant practical mood, and yet if it is left unchecked by scientific discipline and artistic culture, it tends to run riot. The prac- tical man elects to do, not know, but action without knowledge is often our undoing. Ignorant practice may be more dangerous than any dogma. The prac- tical man will have " nothing to do with sentiment," though he prides himself in. keeping close to the facts ; he cannot abide any theory and yet he is im- bued with a Martin Tupperism which gives a false simplicity to the problems of life ; he will live in what he calls "the real world," and yet he often hugs close to himself the most unreal of ideals. Secondly, there is a man of dominantly artistic mood, which seems to find expression in Schiller's words : — " 0 wunderschon ist Gottes Erde, und schon auf iTir ein MenscJi zu sein; " " How beautiful is God's earth, how good it is to live a man's life upon it." From man's first emergence until to-day, the drama of nature has doubtless appealed to human emotions. Especially, perhaps, as he gained firmer foothold in the world, secured by his wits against stronger rivals and a careless environment, did the emotional tone rise into dignity as a distinct mood, finding its ex- pression in painting and carving, song and story, music and the dance. The herbs and the trees, the birds and the beasts, sent tendrils into the human heart, claiming and finding kinship. Like the practical mood, so the emotional mood has its obvious virtues. It is part of the salt of life. In a noisy world it helps to keep us aware of the har- mony in the heart of things. THE SCIENTIFIC MOOD. 5 Yet it has its vices; if unruled or uncorrelated, if uncurbed by science, if unrelated to the prac- tical problems of life, it tends to become morbid, mawkish, mad. There may be over-feeling, just as there may be over-doing. Most serious consequences of feeling without knowledge, of sympathy without synthesis (in the language of the learned), are well known in the practical aff airs of to-day. On the other hand, we must not be slow to admit that just as the practical man has some justification when he reacts from science, because, as he says, it is too theoretical, so the artist, poet, or man of feel- ing has some justification when he recoils from science because it is disproportionately analytic. It must be granted that science, like a child pulling a flower to bits, is apt to dissect more than it re- constructs, and to lose in its analysis the vision of unity and harmony which the artist has ever before his eyes. Perhaps, however, if the artist had pa- tience, he would often find that science restores the unity with more meaning in it than before. Thirdly, there is the dominant scientific mood. To this mood the world-picture is no phantasma- goria, but a scene in an ordered drama; even its beauty is not kaleidoscopic but rather of growth. To the scientific mood it is plain that through the mul- tiplicity of items great likenesses are observable, which admit of being summed up in brief descrip- tive formulae — laws of motion, gravitation, in- destructibility of matter, conservation of energy, development from the apparently simple to the ob- viously complex evolution. Although science has some of its roots in practice, and often receives stimulus from the actual needs of the day, it is not practical either in main inten- 6 PROGRESS OF SCIENCE IN THE CENTURY. tion or in main result. Its main intention is to describe in the simplest possible formulae, to classify and inter-relate sense-impressions, to interpret the known world ; its main result is an intellectual system and the development of a certain way of looking at things. Similarly, though emotion has influenced the growth of natural knowledge not a little both for good and ill, and though scientific discoveries have in turn given nutriment to emotion, science is certainly in itself non-emotional. The student of science seeks, not like the practical man, to realise the ideal, but rather to idealise [con- ceptualise] the real, or those fractions of reality which constitute his experience. He elects pri- marily to know, not do. He would make the world translucent, not that emotion may catch the glimmer of the indefinable light that shines through, but for other reasons, — because of his inborn inquisitiveness, because of his dislike of obscurities, because of his craving for a system — an intellectual system in which phenomena are provisionally unified. Like the other moods, the scientific mood has its virtues of method and ideal. It is painstaking, pa- tient, precise; it is careful, conscientious, contriv- ing; it aims at making a working thought-model of the universe. But it has also its vices,— of over-knowing, of ranking science first and life second (as if science were not after all for the evolution of life), of ignor- ing good feeling (as if knowledge could not be bought at too dear a price), of pedantry (as if science were a " preserve " for expert intellectual sportsmen, and not an education for the citizen), of maniacal muck- raking for items of facts (as if facts alone consti- THE SCIENTIFIC MOOD. 7 tuted science). But it is a natural and necessary ex- pression of the developing human spirit, and supplies the foundation without which practice is merely em- pirical and emotion superstitious. CHABACTEEISTICS OF THE SCIENTIFIC MOOD. In his stimulating presidential address at the meet- ing of the British Association at Dover in 1899, Sir Michael Foster raised the question of the dis- tinctive features of the scientific spirit " What are the qualities," he asked, " the features of that scientific mind which has wrought, and is working, such great changes in man's relation to nature ? " And his answer was that the features of the fruitful scientific mind are in the main three.* " In the first place, above all other things, his nature must be one which vibrates in unison with that of which he is in search ; the seeker after truth must himself be truthful, truthful with the truthful- ness of nature; which is far more imperious, far more exacting than that which man sometimes calls truthfulness. " In the second place, he must be alert of mind. Nature is ever making signs to us, she is ever whis- pering to us the beginnings of her secrets ; the scien- tific man must be ever on the watch, ready at once to lay hold of Nature's hint, however small, to listen to her whisper, however low. " In the third place, scientific enquiry, though it be pre-eminently an intellectual effort, has need of the moral quality of courage — not so much the cour- age which helps a man to face a sudden difficulty as the courage of steadfast endurance." * Report British Association for the Advancement of Science, 1899, pp. 16-17. B 8 PROGRESS OF SCIENCE IN THE CENTURY. To the obvious objection that these three qualities of truthfulness, alertness, and courage, though, let us hope, possessed by the scientific man, are not in any way peculiar to him, but " may be recognised as belonging to almost every one who has commanded or deserved success, whatever may have been his walk in life," Sir Michael answered : " That is exactly what I would desire to insist, that the men of science have no peculiar virtues, no special powers. They are ordinary men, their characters are common, even commonplace. Science, as Huxley said, is organised common sense, and men of science are com- mon men, drilled in the ways of common sense." Let us endeavour to make the diagnosis of the scientific mood a little more definite. The follow- ing has at least the interest of having been almost entirely written before the delivery of Sir Michael Foster's stimulating address. (a) As a first characteristic of the scientific mood — corresponding to what has been above referred to as " truthfulness," we may note a passion for facts. 'And what are more difficult to catch than facts ; they are more elusive than ideas. How difficult it is even in regard to simple problems to get a grip of the facts of the case ! How difficult it is for any one with even a dash of the artistic mood to relate an occurrence accurately ! Most of us are Munchausens in a small way, but with less sense of humour. Just as we may distinguish carpenters who can work to this or that fraction of an inch of accuracy; so we must distinguish one another as able to observe or to record to this or that degree of truthfulness. " Man, unscientific man, is often content with ' the nearly ' and ' the almost.' Nature never is. It is not her way to call the same two things which THE SCIENTIFIC MOOD. 9 differ, though the difference may be measured by less than the thousandth of a milligramme or of a millimetre, or by any other like standard of minute- ness. And the man who, carrying the ways of the world into the domain of science, thinks that he may treat Nature's differences in any other way than she treats them herself, will find that she resents his conduct ; if he in carelessness or in disdain overlooks the minute difference which she holds out to him as a signal to guide him in his search, the projecting tip, as it were, of some buried treasure, he is bound to go astray, and, the more strenuously he struggles on, the farther will he find himself from his true goal."* Many people — most excellent in virtues — seem constitutionally incapable of accurately reporting an occurrence ; many more seem quite unable to see the difference between an observation and an inference. The scientific worker is himself well aware that, in measurements and observations, only an approximate accuracy can be attained, and that the degree of approximation varies with the indi- vidual. But this relativity of accuracy is far from being generally recognised, and scientific state- ments often get credit for a precision which they do not claim. The personal equation has been for a long time frankly recognised and allowed for in astronomy; it is also sometimes estimated in chem- istry and physics,f but we hear too little of it in the less exact sciences such as biology and psy- chology. Even apart from intellectual training, may it not be claimed that the discipline of the chemical balance, * Sir Michael Foster, loc. cit. p. 16. f See Ostwald's Text-book of General Chemistry. 10 PROGRESS OF SCIENCE IN THE CENTURY. of analysis, of dissection, of faithful drawing, is one of the most effective factors in the evolution of truthfulness? Many will agree with Agassiz that some training in natural science is one of the best preparations a man can have for work in any depart- ment of life where accurate carefulness and ad- herence to the facts of the case means much. Long ago Bacon said : " We should accustom ourselves to things themselves," and this — to distinguish between appearance and reality — is what the scientific mood seeks after. It was Huxley who spoke of " that enthusiasm for truth, that fanaticism of veracity, which is a greater possession than much learning; a nobler gift than the power of increasing knowledge." It is one of the motive forces of scientific progress. If every virtue has its vice and every function its disease, so danger may lurk in this precious posses- sion,— a passion for facts. It may become a mania for information and an intellectual intemperance. Unskilful teaching or careless learning may result in mere fat without muscle, or in the matter-of-fact man — one of the most unscientific of persons — who ignores one of the biggest of all facts, the reality of ideas. Any mood may in extreme development become vicious, and the passion for facts may become so pre- dominant that it implies violence to emotional sanity and disloyalty to the ideal of a full and healthy hu- man life. Take an illustration from real life. The great embryologist Von Baer once shut himself up in his study when snow was upon the ground, and did not come out again until the rye was in harvest. He was filled, he tells us, with uncontrollable pathos at the sight. " The laws of development may be THE SCIENTIFIC MOOD. H discovered this year or many years hence — by me or by others — what matters it? — it is surely folly to sacrifice for this the joy of life which nothing can replace." Indeed life is not for science, but science for the development of life. These are days of popularising, in magazine ar- ticles and on lecture platforms, and much of this is justifiable and healthy, for science can no longer be defined off as a preserve for the learned. Yet there is the risk of giving a false simplicity to problems, or of suggesting that there are royal roads to learn- ing; the sin easily besets us of depreciating the dig- nity of a hard-won fact. Therefore at the risk of ex- ceeding triteness, we would emphasise that a genuine passion for facts implies a certain seriousness, a rever- ence for what is beneath (in Goethe's words), a re- spect for facts when one gets them. Though we need not be always in the scientific mood — for which we are truly thankful — we must be scientific when we propose so to be. " Science," Bacon said, " is not a terrace for a wandering and variable mind to walk up and down with a fair prospect." What we mean by saying that we need not be always scientific is simply that the scientific mood is sometimes unnatural and irrelevant. To botanise upon our mother's grave is the classic illustration, and for another we may refer to the medical man's discovery that Botticelli's " Venus," in the Uffizi at Florence, is suffering from consumption, and should not be riding across the sea in an open shell, clad so scantily. (6) Following from the passion of facts, is a second characteristic of the scientific mood, namely, cautiousness, or distrust of finality and dogmatism of statement. Scotsmen have done well for the ad- 12 PROGRESS OF SCIENCE IN THE CENTURY. vancement of science ; they are said to stand far above the average in the nineteenth century; perhaps this is in part because they are so " canny," so unwilling to commit themselves unless they are sure. It may even be that the excessive changeableness of Scotch weather has helped to engender the characteristic mood of caution. Sometimes, indeed, the cautious- ness becomes almost morbid, when three saving clauses are inserted in a single sentence. One recalls Stevenson's story of the sailor : — " Bill, Bill," says I, " or words to that effect" No doubt the scientific mood is continually making hypotheses or guesses at truth; the scientific use of the imagination is part of our method. But what we have to guard against is the insidious tendency to mistake provisional hypotheses for full-grown theories, and, still worse, for dogmas. As Prof. W. K. Brooks says in his Foundations of Zoology : " The hardest of intellectual virtues is philosophic doubt, and the mental vice to which we are most prone is our tendency to believe that lack of evidence for an opinion is a reason for believing some- thing else. . . . Suspended judgment is the greatest triumph of intellectual discipline." As Huxley said — and who has had the scientific mood more strongly developed — " The assertion that outstrips the evi- dence is not only a blunder but a crime." Just as burnt bairns dread the fire, so the scientific mood, often deceived by hearsay evidence, by incomplete induction, by the will-o'-the-wisp glamour of a seduc- tive idea, by inference mixed up with observation, and even by wilful falsehood, becomes more and more cautious, distrustful, " canny." Another aspect of the quality of cautiousness which characterises the scientific mood is distrust of THE SCIENTIFIC MOOD. 13 personal bias in forming judgments. It should always be possible to eliminate opinion from all scientific conclusions ; their validity, in fact, depends upon this. " The scientific man has above all things to strive at self -elimination in his judgments, to pro- vide an argument which is as true for each individual mind as for his own. The classification of facts, the recognition of their sequence and relative signifi- cance, is the function of science, and the habit of forming a judgment upon these facts, unbiassed by personal feeling, is characteristic of what may be termed the scientific frame of mind." * " The world," Faraday writes, " little knows how many of the thoughts and theories which have passed through the mind of a scientific investigator have been crushed in silence and secrecy by his own severe criticism and adverse examination; that in the most successful instances not a tenth of the suggestions, the hopes, the wishes, the preliminary conclusions have been realised." As a complementary statement, another quotation from the same great authority may be permitted : — " The philosopher should be a man willing to listen to every suggestion, but determined to judge for himself. He should not be biassed by appearances; have no favourable hypotheses; be of no school, and in doctrine have no master. He should not be a respecter of persons, but of things. Truth should be his primary object. If to these qualities be added industry, he may indeed hope to walk with- in the veil of the Temple of Nature." (c) A third characteristic of the scientific mood is dislike of obscurities, of blurred vision, of foggi- ness. We instinctively discount the scientific abili- * Karl Pearson, Grammar of Science, rev. edition, 1900, • p. 6. 14 PROGRESS OF SCIENCE IN THE CENTURY. ties of the student who always has his microscope wrongly focussed and is satisfied with the ill-defined image, or of the other whose dissection is invariably either a mince or a tangle, or of the other who is never quite sure whether he knows a thing or not. Ignorance in itself is no particular reproach; the point is to know when we know and when we don't, and it is one of the characteristics of the scientific mood that it will have yes or no to this question. Those of the scientific mood are mainly trying to construct a working-thought-model of the outer world, to form a mental image which will be a living picture, — an intellectual cinematograph. In other words they would make the world translucent, as translucent as the human body becomes to the skilled anatomist. Clerk-Maxwell's boyish question — " What is the go of this ? " — and, when put off with some verbal- ism, " What is the particular go of this ? " is a ques- tion characteristic of the scientific mood, which may be applied to any order of facts. The mole has a sort of half-finished lens, which is physically incapable of throwing a precise image on the retina. If there is any image, it must be a blurred tangle of lines. In our busy lives, we tend to acquire mole-like lenses in regard to particular orders of facts ; we see certain things clearly, others are blurs; but the scientific mood is in continual protest against obscurities, insisting upon lucidity. Thus we feel the force of one of Bacon's most historically true aphorisms, which declares " Truth to emerge sooner from error than from confusion." It is a great step when a false notion is formulated. The definitising of error has been the beginning of its disappearance. As soon as the evil genie of the THE SCIENTIFIC MOOD. 15 Eastern tales took on some definite bodily form there was some chance of tackling him ; as a mere wraith he was invulnerable. (d) A fourth characteristic of the scientific mood is a sense of the inter-relations of things. The real- isation of nature as a great inter-connected system is, indeed, one of the ends of science; to be on the out- look for inter-relations is diagnostic of the mood. As long as the collection and registration of facts preoccupies the energies and attention, scientific enquiry has hardly begun. As Mr. Pearson says, " The classification of facts, the recognition of their sequence and relative significance is the function of science." To put it more concretely, the student of biology, for instance, has hardly caught on at all unless he has some realisation of the web of life, the correla- tion of organisms. He must have some apprecia- tion of the " system of nature," of the links between old maids, cats, bees, and clover crop ; between earth- worms and the world's bread-supply; between mos- quitoes and malaria ; between white ants and African agriculture; between ivory ornaments and the slave trade. To sum up: the scientific mood, whose diffusion through wide circles has been one of the achieve- ments of the latter half of the nineteenth century, is characterised by a passion for facts, an alert cau- tiousness, a striving after clearness of vision, and a sense of inter-relations. To which, as will be after- wards made plain, it should perhaps be added that the consistent scientific mood does not at all concern itself with metaphysical problems or ultimate inter- pretations. These may be legitimately complemen- tary to science, but if the word is to retain its present meaning, they are beyond its scope. 16 PROGRESS OF SCIENCE IN THE CENTURY. THE AIM OF SCIENCE. Briefly stated, the primary aim of science in- cludes the observation, description, and interpreta- tion of the knowable universe. Concerning the need for careful observation and accurate description, enough has been said in our ex- position of the characteristics of the scientific mood ; it is necessary, however, to give particular attention to the nature of a scientific interpretation, — in re- gard to which misunderstanding is rife. The man of scientific mood becomes aware of cer- tain fractions of reality which interest him; he pro- ceeds to become more intimately aware of these ; i.e., to make his sensory experience of them as full as possible. He seeks to arrange them in ordered series, to detect their inter-relations and likenesses of sequence; he tries to reduce them to simpler terms or to find their common denominator; and finally, he endeavours to sum them up in a general formula, often called a " law of nature." Aristotle defines the aim when he says, " Art [or as we should say, Science] begins when, from a great number of experiences, one general conception is formed which will embrace all similar cases." Similarly the nature of scientific explanation is sug- gested by Kirchhoff's definition of mechanics, as the science of motion, whose object it is " to describe completely and in the simplest manner the motions that occur in nature." With the advance of clear thinking our way of looking at facts has altered not a little, and even when we use the same words as our forefathers did we do not always mean the same thing. Thus whea THE SCIENTIFIC MOOD. 17 the lecturer says that a gas " obeys Boyle's Law," he is using the language of the past and suggesting a conception of the order of nature which is no longer current. " We must confess," says Prof. J. J". Poynting,* " that physical laws have greatly fallen off in dignity. No long time ago they were quite commonly described as the Fixed Laws of Nature, and were supposed sufficient in themselves to govern the universe. Now we can only assign to them the humble rank of mere descriptions, often tentative, often erroneous, of similarities which we believe we have observed." Prof. Poynting goes on to say that a " law of na- ture explains nothing — it has no governing power, it is but a descriptive formula which the careless have sometimes personified. There may be psycho- logical and social generalisations which really tell us why this or that occurs, but chemical and phys- ical generalisations are wholly concerned with the how." In other words, if we may condense a little of Poynting's admirable discourse, concurrently with the change in our conception of physical law has come a change in our conception of physical expla- nation. The change is in our recognition that " we explain an event not when we know ' why ' it hap- pened, but when we know ' how ' it is like something else happening elsewhere or otherwise — when, in fact, we can conclude it as a case described by some law already set forth. In explanation we do not account for the event, but we improve our account of it by likening it to what we already knew." In short, the notion of antecedent purpose — which rises * Address, Section A, Report of British Ass. for 1889, pp. 616-17. 18 PROGRESS OF SCIENCE IN THE CENTURY. at once in our minds when we try to explain human action — is irrelevant in physical seienca On the same subject, Dr. J. T. Merz writes as follows in his impressive history of scientific thought in the nineteenth century : " A complete and simple description — admitting of calculation — is the aim of all exact science. . . . We shall not expect to find the ultimate and final causes, and science will not teach us to understand nature and life. . . . Science means * the analysis of phenomena as to their ap- pearance in space and their sequence in time.' " * Thus the common assertion that science gives ex- planations of nature is a misunderstanding, if the word explanation is taken to mean more than a de- scriptive formula. The word ultimate does not oc- cur in the scientific dictionary. The biologist draws cheques, but they are all backed by such words as protoplasm and germ-plasm; and a little enquiry suffices to show that these words imply conceptual hypotheses invented to express the facts and war- ranted by the success with which they fit these. The physicist's bills, similarly, are accepted on the credit of the ubiquitous ether, the mighty atom, or the like, but these again are conceptual hypotheses invented to summarise the sequence of phenomena. Let us take a concrete case. " The law of gravi- tation is a brief description of how every particle of matter in the universe is altering its motion with reference to every other particle. It does not tell us why particles thus move; it does not tell us why the earth describes a certain curve round the sun. It simply resumes, in a few brief words, the relation- * J. T. Merz. A History of European Thought in the Nine- teenth Century. Vol. I., Introduction — Scientific Thought, Part I., 1896, pp. 382-3. THE SCIENTIFIC MOOD. 19 ships observed between a vast range of phenomena. It economises thought by stating in conceptual short- hand that routine of our perceptions which forms for us the universe of gravitating matter." SCIENTIFIC METHOD. From what we have already said it should be plain that science has no mysterious methods of its own. Its method is the method of common sense. In his little book on scientific thinking, f Dr. Adolf Wagner points out with great vivacity that science is characterised as an intellectual attitude; it is not any particular body of facts; it has no peculiar method of inquiry; it is simply sincere critical thought, which admits conclusions only when these are based on evidence. Let us, however, briefly indi- cate some of the chief steps in the scientific treat- ment of a given problem. (a) Observation of Facts. — The first step is to make sure of the facts concerning which a problem has been raised in the inquisitive mind. Here the fundamental virtues are precision, caution, clear- ness, and impartiality. The rough and ready record, the second-hand evidence, the vague impres- sion, the picking of facts which suit must be elimi- nated. Hence, since the observer is a fallible mortal, the importance of co-operation, of independent ob- servation on the same subject, of instrumental means of extending the range and delicacy of our senses, and of automatic methods of registration, such as photography supplies. •Karl Pearson, The Grammar of Science, rev. ed., 1900, p. 99. t A. Wagner, Studien und Sklzzen aus Naturwissenschaft und Philosophic. I. Ueber toissenschaftliches Denken und tiler populdre Wissenschaft, Berlin, 1899, p. 79. 20 PROGRESS OF SCIENCE IN THE CENTURY. (&) Classification of Facts. — In many cases after the accumulation of data, much time must be spent in their arrangement. A careful worker at the prob- lem of migration in birds, like Mr. Eagle Clarke, may require for the classification of his data a longer time than was spent in their collection. If the facts are to form part of the body of science, they must be made readily available, and this process of diges- tion is often slower than that of ingestion. If the aim be to detect similarities of sequence the facts must be grouped in ordered series. Here, in many cases, the use of graphs, curves, and mathematical methods has proved itself invaluable, notably, for in- stance, in Galton's work on inheritance, or in the re- cent statistical studies on variation. It has been a common experience in the arrange- ment of data that some minute discrepancy has re- vealed itself, and that the following of this at first perhaps puzzling occurrence has led to the elucida- tion of the whole problem. Thus it has become a maxim in science that no apparent departure from the rule or general sequence should be treated as trivial, and no minute discrepancy disregarded. That nitrogen obtained from chemical combinations should be about one-half per cent, lighter than that obtained from the atmosphere, may seem a very minute fact, but it led Lord Rayleigh and Professor W. Ramsay to the discovery of Argon. (c) Analysis. — With scientific problems of a cer- tain order, there is often need for a preliminary process of analysis before the desired data can be obtained. Whenever we get below the surface phe- nomena of life — patent to the observer — we have to dissect, to cut sections, to take advantage of chemical analysis and so on. The end desired is a re-state- THE SCIENTIFIC MOOD. 21 ment in simpler terms, or in another sense, in more generalised terms; and to effect this analysis is in itself a scientific problem. (d} Hypothesis. — There is no doubt that some conclusions have arisen in the mind as if by a flash of insight, but even these have perhaps been due to processes of unconscious cerebration. In the ma- jority of cases, the process is a slower one, the scien- tific imagination devises a possible solution — an hy- pothesis— and the investigator proceeds to test it. In other words, he forges intellectual keys and then tries if they fit the lock. If the hypothesis does not fit, it is rejected and another is made. The scientific workshop is full of discarded keys. Nor can it be forgotten that even those conclusions which com- mend themselves at first sight have to submit to the process of testing like those which were tried with less confident fingers. It matters little, except to the logician, whether the hypothesis was reached as an induction from many particulars or as a deduc- tion from some previously established conclusion; in either case the result is a provisional hypothesis, which has then to be tested. Newton said in his Principia that he did not make hypotheses (Hypotheses non fingo}, and yet he, like all great scientific workers, certainly did, for in- stance in his corpuscular hypothesis of light, which has turned out to be erroneous. The fact is that there are different kinds of hypotheses, — there are guesses at truth which have no experimental basis, which are usually prompted by some big conclusion dominating the mind of the guesser, such as Sweden- berg's nebular hypothesis; and there are scientific hypotheses which are more or less carefully con- structed systems, harmonised with existing knowl- 22 PROGRESS OF SCIENCE IN THE CENTURY. edge, and projected upon nature to satisfy our desire for continuity. They relate to what lies beyond the range of observation, beyond the range of our sense- impressions. An interesting method of testing the accuracy of a formula is to use it as a basis for prediction. Many observant people are familiar with a mild form of scientific prophecy in connection with the weather. After long observation they hazard a generalisation, in private, if they are wise; and they test this by a prediction. As this is usually wrong, they conclude that their generalisation had not a sufficiently wide basis. But better examples may be found in the prediction of Neptune by Adams and Leverrier (from calculations based on the gravitation-formula) and the subsequent discovery of that planet by Galle; or in the prediction of the element german- ium by Mendelejeff and its discovery by Winkler. (e) Test Experiments and Control Experiments. — The distinction between an observation and an ex- periment seems quite artificial, the point of contrast being that in the former we study the natural course of events, while in the latter we arrange for the oc- currence of certain phenomena. In studying the effect of electric discharges on living plants we might wait for the lightning to strike trees in our vicinity ; but as this would be worse than tedious, we prefer to mimic the natural phenomenon in the laboratory. This is obviously a distinction without a difference, and instead of calling the first step (a) observation, as we have done, we might equally well have used the word experiment. On the other hand, at a later stage in the scientific treatment of a problem, our opportunities for experi- ment can be profitably used, not for accumulating THE SCIENTIFIC MOOD. 23 more data, but for putting our hypothesis to the proof. We allude to what are called test or crucial experiments and control experiments. Much of the success of a scientific worker may depend on his ingenuity in thinking out crucial experiments and on his rigorous use of control experiments. When bacteriology was in its infancy, Pasteur put his theory that putrefaction was the result of the life of micro-organisms to a crucial test when he steri- lised readily putrescible substances, and, having her- metically sealed the vessel, kept them for years with- out the occurrence of any putrefaction. When Yon Siebold and his fellow-workers had gradually convinced themselves that certain bladder- worms in various animals used as food were the young stages of certain tapeworms occurring in man, they made the crucial experiment of swallowing the bladderworms and proved the accuracy of their con- clusion by becoming shortly afterwards infected with tapeworm. The control experiment is closely akin. A cray- fish is known to have a sense of smell. Various rea- sons lead the enquirer to conclude that this sense has its seat in the antennules. He may verify this by observing that a crayfish without these appendages will not respond to a strong odour, but he would not be satisfied unless he had shown that in exactly the same conditions and to exactly the same stimulus an- other crayfish with its antennules intact did actively respond. Having gone so far, he would proceed to localise the sense more precisely; microscopic re- search would direct his attention to peculiarly shaped bristles on the antennules. By shaving these off, and observing that response to strong odours ceased, he would prove his point, but again, in view of possible 24 PROGRESS OF SCIENCE IN THE CENTURY. error, he would confirm his conclusion by control ex- periments with normal animals. The above case illustrates a combination of the method of exclusion with the use of control experiments. (/) Formulation and Incorporation. — The final step is to sum up what has been attained in terms as clear and terse as possible, and to add the dis- covery to what has been already established. The digested data are absorbed into the body of science. If the discovery is one of magnitude it will be expres- sible as a formula, which should have the criterion of universal validity in the minds of all who are able to estimate the evidence. But even here, in our judgment, there should arise the final question of considering how the new generalisation consists with others, or in wider terms, how it is related to the sum of human experience. Should it be markedly inconsistent, as the evolution-formula seemed at first to so many, there may be need for re-consideration. The body may have to adapt itself — possibly not without pain — to its new food. Finally, to quote once more from Prof. Karl Pear- son : " The scientific method is marked by the fol- lowing features: — (a) careful and accurate classi- fication of facts and observation of their correlation and sequence; (6) the discovery of scientific laws by aid of the creative imagination; and (c) self- criticism and the final touchstone of equal validity for all normally constituted minds." CHAPTER IT. THE UNITY OF SCIENCE. C:LASSIFICATION OF THE SCIENCES. SINCE science presumes to take the whole uni- verse for its province, and faces the immense prob- lem of the order of nature, it is not surprising that a division of intellectual labour has been found con- venient, and that separate sciences have been defined off, each with particular problems and special meth- ods. This is an adaptation to the shortness of hu- man life and the limitations of human faculty, for while there is nothing but laziness and mis-education to hinder an intelligent citizen from having scientific interest in all orders of facts, the long discipline which a science requires renders it impossible that any average man will succeed in gaining masterly familiarity with more than one department of knowl- edge. The title of the old Scotch professorships of " Civil and Natural History " perhaps expressed more than one good idea, — for instance, that man must be studied in relation to his environment, or, again, that the history of non-human organisms might have some light to throw upon the history of mankind, but the ideal suggested was too ambitious for ordinary mor- tals. The fact is that a compromise has to be made between two desirabilities. On the one hand, the 26 PROGRESS OF SCIENCE IN THE CENTURY. aim of science-teaching, which is a culture of the scientific mood and an appreciation of scientific method, seems more likely to be attained by a thorough study of some one order of facts than by an intellectual ramble through the universe; on the other hand, the true dignity and value of science can- not be appreciated if the unity of nature and of knowledge be practically denied. Superficiality re- sults from lack of specialisation, and pedantry from too much of it. Let us briefly consider some of the classifications which have been found convenient. Francis Bacon (1561-1626) recognised three de- partments of human learning: (1) History (based on memory) both "natural" and "civil"; (2) Poesy (based on imagination) ; and (3) Philosophy or the Sciences (based on reason), including Divin- ity, which has to do with revelation, and Natural Philosophy, which deals with God, Nature, and Man! There is little in this classification which can be of service to us to-day in mapping out the territory of science, but it is interesting (as Karl Pearson points out) to notice the suggestion that " The divisions of knowledge are not like several lines that meet in one angle, but are rather like branches of a tree that meet in one stem." Auguste Comte (1798-1857) recognised six fun- damental sciences: Mathematics, Astronomy, Phys- ics, Chemistry, Biology, Sociology — and a supreme or final science of Morals. He sought to eliminate from his system all that is not based on experience, and he introduced the important conception of a hierarchy of knowledge, that is to say the idea that one department of science is dependent on another, sociology on biology, biology on chemistry, chemis- try on physics, and so on. Without pretending that THE UNITY OF SCIENCE. 27 the facts of life can be re-stated in terms of chemistry and physics, or that the biologist has given into his hands the key to the problems of human society, we may profitably recognise that an understanding of the organism is facilitated by the results of chemical and physical science, and that the data of biology are full of suggestion to the sociologist. It may be true — many would call it obvious — that life transcends the categories of mechanism, or, in other words, that the formula of physics do not suf- fice to re-express the facts of life. Yet it must be admitted that vital phenomena have become more in- telligible— more readily dealt with in thinking — since Biology began to avail itself of the aid of Chem- istry and Physics. It may be true that man tran- scends the categories of Biology, and it seems to many that man as compared with the Amoeba expresses an entirely new synthesis, just as the Amoeba does in relation to a mineral, and that the secret of both new syntheses remains as yet hidden. Yet it must be admitted that human life has become more intel- ligible— more readily dealt with in thinking — since Psychology and Sociology condescended to listen to the suggestions, confessedly still immature, offered by Biology. On the other hand, it seems historically true that such valuable ideas as division of labour and evolution were made clear in regard to human affairs before they were transferred to and re-illustrated in the study of organisms. There is a sense in which the Amoeba may be said to be of use in the interpreta- tion of man; but it is also true that the study of man has reacted upon the biological interpretation of the Amoeba. Similarly great advances were made by Chemistry when attention was extended from in- organic to organic substances, and there are at least 28 PROGRESS OF SCIENCE IN THE CENTURY. hints that the application of the Evolution-idea to the problems of the inorganic will make for progress. It was this idea of the interdependence of different scientific disciplines which especially marked Comte's classification. Herbert Spencer (1864) "combined the ' tree ' system of Bacon with Comte's exclusion of theology and metaphysics from the field of knowl- edge," * and he focussed the distinction between the Abstract sciences of Logic and Mathematics (which deal with our methods of conceptual description) and the Concrete sciences which are conceptual de- scriptions of phenomena. In other words, f the abstract sciences deal with modes of perception, the concrete sciences with the contents of perception. Eor the most detailed map of science as yet worked out, we may refer to the concluding chapter of Karl Pearson's Grammar of Science, noticing only: (1) that it has been almost unanimously recognised as convenient that the sciences dealing with organisms (Biology, Psychology, Sociology) should be distin- guished from those which deal with inorganic phe- nomena (Chemistry and Physics) ; and (2) that different departments are bound together, e.g., ap- plied mathematics linking the abstract to the concrete, chemical physiology linking the study of the in- organic to that of the organic. Thus, the broad lines of the scientific map may be represented in a scheme like this : — * Karl Pearson, Grammar of Science, rev. ed.f London, 1900, p. 513. t Ibid., p. 515. THE UNITY OF SCIENCE. 29 ABSTRACT AND CONCRETE SCIENCES. LOGIC. | ( SOCIOLOGY 2 0) c I E [METHODS OF •ia r. . ~ - \ PSYCHOLOGY 0 C DISCRIMINATION !l ( Botanv o o GENERALLY.] i ^ooloffy^j GwCt • ^ ~ V O«/ 0 £ 8 ^ "S -3 'o .2 C - CHEMISTRY ( Astronomy, — MATHEMATICS. co "• AND •} Geology, Jrf 3 PHYSICS. ( Meteorology, etc. - 3 THE COKKELATION OF KNOWLEDGE. Verworn speaks of Johannes Miiller (1801-1858) as " one of those monumental figures that the history of every science brings forth but once. They change the whole aspect of the field in which they work, and all later growth is influenced by their labours." When we enquire into the secret of Miiller's achievements, we find that he combined genius with unsurpassed working-power, but it is important to notice more definitely what we may call his sense of the correla- tion of knowledge. " He did not recognise one physiological method alone, but employed boldly every mode of treatment that the problem of the moment demanded. Physical, chemical, anatomical, zoological, microscopic and embryological knowledge and methods equally were at his disposal, and he employed all of these whenever it was necessary for the accomplishment of his purpose at the time." * If we take Pasteur (1822-1895) as another repre- sentative figure in nineteenth century science, we may * Max Verworn, General Physiology, trans. 1899, p. 20. 30 PROGRESS OF SCIENCE IN THE CENTURY. read the same lesson. Far from being pre-occupied with vivisection and inoculation, as the commonplace summary too often suggests, he passed in an ever- widening spiral of scientific investigation from his rural centre upwards, from tanpit to vat and vintage, from manure heaps, earth-worms, and water-supply to the problems of civic sanitation. On each radius on which he paused he left either a method or a clue, and set some other enquirer at work. Biologist and brewer, chemist and physician, agriculturalist and surgeon, — and how many more — have all felt the influence of his achievements, and part of the secret of these lay in his sense of the correlation of knowl- edge, in his grasp of the fact that workers in different departments of science have much to say to each other.* Another, and again a different illustration may be found in the work of Darwin. It was natural that one who discerned so vividly the correlation of or- ganisms should also realise the correlation of knowl- edge. We see this, for instance, as we turn over the pages of The Origin of Species, The Descent of Man, Variation under Domestication, and his other great works, and infer from the foot-notes something of the range of the fields in which he gleaned. We see it in his recognition of the far- reaching scope of the doctrine of descent, that it be- longs not merely to the biologist, but affects psychol- ogy and sociology, the whole life of man and society. He once expressed satisfaction that he had not been permitted to become a " specialist " ; it is hardly too much to say that there is no specialism in concrete organic science which he has left unaffected. * P. Geddes and J. Arthur Thomson, " Louis Pasteur," Contemporary Review, Nov., 1895, pp. 632-644. THE UNITY OF SCIENCE. 31 Let us take an illustration from the history of astronomy. Apart from pioneer suggestions, as- tronomy was till the middle of the century a science descriptive of the movements of the heavenly bodies. But the establishment of spectroscopy by Kirchhoff and Bunsen was the beginning of a close correlation between astronomy and other sciences. Formerly " it was enough that she possessed the tele- scope and the calculus. Xow the materials for her inductions are supplied by the chemist, the elec- trician, the enquirer into the most recondite myster- ies of light and the molecular constitution of matter. She is concerned with what the geologist, the meteor- ologist, even the biologist, has to say ; she can afford to close her ears "to no new truth of the physical or- der. Her position of lofty isolation has been ex- changed for one of community and mutual aid." * XEED FOB CRITICISM OF SCIENTIFIC WOBK. A large part of the scientific work done year after year is instinctive and spontaneous rather than delib- erate and controlled. It is done because the doers have delight in it, a " natural taste," as they say, and thus self-criticism as to the value of it is silenced. The result is an enormous waste of mental energy. Big-brained men often fritter away their intelligence on the study of trivialities, which may be admirable as what used to be called an " elegant amusement," but represents a great loss to science. It is perhaps useful at times to stand by and calmly watch the succession of gifts laid upon the altar of science. There are the well-finished offer- ings of those who have what seems to some of us so in- * A. M. Clerke, History of Astronomy in the Nineteenth Century, 1885, p. 183. 32 PROGRESS OF SCIENCE IN THE CENTURY. estimably precious — the leisure to work thoroughly undisturbed ; there are the ill-finished offerings of the impetuous, and enthusiastic, and hard-driven; there are humble offerings which have involved years of self-denial; there are brilliant offerings which have meant but a few flashes of clear insight; there are tarnished offerings which have been gained illegiti- mately ; there are heroic offerings which are received in absentia from those who have died to know ; there are epoch-making offerings, like those of Newton or of Darwin, which set the whole altar aflame. One cannot see this vision of the altar of science without being impressed. There is a majesty in the advancement of knowledge, and a sublime patience in research. But it is difficult to tell how much of the work would be regarded as effective expenditure of energy by a sufficiently wise judge, wise for science and wise for humanity. The only sufficiently wise judge is Time, whose decisions are often very slow. That contemporary appreciation of an offering has often been far from just is one of the most obvious facts in the history of science. But as one lingers near this " altar of science," one must be much absorbed if one does not hear a murmur of dissentient voices. The practical man growls over the time spent in the classification of seaweeds when " what we want is more wheat," over embryological research instead of fish-hatching, over the theoretical puzzles of geology instead of the search for more coal and iron. When the practical man supports the scientific worker, he has doubt- less some right to control the direction of his activities, though it is not very clear that much good has ever come of this. Man does not live by bread alone, and some of the most important practical THE UNITY OF SCIENCE. 33 results, such as the use of antiseptics, have been reached by very circuitous paths. It did not seem a very promiseful beginning which Pasteur found in the study of tartrate crystals, and yet what a begin- ning it was ! It is long since Bacon replied to the objection of the practical mood which we have just noted. We may recall his vindication of investigations which are light-giving (lucifera) against those which are of direct practical utility (fructifera) ; and the deliver- ance " Just as the vision of light itself is something more excellent and beautiful than its manifold uses, BO without doubt the contemplation of things as they are, without superstition or imposture, without error or confusion, is in itself a nobler thing than a whole harvest of inventions." But there are many other dissentient voices. The humanitarians mutter " cruelty," " inhuman curios- ity," " barbarous inquisitiveness," " triviality." The scholars say with a smile, " We would rather know the thoughts of Plato and Aristotle than pore over the entrails of an antediluvian frog," — " a Kindergar- ten study at the best is your Natural Science." The poets and artists laugh and say, " Grubbers among dust and ashes, besmirching the wings which might lift you as eagles," " a botany which teaches that there is no such thing as a flower," " a biology which has become necrology," " a chemistry which has flooded the world with aniline dyes," " a physiology which has made a debased — not kailyard, but mid- den-heap— literature possible," and so on. These and a hundred other criticisms reach the ear, and though a retort may readily be made to each, the feeling remains that there is some justice in most of them, that scientific industry is not always suf- 34 PROGRESS OF SCIENCE IN THE CENTURY. ficiently self-critical. To rise above particular criti- cisms to a general basis of criticism would be a great gain, and perhaps this may be found in a recognition of what may be called The Three Unities. UNITY OF LIFE. The first of these unities is the Unity of Life. We have already referred to the three main moods or atti- tudes of mind observable in human relations to na- ture— practical, emotional, and scientific. They find expression in doing, feeling, and knowing; in prac- tice, in art, and in science; they may be symbolised by hand, heart, and head. We are not of course supposing the existence of altogether separable faculties, or nonsense of that sort; we do not say that there are any purely prac- tical, or exclusively emotional, or solely scientific men; we simply note what appears to be a fact of life that we can practically distinguish around us the doers, the feelers, and the knowers. And as one of the moods often has temporary dominance, we are all apt to err in over-doing, or over-feeling, or over-knowing. It is believed by most comparative physiologists that the ears of many of the simpler animals are not hearing ears, but rather directive organs, impor- tant in balancing, equilibrating, and orientation. It is such an equilibrating organ that we all need to help us to adjust the balance of our moods. Our thesis then is that some measure of complete- ness of life — in ideal at least — is the condition of sanity in human development. A thoroughly sane life implies a recognition of the trinity of knowing, feeling, and doing. It spells health, wholeness, holi- ness, as Edward Carpenter has said. THE UNITY OF SCIENCE. 35 Contrariwise, non-humane activity, whether prac- tical, emotional, or scientific, implies primarily a denial of the trinity referred to, a violence to the unity of life. The one-sided man has let at least two of the lights of life die out. To be wholly practical is to grub for edible roots and see no flowers upon the earth nor the stars over- head; to be wholly emotional is to become unreal and effervescent ; to seek only to know is to deny our birth-right and birth-duty as social organisms. The various sins of our relations to nature — sins of ignorance, indifference, irreverence, cruelty, ob- scurantism, and so on — all imply some denial of the trinity. Science for its own sake requires to be continu- ally moralised and socialised, oriented, that is to say, in relation to other ideals of human life than its own immediate one of working out an intellectual cosmos. Our science requires to be kept in touch at once with our life and with our dreams; with our doing and with our feeling; with our practice and with our poetry. Synergy and sympathy are needed to complete a synthesis. If the above be a reasonable position, it suggests that the scientific way of looking at the world is not the only one. There are many whose outlook ex- presses quite a different mood. As we have seen, the student of science does not pretend to explain the order of nature, he simply tries to re-describe it in general conceptual formulae, and he believes that his task is justified by the results — intellectual, emotional, and practical. He has a right to insist on being heard as to the aim of his own industry, but it does not follow that his statements are of equal value when he speaks of other than scientific 36 PROGRESS OF SCIENCE IN THE CENTURY. expressions of the developing human spirit. Irri- tated by the way in which others misunderstand him, he often misunderstands them. Thus as an expres- sion of the recoil of the scientific mood from meta- physical speculation — a recoil which seems to us largely due to misunderstanding of aims — we may quote what Liebig said of Schelling: "I myself spent a portion of my student days at a university where the greatest philosopher and metaphysician of the century charmed the thoughtful youth around him into admiration and imitation; who could at that time resist the contagion ? I too have lived through this period — a period so rich in words and ideas and so poor in true knowledge and genuine studies; it cost me two precious years of my life." * The above citation expresses the opinion of many scientific workers, and yet is it not, to say the least, arrogant, to attempt to ignore the attempts which have been made throughout all the ages to re-express the order of nature in transcendental or metaphysical terms? " The search after ultimate causes," says Dr. Merz, " may perhaps be given up as hopeless ; that after the meaning and significance of the things of life will never be abandoned : it is the philosophi- cal or religious problem." We cannot readily understand a phenomenon which seems to occur — that of an active and well- disciplined brain in which there are, so to speak, idea-tight compartments, the contents of which are prevented from mutual influence. The mental like the bodily life should be a unified system of correla- * Veber das Studium der Naturwissenschaften. On the Study of the Natural Sciences, 1840, cited by E. von Meyer, History of Chemistry, 1891. THE UNITY OF SCIENCE. 37 tions. It cannot be normal that a man should cher- ish incompatible ideas. But that is not to say that he may not be both scientific and metaphysical, or both scientific and poetical. These are indeed different moods, but complementary rather than in- compatible, and disharmony results only when they are allowed to mix with one another in verbal state- ments, or when the particular concrete expressions given to the poetic or philosophic activity happen to be at variance with sound science. Between the moods there is no variance. The different moods express different ways of looking at things, and use as it were words of different languages. The evolutionist postulates a beginning somewhere, — an initial order of nature instituted in some fashion quite unknown and implying the potentialities of the future in some fashion quite unknown; the creationist gives in non-scientific or transcendental terms some account of the institution of the order of nature; the ideas are not antithetical, they are incommensurable. Moreover, if we may take an- other point of view for a moment, the teaching of the history of science leads us to a strong feeling of gratitude to the deductive or a priori thinkers. They were at least thinking — often with a broad perspec- tive— and that cannot always be said of researchers. They may have interpolated fanciful ideas where facts alone are decisive, their deductions may have led induction off the scent, they may have blinded vision by preconceptions and deranged reasoning by preju- dices, they may have caused confusion by mixing up objective and subjective terms, and done many other evil things ; but it is a historical fact that astrology led on to astronomy, alchemy to chemistry, cosmolo- gies to geology, and superstitious medical lore to 38 PROGRESS OF SCIENCE IN THE CENTURY. physiology. Even the frequent break-downs of the a priori methods prompted a posteriori enquiry. UNITY OF SCIENCE. The second unity — a recognition of which makes for sanity — is the unity of science or knowledge. The sciences in the broadest sense form one body of truth. Blocked apart for practical convenience, treated of in separate books, expounded by different teachers, investigated in different laboratories, they are parts of one discipline, illustrations of one method, expressions of one mood, and attempts to make clear — if never to solve — the one great prob- lem of the Order of Nature. The sciences have their ideal completeness only when inter-related. This is the ideal alike of the philosopher's stone, of the en- cyclopaedic movement, and of the most modern scien- tific synthesis. This note of the unity of the sciences is sounded — though so often quickly silenced — in the word Uni- versity. Its value is demonstrated by the history of the sciences, which shows how often a fresh contact between two departments has led to great advances. It becomes insistent when we consider a big subject like the physiology of marine organisms, which there is no hope of understanding except through the com- bined efforts of chemist and physicist, botanist and zoologist, meteorologist and geographer. Whether we take a hint from the term " Natural History," or from the word " Organisata," which Linnaeus used to include both animals and plants, or from Comte's hierarchy of the sciences, or from Caird's essay on the unity of science, or from Spencer's Synthetic Philosophy — we have purposely chosen incongruous examples — we hear the same note THE UNITY OF SCIENCE. 39 of unity. It is the end towards which our teaching and learning must move, even if the curve be asymp- totic. As we have already noted, the study of living crea- tures stands midway between the chemical and phys- ical sciences, which are in a sense beneath it, and the mental and social sciences, which are in a sense above it ; there are lights from below and lights from above; and to attempt to shut out either means un- necessary obscurity. The living organism is a syn- thesis, whose secret has certainly not been solved, but we are surely saved from some misunderstand- ings of it by the results of other sciences than Bi- ology. Thus, there comes to the aid of the biologist or any other scientific worker, this criterion: Am I, as a thinker, teacher, and investigator, recognising, respecting, doing no violence to, the unity of science? 'Am I recognising other disciplines, other bodies of thought, as I wish that they should recognise mine I Even more positively, the criterion might read: Does this piece of work in any way tend to the real- isation of the Unity of Science ? UNITY OF NATURE. A third unity may perhaps be spoken of as the unity of nature — by which we mean to refer both to the unity of the particular subject of scientific enquiry, and to the unity of the whole system of things. To the psychologist, the unity which must not be lost sight of is that of the person- ality which he is studying. To the biologist, the unity which cannot be ignored without fallacy is the unity of the organism. But besides these there is the unity of the whole system of nature in which D 4:0 PROGRESS OF SCIENCE IN THE CENTURY. part is linked to part by sure, though often subtle bonds in which nude isolation is as rare as a vacuum. In regard to all matters we have many questions to ask, each difficult, each interesting, each often re- quiring special methods of investigation, and in the search of answers we are sometimes apt to forget the unity of the subject. There can be no doubt, for in- stance, that in the eager pursuit of comparative anatomy, or chemical physiology, or any other par- ticular line of biological enquiry, the unity of the organism is often forgotten. The same is true, though perhaps less markedly, in other sciences, where the fascination of some one aspect or method causes the investigator to lose his sense of the unity of his subject. Specialism of enquiry is necessary and valuable, but it loses its virtue if the specialist remain like a beetle in a rut, the sides of which form the horizon. Thus we reach a third criterion of scientific work and thought ; we must force upon ourselves the ques- tion— Am I studying this — whatever it is — as I would have myself studied, as a whole, as a unity, and moreover as a part in the great system of things which we call Nature, which is also a Unity ? To sum up, there are a certain number of 'isms which we scornfully call fads. They express a loss of perspective, — intellectual, emotional, or practical, the dominance of some fixed idea which distorts or obscures vision. It is easy to scoff at one or other of these fads, but the chances are that we are ourselves victims. It is more in the line of progress to study their meaning, and then we see that they are often reactions against some denial of the unity of life, the unity of science, the unity of nature, or some greater unity than these. CHAPTEE III. PBOGRESSIVE^ESS OF SCIENCE. THE FIBST CONDITION OF SCIENTIFIC PKOGBESS. ~No one who has watched a colony of ants with anv precision will find it easy to agree with the ancient proverbialist that the " little people " are " exceeding wise," if we mean by " wise " to imply anything like " knowing " or " scientific " in the hu- man connotation of these terms. Ants are marvel- lous creatures of routine, but they are foolish before the new. Their little complex brains are well-stocked compendia of ready-made nervous mechanisms, but they are eminently non-educable. It is very difficult to prove that the little people are able to profit by experience at all. Therefore, if one were inclined to give a lifetime to the education of insects, one would not begin with ants. Their brains are too much " set," or stereotyped, to be readily docile. It would be unwise to be dogmatic regarding this difficult prob- lem, but the general verdict of present biological and psychological research on the behaviour of ants is, that their marvellous powers are not acquired by the individual in relation to the particular needs of its life, are not readily modifiable to suit novel contingencies even of a simple kind, are not, in the strict sense, intelligent, but are hereditary instincts which have arisen in the course of a long series of generations by the action of natural selection on germinal variations. If a disaster befell the ant-hill and reduced the 42 PROGRESS OF SCIENCE IN THE CENTURY. community to the minimum number necessary to avoid extinction — say to a fertile queen with two or three workers to look after her — there seems no reason to doubt that in a short time the whole ant- hill would contain a population as effective as before. Their powers are implied in their brain-inheritance ; their capabilities of effective response to their en- vironment have little or no external registration. It is possible that in some animals, where a social life is sustained generation after generation, there may be something corresponding to tradition which gradually grows larger in its content, which forms what may be called an external heritage as contrasted with a natural or organic inheritance. It is also to be noted that some of the higher ani- mals seem to have words — particular sounds in- dicative of certain things or expressive of definite emotional states — and it can hardly be doubted that the existence of these will facilitate mental processes. In some cases, too, the permanent products which animals make — dwellings, nests, roads, and the like — may become suggestive symbols, and may be of some importance as stimuli to successive genera- tions. Yet after all these admissions are made, it re- mains as a great contrast between man and animals that our possession of language and methods of re- cording conclusions makes the progress of science possible. In the case of ants it seems as if the brain had evolved in the direction of a more and more per- fect automaton ; in the case of man, the existence of external means of registration has made it possible for the brain to be born more and more plastic, less weighted by an inheritance of ready-made powers, in a word, more educable. " To the educable animal — PROGRESSIVENESS OF SCIENCE. 4.3 the less there is of specialised mechanism transmitted by heredity, the better. The loss of instinct is what permits and necessitates the education of the receptive brain." * In this book-ridden age when the student so often laboriously uses another's eyes instead of lifting his own, and when many, as a stern critic has said, " seem unable to cerebrate except in the presence of print," the hasty wish has sometimes been expressed that all books could be burned. But, however, in- teresting the century succeeding the conflagration might be — with enthusiastic reconstructing of the classics from reminiscences and with uninhibited in- dependence of inquiry — it is probably safe to say that men would return to the conclusion which we are now expounding, that the first condition of the progressiveness of the sciences is in permanent methods of external registration. Extraordinary, indeed, would be the calamity if the Temple of Science should fall like the Tower of Babel, if all the living embodiments of science should suddenly disappear, if all the instruments and inventions which are suggestive symbols of hard-won generalisations should be lost, if all the phrases which condense discoveries into formulae should be wiped out of human language — then we should have to begin at the beginning again. The prime condition of the progressiveness of science is in external modes of registration, — in words and formulae, symbols and instruments. THE FACT OF PEOGEESS. In an eloquent lecture on " The Progressive- ness of the Sciences," the late Principal John *E. Ray Lankester, Nature, LXL, 1900, p. 625. 44 PROGRESS OF SCIENCE IN THE CENTURY. Caird spoke as follows : " The history of human knowledge is a history, on the whole, of a continu- ous and ever-accelerating progress. In some of its departments this characteristic may be more marked and capable of easier illustration than in others. External accidents, affecting the history of nations, may often have disturbed or arrested the on- ward movement, or even, for a time, seem to have altogether obliterated the accumulated results of the thought of the past. But on the whole the law is a constant one which constitutes each succeeding age the inheritor of the intellectual wealth of all pre- ceding ages, and makes it its high vocation to hand on the heritage it has received, enriched by its own contributions, to that which comes after. In almost every department of knowledge the modern student begins where innumerable minds have been long at work, and with the results of the observation, the experience, the thought and speculation of the past to help him. If the field of knowledge were limited, this, indeed, would, from one point of view, be a discouraging thought; for we should in that case be only as gleaners coming in at the close of the day to gather up the few scanty ears that had been left, where other labourers had reaped the substantial fruits of the soil. But, so far from that, vast and varied as that body of knowledge which is the result of past research may seem to be, the human race may, without exaggeration, be said to have only en- tered on its labours, to have gathered in only the first fruits of a field which stretches away interminably before it." * It is one of the aims of this volume to illustrate * A lecture delivered In 1875. Reprinted in Lectures and Addresses, 1899. PROGRESSIVENESS OF SCIENCE. 45 the progress of the sciences within a century, and there are many ways in which the impression of progressiveness may be made vivid. Many of the articles in the older Encyclopaedias are splendid pieces of intellectual workmanship, but to read one of them and then its correspondent in a modern encyclopaedia is like a sudden transition from an incipient spring to midsummer. And yet we know that, to our successors, this modern article will soon seem quite vernal. There have been scientific works like those of Aristotle, Pliny, and Galen which lasted in varied forms through centuries ; and there are masterpieces, like the books of Euclid, and Newton's Principia, which in some form will be text-books while learning lasts; but every one knows that nowadays even the best of text-books is very short-lived. If we take a survey of the sciences, from astron- omy to sociology, how striking are the changes, alike as to facts and ideas, in the last hundred years. He must be indeed blase or callous who does not feel ex- hilaration in the thought of the advance in the in- terval between Laplace and Lockyer ; between Count Rumford and Lord Kelvin; between Hutton and Playfair and the Geikies; between Richard Owen and Louis Agassiz on the one hand, Cope and Zittel on the other; between Cuvier and Huxley; between Lamarck and Ray Lankester ; between Von Baer and Francis Balfour; between Bichat and Sir Michael Foster; between Erasmus Darwin and his grand- son ; between Reimarus and Romanes ; between Prich- ard and Taylor; between Adam Smith and Herbert Spencer. To any one who knows even a little con- cerning the history of science the contrasts of these coupled names must stimulate afresh the impression 46 PROGRESS OF SCIENCE IN THE CENTURY. that there are few facts more marvellous and inspir- ing than the advancement of science. ITS NECESSITY. The primary reason for the progressiveness of science is simply that the scientific mood is a natural and necessary expression of the developing human spirit. It may be thwarted, discountenanced, even banned, as it was during the early mediaeval cen- turies, but stifled it cannot be. The innate inquisi- tiveness, the passion for facts, the active scepticism, the desire after lucidity, and the other qualities to which we have referred as characteristics of the scientific mood, may be widespread or confined to small circles of enquirers, may be exhibited in re- gard to all orders of facts or restricted to a single department, but the scientific mood is essential to man's nature, and science will not cease to progress until both practice and poesy have likewise come to an end. There is no doubt that many pieces of scientific research are entered upon with the set purpose of solving practical problems; on the other hand much scientific activity is as spontaneous and instinctive as a great part of artistic activity is : in other words, it is a natural expression of the man. In evidence of this, at a time when the pursuit of science is so often a " profession " and a " Brodwissenschaft" one may recall that up and down through the country one finds many obscure enthusiasts pursuing in their lei- sure hours, or in hours when others sleep, some path of scientific enquiry — astronomical, geological, botanical, zoological, or otherwise — in most cases without hope of or wish for reward, without desire PROGRESSIVENESS OF SCIENCE. 47 for publicity or publication, for they are genuine amateurs in the literal sense. Another way of illustrating the ineradicable sci- entific mood is to consider a few biographies of eminent workers, and to notice how often the environ- mental conditions were the very reverse of propi- tious. The " Pursuit of Knowledge under Difficul- ties " is a well-worn theme, — of considerable interest to those who have had experience in the task of try- ing to induce uninterested students to pursue knowl- edge under the most favourable conditions. It may perhaps be argued that although the sci- entific mood is characteristically human and must therefore persist, while man as we know him does, yet the subjects of enquiry are limited and the range of our sense-experience is not infinite. Therefore there must be an end to the progress of science, and a time must come when the confession ignoramus will be no longer heard in the land, for all the prob- lems that have not been solved will be insoluble, and ignorabimus will remain as the only word of intel- lectual modesty. It can hardly be said that this question of the completion of scientific enquiry is one of practical politics, but it may not be unprofit- able to consider it for a little. It was surely a momentary aberration which led a great zoologist to suggest not long ago, in the enthusiasm of a retrospect, that it was now about time for us to be making a list of the things we did not know. A very different suggestion was made in a remarkable sentence in the presidential address delivered by the late Dr. Edward Orton at the 1899 meeting of the American Association for the Ad- vancement of Science. " The founders of the As- sociation, fifty years ago, clearly saw that they were 48 PROGRESS OF SCIENCE IN THE CENTURY. in the early morning of a growing day. The most unexpected and marvellous progress has been made since that date, but as yet there is no occasion for, and no prospect of modifying the title (Association for the Advancement of Science). We are still la- bouring for the advancement of science, for the dis- covery of new truth. The field, which is the world, was never so white unto the harvest as now, but it is still early morning on the dial of science." It is this last sentence which should be pondered over by any one who is inclined to speak or think or act as if it were already late afternoon ! The fact is, that to whatever department of scien- tific enquiry we turn, we find an embarrassment of unsolved problems. Everywhere there is a widening outlook, a more and more intensive analysis, but never a hint of finality. Everywhere we hear the words, " for leagues and leagues beyond, and still more sea." It might seem to some that an old-established and persistently prosecuted department of science like human anatomy must be now almost exhausted, but among the experts the suggestion would be received with derision. It might seem to some that a little animal like the lancelet, every millimetre of whose body has been subjected to the scrutiny of the keen- est zoological observers, must be now almost com- pletely known, but the suggestion is one that only an outsider could make. We have not nearly fin- ished with this one animal, and is it not a little one? The animal cell has been studied with the most assiduous carefulness, with gradually perfected microscopes, with ingenious devices of fixing and staining and cutting, for more than three-quarters of a century, and yet it remains very imperfectly known. We may recall, for instance, that the dia- PROGRESSIVENESS OF SCIENCE. 49 covery of the central corpuscles or centrosomes — somewhat enigmatical, apparently very important, and practically constant components of the animal cell — members of the " cell-firm " — dates from only a few years ago. N"or should it be forgotten that we live in a world of change, in which a process of evolution is going on, and that, therefore, the subject-matter of a sci- ence is developing just as the science is. We hear of stars that die and of others that are a-making (we may use the present tense though the events are, of course, vastly older than our observation of them) ; even in a human lifetime — the minutest moment compared with the earth's age — the features of a countryside may change perceptibly, indeed a shors may get a new face in a single storm; the distribu- tion of plants and animals is in process of rapid flux; the characters of organisms, including our- selves, are slowly but surely changing. Thus with an evolving subject-matter before our eyes, we need say little about the prospect of — completed science. SCIENTIFIC CONCLUSIONS OF THE FIRST MAGNITUDE. We hear so much nowadays in regard to the rapid progress of science that there seems some dan- ger lest our impression become exaggeratedly san- guine. In more critical moods, however, the suspi- cion arises that in spite of the rapid accumulation of natural knowledge, information often proves itself the death of wit ; and that in spite of the remarkable diffusion of the scientific mood throughout wide cir- cles in our community, the growth of scientific in- sight is really very slow. That this suspicion is not unfounded becomes clear 50 PROGRESS OF SCIENCE IN THE CENTURY. when we consider the small number of scientific gen- eralisations which we can venture to describe as of the first magnitude. We begin to count these: The doctrine of the indestructibility of matter, foreseen by Democritus, but for practical, scientific purposes only about a century old — dating from Lavoisier; the doctrine of the conservation of energy, with its corollaries of transformability and dissipation; the theory of gravitation, with its far-reaching applica- tions; and the theory of organic evolution which will be linked for ever with the name of Charles Darwin. But after we have enumerated these, we begin to hesitate. Are there any others on the same plane, which thoughtful men accept without hesitation and without saving clauses, to lose any of which would spell intellectual disaster? Should we include, for instance, what is grandiloquently called the Law of Biogenesis — which states that, so far as we know, every living creature has its parentage in another living creature or in two other living creatures ? This is a big fact, no doubt, but it is hardly more than an empirical fact, and there are many who suppose from foreshadowings which they see that the coming events of the next quarter of a century will con- vince us that this at present unimpeachable conclu- sion will be shown to be fallacious, not in itself per- haps, but in its suggestion of an impassable gulf between the not-living and the living. Or should we include the " biogenetisches Orunagesetz " — the Re- capitulation Doctrine — that the individual develop- ment recapitulates the racial evolution, or that the organism in its becoming climbs up its own genea- logical tree ? but there are many who will agree with Mr. Sedgwick — the eminent zoologist of Cam- PROGRESSIVENESS OF SCIENCE. 51 bridge — that before this recapitulation doctrine can be accepted it must be subjected to emendations so serious that it comes to resemble a shoe cobbled so often that almost nothing of the original structure remains. We read of the stuffed horse of Wallen- stein at Prag which has " only the head, legs, and part of the body renewed," and the " biogenetisches Grundgesetz " seems much in the same state at present. In revised form it must prove its power of survival a little longer, before we can admit it to a place of honour among the scientific generalisations of the first magnitude. A recent paper on the cardinal principles of science reminds us that we have overlooked " The Uniform- ity of Nature," which states that in similar condi- tions similar things are likely to happen, and also the platitudinarian doctrine of " The Responsivity of Mind," which asserts that minds react in similar ways to similar stimuli. "With every wish to be generous, we cannot throw these in, for the first seems a platitude — a fallacious platitude — and the second, well, it is a corollary of the first. Huxley gets credit for the phrase " The Uni- formity of Nature," which has been called a cardinal principle, indeed the cardinal principle of science. But if Huxley made the phrase, which we doubt, it does not seem so happy as some others that he minted. It is difficult to state clearly what the so- called principle means. That there are uniformities of sequence in the world around us all will admit, —else there were no science possible — but what the uniformity is remains obscure. We believe that the gravitation formula fits wherever it can be applied, that is one uniformity; we find no evidence to warrant our doubting that what we call matter and 52 PROGRESS OF SCIENCE IN THE CENTURY. energy always persist however their forms of expres- sion may change, here are two other uniformities — or, perhaps, the two are one ; but there are not many other conclusions which admit of the same univer- sality of application and verifiability of accuracy. We know the " law of biogenesis," omne vivum e vivo, to mean that, so far as our experience goes, every living creature springs from some other living crea- ture; we do not know of any exception to the state- ment, but we see no warrant in this for asserting that the so-called law was, or will be, or even is always true. And the same doubt, which becomes more as- sertive when we consider this last instance, is not silent even in regard to the alleged indestructibility of matter or the alleged indestructibility of power. It does not seem particularly forcible to retort that " one cannot conceive of the reverse happening," for it is not so long since a belief in spontaneous genera- tion was widespread, or since the idea that the earth was not the hub of the universe was deemed by many — and these not small-brained men — " quite incon- ceivable." And these were the very words of Mother Grundy when she first heard of the Doctrine of De- scent. In short, is there not a radical fallacy in the phrase " The Uniformity of Nature," since our so-called natural laws are only descriptive formulae of what is seen and known in given conditions of space and time, neither " governing nature," nor " explaining nature " ? As descriptive formulae of observed phe- nomena, presumably descriptive of similar unobserved phenomena, they make it easier for us to look out upon the world without intellectual biliousness — in- deed with the greatest of joy, to follow the course of events with some appreciation of their orderliness, PROGRESSIVENESS OF SCIENCE. 53 to utilise them for our practical purpose ; but, surely, it is time that we ceased supposing that they enable us to explain, to see the ultimate causes, the " real inwardness," of what we observe. But even if the reiterated distinction between descriptive formulae and explanations be not admitted — its vindication will be found in Karl Pearson's Grammar of Science, — it may perhaps be granted that the less we say about the Uniformity of Nature the better for the consistency of our scientific mood. Is not the whole point expressed in Bacon's aphorism ? — " Man, as the minister and interpreter of nature, does and understands as much as his ob- servations on the order of nature, either with regard to things or the mind, permit him, and neither knows nor is capable of more." It is difficult, perhaps, to say what the word " understand " means in this aphorism, but if it mean " redescribe in simpler terms," it expresses our present position. There is another consideration which should per- haps give us pause in our talk about the Uniformity of Nature. It may be illustrated by the following quotation from a paper by Winkler.* " Four hundred years ago Nicholas Copernicus left, as a young master of philosophy and of medicine, the old university of Ulica St. Anny, at Cracow, to go to Bologna and to Rome for the purpose of con- secrating his talents as a mathematician to the study of astronomical sciences. There, attacking the enigma of the firmament, he finally attained the certainty that the earth was not, as had been hitherto believed, a central fixed world, but a sphere suspended freely in space, a planet similar to the other planets, * Transl. in Rep. Smithsonian Inst. for 1897, pp. 237-246. 54 PROGRESS OF SCIENCE IN THE CENTURY. turning around the sun and having a movement of rotation around its own axis under the action of gravitation. It was, indeed, a true revolution in the theories that had been hitherto held, this theory that fixed the sun in the firmament in spite of its daily ascent and disappearance; an idea that, at the present day, has become familiar to us. And fur- ther, we now know that neither is the sun itself fixed, but that it is drawn with all its cortege of planets along a course without end, across space with- out limit. Whence comes it and whither goes it? Properly speaking, we know nothing about it, and doubtless we will never know either its origin or its end; but as the earth turns around this movable sun, it hence results that our planet does not describe a closed path, but a sort of spiral, and that it never returns to a spot that it has once quitted. Each second takes our planet to a new point in the universe, and from this incessant displacement it ought to follow that no phenomenon or event can ever reproduce exactly any anterior phenomenon. Clouds may resemble each other, as one sunrise re- sembles another, but there is never an absolute coin- cidence, and it would seem that these variations ought to be perpetuated throughout the course of time that is embraced by the history of humanity. " It would be useless to push further these con- siderations, they are merely speculations; but they lead to this thought, which, although unsupported, continually recurs to our mind — the possibility of a progressive transformation of matter in a given direc- tion, in that they show that everything that is with us is drawn along in a dizzy course across an un- known immensity." Let us return, however, to our particular point PROGRESSIVENESS OF SCIENCE. 55 in this section, which was the small number of scientific generalisations of the first magnitude. What, some one may indignantly ask, what of the atomic theory, the periodic law, the kinetic theory of gases, the mechanical theory of heat, the un- dulatory theory of light, the cell-theory, Weber's law, and so on ? To which we would answer that while these are doubtless of importance, they lack the generality and the intellectual influence of the four great generalisations already mentioned — the in- destructibility of matter, the conservation of energy, the formula of gravitation, and the theory of organic evolution. What impresses one then is, that scientific generalisations of the first magnitude are few, and therefore that the scope for progressive science kas at present no visible boundaries. FACTOES IN FURTHER PROGRESS. (a) Growing Intensity of the Scientific Mood. — It cannot be doubted that serious scientific study is now common in circles where half a century ago it was rare; this means an increasing body of observ- ers, critics, and formulators. It is also certain that scientific methods are now being applied to orders of phenomena which half a century ago were observed and discussed in a very easy-going and light-hearted fashion. Some one has said rather bitterly that every science must pass through three periods: of presentiment or of faith, of sophistry, and of sober research ; but it may be confidently asserted that most departments of science have now entered upon the third period. It is not long since comparative psychology was, 56 PROGRESS OF SCIENCE IN THE CENTURY. apart from a few classical works, for the most part anecdotal. Precision of observation and record was blurred by fancies; facts and inferences from facts were subtly intermingled ; experiment was almost un- known, indeed scarcely thought of; and transcen- dental preconceptions prejudiced the whole outlook. But these blemishes are rapidly disappearing, and we see the rise of a young science, — careful, pains- taking, precise, given to measuring and experiment- ing. To take another illustration. It is well known that one of the master-keys to evolutionist problems is labelled " variation" by which is usually meant the process or the result of innate or constitutional change which renders a living creature from birth onwards more or less different from its parents. Since the process of variation furnishes a great part, if not the whole, of what may be called the raw material of progress, its importance is obviously fundamental. And yet the post-Darwinian history of biological activity in reference to variation has only recently begun to be creditable to science. Let us quote a few sentences from Mr. Bateson's Materials for the Study of Variation (1894) — a work which has done much to lift our feet out of the mire. " We are continually stopped by such phrases as, ' if such and such a variation then took place and was favourable/ or, ( we may easily suppose circum- stances in which such and such a variation if it oc- curred might be beneficial,' and the like. The whole argument is based on such assumptions as these — assumptions which, were they found in the arguments of Paley or of Butler, we could not too scornfully ridicule. ' If,' say we with much circumlocution, ' the course of Nature followed the lines we have PROGRESSIVENESS OF SCIENCE. 57 suggested, then, in short, it did.' That is the sum of our argument. . . . Surely, then, to collect and codif y the facts of Variation is the first duty of the naturalist. This work should be undertaken if only to rid our science of that excessive burden of contra- dictory assumptions by which it is now oppressed. ... If we had before us the facts of Variation there would be a body of evidence to which in these matters of doubt we could appeal. We should no longer say ' if Variation take place in such a way/ or ' if such a variation were possible ' ; we should on the contrary be able to say, ' since Variation does, or at least may take place in such a way,' ( since such and such a Variation is possible,' and we should be ex- pected to quote a case or cases of such occurrence as an observed fact." It was in this mood that Bateson compiled his invaluable work, which, though still represented by only Part I., has been a big stride towards a more scientific basis for the study of organic evolution. It has been followed by numerous statistical studies of actually occurring variations, by experimental at- tempts to distinguish between germinal variations and bodily acquired modifications (due to the in- fluence of functions and environment), and so on. The point is, that here, as in many other cases, an over-impetuous, undoubtedly too easy-going science, has had to retrace its steps, and to begin again where science always begins, in precise and unprejudiced observation and recording of facts, in measurement, and in experiment. (6) A Fuller Recognition of fhe Unities. — When we recall the fact that qualitative advance is very slow, while quantitative advance is exceedingly rapid, we are led to enquire whether there may not be some 58 PROGRESS OF SCIENCE IN THE CENTURY. deep reason for this. Perhaps the chief reason is the limitation of human faculty which so readily leads to a disregard of what we have called the Three Unities. The limitation is partly the result of mis- education, the persistent tendency to fill the mind instead of evolving it, to set it in grooves instead of allowing it free scope. It is also due to the pressure of social conventions, which nip the buds of individu- ality, frown down idiosyncrasies, and allow no elbow room (Abanderungsspielraum} to novel variations, which are, after all, the potentialities of progress. It is also due to the pressure of the struggle for exist- ence, which forces the young enquirer to premature specialism, that he may thereby make a name and a position for himself. " Er will sich nahren, Kinder zeugen" and so on. If we may define a genius as one who has by inheritance and appropriate culture an unusual complement of powers all in strong devel- opment,— poetic as well as scientific, or practical as well as philosophical, or otherwise, — there are many facts within our experience which suggest the sad conclusion that for one genius who makes himself felt, there are perhaps nine whose light is hidden under a bushel. It is for this reason that many who are under no delusion as to the equality of men or the triumph of democracy would favour every measure which opens the portals of learning — let us say, the gates of our Universities — more widely to all sorts and conditions of men.* There remains, however, another reason, that when the scientific student, who has retained an open and sympathetic mind, finds himself in his maturity more than ever aware of the need for correlation in knowl- * This is now pecuniarily possible in Sootland, thanks to Mr. Carnegie's magnificent gift. PROGRESSIVENESS OF SCIENCE. 59 edge-making or for co-operation in science, he is also likely to find himself pre-occupied with his own problems, mastered by his strongest personal interests, burdened by immediate duties, with neither time nor energy left for that effort which an active reali- sation of the unities implies. For lack of sympathy in some cases, for lack of synergy in other cases, the progress of synthesis is sluggish. For this reason we emphasise our thesis that the progressiveness of science depends first on a realisa- tion of the Unity of Life. The scientist, by which we mean the student of the order of nature, is incomplete in his arm-chair; he is even incomplete in his laboratory. He must be, in some measure, also a citizen, a man of feeling, and a philosopher! That even his science will suffer from his practical denial of the trinity of doing, feel- ing and knowing, is our argument, and this the slow progress of science seems to us to bear out. One might appeal to biologists who have because of their expert knowledge been appointed to serve on gov- ernmental commissions, dealing with practical prob- lems of life, and ask whether, after allowing for the delay of their personal investigations, they did not return to these with new zest, widened outlook, and fresh insight. The German government digni- fies prominent scientists with the title of GeJieimrath or Privy Councillor, and in many cases there is an honour conferred, and that is all. But behind the honorary title, there is the suggestion — sometimes realised — that the expert advice thus dignified is at the service of the government in critical situations, — a plague, a famine, an exploitation of new territory and so forth. That the same sort of expert advice should be at the command of all nations who nurture 60 PROGRESS OF SCIENCE IN THE CENTURY. scientific academies and scientific professors, seems sound common sense, and that it would be the better for science, as well as for the community, if this were oftener called into exercise seems equally obvious. We have illustrated our point by reference to the need for contact with the practical problems of life ; but a strong case could also be made for the advantage which science would gain by endeavouring at least to understand the point of view of the artist and the philosopher. Secondly, the progressiveness of science depends upon a fuller realisation of what we have called the unity of science. Mineralogy and petrography have acquired new vitality and greatly enhanced impor- tance since they became definitely chemical; the method of spectrum analysis has brought astronomy from a position of isolation into intimate contact with chemistry and physics; the recent development of physical chemistry is another instance of happy and fruitful union; since physiologists called chemists to their aid physiological chemistry has be- come so important that what used to be relegated to an appendix in a physiological treatise now pervades the whole book ; psychology has listened to biological results; and the indebtedness of social science to biology and the physical sciences is admitted by most to be of value, though the contact is still only in- cipient. But while this and more may be said of actual co- operation, it remains necessary to point out that many workers, and many departments of this or the other science, continue to flounder along, where- as they might swim swiftly if they condescended to take assistance and instruction from their fellow- travellers. After all, the current is not so swift, PROGRESSIVENESS OF SCIENCE. 61 that there is no time for mutual consultation by the way. Thirdly, the progress of science depends upon a recognition of the unity of the subject, which extends itself to a recognition of the unity of nature. A great part of scientific work is analytic; we take things to pieces — social institutions, man, the animal, the plant, the earth, the piece of matter — just as the boy dissects the watch. And this analysis is neces- sary, as well as fascinating. The danger is lest we forget that it is only a means to an end — namely, that we may put the things together again with a better understanding of the unity which we have dissolved. It is plain that in anatomy, for instance, we make an abstraction necessary for the purpose on hand, but still an abstraction — for we leave the life out of consideration. Our point is, that the analytical work of the anatomist only fulfils its function when the results are brought as a contribution towards a fuller understanding of the unity of the organism. In the same way, to take another illustration, the comparative physiologist concerns himself mainly with an analysis of the activities or functions of or- gans, tissues, and cells in different kinds of creatures ; and his work, still very young, has been rich in im- portant results, and is full of promise. But, again, for purposes of research, abstractions are necessary, the living creature is abstracted not from its life — for the physiologist is always concerned with activity — but from its full life as it is lived in nature. Our point is, that physiology does not fulfil itself until its results are brought as a contribution to a fuller understanding of the life as a whole — of what is in some sense a personality with character and habits, with a complex life in a complex environment, a 62 PROGRESS OF SCIENCE IN THE CENTURY. member of a family, a unit in a fauna, a thread in the web of life. And although we have taken our illustrations from biology, the same condition of progress applies to the other sciences. That man cannot be studied to much purpose, if he is persistently held in artificial isola- tion, is as certain as is the impossibility of under- standing the earth apart from the solar system. To sum up, three important factors in the progress of science are: a fuller recognition that science is for life and not life for science, a more practical ap- preciation of the benefits of co-operation between different disciplines, and a frank acknowledgment that analysis is a means not at end. But there is another important factor ; namely, the improvement of methods, — of devices by which we not only extend the range of our sense-experience but intensify our powers of precision. To give an ac- count of the development of methods would be to write half of the history of science, and we must refer for illustration to the separate chapters of this book. But how much progress is suggested when we recall the methods of quantitative analysis in chemistry, of measuring the different forms of energy in physics, of spectrum analysis in astronomy, of microscopic technique in biology, of experiment in psychology. Apart altogether from instrumental devices, the in- creasing use of mathematical and statistical methods in dealing with the problems of biology furnishes a good illustration of the fact that the rate of progress is partly dependent on the methods employed. JUSTIFICATION OF SCIENCE. If science be a natural and necessary expression of PROGRESSIVENESS OF SCIENCE. 63 the developing human spirit, this is justification enough. Yet a more detailed justification may be de- manded, not only by critics who object to the vast ex- penditure of time and money, labour and life, which the pursuit of knowledge involves, but also by those who at times lose confidence and enthusiasm, and are inclined to cry " Vanity " with the Preacher. Great conclusions are few and far between, practical dis- coveries bring curses as well as blessings, increase of knowledge often means increase of sorrow ; and there is the endlessness of it, like that of an asymptotic line always approaching nearer a given curve but never reaching it. " Advance brings us no nearer the end of our labour, for the more we know the more we see of what remains to be known. Every problem laid at rest gives birth to two new problems which did not present themselves to the mind before." * If we can suppose a science — Biology, for in- stance— arraigned before the bar of Humanity, as it should for its own sake feel itself arraigned, the lines of defence might be briefly summed up as fol- lows : f First, Biology is, like the other sciences, like art and poesy, a natural expression of human activity, at once a development and discipline of man. To cease to be scientific is to abdicate manhood. Along certain lines even the so-called savage is scientific. Second : and " without prejudice," Biology is jus- tified by practical results. In spite of many mistakes, it has made valuable contributions in relation to hu- man health, the supply of food and other necessaries, •Alex. Hill, An Introduction to Science, London, 1900, p. 41. t See my lecture. " The Humane Study of Natural His- tory," in Humane Science Lectures, London, 1897. 64 PROGRESS OF SCIENCE IN THE CENTURY. the use of animals, and so forth. We say " without prejudice," since we cannot, for a moment, allow that a science, as a science, should ever submit to the practical man's canon which makes immediate utility a stringent criterion of worthiness. Third, while the partial pursuit of certain paths may sometimes have dulled or even played false to healthy emotion, the general result of Biology is to deepen our wonder in the world, our love of beauty, our joy in living. The modern botanist is, or at least ought to be, more aware of the Dryad in the tree than the Greek poet could be. Fourth, Biology has partially worked out certain general conceptions of life and health, of growth and development, of order and progress, — centred in the idea of evolution, — which are not only attempts to see more clearly what is true, but which make for finer feeling and for the betterment of life. No doubt there have been impetuous attempts to apply immature biological results to the problems of hu- man conduct; no doubt the sociologist has some- times tried unwisely to force the biologist's hand; but one may still maintain with confidence that biology has justified itself in contributing to the ascent of man. In the introduction to his Grammar of Science* Prof. Karl Pearson has admirably expounded tho claims of science in general, and his summary may be quoted : " The claims of science to our support depend on: (a) The efficient mental training it provides for the citizen; (6) the light it brings to bear on many important social problems; (c) the * The author's statement was written some years before reading the work cited. PROGRESSIVENESS OF SCIENCE. 65 increased comfort it adds to practical life; (d) the judgment." Just as Huxley expressed himself at one with Descartes in declaring as his fundamental motive in scientific study " to learn how to distinguish truth from falsehood, in order to be clear about my actions, and to walk sure-footedly in this life," so, it should be noted, Pearson lays most stress upon the permanent gratification it yields to the aesthetic, the educational side of science : " Modern science, as training the mind to an exact and impartial analysis of facts, is an education specially fitted to promote sound citizenship. . . . This first claim of scientific training, its education in method, is to my mind the most powerful claim it has to state sup- port. I believe more will be achieved by placing instruction in pure science within the reach of all our citizens, than by any number of polytechnics de- voting themselves to technical education, which does not rise above the level of mutual instruction." SCIENCE AXD PRACTICAL UTILITY. Science and practice act and react upon one an- other. On the one hand, historical enquiry shows that a science may arise out of practical lore and that it may receive fresh stimulus in every fresh application to practical problems. In gathering herbs man gath- ered knowledge, and in cultivating his garden he laid the foundations of the science of botany; to their gathering and gardening most teachers of botany still return with pleasure and profit. The lore of the hunter and the fisher is older than all zoology, and many will agree that the vitality of the science depends upon a periodic return to the study of the 66 PROGRESS OF SCIENCE IN THE CENTURY. actual life of animals as it is lived in nature. It may be going too far to say with Espinas, — "La, pratique a partout devance la theorie/' but there is no doubt as to the progressive impulse which comes to a science from its corresponding art. On the other hand, an exaggeration of the impor- tance of contact with practical problems and of im- mediate practical results, is, we believe, disastrous to the welfare of science, and it may not be out of place to enter a brief protest. " The fundamental importance of abstruse re- search receives too little consideration in our time. The practical side of life is all absorbent ; the results of research are utilised promptly, and full recogni- tion is awarded to the one who utilises, while the in- vestigator is ignored. The student himself is liable to be regarded as a relic of mediaeval times. . . . The foundation of industrial advance was laid by workers in pure science, for the most part ignorant of utility and caring little about it. ... The investi- gator takes the first step, and makes the inventor possible. Thereafter the inventor's work aids the investigator in making new discoveries, to be utilised in their turn."* In his admirable Introduction to Science (1900) Dr. Alex. Hill says: "Great ad- vances have been made by investigators whose object was wholly technical. Yet, if the history of science were written, it would be found that the first step in advance, the germ of the discovery which became fruitful in the hands of the practical chemist, the mechanician, the pathologist, was discovered by the investigator, for whom science lost its interest as soon "John J. Stevenson, "The Debt of the World to Pure Science," Pres. Address, New York Acad., February, 1898, Science, March 11, 1898; Rep. Smithsonian Institute for 1897, pp. 325-336. PROGRESSIVENESS OF SCIENCE. 67 as it could be put to practical use." He instances the discoveries preceding the use of antiseptics and of Rontgen rays. Undue insistence on practical results is apt to be unjust, partly because no one is wise enough to pre- dict the outcome of a research, and partly because secure progress in science is often extremely slow. The twitching legs of Galvani's frog were studied as a theoretical curiosity ; who could have told that they pointed to the flicking needle of the telegraph? It was not for practical ends that William Smith plodded afoot over England, neither resting nor hur- rying in his exploration of the strata, but how much of the exploitation of Britain's mineral resources had its origin in his maps ? Or who can say that the series of discoveries which found the open sesame of coal-tar and brought forth its treasures had at first any practical outlook ? One use which a volume like this may have is to curb the impatience of the practical man in regard to experiments whose outcome he regards as useless, and to prompt him to a more generous support of scientific research. A little knowledge of the history of science may not be altogether a dangerous thing, if it suggests that from apparently inauspicious be- ginnings and from apparently unpromising items of honest work, great results may follow. Spectrum analysis— a method of very great importance to astronomer and physicist, chemist and physiologist — had its beginning in some apparently insignificant observations by Marcgraf, Herschel, and others. Pasteur's at first sight extremely theoretical re- searches on the hemihedral facets of tartrate crystals were logically as well as actually connected with his practical researches on fermentation. 68 PROGRESS OF SCIENCE IN THE CENTURY. Over and over again in the course of the history of science we find illustrations of the long gestation of scientific truth. Minerva-like birth is rare. " Dis- coveries which proved all important in secondary re- sults do not burst forth full grown; they are, so to say, the crown of a structure raised painfully and noiselessly by men indifferent to this world's affairs, caring little for fame and even less for wealth. Facts are gathered, principles are discovered, each falling into its own place, until at last the brilliant crown shines out, and the world thinks it sees a miracle." * But it was after waiting and working for almost a score of years that Darwin published his theory of natural selection. Another good illustration of the gradual emergence of an important conclusion is to be found in the history of the kinetic theory of gases. We usually, and rightly, associate this conception with the names of Joule and Clausius, and fix the date about 1857, but " the researches of Paul du Bois-Reymond and others have unearthed a whole list of authors who, in more or less definite ways, had resorted to the hypo- thesis of a rectilinear translatory motion of the molecules in order to explain the phenomena of pres- sure and other properties of gases. Among these Daniel Bernouilli (in his Hydrodynamica, 1738), seems to have expressed the clearest views, and he is usually now named as the " father of the hypoth- esis." f While then we hold firmly that science is for life and not life for science, we protest against a narrow rendering of the words " for life." The practical man's impatient "What's the use of it ? " often reveals * J. J. Stevenson, Rep. Smithsonian Inst., for 1897, p. 325. t J. T. Merz, History, 1896, p. 433. PROGRESSIVENESS OF SCIENCE. 69 a vulgar materialism. " Truer relations of science to industry are implied in Greek mythology. Vul- can, the god of industry, wooed science, in the form of Minerva, with a passionate love, but the chaste goddess never married, although she conferred upon mankind nearly as many arts as Prometheus, who, like other inventors, saw civilisation progressing by their use while he lay groaning in want on Mount Caucasus." * * Sir Lyon Playfair, Pres. Address, Rep. Brit. Ass., 1885, p. 17. BOOK TWO. MATTER AND ENERGY. CHAPTER IV. A CENTUEY OF CHEMISTRY. SEARCH FOR THE ELEMENTS. Different Kinds of Things. — An inquisitive out- look on the world at once gives us the impression of an enormous number of different kinds of things — different in substance or composition as well as in form and activity — and we feel the need of arranging these in some order. If we continue our inquisitive outlook we soon per- ceive that no small part of the apparent variety of the things we see around us is due to the fact that different stuffs or kinds of matter occur mixed up together. If we take a handful of coarse sand from the shore, we can, by working for a few hours, put it into some order, placing fragments of lime shells in one corner and pieces of quartz in another, and so on. But this sorting out is easy work, and can be done by a machine ; it is not the chemist's problem, — he deals with, the changes in the nature of substances which are not mixtures. Among these not-mixtures it is necessary to distinguish (1) a certain number of definite kinds of matter which cannot be separated by any known means into unlike parts, such as iron and A CENTURY OF CHEMISTRY. 71 carbon; and (2) others which, by heating or other- wise, can be broken up (not sorted out) into unlike parts, such as sugar and salt. In other words, after sorting out the heterogeneous mixtures the chemist has to do with the two sets of homogeneous stuffs to which we have just referred — which are familiarly known as Elements and Compounds. Though many of the elementary substances, such as copper, gold, iron, lead, silver, tin, zinc, sulphur, have been known from remote antiquity, the recogni- tion of elements as such — i.e., as substances which cannot, so far as we know at the time, be resolved into other kinds of matter — practically dates from Robert Boyle, the author of The Sceptical Chymist (1680). A hundred years later, Lavoisier, who first made the conception of elements practically useful in scientific research, enumerated thirty-three (includ- ing light and heat), but the list increased by leaps and bounds during the nineteenth century. Thus Sir Humphry Davy discovered six new metals between 1808 and 1810, and the Swedish chemist Berzelius added an equal number in about the same time. As was to be expected, the practical interests of miner- alogy and metallurgy, especially in Sweden and Germany, gave zest to the search after elements, and led Scheele and others to many discoveries. By 1830, Lavoisier's list was nearly doubled, and it is still being added to. Interactions of Elements. — Another impression that we get from our outlook is that things are changeful. We see stones weathering and crum- bling, shells being dissolved away, iron rusting, coal burning, and thousands of other changes, which ex- cite curiosity and offer problems to be solved. 72 PROGRESS OF SCIENCE IN THE CENTURY. A moment's reflection will show that two some- what different sets of changes go on around us. In the frosty night water changes into ice; the sun rises, and the ice changes into water; in the bright sunshine the water may even pass into the air as vapour. Here we have one of the most familiar instances of a change of state, but the water remains in a real sense water all the time. There is no change in the nature of the stuff, and it is with changes in the nature of the stuff that chemistry has primarily to do, with the change, for instance, which occurs when, by an electric current, water is decomposed into its two constituents, hydrogen and oxygen. The chemist has as his fundamental prob- lem, not merely the recognition and isolation of elements, but their affinity in relation to one an- other, their capacity of exerting chemical action or inducing chemical change. Detection of an Element. — The question natur- ally rises in the mind, how does the chemist know when a given substance is an element or not ; and the only scientific answer is that all substances should be assumed to be compounds until all known methods of decomposing them have been tried without suc- cess. " If the products we obtain always weigh more than the substance itself and never less, no matter to what changes it has been subjected, then, provided each change is complete and accompanied by no loss of substance through our imperfect methods, we are constrained to regard that substance as an ele- ment," * Thus the chemical conception of an element is simply that of an undecomposed — not necessarily * Ostwald, Outlines of General Chemistry, trans. J. Walker. 1890, Chap. II., " The Elements," p. 9. A CENTURY OF CHEMISTRY. 73 undecomposable substance — since we must always bear in mind that an increased perfection of method may result in the decomposition of what was pre- viously regarded as elementary. Recent Discoveries of New Elements. — During the last quarter of a century the number of known elements has been very rapidly increased. In a gen- eral way, it may be said that analysis has become more penetrating, but there are several particular reasons for the increase. (1) It was by the electro- lytic decomposition of alkaline earths that Davy dis- covered potassium and sodium; this was about the beginning of the century, and the discoverer had at his command only a feeble Voltaic pile; now in- tensely powerful currents are utilised, and it was by these that Moissan, for instance, was able to isolate fluorine from its combinations. (2) Spectrum analysis has shown the existence of a series of ele- ments with characteristic spectra, and it is a remark- able fact that one of these, helium, was known from the sun before it was discovered in the earth. (3) Certain theoretical conceptions, such as Mendelejeffs periodic law, have led chemists to look out for and to find elements whose existence was predicted on a priori grounds. Thus Xilson in 1879 discovered scandium which Mendelejeff had foretold. Gallium, discovered by Lecoq de Boisbaudran in 1875, and germanium, discovered by Winkler in 1886, are other famous examples. Argon. — Two of the latest additions to the list of elements deserve special notice. In 1892, Lord Eayleigh directed attention to the fact that nitrogen obtained chemically was about one-half per cent, lighter than that got from the air, and it was this minute discrepancy which led him to look for and 74 PROGRESS OF SCIENCE IN THE CENTURY. discover a heavier gas in the atmosphere. In tho meantime, and independently, Prof. W. Ramsay dis- covered the same gas by removing the nitrogen by means of red-hot magnesium. Combining their re- sults, the two investigators published their memoir on Argon, " which will go down to posterity among the greatest achievements of an age renowned for its scientific activity " (Meldola). Argon is an extraordinarily inactive or chemically indifferent gas of great density; occurring along with atmospheric nitrogen, forming about 8 or 9 per cent, of the volume. It can be separated by incandescent magnesium or by the continued action of the electric spark, and in the latter way Cavendish seems actually to have produced it a hundred years ago ! Alone or along with helium it has been found in natural waters, in minerals, and in a meteorite. It is not known to form combinations, and it does not fit in well with the periodic system, so that its real nature remains the subject of enquiry. That it is truly an element is suggested by the distinctness of its electric spark spectrum and by the discovery that the molecule is monatomic, but the possibility re- mains that it is a mixture of monatomic gases. Helium. — The facts in regard to the discovery of helium are not less interesting. In 1868 Frankland and Lockyer had observed a particular line D in the solar spectrum which they attributed to the presence of an element — helium — then unknown upon the earth. It was also recognised in the spectrum of Orion and other fixed stars. Subsequently the line of helium was seen by Palmieri (1882) in the lava of Vesuvius, and Hildebrand observed in 1891 what were probably its lines in a spectrum of the nitrogen gas which he got by heating or otherwise treating A CENTURY OF CHEMISTRY. 75 uranium ore. While demonstrating argon in the nitrogen gas obtained from Cleveite, Prof. Kamsay observed in 1895 another bright yellow line, and this Sir William Crookes recognised as the D line of helium. Helium has now been found in many ores, in mineral waters, and in very minute quantities in the air. It is the lightest of all the gases except hydrogen, and Dr. Johnstone Stoney has suggested that this may explain the paucity of these two ele- ments in a free state upon the earth while they are abundant in the universe. As Winkler puts it, " the comparatively small force of the earth's gravitation does not form a sufficient counterpoise to the velocity of their molecules, which therefore escape from the terrestrial atmosphere unless restrained by chemical combination. They then proceed to reunite around great centres of attraction, such as fixed stars, in whose atmospheres these elements exist in large quantities." * Helium, like argon, is believed to be monatomic, and it is not known to enter into chemical combina- tion. There remains much uncertainty in regard to its position, some maintaining, for instance, that it is composed of two gases. SUMMARY. — It is the business of chemistry to distinguish the different kinds of matter, and to study their transformations. Heterogeneous mix- tures have to be distinguished from homogeneous com- pounds and elements. A homogeneous substance which cannot be decomposed by known means is called an element. Careful searching and more ac- * Trans, of a paper in Rep. Smithsonian Inst. for 1897, p. 244. 76 PROGRESS OF SCIENCE IN THE CENTURY. curate methods have resulted in an enormous increase in the list of elements in the course of the nineteenth century. Special interest is attached to the recent discovery of argon and helium. THEOBY OF COMBUSTION AND THE CONSERVATION OF MATTER. Theory of Combustion. — Since the science of chemistry has to do with the changes in the nature of substances when they combine or separate, and since burning is one of the most obvious of these changes, it is natural that we should give prominence to the theory of combustion. But there is another reason why we should do so here, namely, that some under- standing of combustion marks the beginning of the century-period with which our brief historical sketch deals. It is hardly too much to say that modern chemistry dates from the time when the burning fire began to be in some measure intelligible, or, what conies almost to the same thing, from the time when, oxygen and carbonic acid gas having been discovered, it became possible to measure the changes which take place in a combustion. It is interesting, as we sit by the fireside, to think of the different ways in which the familiar sight has been regarded by successive generations of men, from the time when the four elements were first vaguely imagined to the days of " phlogiston " and " principles of combustion," and thence to tho present day, — a long story of changing ideas. But it is sufficient for our purpose here to recall, that it was not until about a century ago that there was anything approaching to a scientific vision of the burning fire. A CENTURY OF CHEMISTRY. 77 The Greeks and Romans who accepted the four elements of Empedocles — fire, water, earth, and air — regarded fire as a material substance, and combus- tion as the separation or liberation of the fire-stuff from other material. In the seventeenth century, Becher and Stahl regarded combustion as the separa- tion of " inflammable earth," or the escape of "phlogiston," a compound substance; for "only compound substances can burn." For a long time this Phlogiston theory was generally accepted, and proved a useful stimulus to research. But the re- peated demonstration of increase of weight on com- bustion, the evidence that part of the air is absorbed during the burning, Newton's suggestion that fire was not a special substance at all, and, especially, the discovery of oxygen, hydrogen, carbon-dioxide, and other gases, seriously affected the vitality of the theory, and finally shattered its constitution. It be- came the subject of most ingenious doctoring, and died a lingering death in the end of the eighteenth century. What John Mayow, with penetrating insight, had almost discerned more than a century before, that burning means a union of something in the air with inflammable particles in the stuff that burns, became clearer when Priestley discovered oxygen in 1771, when Lavoisier interpreted combustion as oxidation in 1775, and when Cavendish showed that water was a combination of hydrogen and oxygen in 1784. It is interesting to notice that although Priestley had discovered oxygen and supposed that air sup- ports combustion in virtue of the oxygen which it contains, he died a believer in phlogiston; and that although Scheele — " the ideal of a pure experimental chemist, the discoverer of numberless substances, who 78 PROGRESS OF SCIENCE IN THE CENTURY. possessed in the highest degree the faculty of obser- vation " — had also discovered oxygen, he was unable to free himself from the bondage of phlogistic theory. The same was true of many others, and it is to Lavoisier (1743-1794) that we must give the credit of destroying the old theory by replacing it with a better. Here we have one of the many instances which lead us to say with confidence that to destroy effectively one must replace. It is true that Lavoisier stood on the shoulders of other workers, but his own experiments were not less in- genious, and, more than any of his predecessors or contemporaries, he reached the importance of precise quantitative measurement. Thus he was led to state about 1777 the fundamental conclusion that in the process of combustion, the burning substance unites with oxygen, whereby an acid is usually produced; and that the increase in weight of the substance burned is equal to the loss in weight of the air. His researches also led him to the general proposition that in all chemical reactions it is only the kind of matter that is changed, the quantity remaining constant ; and to the brilliant idea that " heat is the energy which results from the imperceptible move- ments of the molecules of a substance." The Conservation of Matter. — One of the foun- dation-stones of chemistry — which every worker builds upon with unquestioning confidence — is the conservation of matter. We can neither create nor destroy the smallest particle; the elements which enter into the composition of the soap-bubble film are as lasting as those which form the granite rocks. The state of the matter may wholly change — from solid to gaseous, or in other ways, the particular com- binations of the elements may wholly change as they A CENTURY OF CHEMISTRY. 79 do when the barrel of gunpowder explodes, but the total amount of matter is the same in the end as it was in the beginning. The doctrine of the Conservation of Matter states, as Ostwald puts it, that "the total mass of the sub- stances taking part in any chemical process remains constant." And since masses of bodies are at any one place proportional to their weights, the doctrine may read that in any chemical process the weight remains constant. If we change the contents of a sealed vessel by heating, or by mixtures brought about through shaking, or otherwise, we find that the weight at the end equals the weight at the begin- ning.* Although the recognition of the conservation of matter was brought about by the work of many, it may be particularly associated with Lavoisier. For one of his earliest investigations, on the sup- posed conversion of water into earth, he constructed what was at the time the most accurate balance in existence, and he reaped the usual reward of the accurate measurer. When he passed water vapour over red-hot iron turnings and collected the resulting hydrogen, he weighed everything — the water, the iron before and after, and the hydrogen. It was by such typical experiments that " with the balance in his hand, he vindicated the universality of the prin- ciple of the conservation of matter." f The establishment of the general fact of the con- servation of matter was of much more than theoreti- cal interest; it was not only a foundation-stone, but a * W. Ostwald, Outlines of General Chemistry, trans, by James Walker, 1890, Chap. I. t A. Ladenburg, History of Chemistry, trans, by L. Dob- bin, 1900. p. 21. 80 PROGRESS OF SCIENCE IN THE CENTURY. touch-stone for chemistry; it supplied a quantitative test by which the accuracy of research could be con- tinually judged. THE ATOMIC THEORY. Before Dalton. — The great chemist Berzelius, following his predecessor Richter, quotes on the first page of his classic treatise on Chemical Proportions the verse from the Book of Wisdom which says : — Omnia in mensurd et numero et pondere disposuisti. Thou hasfc ordered all things in measure and number and weight. — Sap. XI. 21. This may be regarded by some as expressing a re- markable prevision of one of the great results of chemical science, — that exact quantitative relations are always implied in qualitative changes of sub- stance. But whether it was a prevision or not, the verse quoted found no scientific commentary till towards the end of the eighteenth century, and the commentary then begun is still in progress. The invention of accurate balances — like Lavoi- sier's— made it possible to pass beyond the detection of chemical elements to some understanding of ma- terial architecture. And there seem to have been many who were simultaneously pondering over the problem. Thus Jeremias Benjamin Richter, a math- ematical chemist born before his time, published in 1792-1794 a treatise on Stoicheiometry, or " the art of measuring chemical elements," in which he showed that acids and bases combine in definite quantita- tive proportions to form neutral salts. About the same date Proust drew the familiar distinction be- tween chemical mixtures and chemical compounds, pointing out that the latter are characterised by quite definite proportions, whether formed artificially in A CENTURY OF CHEMISTRY. 81 the laboratory or found in nature. In 1802 Fischer made the first table of " chemical equivalents," show- ing what quantities of the different alkaline bases are neutralised by the same quantity of an acid, and con- versely for the acids. But while it is important even in a short historical sketch to observe that scientific discoverers have very rarely a Minerva birth, we must not obscure the fact that though Richter, Proust, and others were work- ing towards a big conclusion, it is to John Dalton that we are indebted for the clear statement of the fundamental fact regarding chemical combina- tion:— that substances, both simple and compound, always combine in definite proportions of their weights. In whatever way one substance is trans- formed into another, the masses of the two substances always bear a fixed ratio. Even if several substances react together, their masses and those of the new bodies are always in fixed proportions. These facts almost necessarily lead to the atomic conception. Dalton. — The doctrine of the Quaker chemist de- pended partly on the following results of experi- ence : — " Ko new creation or destruction of matter is with- in the reach of chemical agency. We might as well attempt to introduce a new planet into the solar sys- tem, or to annihilate one already in existence, as to create or destroy a particle of hydrogen " (Dalton, after Lavoisier). In a chemical compound the different constituents are always present in invariable proportions (Dal- ton, after Proust). In the interactions of acids and bases, etc., the quantity by weight of an element, or of a compound which takes active part in the chemical change is al- 82 PROGRESS OF SCIENCE IN THE CENTURY. ways expressible by a fixed number or by a whole multiple of that number. When elements unite with one another in several different proportions — e.g., oxygen and nitrogen — these proportions are related to one another in a simple way. In other words, " If two substances, A and B, form several compounds, of which the compositions are all calculated with re- spect to the same quantity of A, then the quantities of B combined with this stand to each other in a simple ratio " * (Law of constant equivalents and multiple proportions). " Thou knowest no man can split an atom " was one of Dalton's sayings, but it should be noted that he meant by an atom the smallest conceivable particle which exhibits the essential properties of the sub- stance in question. Thus he spoke of an atom of water (a compound, H2 O), just as he spoke of an atom of carbon. With a vision of the grained structure of matter clearly before him, he supposed in his theory that while every atom of a given simple substance is like every other atom of that substance, the atoms of dif- ferent substances have different weights; that in chemical union of elements there is a grouping of definite numbers of elemental atoms into more com- plex atoms of compounds, and contrariwise in chemi- cal decompositions ; and that the elements combine in the proportions indicated by the relative weights of their atoms or in multiples of these. This is the atomic theory " which at once changed chemistry from a qualitative to a quantitative science " (Ros- coe). An examination of some of Dalton's manuscripts has led Koscoe and Harden to the conclusion that * Ladenburg, p. 55. A CENTURY OF CHEMISTRY. 83 he was led to adopt the atomic theory in chemis- try in the first instance by purely physical considera- tions, in opposition to the view generally held that the discovery of combination in multiple proportions led him to invent the atomic theory as an interpreta- tive formula. It seems that Dalton, who was not well aware of contemporary continental work, was led to his great doctrine, not by making an induction from his laborious experiments and measurements, but by a deduction from a theory of the constitution of matter which he devised to account for some of the physical properties of gases. As in many other in- stances in the development of natural knowledge an important conclusion was reached deductively and then verified inductively. The way in which Dalton reached his conclusion explains why he gave it the extremely generalised form to which we refer when we speak of the atomic theory. While he was thinking about the definite and fixed quantitative proportions observed in chem- ical combinations, he was also experimenting with gases (about 1790), and he had visualised these as consisting of distinct particles : — " A vessel full of any pure elastic fluid [that is, gas] presents to the imagination a picture like one full of small shot." The idea that bodies are formed of distinct parti- cles was not of course Dalton's, but the chemical ap- plication was. The idea had been suggested in New- ton's Queries, and had been used by Boyle, Boer- have, Higgins, and others; it was indeed one of the legacies with which ancient philosophy endowed modern science. Atomic Weights. — But Dalton was not content to leave the atomic conception in this vague form, he proceeded, in a manner epoch-making though imper- 84: PROGRESS OF SCIENCE IN THE CENTURY. feet, to determine the relative weights of his hypo- thetical ultimate particles, and drew up what would now be called a table of atomic weights. To do this he required a unit of comparison, and he chose hydrogen, the lightest kind of matter known. The weight of an atom of hydrogen was called one. Then, as 8 parts by weight of oxygen combine with 1 part by weight of hydrogen to form water (combining weights), Dalton argued that the atom of oxygen weighed 8 times more than that of hydrogen. And so on for other elements. It must be borne in mind that the atomic weights were determined with reference to an arbitrary standard, and that they had at first only approximate accuracy. Summary. — Through the aid of many, but notably through the pioneering genius of Dalton, the atomic theory has won a place among the conceptual for- mulae of chemistry. It cannot be said to be proved ; indeed, neither "proved" nor "disproved" is an ap- propriate word to use in regard to these hypotheses. The tests are convenience, comprehensiveness, and consistency (at once with facts and with other con- ceptions), and the atomic theory has stood these tests. Forestalling the history a little, we may sum up the general idea in Ostwald's words : " All substances consist of discrete particles of finite but very small size — of atoms. Undecom- posable substances or elements contain atoms of the same nature, form, and mass. If chemical combina- tion takes place between several elements, the atoms of these so arrange themselves that a definite and usually small number of atoms of the combining elements form a compound atom which we call a molecule. Every molecule of a definite chemical A CENTURY OF CHEMISTRY. 85 compound (chemical species) contains the same num- ber of elementary atoms arranged in the same way. If the same elements can unite to form different compounds, the elementary atoms composing the molecules of the latter are either present in differ- ent numbers, or if their number be the same, they are differently arranged" * DEVELOPMENT OF THE ATOMIC THEOEY. Dalton's atomic theory, though not final, was fructifying. It prompted a long series of researches which led, after some vicissitudes, to the establish- ment of the atomic view of nature on a firmer and broader basis. Among the steps of importance, we may especially notice (1) the more accurate deter- mination of atomic weights, (2) the conception of molecules, (3) the kinetic theory of gases, and other physical theories as to the different states of matter, and (4) the development of organic chemistry. The general problem was to form conceptions of material architecture which would harmonise with the facts of chemical change. Determination of Atomic Weights. — It is well known that each element is conventionally de- noted by the first letter or letters of its Latin name, and that with each element a certain number is associated; e.g., 16 with oxygen, 14 with nitrogen, 12 with carbon. This number, or some multiple of it by a whole number, expresses the relative quantity of the given element which enters into compounds. It is the combining mass (or weight, though weight must vary with place), or on Dalton's theory, the atomic mass or weight, *W. Ostwald, General Chemistry, trans. 1890. 86 PROGRESS OF SCIENCE IN THE CENTURY. It has also been noticed that in estimating these numbers, hydrogen is taken as a unit, because it enters into compounds in relatively the smallest weight. The other elements and compounds are tabulated according to the relative amounts of their weights in forming compounds with hydrogen, or with some other element whose equivalent with hy- drogen has been already estimated. When one and the same substance combines in several proportions with another, as nitrogen, for instance, does with oxygen, the smallest number according to which the substance forms combinations is taken, the other numbers relating to the same substance being found to be exact multiples of the smaller. So far the Dal- tonian rules. What Dalton began was continued by Berzelius, Turner, and others; but we cannot enter into the record of toil. Only two or three points of interest can be indicated. The process of determining the atomic weight of an element involves: (1) finding the combining proportion or equivalent, and (2) multiplying this by a factor (1 — 4) decided by the measurement of the vapour density (Avogadro's Law), or by finding the specific heat whose product by the atomic weight is practically constant (Law of Dulong and Petit), or by some other consideration. Berzelius in his determinations utilised Gay-Lus- sac's law of volumes (1808) (that two gases always combine in simple proportions by volume), the law of Dulong and Petit (1819), and furthermore the aid furnished by Mitscherlich's discovery of isomor- phism (1820). " Mitscherlich established the fact that the corresponding phosphates and arseniates, with the same number of atoms of water, possess the same crystalline form, so that even the secondary A CENTURY OF CHEMISTRY. 87 forms coincide. Even at that time, the same number of atoms was assumed to be present in both acids, and thus Mitscherlich arrived at the idea that it was similarity of atomic constitution which gave rise to identity of form." * This discovery was utilised by Berzelius in the following rule : — " When one substance is isomor- phous with another in which the number of atoms is known, then the number of atoms in both is known, because isomorphism is a mechanical consequence of similarity of atomic construction." " The chemical edifice which Berzelius erected was a wonderful one, as it stood completed (for in- organic substances) at the end of the third decade of the century. Even if it cannot be said that the fun- damental ideas of the system proceed exclusively from himself, and if he is indebted to Lavoisier, Dalton, Davy, and Gay-Lussac for a great deal, still it was he who moulded these ideas and theories into a connected whole, adding also much that was origi- nal. His electro-chemical hypothesis no doubt had points of similarity with that of Davy, but, in spite of that, it was essentially different from it. Besides, the first method of atomic weight determination, of moderately general applicability, proceeded from Berzelius; and this method was so extraordinarily serviceable that it rendered possible the fixing of these most important numbers, so that alteration was nec- essary in only a few cases. " f It is important to notice, however, that about 1840 an error of about 2 per cent, was discovered in the estimate which Berzelius had made of the atomic weight of carbon. This raised suspicions and further * L,adenburg, 1900, p. 96. fLadenburg, 1900, pp. 101-102. Q 88 PROGRESS OF SCIENCE IN THE CENTURY, inaccuracies were discovered. A revision became imperative, in which Liebig, Dumas, Stas, and others took part. Different methods of determination were discovered, one method was used to check another, stimulus in the arduous task came at different periods from the vision of supposed or real regularities con- necting the different numbers (Prout and Meinecke to Mendelejeff and Meyer), and gradually a well- established, well-criticised system of atomic weights was worked out. To Cannizzaro (1858) in particu- lar credit is due for utilising the specific heat method as a check on the others, and Mendelejeff's periodic law furnished, as will be seen, another valuable cor- rective. It is a remarkable historical fact, however, that owing to the relative unreliability of the methods for determining the atomic weights, the conception of the chemical atom fell for a time into general disrepute. " At the end of the fourth decade of the century, we find the atomic theory — the most bril- liant theoretical achievement of chemistry — aban- doned and discredited by the majority of chemists as a generalisation of too hypothetical a character." It was reserved for organic chemistry to re-vindicate it, and for physical researches, especially on gases, to place it on a yet firmer basis. Physical Enquiries and the Concept of the Mole- cule.— It is now necessary to allude to a path of physical investigation which had a most important influence on the atomic theory, especially through Avogadro's Law and the kinetic theory of gases. In 1662, Boyle had stated, as Mariotte did some years afterwards (1679), that the volume of a gas, at the same temperature, is inversely as the pressure. When the pressure increases, the volume diminishes A CENTURY OF CHEMISTRY. 89 in inverse ratio. In 1802, Gay-Lussac, whose work touched almost every department of chemistry with important results, stated what had been foreseen (as he says) by Charles fifteen years earlier, that equal volumes of different gases change their volumes equally with equal rise of temperature. Dalton also had perceived this conclusion (the law of Charles) that all gases expand in the same proportion for the same increase of temperature. It should be noted that both these laws (Boyle's and Charles') are ideal formulae which only approximately fit the facts. In 1805, along with Alexander von Humboldt, Gay-Lussac observed that exactly two volumes of hydrogen unite with one volume of oxygen to form water. From this starting-point he went on to show (1808) that similarly simple volumetric relations hold true in regard to all gases which combine chemically with one another, and that the volumes of the gaseous products formed always have a simple relation to the volumes of their components (all be- ing measured, of course, at the same pressure and temperature). "Having concluded from their simi- lar behaviour with regard to changes of pressure and temperature that all gases possess a like molec- ular constitution, Gay-Lussac deduced from his re- searches (above referred to) the following impor- tant law: — The weights of equal volumes of both simple and compound gases, and therefore their den- sities, are proportional to their empirically found combining weight, or to rational multiples of the lat- ter." In other words, if gases, like other bodies, combine according to definite proportions of their weights (Dalton's law) ; and if gases (under the same pressure and at equal temperatures) combine * E. YOU Meyer, History of Chemistry, trans. 1891, p. 202. 90 PROGRESS OF SCIENCE IN THE CENTURY. in definite proportions of their volumes (Gay-Lus- sac's law) ; then, since density of a gas means the amount of matter measured by weight in the same volume, it follows that the combining weights of gases bear a simple numerical proportion to their densities. Avogadro's Law. — Another important and closely related result was expressed in 1811 by the Italian chemist, Amadeo Avogadro (1776-1856). He was impressed by the fact that, when there is chemical interaction between gases, there is observable a very simple relation between the volumes concerned. A pint of oxygen combines with two pints of hydrogen to form two pints of steam. Such a simple fact, com- bined with others relating to the physical properties of gases, led him to suggest that a given volume of any gas (elementary or compound) contains the same number of molecules as the same volume of any other gas measured at the same temperature -and pres- sure. Equal volumes of gases, equal numbers of molecules is Avogadro's law, — another foundation- stone of modern chemistry. It should be noted that similar views were stated by Ampere in 1814, but neither he nor Avogadro found contemporary recogni- tion or even attention. Avogadro distinguished between molecules inte- grantes and molecules elementaires, or, as would now be said, between molecule and atom. " The physi- cal properties of the gases (especially the similarity in their behaviour towards changes of pressure and of temperature) led Avogadro to assume in equal volumes of all gases the same number of molecules; and the distances of the latter from one another he considers to be so great in proportion to their masses, that they no longer exercise any attraction upon one another. These molecules are not sup- A CENTURY OF CHEMISTRY. 91 posed, however, to constitute the ultimate particles of matter, but are assumed to be capable of further subdivision under the influence of chemical forces. According to Avogadro, therefore, substances (ele- ments and compounds alike) are not converted, in passing into the gaseous state, into indivisible par- ticles, but only into molecules integrantes, -which in turn are composed of molecules elementaires" * The conception of a molecule is that of the smallest portion of a substance which possesses all the prop- erties of that substance; it represents a higher cate- gory than atom ; thus the molecule of water is repre- sented by the symbol H2O, which means, in part, that the smallest particle of water consists of two atoms of hydrogen united with one atom of oxygen. Avogadro's generalisation has furnished one of the main grounds for determining the atomic weights of the elements; and it went far to reconcile Gay- Lussac's discoveries as to gases with Dalton's atomic theory. "We have only space to mention that another ground for the determination of atomic weights was furnished by the researches of Dulong and Petit (1818), who showed the close relation between the specific heats of the elements and their atomic weights, and concluded that the atomic heats of all elements (specific heats multiplied by atomic weights) are practically identical ; i.e., that all atoms have the same capacity for heat. Avogadro's recognition of the proportion between the specific gravity of a gas and its molecular weight was slowly appreciated,! but it has borne much fruit. * Ladenburg, 1900, pp. 61-62. t Dr. J. T. Merz notes in regard to this belated recogni- tion that Avogadro's hypothesis (1811) is not mentioned in \Vhewell's History, nor in Kopp's (1843-1847), nor in Pog- gendorfs Dictionary (1863). 92 PROGRESS OF SCIENCE IN THE CENTURY. By improved methods of determining the specific gravity of gases and vapours, " the all-important knowledge of the relative weights of the atoms and molecules of elements and compounds has been im- mensely advanced" (E. von Meyer, p. 441). From the study of anomalous vapour-densities, H. de St. Claire Deville discovered in 1857 the fact of " dis- association " or the gradual decomposition of a com- pound with rise of temperature, — the starting-point for another series of important investigations. Though confirmed by similar conclusions (Davy, 1812, Ampere, 1814), Avogadro's hypothesis: " Equal volumes, equal number of particles " was not appreciated until the establishment of the kinetic theory of gases (q.v.), and " no substantial chemical reasons for its adoption were adduced until the year 1846, when Laurent published his work on the law of even numbers of atoms and the nature of the ele- ments in the free state." * Further Influence of Physical Researches. — When the century was about half over, the doctrine of fixed and multiple proportions was generally ac- cepted (with some saving clauses for not-solid com- pounds), but the conception of atoms which lay be- hind this doctrine was looked at more cautiously. The careless may have believed in the physical exist- ence of these smallest indivisible particles, but this was certainly not the general belief. And even as a symbolism, as an alphabet, as a means of notation, there were many chemists who doubted if the atom- concept was indispensable or even legitimate. Cor- roboration had to come from an independent source, and it came from the physicists, more especially * Prof.. R. Meldola, Address, Section B, Rep. Brit. Ass. for 1895, p. 639. A CENTURY OF CHEMISTRY. 93 from the kinetic theory of gases, taken in connection with Avogadro's law. Kinetic Theory of Gases. — As facts began to ac- cumulate showing a remarkable uniformity in the behaviour of different gases to the same changes of temperature and pressure, the need for some concep- tion of the nature of a gas made itself felt in many minds. The early suggestions of Daniel Bernouilli (1738) and of Waterston, Graham's discovery of the law of diffusion, the work of Herapath, Joule and Kronig, the achievements of Clausius (1857-1862) and Clerk Maxwell (1860-1867), are some of the steps in a long history — the history of the kinetic theory of gases, one of the revolutionising concepts of modern science. According to this theory, a gas consists of innumerable particles moving with high velocity, overflowing into any free space which is available, thus securing that there is the same aver- age number in every unit of volume, impinging on the contained walls, if there are any, and thus caus- ing pressure which must obviously increase with the number of the molecules and the mass and velocity of each. Such is at least a suggestion of the view which gave new life to the atomic theory, and that at a time when it was much in want of support. When it was shown that precise and workable conceptions could be formed of the rectilinear movements of molecules in a gas, when the internal motion of the atoms composing the molecules was shown to be a needful assumption, when the rate of velocity of a particle of hydrogen gas was actually calculated, when the laws of Boyle, Gay-Lussac, and Avogadro were brought into harmony, and so on, — chemistry became, in a more real sense than before, a study of the changes of equilibrium in atoms. 94 PROGRESS OF SCIENCE IN THE CENTURY. Extension of the Atomic Conception. — Here it must be recalled that while physical enquiries into the constitution of matter [or attempts to form a conception of molecular motion] were mainly con- cerned with gases, the solid and liquid states were also studied. The solid state, where the mass has a proper volume and a proper form, more or less dif- ficult to change, began gradually to be conceived of as one in which the relations of the molecules are such that mutual displacement is not easy. En- quiries into crystallisation begun by Steno (1669), re-stimulated by the genius of Hauy (1781), con- tinued by many workers (Weiss, Von Lang, etc.), also proved suggestive, notably, for instance, when Mitscherlich (1820) elaborated what Klaproth (1Y98) had observed that the same substance might have different crystalline forms (e.g., calc spar and arragonite). Gradually, too, the atomic conception was extended to liquids which differ from gases in occupying a definite volume and from solids in having no proper form and much less internal friction. Especially through enquiries into the phenomena of osmosis and of solution, the theoretical conception of gases was applied to liquids. But this was hardly realised till towards the end of the century; indeed it may be associated with the work of Van't Hoff (1887). Instead of trying to follow the multitudinous lines of research, we propose to take a single illustration — the liquefaction of gases — which may serve to sug- gest the unity of the different states of matter. Liquefaction of Gases. — Erom the time of Fara- day's researches in 1823 to the recent work of Dewar, popular imagination has been impressed by the re- peated announcement, that such and such a gas had A CENTURY OF CHEMISTRY. 95 yielded to the combined effects of high pressure and low temperature, and had been obtained in liquid or solid form. Andrews, Mendelejeff, Pictet, Caille- tet, Wroblewski, Olszewski, and many others have contributed to the striking series of experiments. By a long series of researches, extending through the century, it has been made clear that all ponder- able matter may be thought of as essentially of the same nature, irrespective of what its state — solid, liquid, vaporous, or gaseous — may be. The differ- ences of state are conceived of as due to the way in which the relations of the component particles are affected by the greater or less relative activity of the attractive molecular forces and the dispersive ther- mal motions. As every one knows, water may occur as a solid, a liquid, a vapour, or a gas (saturated steam above 720.6° C.). " Above 30.92° 0. carbonic acid is a true gas; no pressure will then liquefy it; but at 30.92° C. a pressure of 77 atmospheres, and below 30.92° C. progressively smaller pressure will condense it; at and below that temperature (An- drews' Critical Temperature) gaseous carbonic acid is a ' vapour,' condensable by pressure alone." It may also be procured as a solid. Endless examples might be given, for the idea of necessary permanence of state has now disappeared, — and .theoretically no case is more striking than another, though technical difficulties have enhanced the interest of some par- ticular instances. It was about the beginning of the century that !N"orthmore and others liquefied sulphurous acid gas by pressure, but progressive research on the subject began with the work of Faraday and Davy in 1823. They used the method of " enclosing materials from * Article " Gas," by Daniell, Chambers's Encyclopaedia. 96 PROGRESS OF SCIENCE IN THE CENTURY. which the gas can be generated within a tube strong enough to resist the pressure of the gas as it accumu- lated," and thus chlorine, muriatic acid, carbonic acid, ammonia and many others were liquefied, es- pecially through the energetic work of Faraday.* In 1835, Thilorier published an account of an experiment, now familiar to students of chemistry, in which he allowed a jet of liquid carbonic acid to escape into a receiver where the evaporation of part of the liquid produced a temperature so low that the rest was frozen into fine snow. In 1845 Faraday combined the method of low temperatures with that of high pressures in the hope of conquering the so- called permanent gases, such as oxygen, hydrogen, nitrogen. But these, along with nitric oxide, carbon monoxide, and methane, resisted his efforts. In 1869, Andrews expounded his definition of the "critical point," — the temperature (30.92° C. for carbonic acid) above which no amount of pressure produces visible liquefaction, but below which lique- faction occurs when the pressure is sufficient. " A vapour is a gas at any temperature below its critical point." This step towards clearness led experi- menters to recognise that the reason why oxygen, nitrogen, etc., proved intractable was that sufficient low temperatures (below their critical points) were not available. In 1875-Y, by devices securing lower tempera- tures, Raoul Pictet and Louis Cailletet succeeded in liquefying oxygen. Carbonic oxide, marsh gas, nitric oxide, and others also yielded to the " Caille- tet pump," and only nitrogen and hydrogen remained unsubdued. In 1883, nitrogen was liquefied by two Polish workers, Wroblewski and Olszewski. Finally * Tilden, Short History of Chemistry, p. 240. A CENTURY OF CHEMISTRY. 97 in 1898, after years of preparation, Professor Dewar produced liquid hydrogen, — a clear, colourless liquid, about one-sixth the density of liquid marsh gas, or about one-fourteenth the density of liquid water at 0°. As Prof. Tilden remarks : " It was both inter- esting and gratifying that the final victory which crowned the long series of successful attacks upon the apparently impregnable position of the perma- nent gases should have been recorded in the labora- tory of the Royal Institution, where the first suc- cesses in this field were won by Faraday." DEVELOPMENT OF ORGANIC CHEMISTRY Organic and Inorganic Chemistry. — The distinc- tion between the substances found in plants and animals and those in the not-living world is an old- standing one. Rooted in the belief that the sub- stances composing or formed by living creatures were under the domination of a specific vital force, the distinction was for a time accented by the complex- ity of most of the substances in question, by the fact that they were often difficult to isolate and very ready to change, and by the absence of a secure method of analysing their composition. Later on, the generalisations reached by the students of inor- ganic substances did not seem to fit in well with what was known in regard to the organic, and the breach was widened. It was thus to a large extent inde- pendently that organic chemistry developed, until it became strong enough to react upon the study of the inorganic with a potent and progressive influence. " At the beginning of the century, when qual- * For a brief account of the subject the reader is referred to Chapter IX. of Tilden's Short History of the Progress of Scientific Chemistry, London, 1899. 98 PROGRESS OF SCIENCE IN THE CENTURY. itative analysis had already attained a high degree of accuracy, and even the quantitative method had found excellent exponents in Proust, Klaproth, and iVauquelin, Lavoisier's experiments with alcohol, oil, and wax were the only ones in existence, designed to ascertain the composition of organic compounds ; and these, it may easily be understood, were not very ac- curate." * Some Factors in the Development of Organic Chemistry. — The development of organic chemistry which has been characteristic of the latter half of the century has been influenced in many ways: — by the elaboration of more perfect methods of determining the composition of organic substances (Gay-Lussac, Liebig, Wohler, Bunsen, Dumas, and many others) ; by the clear recognition, which may be associated with the name of Berzelius, that organic compounds could not be separated by any hard and fast line from inorganic compounds, but illustrated similar laws, and might in many cases be profitably regarded as derivations of inorganic compounds; by the fasci- nation of the methods of synthesis which gave the chemist an almost creative power; and by the enor- mous practical interests involved, in connection, for instance, with coal-tar products, one of the most fa- miliar of the many possible illustrations. We may pause here for a moment to note the fine instance of gradual discovery which the utilisation of coal-tar affords. " Sixty years ago an obscure German chemist obtained an oily liquid from coal- tar oil, which gave a beautiful tint with calcium chloride ; five years later another separated a similar liquid from a derivation of coal-tar oil. Still later, Hofmann, then a student in Liebig's laboratory, in- * Ladenburg, 1900, p. 112. A CENTTTR r OF CHEMISTRY. 99 vestigated these substances and proved their identity with an oil obtained long before by Zinin from indigo, and applied to them all Zinin's term, Anilin. The substance was curiously interesting, and Hof- mann worked out its reactions, discovering that with many materials it gives brilliant colours. The prac- tical application of these discoveries was not long de- layed, for Perkin made it in 1856. The usefulness of the dyes led to deeper studies of coal-tar products to which is due the discovery of such substances as antipyrin, phenacetin, ichthyol, and saccharin, which have proved so important in medicine." Wohler's Synthesis of Urea. — As analyses of or- ganic substances accumulated, it became perfectly clear that the stuffs composing and formed by living creatures did not contain any peculiar elements. It was seen that they consisted of compounds of carbon with hydrogen, oxygen, nitrogen, and other elements familiar in the organic world. Those who thought it important to emphasise the distinctions between the living and the not-living then fell back upon the assertion that it was in the arrangement of the elements that the uniqueness of organic substance lay. It was an architectural not a material distinction, and the architect was Vital Force. It was in the midst of these opinions that Wohler in 1828 effected the synthesis of urea — the character- istic waste product of higher animals. Starting with cyanic acid, which he had discovered in 1822, he found that urea was formed upon the evaporation of a solution of its ammonium salt. Without the aid of vital force he had formed from a simpler sub- stance a characteristic organic product. It should * J. J. Stevenson, Rep. Smithsonian Inst. for 1897, p. 330. 100 PROGRESS OF SCIENCE IN THE CENTURY. indeed be noted that he did not build up urea from its elements, but started with cyanic acid, which would now be classed as an organic compound. Professor Meldola has called attention * to the his- torical fact that Henry Hennell deserves a place among the pioneers of chemical synthesis, for in 1826-1828 he effected the synthesis of alcohol from ethylene. Though neither synthesis was complete, the steps were very important. They indicated the beginning of the end of vital force as a chemical factor, the beginning, too, of a remarkable series of synthetic achievements, — trichloracetic acid (Kolbe), formic acid and alcohol (Berthelot), indigo, grape-sugar, and many more — about 180 in all — all of which have been artificially produced. Isomerism. — Wohler's synthesis of urea did not quickly find the recognition it deserved, but it doubt- less helped to break down the arbitrary distinction between inorganic and organic chemistry, and to further the progress of the latter, which began to be spoken of as the chemistry of the carbon compounds. But Wb'hler was also concerned in other steps hardly less significant. The first of these steps is indicated by the word isomerism. Even Dalton had called attention to the existence of substances of identical chemical com- position, but with different properties, and had sug- gested that this might be explained by different or multiple arrangement of the constituent atoms. But little notice was taken of this. In 1823 Wb'hler dis- covered the composition of cyanic acid ; in the follow- ing year Liebig reported the same composition for fulminic acid. These two bodies have the same *Rep. Brit. Ass. for 1895, p. 649. A CENTURY OF CHEMISTRY. 101 composition, but are very different in character. In 1825 Faraday showed that butylene has the same composition as ethylene (olefiant gas), though the former has twice the specific gravity of the latter. In 1830 Kestner showed that racemic acid has the same composition as tartaric acid, and hundreds of such cases are now known. These facts at first served to complicate matters; they showed that compounds with widely different properties may contain the same constituents and in the same proportions. Berzelius, in labelling the puzzle with the term isomerism, suggested, as Dumas also did, that the component atoms must " be placed together in different ways " in the various isomers, which were the same in com- position and yet different in properties. The sug- gestion seems an easy one, especially when we note that " one chemical compound, a hydrocarbon con- taining thirteen atoms of carbon combined with twenty-eight atoms of hydrogen, can be shown to be capable of existing in no less than 802 distinct forms" (Roscoe). Indeed, possible substances have been repeatedly predicted, and afterwards discov- ered or made. But for forty years from Berzelius and Dumas there has been a succession of attempts to show how we may reasonably conceive of compo- sition being the same while the constitution and re- sulting properties are different. It seems likely that the solution is to be found in the modern develop- ment which is called " Chemistry in Space." Radicals. — But another step with which Wb'hler •was associated, along with Liebig, Bunsen, Dumas, and others, was the formulation of the radical theory. It was well known that salts are formed from an acid and a base and can be decomposed into these two constituents. For an understanding of the 102 PROGRESS OF SCIENCE IN THE CENTURY. salt it is more important to recognise its two constitu- ents than to know the quantitative proportions of its component elements. This may suggest the idea, which has been of enormous importance in organic chemistry, that in the usually complex substances in- volved there exist groups of elements which because of their stability of union, may be said to play the part of an element. Such a group is called a com- pound radical. To take a concrete case, in their re- searches on bitter almond oil and the allied com- pounds, Wohler and Liebig " showed that we may as- sume the existence, in these substances, of an oxygen- ated group which remains unchanged in the majority of the reactions, and therefore behaves like an ele- mentary substance. On this account, they called it the radical of bitter almond oil." * In 1837, Liebig wrote : " We call cyanogen a radical (1) because it is a non-varying constituent in a series of compounds, (2) because in these latter it can be replaced by other simple substances, and (3) because in its compounds with a simple sub- stance, the latter can be turned out and replaced by equivalents of other simple substances." The idea may seem to the outsider far off and theoretical, but there can be no doubt that the formulation of the radical theory not only introduced new clearness into chemistry, but was most provocative of research, some of the results of which have had no small influence on practical human affairs. SUMMARY. — Just as it had been shown (Ampere, 1816) that the salts of ammonia can be conveniently discussed and studied by regarding them as salts of a compound clement (NH*) so Berzelius, Dumas, Wohler, Bunsen, Liebig and others sought to work * Ladenburg, 1900, p. 109. A CENTURY OF CHEMISTRY. 103 out the idea that organic compounds might be brought into line with inorganic compounds by sup- posing that they contained compound radicals, like cyanogen, which 'behaved like elements. In mineral substances the radicals are simple; in organic sub- stances they are compound. Substitution. — About 1840, Dumas' idea of " sub- stitution " was added to the conceptual formulae of the organic chemist. " It was found that one or more atoms in an organic compound, notably of hydrogen, might be replaced by an equal number of atoms of other elements, and that such products of substi- tution retained similar qualities, and could be mutu- ally converted into each other, the type of the com- pound remaining the same." * Dumas showed that chlorine may replace hydrogen, atom for atom, in many organic compounds, and " it may be easily imagined how distasteful such a dis- covery would be to Berzelius and the school of electro- chemists, involving as it does the idea that a negative element may be exchanged for a positive element, without a fundamental alteration in the chemical character of the resulting compound." f According to Roscoe, the idea of substitution was the germ of Williamson's researches on etherification and those of Wurtz and Hofmann on the compound ammonias — investigations which lie at the base of the structure of modern chemistry — and had also a pro- found influence on the development of organic synthesis. Nuclei and Types. — The older radical theory, in- fluenced by the facts of substitution, gave place to the " type theory " of Laurent and Gerhardt and the * Merz, History, Vol. I., p. 410. t Tilden, Short History, p. 15. H 104 PROGRESS OF SCIENCE IN THE CENTURY. conception of " nuclei." " The radical, as the per- manent constituent in organic compounds, — cor- responding to the elements in inorganic chemistry, — gave way to the changeable nucleus, which only pre- served its form ; the unchangeable principle was found in the form, the structure or type, instead of in the substance of the simple or composite consti- tuents." Valency. — Time and ability alike fail us to dis- cuss how the endeavour after systematisation and simplicity was continued by Kekule (1829—1896), Kolbe (1818-1884), A. W. von Hofmann (1818- 1892),"Wurtz (1817-1884), and many others. The radical theory was characteristically German, the type theory, French ; and now we have to notice a more distinctively British contribution, — the idea of the " atomicity " or " valency " of chemical substan- ces, whether elements or compounds. With this idea the name of Frankland (1852) ought perhaps to be particularly associated. The conception of " valency," or the capacity of saturation of the atoms, was used with great effect by Kekule. Almost simultaneously, in 1858, he and Couper suggested that the carbon atom should be con- sidered as quadrivalent; i.e., able to unite with four univalent atoms or radicals (such as can replace one atom of hydrogen), but not with more. Kekule found in this a key to the constitution of many car- bon compounds. " We have chiefly," Ostwald says, " to thank Kekule for carrying through this idea. In the theory of valency, which is at the present time the prevalent one, it is assumed that each atom pos- sesses a definite limited capacity for combining with other atoms. This capacity is called the valency, A CENTURY OF CHEMISTRY. 105 and the atoms that can combine with one, two, three or four atoms (or equivalent atoms or radicals) are said to be univalent, bivalent, trivalent, or quadri- valent respectively. Thus marsh gas CH4 illustrates the quadrivalent character of carbon, and water OH2 the bivalent character of oxygen. Another development, foretold by Wollaston, but practically beginning about 1858, when Pasteur founded " stereochemistry " and Kekule stated his theory of chemical structure, attained epoch-making expression in 1875, when Van't Hoff published his work entitled La Chimie dans I' E space * — an at- tempt to formulate a geometrical conception of the manner in which the hypothetical atoms may be sup- posed to be placed in space. Along with Le Bel, he formulated what is called the theory of " the asym- metric carbon-atom " f and initiated what may be de- scribed as a mechanical theory of valency, which has been further strengthened by the work of Wislicenus (1887), and other masters of the chemist's craft. SUMMARY. — The development of organic chem- istry on its theoretical side affords a fine instance of the gradual specialisation of an hypothesis as the facts require it. The steps indicated by theories of radicals, types, nuclei, and valencies are steps to- wards a conception of material architecture which will consist with the facts of chemical change. The concept of the atom was in its first form too simple; the study of gases showed the necessity of recognising the molecule; the development of or- ganic chemistry enlarged the concept by the sug- gestion of radicals and nuclei, equivalents and val- * J. H. Van't Hoff. Chemistry in Space, trans, and ed by J. E. Marsh, Oxford, 1891. t One whose four valencies are satisfied by four atoms or radicals of different kinds. 106 PROGRESS OF SCIENCE IN THE CENTURY. encies; the phenomena of right and left handedness led on to ideas of definite geometrical arrangement within the molecule; in these and other ways the atomic theory in its chemical applications has be- come more and more specialised. " The present position of structural chemistry may be summed up in the statement that we have gained an enormous insight into the anatomy of molecules, while our knowledge of their physiology is as yet in a rudi- mentary condition" (Meldola, 1895). THE PERIODIC LAW. A General Statement by Mendelejeff. — " Many natural phenomena," Mendelejeff says, " exhibit a dependence of a periodic character. Thus the phe- nomena of day and night and of the seasons of the year, and vibrations of all kinds, exhibit variations of a periodic character in dependence on time and space. But in ordinary periodic functions one variable varies continuously, while the other increases to a limit, then a period of decrease begins, and having in turn reached its limit, a period of increase again begins. It is otherwise in the periodic function of the ele- ments. Here the mass of the elements does not in- crease continuously, but abruptly, by steps, as from magnesium to aluminium. So also the valency or atomicity leaps directly from 1 to 2 to 3, etc., without intermediate quantities, and in my opinion it is these properties which are the most important, and it is their periodicity which forms the substance of the periodic law. It expresses the properties of the real elements, and not of what may be termed their mani- festations usually known to us. The external proper- ties of elements and compounds are in periodic de- pendence on the atomic weights of the elements only A CENTURY OF CHEMISTRY. 107 because these external properties are themselves the result of the properties of the real elements forming the isolated elements or the compound. To explain and express the periodic law is to explain and express the cause of the law of multiple proportions, of the difference of the elements, and the variation of their atomicity, and at the same time to understand what mass and gravitation are. In my opinion this is now premature. But just as, without knowing the cause of gravitation, it is possible to make use of the law of gravity, so for the aims of chemistry it is possible to take advantage of the laws discovered by chemistry without being able to explain their causes. The above-mentioned peculiarity of the laws of chemistry respecting definite compounds and the atomic weights leads one to think that the time has not yet come for their full explanation, and I do not think that it will come before the explanation of such pri- mary laws of nature as the law of gravity." * The general idea of Mendelejeff's periodic law is that the properties of the elements are periodic func- tions of their atomic iveights, but while this is a simplifying concept it is not in any way an expla- nation. The Problem of Chemical Classification. — The desire for orderly grouping is one of the mainsprings of scientific work. Even artificial classifications — like the grouping of flowers according to the number of their stamens — have often justified themselves, though they are apt to outlive their usefulness. It is plain that natural classifications — based on deep- seated resemblances — must economise thought and make our outlook on the world clearer. Therefore * D. Mendelejeff . The FYinc-iples of Chemistry, trans. 1897, Vol. II., pp. 20-21, foot-note. 108 PROGRESS OF SCIENCE IN THE CENTURY. it has often been felt that the boon would be great if we could arrange the different kinds of matter in groups or series corresponding in some measure to the classes, orders, families, etc., in which we ar- range plants and animals. It is therefore hardly necessary to say that Men- dele jeff was not the first to be attracted by the possi- bility of detecting serial relations among the chem- ical elements. Apart from the speculations of the ancients and of the alchemists, glimpses of a sup- posed orderly relationship of the various elements seem to have been frequent in the history of chem- istry. Particularly noteworthy was the idea of a fun- damental substance, " protyle " or " prothyle," often identified with hydrogen, of which the other elements were supposed to be derivatives. Prof. Tilden sums up the idea in the quotation : — ' ' All things the world which fill Of but one stuff are spun." More concretely, the hypothesis was hazarded anony- mously by Prout (1815) that the atomic weights of the gaseous elements are all whole multiples of hydrogen. And with this view, supported by Mei- necke (181Y), was involved the suggestion that the various elements might turn out to be derivatives of one primary form of matter, such as hydrogen, or something of which hydrogen was an atomic multiple. It was an evolutionist speculation, but born before its time. It has been buried and res- urrected several times throughout the century. De- fended in Britain by Thomson, scouted by Berzelius, revived by Dumas, it was once more sent to rest about 1860 by Stas, a Belgian chemist, who did splendidly accurate work, from 1860 onwards, in A CENTURY OF CHEMISTRY. 109 confirming the doctrine of the regularity of chemical proportions in all combinations. Others again, without accepting any protyle-hy- pothesis, pointed out the existence of serial regular- ities in the atomic weights of the elements, (Lens- sen 1857, Pettenkofer 1850, Dobereiner 1817, and even before the atomic theory, J. B. Kichter 1798). Dobereiner pointed out that a number of elements could be arranged in groups of three, or triads ; e.g., calcium, strontium, and barium, the members of each triad having analogous properties and displaying a certain regularity in the relations of their atomic weights. This idea of family characteristics was afterwards extended by Dumas. Most noteworthy, however, was the work of New- lands (1863-4), who showed that when the elements were arranged according to the magnitude of their atomic weights, " similar elements were found at approximately equal distances in the series; count- ing from any one element, every eighth was in gen- eral more similar to the first than the other ele- ments." * As the eighth element, starting from a given one is a kind of repetition of the first, like the eighth note of an octave in music, he called the regularity " The Law of Octaves." He did not succeed, however, in fully carrying out his idea. In the same year (1864), Dr. Odling also published a suggestive pa- per on " The Proportional lumbers of the Elements and their Serial Relations." Independent Discovery by Meyer and Mendelejeff. — We accept the conclusion of expert authorities that in 1869 Lothar Meyer and D. Mendelejeff inde- * Ostwald, General Chemistry, trans, by Walker, 1890, y. 35. 110 PROGRESS OF SCIENCE IN THE CENTURY. pendently reached the same conclusion: — That the properties of the elements are periodic functions of their atomic weights. " If all the elements be ar- ranged in the order of their atomic weights a peri- odic repetition of properties is obtained. This is ex- pressed by the law of periodicity; the properties of the elements, as well as the forms and properties of their compounds, are in periodic dependence, or, ex- pressing ourselves algebraically, form a periodic function of the atomic weights of the elements." " If all the elements are arranged in the order of their atomic weights in a series, their properties will so vary from member to member that after a definite number of elements has been passed either the first or very similar properties will recur." f This was the conclusion which Mendelejeff and Meyer ex- pounded. Let us state the general idea once more. When the elements are arranged according to the magnitude of their atomic weights, " the elements following one another show apparently no regularity in properties, but after the lapse of a certain period the chemical and physical behaviour of the elements now suc- ceeding each other strongly recall that of the previ- ous group, in fact, repeat it. The elements which resembled one another were therefore united into groups or natural families, and these in their turn were distinguished from the periods, which com- prised the elements whose atomic weights lay be- tween those of two successive members of a natural family." J Scientific Justification of the Periodic Law. — It * Mendelejeff, Principles of Chemistry, Vol. II., trans, by Kamensky and Greenaway, 1891, p. 16. f Ostwald, General Chemistry, trans, p. 35. j E. von Meyer, History of Chemistry, trans. 1891, p. 347. A CENTURY OF CHEMISTRY. HI may be said in a sentence that the general result of chemical work, since Mendelejeff and Meyer stated the Periodic Law in 1869, has been to show that " al- most every well-defined and comparable property of the elements appears as a periodic function of the atomic weights" (Ostwald). The atomic volume shows the periodic variation most clearly (Meyer), the melting point of the elements varies periodically (Carnelley), the same holds true of the specific gra- vities, the magnetic properties of elements depend on the position occupied in the periodic system (Carnel- ley), there is also a periodicity in the amount of heat developed in the formation of the chlorides, bromides, and iodides (Laurie) ; these must serve as illustra- tions of the manifold justification which the theory has received. The Test of Prophecy. — In regard to vital phenom- ena where the operative factors are usually complex and numerous, there are few who would be willing to submit their favourite generalisations to the severe test of using them as a basis for prophecy, as the as- tronomer, for instance, can do with some security. But this severe test Mendelejeff did apply to his periodic law. In his arrangement of elements into groups and series, Mendelejeff was compelled to leave certain blanks. He asserted that these would be filled up by the discovery of new elements. " He was able to foretell the atomic weights and other properties of these elements from their posi- tion in the system, with the aid of the properties ob- served in the groups and series, which, like a system of co-ordinates, could be called in to assist. Three such blanks occurred in the first five series, and these he indicated as representing the positions of eka- 112 PROGRESS OF SCIENCE IN THE CENTURY. boron (at. wt. 44), eka-aluminium (at. wt, 68), and eka-silicon (at. wt. 72). Since that time, these three elements have been discovered, and they have been found to possess, approximately, the properties pre- dicted by Mendelejeff. They are: scandium, discov- ered by Mlson, with atomic weight 44.1; gallium, discovered by Lecoq de Boisbaudran, with atomic weight 70; and germanium, discovered by Winkler, with atomic weight 72." * To sum up: " The periodic law has not only embraced the mu- tual relations of the elements and expressed their analogy, but has also to a certain extent subjected to law the doctrine of the types of the compounds formed by the elements; has enabled us to see a regu- larity in the variation of all chemical and physical properties of elements and compounds, and has ren- dered it possible to foretell the properties of ele- ments and compounds yet uninvestigated by exper- imental means; it therefore prepares the ground for the building up of atomic and molecular me- chanics." f Inorganic Evolution. — An alluring, but perhaps il- lusory, idea has occurred to many chemists who have pondered over the relations of the elements to one another, — the idea that chemically analogous ele- ments may be related in a real, i.e., genetic, sense, or that they may be derivatives of a common stock. The historians of chemistry have shown that this is an ancient and frequently recurrent idea. Some of the early Greeks imagined one primeval substance developing into all the different kinds of matter; * Laclenburg, 1900, p. 313. t Mendelejeff, Principles of Chemistry, Vol. II., trans., p. 34. A CENTURY OF CHEMISTRY. 113 Boyle spoke of " one universal matter common to all bodies;" Dalton said, "We do know that any of the bodies denominated elementary are absolutely indecomposable ; " Graham suggested as conceivable, " that the various kinds of matter now recognised as different elementary substances may possess one and the same ultimate or atomic molecules existing in different conditions of movement." * Many other examples might be given, and we have already re- ferred to the views of Prout, Meinecke, and Thomas Thomson that there is an ultimate relation between hydrogen and the other elements. " In 1888-9 Sir William Crookes again raised the question whether what are called elements may not be compounds, and whether all may not have arisen, by gradual condensation, from hypothetical primitive material which he called protyle. Accepting the suggestion that substances now thought to be elements may turn out to be com- pounds, Lockyer has pictured the possible dissocia- tion of the elements in the fervent heat within the sun's atmosphere. It may be so, but there are no certain facts as yet which alleviate the hypothetical character of these imaginings ; and it seems well to emphasise that Mendelejeff has expressly dissociated his periodic law from speculations as to the deriva- tion of the elements from one prime matter. CO-OPEBATIOX OF CHEMISTRY A3TD PHYSICS. ~No two sciences have entered into a co-operation so close as that which now exists between chemistry and physics. In a way the alliance is almost ancient, for chemistry first became an exact science by adopting * See Sir Henry Roscoe's Pres. Address, Rep. Brit. Ass. for 1887, p. 8. PROGRESS OF SCIENCE IN THE CENTURY. physical methods of weighing and measuring; the balance, which is as familiar an emblem of chemistry as the crucible, is rather a physical than a chemical instrument. But the recognition that chemical and physical properties are inter-dependent and must be studied together, practically dates from Lavoisier, and it has led to a remarkable series of physico- chemical researches which may be said to form a special department of science. Kopp was one of the early workers ; Ostwald is now one of the leaders. Thermochemistry. — A new chapter in the history of chemistry began with Lavoisier's study of com- bustion and with the resulting recognition of the indestructibility of matter. But Lavoisier left the dynamics of combustion untouched, and another new chapter dates from 1843, from Joule's measurement of the mechanical equivalent of heat, and the result- ing recognition of the conservation of energy.* The phenomena of chemical activity assumed a new aspect when it was clearly realised that chemical changes involve only re-distribution, but in no case any destruction of energy or power. This also im- plied that chemical energy might be measured in terms of the heat evolved or absorbed. Let us by means of a quotation from Ostwald gain a clear impression of what the main business of thermochemistry is. " Chemical energy is to us the least known of all the various forms of energy, as we can measure neither it nor any of its factors di- rectly. The only means of obtaining information re- garding it is to transform it into another species of energy. It passes most easily and completely into heat, and the branch of science which treats of the measurement of chemical energy in thermal units is * See the Chapter on the Progress of Physics. A CENTURY OF CHEMISTRY. 115 called thermochemistry. Thermochemistry is thus the science of the thermal processes conditioned by chemical processes. The quantities of heat evolved or absorbed measure the decrease or increase of chemi- cal energy, in so far as other energy is not involved in the processes." * Among the important steps in thermochemistry the following may be noted : The extension of the law of Dulong and Petit by Neumann and later by Regnault (1839) ; the ex- periments of Thomas Andrews (1841) on the heat produced during the combination of acid and bases in aqueous solution; Herman Hess's experimental verification (1840) of the conclusion that "the sev- eral amounts of heat evolved during the successive stages of a process are the same in whatever orderthey follow one another " — a conclusion subsequently re- inforced by Berthelot; Julius Thomsen's vast accu- mulation of data (from 1853 onwards) as to heats of formation and all kinds of chemical change; and Berthelot's equally voluminous researches. We need not, for our purpose, pursue the history further. It is enough to indicate that the aim of discovering the dynamical laws relating to chemical processes is one which has not been lost sight of. At the same time, we have to note the conclusion of an expert like Tilden, that "notwithstanding the labours of half a century, thermochemistry remains for the most part a mass of experimental results, which still await interpretation." The doctrine of the conservation of energy is the foundation of chemical dynamics. Every change in the arrangement of particles is accompanied by a * Ostwald, Outlines of General Chemistry, trans. 1890, pp. 208-209. 116 PROGRESS OF SCIENCE IN THE CENTURY. definite evolution or absorption of heat. The object of thermal chemistry is to measure the energy of chemical changes by thermal methods, and thus to get nearer the fundamental problem of the dynamics of chemical affinity. Photochemistry. — There are few problems more fascinating and more important than those which are raised when we try to follow the transformations of sunlight. Chemical processes in the sun give rise to radiant energy, which is propagated with great ve- locity (3 -f- 1010 cm. per second) through space, with the ether for its hypothetical vehicle. When it reaches the earth, part of it passes into the form of heat and thence into many other forms, while part of it acting on green plants resumes the form of chemical energy. The radiant energy of sunlight is utilised by the green leaves to split up the carbonic acid of the atmosphere and to build up the complex substances which furnish food and fuel, not to speak of the most valuable super-necessaries of life. Nor does the radiant energy affect plants only, it has a subtle influence on many animals, modifying for instance the process of coloration, and above all producing those chemical changes in the retina which are associated with vision. In the volume of this series which deals with Inventions due notice will be taken of photography (Daguerre, 1838), which de- pends on the chemical reactions produced by light on a sensitive surface. But the retina was the first sen- sitive surface, and we may therefore say that it was in the consideration of problems primarily physio- logical and secondarily technical that photochemistry, like thermochemistry, had its beginnings. We have just mentioned the effect of light upon the human eye, and as an illustration from the other A CENTURY OF CHEMISTRY. H7 end of the scale of being we may note the attraction of some micro-organisms to light. ThusEngelmann's Bacterium photometricum — rod-like purple microbes — not only crowd in a drop of water under the mi- croscope to the particular spot on which the smallest possible beam of light is focussed, but when a micro- scopic spectrum is projected on the field " select ' the area whose colour is that which is most absorbed by their minute bodies. One other illustration of the chemical action of light upon living creatures may be given, namely, the destructive effect of light upon many kinds of mi- crobes, both in the air and in culture-solutions. We are accustomed to think of light as life-giving, but it also kills. And the fact is significant and full of practical suggestion that sunlight is the most potent, universal, and economical antagonist of some of our worst enemies. How exactly the light kills the bac- teria remains somewhat uncertain, but it is com- monly believed that it induces too rapid oxidation, that it makes the minute organisms live so fast that they die. Photochemical research has been as yet in great part concerned with different modes of measuring the chemical activity of light. One of the most suc- cessful methods takes advantage of the fact that light induces a mixture of equal volumes of chlorine and hydrogen to form hydrogen chloride (Draper, 1843 ; Bunsen and Roscoe, 1857). This led to the estab- lishment of the conclusions that the chemical action is proportional to the light intensity, that equal chemical effects are produced when the products of light intensity and time of exposure are equal, that substances are affected differently by different rays, and so on. How it is that light induces chemical 118 PROGRESS OF SCIENCE IN THE CENTURY. change we do not know, though hypothetical sugges- tions have been offered. Photochemistry or the study of the effects of radiant energy (light) on chemical processes is still incipient; though its results have led to the develop- ment of photography, the influence of light on the green leaf remains an unread riddle. Electrochemistry. — It is a familiar fact that if a rod of zinc and a rod of platinum "be immersed in dilute sulphuric acid (which does not attack either of them separately), and if the ends of the two rods projecting out of the liquid be apposed or connected by a metal wire, the zinc is dissolved, the hydrogen of the sulphuric acid accumulates on the platinum, and there has come into existence an electric current — a form of energy — which can be made to do work. The source of this energy is in the chemical process, in the heat evolved by the solution of the zinc. By using heat as the common standard of measurement, we are able to prove that a certain amount of poten- tial chemical energy available at the outset is exactly equivalent to the amount of electrical energy pro- duced plus the heat evolved at the seat of the reaction. From the study of comparatively simple experi- ments like that above referred to, always in the light of the doctrine of the conservation of energy, electro- chemistry has evolved into an important and elabo- rate department of science. Faraday distinguished bodies, e.g., metals, which conduct electrical currents without suffering any material change beyond that of heating, from other bodies, such as salts and aqueous solutions of acids and bases, in which the conducted current induces chemical change. " In such conductors of the second class, or electrolytes, the movement of electricity A CENTURY OF CHEMISTRY. 119 takes place so that the metals (or metallic radicals) of the salts and bases, and the hydrogen of the acids, move from the positive part of the current to the negative, while the acid radicals or elements, such as chlorine, bromine, iodine, and also the hydroxyl of bases, move in the opposite direction. These com- ponents, or ions, are set free where the electrolyte is in contact with metal conducting the current " (Ostwald, op. cit. p. 270). In 1833, Faraday for- mulated the general conclusion, fundamental to sub- sequent progress, that equal quantifies of electricity on passing through different electrolytes require equivalent quantities of the ions for their transport. This may be called the foundation-stone of electro- chemistry. It would be interesting to show how the enquiry into the constitution of electrolytes, which must be such that particles charged positively can move in one direction while those charged negatively move in the other, has led through the ideas of Williamson (1851), Clausius (1857), Arrhenius (1887), Planck (1887), to the theory that solutions of salts and of strong acids and bases contain these substances dis- sociated into ions, that a solution of potassium chlo- ride contains in great part single potassium and chlo- rine atoms with enormous electrical charges and with their chemical properties thereby modified. It reads like a romance in the invisible world — far more dar- ing than the biologist has ever ventured with his ids and biophors — and yet it appears to harmonise a large number of observed facts. As Ostwald says, " The assumption that electrolytes contain free ions is not only possible but necessary." It would be interesting also to show how the elec- tric conductivity of electrolytes was measured (Kohl- 120 PROGRESS OF SCIENCE IN THE CENTURY. rausch, 1880), or how the velocity of the migration of the ions was calculated, or how equations have been worked out and confirmed (Willard Gibbs, Helmholtz, Jahn), showing the relation between the chemical energy, the electrical energy, and the altera- tion of the electromotive force (i.e., potential, ten- sion or intensity) with the temperature, such that any one of the three can be calculated if the other two terms are known. But we have said enough to suggest the fruitfulness of the co-operation of chemistry and physics in the department of electro- chemistry, and to suggest how well it will repay the reader to avail himself of the pleasure which is af- forded by modern chemistry, as expounded by mas- ters like Ostwald. THE CIRCULATION OF MATTER. Transformations in Plants. — We have already al- luded to the chemist's power of transforming matter. Out of coal-tar he brings the colours of the rainbow and he makes the rubbish of twenty years ago a source of riches to-day. But any common green plant is the seat of trans- formations of matter not less marvellous. The ele- ments of soil, water, and air are by the touch of life lifted into complexity, united into organic com- pounds, forming part of the capital of a living crea- ture. We are also aware of what Mr. Grove long since called the correlation of the physical forces, what others speak of as the transformations of energy. We know how the energy of the mill-race may drive a dynamo, and we see the energy again in our electric A CENTURY OF CHEMISTRY. 121 lamp. We know that heat, light, and electricity are transformable powers. But any common green plant is the seat of trans- formations of energy not less marvellous. The ener- gies of the sunlight — the undulations of the ethereal waves, according to the student of physics — are so used by the plant that complex organic substances, of which starch is the first to become visible, are built up. The kinetic energy of the sunlight is changed in the potential energy of complex chemical substances, such as wood. We use such potential energy to sup- ply power to our life, to stoke our engines, to warm our hearths. We know of no life which is not life-born, but we know that all the world over, from the red-snow plant of Arctic icebergs to the luxuriant vegetation of the Tropics, from the seaweed on the shore to the Cali- fornian Wellingtonias, the simple so-called dead ele- ments of water, earth, and air are being quickened into life, that is to say, are becoming part of the capital of living plants. On these plants animals feed, and the wealth of the plants is recoined to feed muscle and nerve, and what was once the dust of the wayside may become part and parcel of the brain of a Caesar. Elements in an Organism, — Let us approach the subject in another way. ISTo one knows the chemical nature of living matter, for we cannot isolate what is genuinely alive from associated not-living substance. Moreover, the moment the expert begins his analysis the living matter is dead, and the secret eludes him. But every one now knows the elements out of which the living body is built up, though no one can tell how these elements are arranged in really living stuff nor how they act as they do when thus ar- 122 PROGRESS OF SCIENCE IN THE CENTURY. ranged. The elements cannot escape the chemist, al- though their intricacy of arrangement in many cases does. If we reduce living plants to ashes, and allow nothing to escape undetected, we find a constant pres- ence of twelve elements, carbon, hydrogen, oxygen, nitrogen, sulphur, phosphorus, chlorine, potassium, sodium, calcium, magnesium, and iron. It may be indeed that all the twelve are not present in some of the very simplest forms of life, where the method of ash-analysis is inapplicable. But for ordinary plants which can be burned, the above statement is true. The twelve elements are always present. Had we space, it would be interesting to take each of these elements in turn, to show in what forms they exist in inorganic nature, to follow them from their ab- sorption by root-suckers to their known combinations in plant, animal, or man, and to show how they eventually come back to the so-called dead-state once more. But since it is better to have one definite im- pression than a hundred vague ones, let us confine our attention to nitrogen. Circulation of Nitrogen. — "As is well known, free nitrogen forms about four-fifths of the atmosphere, but the great bulk of this takes no part in vital proc- esses. With certain notable exceptions it is only in the form of compounds that nitrogen can be used by living creatures. Therefore, since nitrogenous food is essential both to plant and animal, the amount of life upon the earth must depend on the amount of fixed nitrogen available.* The commonest circle is the following: Nitrogen is obtained by the plant in the form of nitrates, ni- * Bunge, Text-book of Physiological and Pathological Chemistry, trans. 1890, p. 19. A CENTURY OF CHEMISTRY. 123 trites, or ammonia ; these compounds are used in the elaboration of complex nitrogenous bodies such as proteids. These proteids produced by the plant form the food of animals and become part of their vital capital. As the animals live there is a continual dis- ruption of the complex nitrogenous substances and the formation of less complex nitrogenous waste products. This also takes place in plants, but there is this difference, that while the plant retains its nitrogenous waste, the animal gets rid of it — in the form of urea, uric acid, urates, and the like. These waste products rapidly decompose after they have been excreted, and ammonia is formed — available once more to enter upon the cycle. If the animal or plant die, the agency of putre- factive bacteria brings about decomposition, and the disruption of the nitrogenous materials yields am- monia, nitrates, and the like, which may be again utilised. The availability of nitrogenous material is not thereby affected. On the other hand, as Bunge forcibly points out,* the burning of wood, the crema- tion of an animal, the explosion of gunpowder, in- volve a liberation of nitrogen from its fixed or com- pound form, and a consequent diminution of the available supplies. " It would appear, therefore, that there is a con- tinuous degradation of nitrogen to the elementary condition — a very serious matter if the nitrogen so degraded is finally removed from the sphere of action of organised beings. Are there, then, any other agencies at work to restore the balance, and enable this apparently useless gas to return within the arena of physiological activity ? " f * Bunge, op. cit., p. 21. t F. W. Stoddart, " The Circulation of Nitrogen in Na- ture," Proc. Bristol Nat. Soc.. IX. (1899), pp. 57-74. 124 PROGRESS OF SCIENCE IN THE CENTURY. In the first place, it has to be borne in mind that by electrical discharges in air nitrogen is united with oxygen to form nitric acid, and in a damp at- mosphere the same agency causes nitrogen to combine with water vapour to form nitrite of ammonia (Ber- thelot)- The rain after the thunderstorm brings the products to earth. In the second place, it is stated by Schb'nbein that wherever evaporation occurs minute traces of am- monia are formed in the air. In the third place, the researches of Hellreigel and Willfarth, repeated and confirmed by many, show that leguminous plants can under the influence of partner-micro-organisms, which form root-tuber- cles, utilise (indirectly) the free nitrogen of the air. In the fourth place, the circulation of nitrogen and the increase of availability is furthered by other lilli- putian agencies; namely, those soil-bacteria which convert ammonia into nitrous acid, or carry the oxi- dation further to the level of nitric acid. Foundation of Agricultural Chemistry. — If we wish to associate any particular name with the recog- nition of the fundamental fact of the circulation of matter, it should be the name of Justus Liebig (1803-1873). Himself a student under Gay-Lus- sac, he became the master of one of the greatest schools of chemistry, the initiator of chemical labo- ratories, a pioneer of modern organic chemistry, one of the prompters of chemical physiology, the founder of agricultural chemistry, and the discoverer of many important practical applications. The circulation of elements, of nitrogen for in- stance, from the air or the soil into plants and thence into animals, and thence back to the soil or air again, is a fact of great interest, justifying us A CENTURY OF CHEMISTRY. 125 in speaking of the circulation of matter, — a fact to be associated with Liebig's industry — as not less im- portant than Harvey's theory of the circulation of the blood. The idea marks a new era. CHEMICAL AFFINITY. The Problem of Chemical Changes. — Chemistry has above all to do with changes in the composition of matter, and although in point of time the study of chemical changes was prosecuted, by the alche- mist, for instance, long before there was any sound knowledge of material composition, the understand- ing of the former entirely depends on an understand- ing of the latter. One of the early results of the careful study of these chemical changes or reactions was to show that though the number of possible experiments is endless, the number of kinds of experiment is limited. It began to be seen that substances could be arranged in various groups, the members of each group acting in a similar way in similar circumstances. Thus a number of substances, like oil of vitriol (sulphuric acid) and spirits of salt (hydrochloric acid) exhibit similar properties, or similar reactions in similar conditions, and may be ranked together as acids; another set of substances, like spirits of hartshorn (ammonia) and slaked lime, are most markedly dif- ferent from the acids, and may be ranked together as alkalis; a third set of substances, like chalk, pro- ducible by the reaction of an acid and an alkali, may be ranked together as salts. Thus there arose a clas- sification of compounds based on similarity of reac- tion in similar conditions. It was merely a prelimi- nary step towards order, and it led to many others of greater importance. 126 PROGRESS OF SCIENCE IN THE CENTURY. When two different substances are brought to- gether it frequently happens that changes occur re- sulting in the production of a new substance or sub- stances. Thus an acid and an alkali, as noted above, produce a salt. Since the indestructibility of matter was recognised, and since Dalton made the atomic conception current coin, it has been evident that the change occurs through a separation and re-combina- tion of the component particles of the two substances. As Dalton said : " All the changes we can produce consist in separating particles that are in a state of cohesion or combination, and joining those that were previously at a distance." But after the phenomena of change have been observed, the question is bound to arise — why should the atoms separate and re-com- bine at all ? Is the phenomenon comparable to any- thing else in our experience, or is ' chemical affinity ' an irreducible fact ? Masses attract one another and we can measure the force; is chemical affinity also measurable and does it bear any analogy to gravita- tion ? There is also attraction due to magnetism and different electrical states; has chemical affinity anything to do with this ? Thus arises the inevitable problem of chemical affinity; it is still unsolved, but we may profitably consider for a little some of the suggestions which have been offered. It is part of the work of chemistry to distinguish the different kinds of matter, and we began this his- torical sketch by alludincf to the search for the ele- ments; but a more important problem is to interpret chemical affinity, or the capacity of the elements to exert chemical action. Electricity and Chemical Affinity. — In the long history of attempts to interpret the chemical activi- ties of different kinds of matter in their relations to A CENTURY OF CHEMISTRY. 127 one another, the importance of electrical phenomena has bulked largely. The discoveries of Galvani (1789) and Volta (1792) on the generation of elec- tricity by the use of two metals were not long in being applied to chemistry. Thus in 1800 Nichol- son and Carlisle observed that if an electrical cur- rent be passed through water, the result is a decompo- sition into hydrogen and oxygen, — the two gases, namely, which Cavendish, sixteen years before, had shown (synthetically) to be the constituents of water. In 1803 Berzelius and Hisinger published the results of similar experiments on many different compounds, and showed that hydrogen, metals, alka- lis, metals, etc., possess positive electrical energy, while oxygen, acids, etc., separate at the positive pole. Davy. — Meanwhile Humphry Davy had also turned his attention to similar enquiries ; he con- firmed the results of Hisinger and Berzelius, and made the theoretical suggestion that hydrogen, alka- lis, metals, etc., possess positive electrical energy, while oxygen and the acids are correspondingly nega- tive. As oppositely electrified bodies attract each other, the former substances come off in electrolysis at the negative pole (cathode), and the latter at the positive (anode). From this he went on to the mo- mentous generalisation that chemical affinity is due to difference in electrical condition. Pursuing his decomposition experiments, Davy turned his attention to the alkalis (potash and soda), and found that small metallic globules, burning with brilliancy in air, were formed at the negative pole, while oxygen was evolved at the other. He rightly concluded that the substances he had discovered were the metals Potassium and Sodium, of which the 128 PROGRESS OF SCIENCE IN THE CENTURY. alkalis are the oxides. This important step, checked by the French chemists, seems to have led many for a time to a false expectation. " The idea was arrived at that the substances hitherto known were only compounds', and that the aim of chemistry was now to discover the true elements, which it was supposed would resemble potassium and sodium. . . . The galvanic current, at that period an en- tirely new agent, had accomplished this marvel, and it was itself a marvellous thing. By its aid it had become possible to decompose compounds into their true elements; hence it is not surprising that this agency was regarded as identical with the one which gave rise to combinations ; i.e., with affinity." Berzelius. — The ingenious suggestions of Davy were soon developed by Berzelius into a consistent theory which was then used as the foundation idea of a chemical system. He believed, with Davy, that all chemical reac- tions are produced by electricity, which " thus seems to be the first cause of the activity all around us in nature." But he differed from Davy in his mode of conceiving of the electrical distribution. In his own words, " If the electro-chemical views are ac- curate, it follows that every chemical combination depends wholly and only upon two opposite forces, namely, the positive and negative electricities, and that every compound must be composed of two parts, united by the effects of their electro-chemical reac- tions, since there is not any third force. From this it follows that every compound substance, whatever the number of its constituents may be, can be divided into two parts, of which the one is positively and the other is negatively electrical." * Ladenburg, 1900, p. 67. A CENTURY OF CHEMISTRY. 129 But difficulties soon gathered round this electro- chemical theory. Even as early as 1834, Dumas showed, in stating his " substitution " theory, that in many organic compounds the positive element hydro- gen may be replaced by the negative element chlorine " without a fundamental alteration in the chemical character of the resulting compound." This was practically a deathblow to the theory of Berzelius. Faraday. — About 1833, Faraday was led to con- clude (a) that the chemical power of a current of electricity is in direct proportion to the absolute quan- tity of electricity which passes, and (6) that the proportions of the bodies or ions evolved by an elec- trolytic action (the electro-chemical equivalents of the ions) are the same as their ordinary chemical equivalents or combining proportions. And he re- turned to the theory of Davy, saying that " the forces termed chemical affinity and electricity are one and the same." Sir Henry Eoscoe points out that the great prin- ciple of valency was foreshadowed from a physical point of view in Faraday's law of electrolysis. Faraday showed that the number of atoms electro- lytically deposited is in the inverse ratio of their valencies; Helmholtz in his Faraday lecture ex- plained this by the fact that " the quantity of elec- tricity with which each atom is associated is directly proportional to its valency." lonisation Theory. — It does not seem possible, at present, to be confident in affirming or denying the idea that chemical combination is due to the union of electrically charged atoms; but it is certain that the question is not so simple as it appeared to Davy, Berzelius, and Faraday. To make the matter in any way clear it would be necessary to take account of 130 PROGRESS OF SCIENCE IN THE CENTURY. many researches, notably, for instance, of those con- cerning the nature of solutions. The reader should consult, for instance, the eighth chapter of Professor Tilden's Short History, espe- cially with reference to the theory of ionisation sug- gested by Arrhenius. While the early electro-chemical ideas of Berzelius have been abandoned, a new path of enquiry, es- pecially marked by the work of Svante Arrhenius, continues to be full of promise. Its first milestone bears the date 1884, when Arrhenius proved that def- inite and quantitative relations exist between elec- trical and chemical properties. But to this we must add, as suggestive of one of the most significant steps in modern chemical theory, another quotation from Ostwald. " Research based on a well-defined measure of affinity determinable with numerical exactness only became possible, when, by the development of the electrolytic theory of dis- sociation, the formula was found from which a con- stant of a general character and independent of the dilution could be calculated. This constant has a claim to serve as a measure of affinity." While the nature of chemical affinity remains ob- scure, a mode of measuring it has been attained. If this step is to be associated with any particular name it should be with Ostwald (1889). CHAPTEK V. THE PKOGBESS OF PHYSICS. INTRODUCTORY. Definition of Physics. — " The properties of matter and energy, of energy and ether, and of ether and matter, are the subjects of investigation in physical science." Thus one of the modern masters, Prof. G. F. Fitzgerald,* defined the scope of the science, •whose progress in the nineteenth century will be illus- trated or suggested in this chapter. Although we may note Fitzgerald's statement that physical science is divided from chemistry " by being the study of each kind of matter by itself, while chem- istry studies the actions of different kinds of matter upon one another," we must also note his acknowledg- ment— " of course no real line can be drawn." The physicist has mainly to do with transforma- tions of energy, or, in a word, with motion. Or per- haps it is more accurate to say, with Professor J. J. Poynting : " The range of the physicist's study con- sists in the visible motions and other sensible changes of matter. The experiences with which he deals are the impressions on his senses, and his aim is to de- scribe in the shortest possible way how his various senses have been, will be, or would be affected." f Method of Physics. — The physicist looks out upon nature seeking for similarities of action — likenesses * Science Progress, Vol. I., 1894. p. 3. t Address, Section A, Rep. Brit. Ass. for 1899, p. 615. 132 PROGRESS OF SCIENCE IN THE CENTURY. of motion ; he groups these together if they are seen to be really the same; he uses instruments to enable his senses to detect hidden motions, and to measure these with accuracy ; he tries to find a short descriptive formula of antecedent and sequence which will fit the facts. The so-called laws of motion are " brief descriptions of observed similarities," as Prof. J. J. Poynting expresses it.* As his for- mulae increase in number and precision, he often finds it possible to combine several of them in a more general formulae, which may be so secure, that is so accurate a description, that it affords a basis for safe prediction. Aim of Physics. — " To take an old but never worn- out metaphor, the physicist is examining the garment of Nature, learning of how many, or rather of how few, different kinds of thread it is woven, finding how each separate thread enters into the pattern, and seeking from the pattern woven in the past to know the pattern yet to come. How many different kinds of thread does Nature use ? So far, we have recognised some eight or nine, the number of different forms of energy which we are still obliged to count as distinct. But this distinction we cannot believe to be real. The relations between the different forms of energy and the fixed rate of exchange when one form gives place to another, encourage us to suppose that if we could only sharpen our senses or change our point of view we could effect a still further reduction. We stand in front of Nature's loom as we watch the weav- ing of the garment; while we follow a particular thread in the pattern it suddenly disappears, and a thread of another colour takes its place. Is this a new thread, or is it merely the old thread turned * Address, Section A, Brit. Ass. Report for 1899, p. 616. THE PROGRESS OF PHYSICS. 133 round and presenting a new face to us? We can do little more than guess. We cannot get round to the other side of the pattern, and our minutest watching will not tell us all the working of the loom."11 But since we cannot rest with discon- tinuous descriptions, we construct a hypothetical system as to the constitution of matter and the relation of energy to it, — a system in line with what we do know of visible motions and accelerations, — a system to which we will naturally hold until a more complete knowledge should suggest some im- provement of it, or, it might be, demand its rejection. SUMMABY. — In the main the problem of the phys- icist is to describe and formulate the likenesses of motion which are observed in our outlook upon nature. THE NEWTONIAN FOUNDATION. At the beginning of the nineteenth century, chem- istry was just steadying itself on the foothold afforded by the doctrine of the indestructibility of matter, but Physics had been on sure ground since the publication of Xewton's Principia (1687). It seems necessary to admit that the value of the Xewtonian foundation was not fully appreciated in the eighteenth century, and that many workers left it and built short-lived in- dependent structures, but for the nineteenth century it does not seem too much to say that all stable prog- ress in Physics has been dominated by Newton's con- clusions. " In fact the Newtonian philosophy can be said to have governed at least one entire section of the scientific research of the first half of this period : only in the second half of the period have we succeeded in * Poynting, Address, Section A, Rep. Brit. Ass. for 1899, p. 618. 134 PROGRESS OF SCIENCE IN THE CENTURY. defining more clearly the direction in which Newton's views require to be extended or modified." * As to the import of Newton's work, three points may be distinguished. First, it affords what is probably the most striking instance of the application of scientific method, and part of its influence has been that of an illustrious example. It signalised once for all the contrast between metaphysical contemplation and scientific study. Secondly, in the so-called law of gravitation, which describes " how every particle of matter in the uni- verse is altering its motion with reference to every other particle," Newton not only enlarged the horizon of physics, but gave the world perhaps its finest illus- tration of a focalising " thought-economising " for- mula, whose universality and accuracy seem alike indisputable. Here the science passed beyond ob- servation and description to the recognition of a uni- fying idea. Thirdly, in his laws of motion and other principles Newton gave a marvellous — if still imperfect — pre- cision to the concepts — of force, matter, and the like — with which the physicist works. Some would say with Prof. Ernst Mach f that Newton " completed the enunciation of the principles of mechanics," or with Thomson and Tait that " every attempt to supersede them has ended in utter failure " ; while others would rather say with Karl Pearson that the progress of two centuries has given good reason for trying to modify and restate the Leges Motus, es- pecially in the direction of purifying them, if it be * J. T. Merz, History of European Thought, I., p. 317. \Mechanik in ihrer •' Entwickehmg, 3d ed., 1889, trans. Chicago, by J. T. McCormack, 1893. THE PROGRESS OF PHYSICS. 135 possible, from the metaphysical obscurities which lurk even in their apparent lucidity.* But all will agree that Newton supplied the firm foundation on which, especially during the last hundred years, phys- ical science has gradually grown into a stately edifice. It is doubtless true that Xewton stood on the shoulders of Galilei, but his genius in discerning the unity amid multiplicity was none the less great, and there is no finer instance of a unifying idea than the gravitation-formula. At the same time, it must be recognised that, like other big scientific generalisa- tions, the gravitation-theory raised problems which it did not answer. What we have is a general formula: that every particle or atom or body in the universe at- tracts every other with a force proportional to their masses taken conjointly, and inversely proportional to the square of their distances apart. This may be called the law of gravitation, but is there no theory of the law ? In this respect there has been little ad- vance since the beginning of the nineteenth century. It was then that Lesage of Geneva suggested that in addition to the gross particles of tangible or sensi- ble matter, " infinite as these are in number, there is an infinitely greater number of much smaller ones darting about in all directions with enormously great velocities. Lesage showed that, if this were the case, the effects of their impacts upon the grosser particles or atoms of matter would be to make each two of these behave as if they attracted one another with a force following exactly the law of gravity. In fact, when two such particles are placed at a distance from one another, each, as it were, screens the other from * Grammar of Science, Chapter VIII., " The Laws of Motion." J 136 PROGRESS OF SCIENCE IN THE CENTURY. a part of the shower which would otherwise batter upon it. . . It is necessary also to suppose that par- ticles and masses of matter have a cage-like form, so that enormously more corpuscles pass through them than impinge upon them ; else the gravitation action between two bodies could not be as the product of their masses." * But this speculation is only a pro- visional stop-gap. To the easy-going materialists, if any survive, the ignoramus of one of our leading physicists should give pause : — " Directly we use the term l weight,' we are confronted with the fact that not yet have we any real clew to that astonishing fact of universal gravitation." f SUMMARY. — The foundation of modern physics is in Newton's Principia (1687) whose value is more fully appreciated at the end than it was at the begin- ning of the nineteenth century. CONSERVATION OF ENERGY. The Idea of Energy. — Energy is a convenient term for the power of doing work which is possessed by a material system, or by the ether which modern phys- ics has invented as a hazy background of matter. A stream flowing down a valley illustrates energy of motion, it may turn mill-wheels or bear away bridges ; the reservoir on the plateau illustrates energy of posi- tion, which intention or accident may at any moment bring into operation. These two types of power are, as every one knows, called kinetic energy and poten- tial energy. Whether the kinetic energy be expressed in visible motion, as of the stream, or invisible mo- * P. G. Tait, Recent Advances in Physical Science, 1876, pp. 299-300. t Prof. Oliver J. Lodge, " Modern Views of Matter," Jnternat. Monthly, I. (1900), p. 525. THE PROGRESS OF PHYSICS. 137 tion, as in the particles of a heated bar of iron ; wheth- er the potential energy be expressed in a visible ar- rangement of bodies, as in the stone resting on the roof-edge, or in invisible arrangements, as in the mutual relations of particles in an explosive ; we sum up all the different forms in the one conception of energy or power. The convenience of this concept " Energy " to sum up groups of sense-impressions is obvious, but it must be borne in mind that in using the term we are simply making an abstraction which proves useful in the rapid discussion of the forms or modes of motion which we see and measure. Clerk Maxwell said in his remarkable little book Matter and Motion: " We are acquainted with matter only as that which may have energy communicated to it from other matter, and which may in turn communicate energy to other matter," and again, " Energy, on the other hand, we know only as that which in all natural phenomena is continually passing from one portion of matter to another." But, as Karl Pearson points out, these statements do not carry us far. " The only way in which we can understand matter is through the en- ergy which it transfers. . . . The only way to un- derstand energy is through matter. Matter has been defined in terms of energy, and energy again in terms of matter." " The activity of the material universe," says Prof. Oliver Lodge, " is due to, or represented by, or dis- played in, the continual interchanges of energy from matter to ether and back again, accompanied by its transformation from the kinetic to the potential form and vice versa" * *" Modern Views of Matter," Internal. Monthly, I. (1900), p. 500. 138 PROGRESS OF SCIENCE IN THE CENTURY, Transformations of Energy. — Before methods of measuring the different forms of what we call energy had been elaborated, it was evident that one kind of power was continually being changed into another. Carbon and oxygen have in separation potential energy — the energy of chemical affinity for one an- other, and this is manifested by the heat which they give off when they unite; the heat may be in great part utilised to convert water into steam ; the " expan- sive force " of the steam lifts the piston ; the wheels go round; the energy re-appears partly in the poten- tial form of work done and partly in the heat which results from overcoming friction. The energy of the sunlight enables the plant to build up complex food- stuffs out of simple raw materials ; substances of high potential energy thus result; these become sources of power to man and beast. The energy of chemical separation may be transformed into heat, light, mag- netism, electricity, and so on; or heat, light, and electricity may be used to effect chemical separation. Moreover, all the powers we can employ (except in the case of tidal currents) are directly or indirectly traceable to the energy radiated from the sun, or to stores of potential energy in the earth, which again we have to thank the sun for. Conservation. — These considerations lead us to the doctrine of the conservation of energy, which is one of the foundations of Physics. It is an induc- tion -from experience which states that " the total amount of energy in a material system cannot be varied, provided the system neither parts with energy to other bodies nor receives it from them." There may be degradation or dissipation of energy, as * Article " Energy," Chambers's Encyclopedia, by Dr. W. Peddle. THE PROGRESS OF PHYSICS. 139 when heat passes into the air, but destruction of energy is unknown. Energy is the power of doing work; work is the act of producing a change of configuration in a sys- tem in opposition to resistance; and the doctrine of the conservation of energy is thus expressed by Clerk Maxwell : " The total energy of any material system is a quantity which can neither be increased nor diminished by any action between the parts of the system, though it may be transformed into any of the forms of which energy is susceptible." Dissipation of Energy. — And to this doctrine of conservation there has to be added the corollary, which Sir William Thomson (Lord Kelvin) first focussed into lucidity (1852) — " the principle of dis- sipation or degradation," which is " simply this, that as every operation going on in nature involves a transformation of energy, and every transformation involves a certain amount of degradation (degraded energy meaning energy less capable of being trans- formed than before), energy is becoming less and less transformable." * Foundation of the Doctrine of the Conservation of Energy. — Just as the doctrine of the indestructibility of matter became stable with the perfecting of the balance, so the doctrine of the conservation of energy must be associated with the determination of the mechanical equivalent of heat, — with the experiments of Rumford and Davy leading on to those of Colding and Joule. At the same time, it should be borne in mind that, according to Thomson and Tait, the prin- ciple is clearly implied in Newton's scholium to his third law of motion, — that " if the action of an ex- ternal agent is estimated by the product of its force *P. G. Tait, Recent Advances (1876), pp. 145-6. 140 PROGRESS OF SCIENCE IN THE CENTURY. into its velocity, and the reaction of the resistance in the same way by the product of the velocity of each part of the system into the resisting force, arising from friction, cohesion, weight, and acceleration, the action and reaction will be equal to one another, what- ever be the nature and motion of the system." We have placed the doctrine of the conservation of energy before the dynamical theory of heat because many discoveries were pointing towards the great conclusion of the transf ormability and conservation of energy, before Joule's measurement of the mechanical equivalent of heat made the vaguely foreseen conclu- sion an established doctrine. None the less, however, would we emphasise that the establishment of the general doctrine dates from Joule's success as a measurer of the relation between heat and mechanical work in 1843. For it was then that one of the greatest scientific steps of the century was made. " Clear and unques- tionable experimental proof was given of the fact that there is a definite relation between mechanical work and heat ; that so much work always gives rise, under the same conditions, to so much heat, and so much heat to so much mechanical work. Thus originated the mechanical theory of heat, which became the start- ing point of the modern doctrine of the conservation of energy. Molar motion had appeared to be destroyed by friction. It was proved that no destruction took place, but that an exact equivalent of the energy of the lost molar motion appears as that of the molecular motion, or motion of the smallest particles of a body, which constitutes heat. The loss of the masses is the gain of their particles." * * T. H. Huxley, Essay on " The Progress of Science" (1887), in Method and Results, 1894, pp. 85-86. THE PROGRESS OF PHYSICS. While we have given the foremost place to Joule in connection with the doctrine of energy, we must also recognise the genius of Helmholtz, as expressed in his work on Die Erhaltung der Kraft (the persis- tence of force), published in 1847, in which he showed that this great conclusion follows from New- ton's second interpretation of the third law of motion, if we make the postulate (sufficiently justified exper- imentally) of the impossibility of " perpetual motion." STTMMABY. — "In his determination of the me- chanical equivalent of heat, James Prescott Joule gave to the world of science the results of experiments which placed beyond reach of doubt or cavil the greatest and most far-reaching scientific principle of modern times, namely, that of the conservation of energy." HEAT AS A MODE OF ACTION. Old Theory of Heat as a Kind of Matter. — The theory that heat is a subtle kind of matter was sug- gested by some of the Greek philosophers, and it was a dominant theory in the eighteenth century. In the interpretation of combustion defended by Stahl (1660-1734) a burning body was supposed to give off a substance called " phlogiston." Lavoisier in- cluded heat in his list of elements. Seventeenth Century Theories of Heat as a Mode of Motion. — A more remarkable fact, however, is that in the seventeenth century the modern view was, to say the least, clearly hinted at. As Cajori notes in his History of Physics, " We are surprised to find that Newton's immediate predecessors had antici- pated our modern theory of heat. Heat a Mode of * Sir Henry Roscoe, Pres. Address, Rep. Brit. Ass.. 1887, p. 4. 142 PROGRESS OF SCIENCE IN THE CENTURY. Motion is the title of Tyndall's well-known work (1862), yet Descartes, Amontons, Boyle, Francis Bacon, Hooke, and Newton already looked upon heat as a mode of motion. Of course, in the seventeenth century, this theory rested upon somewhat slender experimental evidence, else the doctrine could hardly have been cast to the winds by the eighteenth cen- tury philosophers." The Fiction of Imponderable Matter. — Even in the eighteenth century, it could not but be noticed, when the habit of weighing began, that a body which had been heated was no heavier than it was before. Therefore a fiction had to be invented, — the well- known fiction of the " imponderables." Heat, or rather " caloric," was a substance, but it was an im- ponderable substance. The further difficulty that heat may be produced in abundance apart from all fire or combustion, — even by rubbing two pieces of ice together, — and that it may in other cases disappear beyond trace, seems to the modern outlook quite fatal to the material theory of heat, but the difficulty does not appear to have oppressed the natural philoso- phers of the eighteenth century. It must be recalled that the doctrine of the indestructibility of matter dates from Lavoisier and that it was not fully ap- preciated till much later. With this and the doctrine of the conservation of energy now clearly before us, the materiality of heat seems like a contradiction in terms, but this is to be wise after the event. Let us therefore consider how the old Newtonian idea was re-habilitated, how it has come to be an elementary fact in physics that heat depends upon motion of the particles of a body, and is a form of energy, not a kind of matter. Rumford's Experiments. — The first strong blow THE PROGRESS OF PHYSICS. 143 which the caloric theory received was dealt it by Ben- jamin Thompson, better known as Count Rumford, who published his observations on the boring of can- non at Munich in 1798. Surprised at the amount of heat given off in the operation, he determined to measure this by its effect in raising the temperature of surrounding water. "At the end of two hours and thirty minutes the water actually boiled ! " and Count Rumford argued : " It is hardly necessary to add that anything which an insulated body, or system of bodies, can continue to furnish without limitation, cannot possibly be a material substance, and it ap- pears to me to be extremely difficult, if not impossible to form any distinct idea of anything capable of being excited and communicated in the manner in which heat was excited and communicated in these experi- ments, except it be motion" The supporters of the idea that heat is a material substance argued that the production of heat by fric- tion or abrasion was due to the fact that the fragmen- tation of the body diminished its capacity for holding caloric; and if, as Prof. Tait points out, Rumford had seen his way to a satisfactory experiment which would have tested the capacity for heat of the abraded metal and of the metal before abrasion, then the fact that heat is not matter would have been established. But the essential experiment — most readily a chem- ical one — did not suggest itself, and this is in part the reason why Rumford's experiments published in 1798 were but little noticed until about 1840. Rumford's argument was on the main line of prog- ress, but his measurement of the heat evolved by fric- tion was rough, and he was unable to make a definite comparison between the energy expended and the work done anfl the heat dissipated. 144 PROGRESS OF SCIENCE IN THE CENTURY. Davy's Contribution. — A more delicate experiment was devised in IT 9 9 by Sir Humphry Davy, who ar- ranged a clockwork for rubbing two pieces of ice against one another in the vacuum of an air-pump, and observed that part of the ice was melted, although the temperature of the receiver was kept below the freezing point. From this he concluded somewhat diffidently that friction causes vibration of the par- ticles, which is heat ; — a conclusion which he strength- ened in 1812 in the statement that " the immediate cause of the phenomenon of heat is motion and the laws of its communication are precisely the same as the laws of the communication of motion." Thomas Young was another of the early supporters of Count Rumford's view. Work of Carnot. — Meanwhile important progress was made, by Dulong and Petit (1815), Haugergues (1822), and others, on the measurement of temper- atures by means of thermometers; by Faraday and others on the liquefaction of gases, and on many other subjects associated with heat : but the next important step in general theory was made by Sadi Carnot (1796-1832), who, in 1824, published his estimate of the amount of work that can be got from a steam- engine, and introduced the fruitful idea of a revers- ible cycle of operations. But this was hardly known until Sir William Thomson called attention to it in 1848. " Without this work of Carnot's, the modern theory of energy, and especially the dynamical theory of heat, could never have attained in so few years its now enormous development." * " The two grand things which Carnot introduced, which were entirely originated by him, and which left * Prof . P. G. Tait's Recent Advances (1876), p. 95. THE PROGRESS OF PHYSICS. 145 him in an almost perfect form, were the idea of a Cycle of Operations and the further idea of a Re- versible Cycle. In order to reason upon the working of a heat-engine (suppose it for simplicity a steam- engine) you must imagine a set of operations, such that at the end of the series you bring the steam or water back to the exact state in which you had it at starting. That is what Carnot calls a cycle of opera- tions, and of it Carnot says, then, and only then, i.e., at the conclusion of the cycle, are you entitled to reason upon the relation between the work which you have acquired, and the heat which you have spent in acquiring it." * " The other grand point with reference to Carnot is this, that he started the notion of a Reversible En- gine,— reversible not in the ordinary technical sense of working its parts backwards, not in the mere sense of backing, but reversible in the sense that, instead of using heat and getting work from it, you can drive your engine through your cycle the other way round, and by taking in work, pump back heat (as it were) from the condenser to the boiler again — a reversing of the whole process, — not a mere reversing of the direction in which the engine is driving. Now, Carnot introduced that notion, and he showed by perfectly conclusive reasoning that if you can obtain a reversible engine, it is the perfect engine; i.e., that it is impossible to get an engine more perfect than a reversible one." f Although he began with a firm belief in the caloric theory, Carnot ended to all intents and purposes as an adherent to the modern dynamical view, and that he had grasped the principle of conservation is evi- dent from his conclusion : " Motive power is in * P. G. Tait, loc. cit., p. 97. t P. G. Tait, loc. cit., p. 98. 14:6 PROGRESS OF SCIENCE IN THE CENTURY. quantity invariable in nature; it is, correctly speak- ing, never either produced or destroyed." Joule and Colding. — Prof. Tait notes that one small chemical experiment would have enabled Rum- ford in 1798 to prove that heat is not matter, just as a little more conclusive reasoning would have brought Davy in 1799 securely to the same conclusion, — which he eventually deduced in 1812. What Seguin and Mayer approached, but, by de- parting from the scientific method, failed to attain, was achieved by Colding of Copenhagen and Joule of Manchester, " the true modern originators and ex- perimental demonstrators of the conservation of energy in its generality." * To Joule in particular, for his experiments were more extensive, his measurements more exact, his con- clusions more generalised than those of Colding, we owe a difficult proof of what Rumford and Davy had foreseen — the First Law of Thermodynamics. In Tait's. statement this reads : " When equal quantities of mechanical effect are produced by any means what- ever, from purely thermal sources, or lost in pure thermal effects, then equal quantities of heat are put out of existence or are generated ; and for every unit of heat measured by the raising of a pound of water 1 degree Fahrenheit in temperature, you have to ex- pend 772 foot-pounds of work." f SUMMARY. — The idea that heat is not material but a mode of motion, a form of energy, is older even than Newton s Principia, yet the foundation of the theory may be fairly dated from the experiments of Joule. But many others contributed to the great conclusion, and still more have furthered its development and ap- plication. * Tait, op cit., p. 567. t Approximately. THE PROGRESS OF PHYSICS. 147 KINETIC THEORY OF GASES. We have had occasion to refer to this important theory in the chapter on Chemistry ; it will be enough to recall two or three of the steps in its develop- ment. Diffusion. — Every one is aware of the rapidity with which an escape of coal-gas makes itself felt through a house. Dalton theorised this in his sug- gestion that a gas consists of particles which are constantly flying about in all directions, spreading as far as they can, and inter-penetrating another gas, or mixture of gases in the case of air, until equilib- rium of pressure is attained. A more precise study of the movements of gaseous particles was subsequently undertaken by Graham, who showed that the relative rates of diffusion of two gases are inversely proportional to the square roots of their densities. Thus hydrogen diffuses four times more quickly than oxygen. Joule's Calculation of Velocity of Particles. — In 1848 and 1857, Joule took another stride forward in determining the mean translational velocity of the particles, basing his calculations on the conclusion that the pressure of a gas is proportional to the energy of motion of its particles. " Thus it may be shown that the particles of hydrogen at the barometrical pressure of 30 inches, at a temperature of 60°, must move with a velocity of 6225.54 feet per second in order to produce a pressure of 14.714 Ibs. on the square inch." In other words, as Sir Henry Roscoe expresses it, a molecular cannonade or hailstorm of particles is maintained against the bounding surface at a rate far exceeding that of a cannon ball. 14:8 PROGRESS OF SCIENCE IN THE CENTURY. It seems that the clearness of the Newtonian view of the movements of the heavenly bodies often sug- gested to chemists and others who thought about atoms and molecules, that these might be bound to- gether in a manner comparable to a planetary system. But the behaviour of gases and the phenomena of heat (so long regarded as a substance) made it nec- essary to suppose that forces of repulsion as well as attraction existed between particles. Gradually the intrusion of what Merz calls " the astronomical view of nature " to support the incipient " atomic view of matter " was found unavailing. The atomic view passed from its static to its kinetic phase, and we may particularly associate this important step with the names of Joule, Clausius, and Clerk Max- well. Although Bernouilli (1738), Herapath, Waterston and many others must find their recognition in learned histories, it was Joule who first gave precise expression to the theory that all particles of gases may be thought of as being in a natural state of rectilinear motion, changed only by their mutual encounters, or by their impinging on containing barriers. It was soon after the half -century (published 1857) that Joule, as we have noted, calculated the velocity of a particle of hydrogen at ordinary atmospheric press- ure and temperature. The calculation presupposed the previous discovery by Eumford, Davy, Mayer, and Joule that heat is not a substance but a mode of motion, and the experimental proof by Joule and Thomson (1853) that in a gas allowed to expand without doing work there is a very slight cooling, due to the energy used up in overcoming the attract- ing forces of cohesion. The general argument is simply that if heat can THE PROGRESS OF PHYSICS. 149 be transformed into the energy of measurable motion of measurably large or molar masses, heat may it- self be " the energy of the directly immeasurable movements of molecular (immeasurably small) masses." Developments. — "By applying calculations simi- lar to those of Joule, but considerably extended by the use of more powerful mathematical methods, such as the methods of the theory of probabilities, Clausius first, and, a little later, but far more profoundly, Clerk Maxwell, and still more recently Boltzmann, have arrived at very valuable results as to the motions of swarms of impinging particles. One of the results arrived at is that in a mass of hydrogen at ordinary temperature and pressure, every particle has on an average 17,700,000,000 collisions per second with other particles ; that is to say, 17,700,000,000 times in every second it has its course wholly changed. And yet the particles are moving at a rate of some- thing like 70 miles per minute. So comes this curious problem — given that the direction of motion of a particle is arbitrarily changed 17,700,000,000 times in every second, and that the particle itself is moving 70 miles in a minute, where would it be at the end of a single minute, having started from any given place? . . . The solution obtained is capable of explaining almost everything that we know with reference to the behaviour of gases, and perhaps even of vapours." * SUMMAEY. — The kinetic theory of gases, the brilliant generalisation which harmonised the nu- merous facts — specific heat, diffusion, friction , etc., — known in regard to the behaviour of bodies in a gaseous state, may be regarded as a corollary of the * Tail's Recent Advances, 1876, pp. 324-5. 150 PROGRESS OF SCIENCE IN THE CENTURY. dynamical theory of heat. " The fundamental idea that a gas was an assemblage of moving particles had been put forward by D. Bernouilli and by Herepath, and Joule had in 1851 made a great step in advance by calculating the mean translational velocity of these particles. . . This idea, in the hands of Kronig and Clausius, gave birth to the modern kinetic theory of gases, which has been so splendidly worked out by Clausius and Maxwell, and since then perfected in detail by Boltzmann, 0. E. Meyer, Van der Waatej and many others." * UNDULATORY THEORY OF LIGHT. The Emission Theory. — Throughout the eigh- teenth century the corpuscular or emission theory of light was almost universally accepted by physicists. The theory was that all luminous bodies emit with equal velocities inconceivably minute elastic corpus- cles which travel at great speed in straight lines in all directions. The Modern View. — Nowadays, however, it is the unanimous view of those who are familiar with the facts that light is not a material substance, but a form of energy, or a mode of motion, in fact the re- sult of ethereal waves. When a body gives forth light, we no longer suppose that it emits corpuscles, as a grain of musk does into the air ; we believe that it sets agoing undulatory movements in the ether. We believe furthermore that the phenomena of light are essentially of the same nature as those of electro- magnetic radiation. The contrast of the theories in the two centuries is characteristic, and it is interest- ing to enquire how the modern view was developed. * E. von Meyer, History of Chemistry, trans. 1891, p. 414. THE PROGRESS OF PHYSICS. 151 "While the corpuscular theory served to interpret a number of the phenomena of light, it failed more or less markedly in regard to others — for instance, the reflection which accompanies refraction, the unequal refrangibility of the different colours of the spec- trum, double refraction, and so on. The result was that subsidiary hypotheses had to be invented to cover the defects of the main assumption. Eventually it became necessary to discard the main assumption altogether. Newton's Position. — The central idea of the un- dulatory theory was suggested by Hooke and others, and was formulated as early as 1678 by Huygens, who interpreted double refraction, but its establish- ment was due to the work of Thomas Young and Fresnel. Although Descartes had suggested that light is produced by waves excited in the subtle mat- ter which pervades the universe (analogous to but different from the non-atomic ether of to-day), and had also ventured the suggestion that the mechanism of light and that of gravitation are inseparable, and although Hooke had made the important suggestion of substituting for the progressive wave of Descartes a vibrating one, we find Xewton weighing the merits of the wave-theory and the emission-theory, finding both unsatisfactory and deliberately refraining from accepting either. Apart from his " theory of fits," — in which he states that the phenomena of thin plates prove that the luminous ray is put alternately in a certain state or fit of easy reflection and of easy transmission — he abstains from taking up a definite position, though " he shall sometimes, to avoid cir- cumlocution and to represent it conveniently, speak of it (the emission) as if he assumed it and pro- pounded it to be believed." It does not seem to be 152 PROGRESS OF SCIENCE IN THE CENTURY. historically justifiable to regard Newton as the founder or even upholder of the emission-theory.* The ray of light, on the emission-theory, was sim- ply the trajectory of a particle in rectilinear motion; the ray of light, as Newton described it, possesses a regular periodic structure, and the period or interval of fits characterises the colour of the ray. This was an important result. It only required a fitter inter- pretation to transform the luminous ray into a vibratory wave, but for this there was a century to wait, and Dr. Thomas Young, in 1801, had the honour of discovering it.f The Wave-Theory of Young. — Thomas Young (1773-1829), whose precocious genius, persisting in manhood, remained, as Tyndall says, " hidden from the appreciative intellect of his countrymen," was led from a study of the eye and its optical properties, to an enquiry into the phenomena of thin plates and " interference," and in the course of this he rehabili- tated the undulatory theory (1801), published in the Philosophical Transactions for 1802. The theory is, in general terms, that light consists of vibrations in an all-pervading elastic ether, and that the vibrations, unlike those of sound, are in di- rections at right angles to the direction of propaga- tion. So far as Young went, the theory was, in simple language, that a homogeneous ray of light is analogous to the wave produced by a musical sound, and that the vibrations of light ought to compose or interfere, like those of sound. " But his hypothesis found no favour; his principle of interference led * A. Cornu, The Rede Lecture: "The Wave Theory of Light: its influence on Modern Physics," Nature, July 27, 1899, pp. 292-297. t From Prof. Cornu's Rede Lecture. THE PROGRESS OF PHYSICS. 153 to this singular result that light added to light could, in certain cases, produce darkness, a paradoxical re- sult contradicted by daily experience." In spite of Young's step, the emission-theory still held the field, and new facts, such as the phenomenon of polarisation discovered by Malus, lent support to it rather than to its rival. Fresnel's Experiments. — In 1816, however, a young engineer, Augustin Fresnel (1788-1827), re- discovered the principle of interference, applied mathematical analysis to the vindication of the un- dulatory theory, and devised the famous two-mirror experiment, by which it was shown that " two rays, issuing from the same source, free from any disturb- ance, produced when they met, sometimes light, some- times darkness." Moreover Fresnel showed that " light is propagated in straight lines because the luminous waves are extremely small, while sound is diffused because the lengths of the sonorous waves are relatively very great," and that " the sound wave cannot be polarised because the vibrations are longitudinal, while light can be polarised because the vibrations are transverse, that is to say, perpendicular to the luminous ray." " Henceforth the nature of light is completely established, all the phenomena presented as objections to the undulatory theory are explained with marvellous facility, even down to the smallest details." f To Fresnel and to Arago, Young " was first in- debted for the restitution of his rights," and it is pleasant to notice the entire absence of any discussion as to priority. But the complete acceptance of the un- dulatory theory was still distant. There followed a * Cornu, loc. cit., p. 295. t Quotations from Cornu. 154 PROGRESS OF SCIENCE IN THE CENTURY. period in which it had still to struggle for existence, when it had to justify itself in application to the phenomena of shadows, double refraction, polarisa- tion, colour, interference, diffraction and so on. With Young, Fresnel, Arago, and others on the winning side, with Laplace, Biot, and Brewster and others championing the older doctrine, a keen, some- times painfully bitter, struggle of opinions continued till the century had run more than a quarter of its course. Joule. — It should not be forgotten that Joule, who contributed so much to the foundation of the dy- namical theory of heat and the kinetic theory of gases, and founded the general doctrine of the con- servation of energy, also made an important experi- ment (1843) bearing on the theory of Light. "He compared the heat evolved in the wire conducting a galvanic current, when the wire was ignited by the passage of the current, with that evolved when (with an equal current, suppose) it was kept cool by immer- sion in water. These experiments showed a small, but unmistakable, diminution of the heat when light also was given out." * Foucault. — It was not, however, till 1850 that an- other crucial experiment in favour of the undulatory theory was announced by Foucault (1819-1868). According to the emission-theory the velocity of light should be greater in an optically denser me- dium ; according to the undulatory theory the reverse should be true. By an ingenious and now familiar device, Foucault, the inventor of the gyroscope and the demonstrator of the Earth's rotation by pendulum experiments, gave the death-blow to the Newtonian * Tait, Recent Advances, 187G, p. 64. THE PROGRESS OF PHYSICS. 155 theory by proving that the velocity of light in water is less than that in air. Fizeau. — The determination of the velocity of light, -which thus became of importance in relation to the general theory, had been previously based, e.g., by Romer and Bradley, on astronomical data, derived from aberration-observations, or from timing the eclipses of Jupiter's satellites when at their greatest and least distances from the Earth, but a direct ex- perimental method was devised by Fizeau (1819- 1896). In 1849, in the suburbs of Paris, he ar- ranged a rapidly rotating cog-wheel which inter- cepted light at regular intervals, and found what speed must be given to the wheel so that it rotated one tooth's breadth while the light travelled to a distant mirror and was reflected back again. Fou- cault modified this method by observing " the posi- tion ultimately assumed by a ray which travels from a source to a rotating mirror, thence to a dis- tant mirror, and thence back to the original mirror, which by this time has been rotated somewhat." * The determination of the velocity of light thus effected by Fizeau and Foucault was revised by Cornu in Paris, by James Young and George Forbes in Britain, but the most accurate determinations are said to be those made by Michelson, Xewcomb, and Holcombe, in the United States. A mean result is that light travels in vacuo at the rate of 186,772 miles per second, and in air at a velocity less than this in the ratio of 10,000 to 10,003. As Professor Alfred Cornu points out in his Rede lecture, to which we have already been much in- debted in this section, the emission theory was a natural but primitive one, with its germ in the ex- * Article Light, by Dr. Daniell. Chaiui^r*' 156 PROGRESS OF SCIENCE IN THE CENTURY. perience of throwing a stone or shooting an arrow into " empty space." The undulatory theory is subtler, space is filled with a continuous elastic medium, in which particles — no longer projectiles — were supposed to oscillate in the direction of propa- gation, like the particles of water in the ripples on a pond. But this conception was insufficient and gave place to Fresnel's idea of waves of transverse vibrations excited in an incompressible continuous medium. Electro-magnetic Theory of Light. — The necessity of admitting the existence of this medium was made clearer by Faraday, and corroborated by his dis- covery of induction, and Clerk Maxwell in his foot- steps ventured to forecast, on theoretical grounds, that light and electro-magnetic radiation are alike due to rhythmical disturbances in the ether, differ- ing only in their wave-lengths — one of the most uni- fying ideas in modern science. Experiments of Hertz. — " But the abstract the- ories of natural phenomena are nothing without the control of experiment. The theory of Maxwell was submitted to proof, and the success surpassed all expectation. ... A young German physicist, Heinrich Hertz, prematurely lost to science, starting from the beautiful analysis of oscillatory discharges by Von Helmholtz and Lord Kelvin, so perfectly produced electric and electro-magnetic waves, that these waves possess all the properties of luminous waves; the only distinguishing peculiarity is that their vibrations are less rapid than those of light. It follows that one can reproduce with electric dis- charges the most delicate experiments of modern optics — reflection, refraction, diffraction, rectilinear, circular, elliptic polarisation, etc." * Cornu. Rede Lecture. Loc. cit. , p. 200, THE PROGRESS OF PHYSICa 157 We owe to Clerk Maxwell, and to Hertz, for experimental corroboration, the image of a plane wave of light as a propagation of an ethereal dis- turbance, in which there is electric and, at the same time, magnetic intensity, varying as a simple har- monic function of the time. In what may seem to be plainer words, we regard light as an electric phe- nomenon, and the term electric light as a tautology. Invisible Light. — From what has been said it may be inferred that light has many forms, and that it is not necessarily visible. Even in sunlight there are components which are not visible to our eyes. One of the most recent additions (1896) is that of an invisible radiation which Becquerel discovered to be emitted by many fluorescent substances and especially by Uranium salts. The radiation can be polarised and by means of it (as by the Rontgen rays) photographs can be obtained through opaque bodies. Moreover, like the Rontgen rays, the Ura- nium-radiation causes an electrified body to lose its charge, whether positive or negative.* SUIIMAKT. — By Young and Fresnel, Fizeau and Foucault and by others the emission theory of light was replaced by the undulatory theory. Light was interpreted in terms of ethereal waves, and Clerk Maxwell and Hertz subsequently showed that it was essentially similar to electro-magnetic radiations. THEOBT OF EiZCTEICITT. Beginnings. — In the last quarter of the eighteenth century, the Italian Galvani — whose name has given our language several new words — had discovered * See J. J. Thomson. Address Section A, Rep. Brit. Asi. for 1896, p. 703. 158 PROGRESS OF SCIENCE IN THE CENTURY. that electrical changes occurred in the contracting muscle of the frog's leg ; in the last year of the same century Volta of Pavia had shown that electricity may be produced by the simple contact of two metals ; but, for a time, little resulted from the discoveries of either of these pioneers. Another impulse was necessary before the wheels of progress began to move, and that was afforded in 1819, by Oersted, who brought the known facts of electricity into touch with those of magnetism, and initiated the movement which has made the word electricity almost as charac- teristic of the nineteenth century as the word evolu- tion. Achievements. — Forestalling the rest of this sec- tion, we may briefly state that the scientific study of electricity initiated by Oersted and also by Ampere, was profoundly influenced by the experimental genius and scientific temper of Faraday, found mathematical or precise formulation in the work of Thomson (Lord Kelvin), and was developed into a provisional dynamical theory by the extraordinary insight of Clerk Maxwell. It is perhaps not too much to say that what Newton did for gravitational phenomena, was done by Clerk Maxwell for electrical phenomena. The study was raised by him and his collaborateurs from the observational and classi- ficatory level to become an integral part of a unified Natural Philosophy. Oersted. — Oersted (1777-1851) may be called the founder of the science of electro-magnetism because he succeeded in proving experimentally (1819) what had been previously surmised, for in- stance from the effect of lightning on compasses, — that electrical and magnetical phenomena are of the eauie nature. In his famous experiment showing THE PROGRESS OF PHYSICS. 159 the disturbance of the magnetic needle by the influ- ence of an adjacent electrical current, he not only made a step of great theoretical import, but pointed forward (as we now recognise) to the invention of the telegraph. Oersted's experiment suggested the possibility of measuring the strength of an electric current by its effect upon an adj acent magnet, and this led Schweig- ger in 1820 to his invention of the galvanometer or electrometer, a fundamental instrument in electrical science. As the history of galvanometers alone is a long one, we must be content here to note that after modifications by Nobili and Pouillet and others, the measuring instrument was brought to great per- fection by Sir William Thomson (Lord Kelvin). Oersted observed the influence of a current on a magnet, and that the latter always tends to set itself at right angles to the direction of the current, but a further step was soon taken by Ampere (1775-1836), who showed (1820) that one current influences an- other, parallel currents in the same direction being attracted, those in opposite directions being repelled by each other. His mathematical theory of these phenomena is still referred to as a masterpiece. Ohm. — To Ohm (1789-1854) the science was greatly indebted for the precision which he gave to the conceptions of electro-motive force, strength of current, electric resistance and conductivity, and for the law (experimentally established in 1826, mathe- matically worked out in 1827) which states that the resistance of a conductor can be measured by the ratio of the electro-motive force between its two ends to the current flowing through it. It appears that this empirical generalisation had been reached in 1781 by Cavendish, but practically its recognition 160 PROGRESS OF SCIENCE IN THE CENTURY. must date from Ohm's work. " Since his day it has been subjected to the severest experimental tests that the scientific mind could imagine, and has stood them all. It is really the basis of our whole system of electrical measurements, and is to electric currents what the law of gravitation is to planetary mo- tions." * The instrumental measurement of resistance which Ohm initiated was subsequently brought nearer per- fection, especially by those concerned in the develop- ment of telegraphy. Thus Charles Wheatstone (1802-18T5) invented what is known as " Wheat- stone's bridge." Here, as in so many other cases, practical requirements led to improvements which stimulated theoretical science and gave it greater possibilities of precision. Faraday. — The next great name is that of Michael Faraday (1Y91-1867), who by common consent is ranked as the greatest experimental genius of the nineteenth century as regards electricity and magnet- ism. Among his numerous achievements three must be specially mentioned. While Oersted had shown the deflection of the mag- netic needle by an electric current, Faraday suc- ceeded in demonstrating the converse; that a magnet reacts upon an electric current. This was the dis- covery of magneto-electricity (1831), and it led him on to another of no less importance, that of induced currents (1831), — that a wire through which an electric current is passing may induce in another adjacent wire a state similar to its own. With Fara- day's discoveries there must also be associated the entirely independent but synchronous work of the * Prof. C. G. Knott. Article, Electricity, Chambers' Ency- clopaedia. THE PROGRESS OF PHYSICS. 161 American Joseph Henry (1799-1878), who also detected the influence of magnetism upon electricity and the phenomenon of induction-currents. Another of Faraday's achievements has already been referred to in the chapter on chemistry, — the discovery of the laws of electrolysis. He showed that the amount of water decomposed or gas set free is strictly proportional to the quantity of electricity passing through, and that equal quantities of elec- tricity decompose equivalent amounts of different electrolytes. In the third place Faraday thought out a dy- namical theory of electricity, which replaced the old two-fluid theory, and has formed the foundation on which Kelvin, llaxwell, Helmholtz, and others have reared an elaborate superstructure. While Coulomb and others had assumed the possibility of " action at a distance," and supposed that electric charges may influence one another without any intervening me- dium, Faraday's ideas were distinctly opposed to this view, for he supposed that electric attraction and re- pulsion were propagated by molecular agitations in the particles of the insulating media which he termed " dielectrics." He found reason to believe that in- ductive influence takes effect along curved lines (" lines of force ") and by the action of adjacent par- ticles in the insulating medium. As the intensity of the electric influence between two charged bodies varies with the nature of the " dielectric," he was led, as Cavendish had been, to the recognition of " specific inductive capacity " — a factor of fundamental im- portance. As Cajori points out, Faraday's theory gave a death-blow both to the old fluid theory and to the assumption of action at a distance. Uaxwdl. — What Faraday had expressed in his 162 PROGRESS OF SCIENCE IN THE CENTURY. symbolism of " lines of force," was re-expressed and further developed in the sterner language of mathe- matics by James Clerk Maxwell (1831-1879), who was also led to conclude on theoretical grounds that electro-magnetic phenomena and light phenomena are alike due to waves of periodic displacement in the same medium (the hypothetical ether), and are, in fact, identical in nature. Hertz. — What Clerk Maxwell had theoretically foreseen was experimentally demonstrated by Hein- rich Rudolf Hertz (1857-1894), who detected tho electromagnetic (electric and magnetic) waves radi- ating into space from the sparks of a Ley den jar or of a Holtz machine, separated the two components, electric and magnetic, and succeeded in reflecting, refracting, diffracting, and polarising the waves. " The object of these experiments," he says, " was to test the fundamental hypothesis of the Faraday- Maxwell theory, and the result of the experiments is to confirm the fundamental hypotheses of the theory." * As Hertz fully recognised, Professors Oliver Lodge and G. F. Fitzgerald were about the same time within sight of the same discovery of the electro-magnetic waves in air. In a review of electrical advance in recent years, Mr. Elihu Thomson notes that the work of Hertz demonstrated " the fact that light of all kinds and from all sources is really an electri- cal phenomenon, differing from ordinary alternate- current waves only in the rate of frequency of vibra- tions. We produce electric waves of about one hun- dred vibrations per second for alternating current work ; and in the waves of red light the rapidity is as * Quoted by Cajori from Hertz's Electric Waves, trans. Ly Dr. E. Jones, London, 1893. THE PROGRESS OF PHYSICS. 163 high as four hundred millions of millions of vibra- tions per second. Hertz and others used waves of some millions per second, and showed how they could transmit signals to distances without wires; these invisible waves being recognised by suitable receivers. The recently announced Marconi wireless telegraph is much the same thing, with certain improvements in detail." * " Hardly had the work of Hertz and others who followed in his footsteps been assimilated, before the truly remarkable, not to say astounding, discovery by Professor Rb'ntgen of what he called the X-rays produced a profound impression not only in the scientific world, but upon the general public as well. The interest of the scientist had a different basis from the popular one of disclosure of objects hidden in opaque structures; for he saw in the discovery a new weapon of attack upon the secrets of nature. This weapon has already proved to be so serviceable as to show that his anticipations were not unfounded. The X-rays, which became at once indispensable to surgery, are the results of electrical actions in certain vacuum bulbs ; and the discovery is properly an elec- trical one." f X and other Rays. — It has long been known that remarkable effects are produced when cathode rays are passed through a highly exhausted vacuum tube. The glass shows bright " phosphorescence," shadows are thrown by opaque bodies, and the rays are de- flected by a magnet. Crookes and Goldstein have been prominent investigators of the phenomena. In 1893, Lenard used a tube with a thin window of aluminium, and found that rays passed through * Ann. Rep. Smithsonian Inst., 1897, p. 135. t Loc. cit., p. 138. 164 PROGRESS OF SCIENCE IN THE CENTURY. this outside the tube, affecting photographic plates and electrified bodies. The rays are also affected by a magnet, and Lenard regarded them as prolonga- tions of the cathode rays. In 1895, Rontgen found that rays issue from the tube which affect a photographic plate after passing through plates, e.g., of aluminium, opaque to ordi- nary light, which pass from one substance to another without refraction and with little regular reflection. These are apparently not affected by a magnet. They are also remarkable in the way in which they alter the properties (especially the electrical proper- ties) of the substances through which they pass. Thus, as Professor J. J. Thomson says,* " we may conveniently divide the rays occurring in or near a vacuum tube traversed by an electric current into three classes ; without thereby implying that they are necessarily distinctly different in physical character. We have (1) the cathode rays inside the tube, which are deflected by a magnet; (2) the Lenard rays out- side the tube, which are also deflected by a magnet; and (3) the Rontgen rays which are not, as far as is known, deflected by a magnet." Two views are held as to the cathode rays: (a) that " they are particles of gas carrying charges of negative electricity, and moving with great velocities acquired as they travelled through the intense electric field which exists in the neighbourhood of the nega- tive electrode"; or (&) that they are waves in the ether. If the nature of the cathode rays is uncertain, so much the more is that of Rontgen's. They differ from light in the absence of refraction, but that may be interpreted as due to the exceeding smallness * Address to Section A, Rep, Brit, Ass, for 1896, p. 701. THE PROGRESS OF PHYSICS. 165 of the wave-length ; and the same interpretation may account for the absence of conclusive evidence of polarisation. SUMMARY. — Of what is meant by an electric charge, the nineteenth century has left us ignorant, but many laws of electrical phenomena have been discovered, and that electrical radiations are best interpreted in terms of ethereal waves is generally conceded. Indeed it has become a question whether all matter may not be resolvable into aggregates of electric charges of opposite sign. But both as regards theory and as regards practical applications, astound- ing as the progress of these has been* the twentieth century is pregnant with possibilities of development. THEORIES OF MATTER. Very early in the history of science the idea arose in the minds of enquirers that matter might consist of an aggregation of invisible particles separated by interspaces. This became a precise scientific hypo- thesis about a century ago, when Dalton developed his Atomic Theory. During the nineteenth century the hypothesis was in several ways developed as fresh facts came to light. When we see water becoming vapour and again be- coming ice, when we see what is usually a gas lique- fied and even solidified, when we watch the crystal of sugar melting away in the teaspoon or a crystal of alum growing in a solution of alum, when we con- sider that many bodies, like iron, expand when heated and contract again as they cool, when we observe that a gas may diffuse through another or even through a * A fascinating exposition of modern views will be found in an article by Prof. Oliver Lodge, International Monthly I. (1900), pp. 483-530. 166 PROGRESS OF SCIENCE IN THE CENTURY. solid; our instinctive desire to visualise what may be going on beyond the limits of the visible, naturally leads us to imagine matter as having a " grained structure," as being made up of minute particles separated by minute intervals which change with the state of the substance, with conditions of temper- ature and pressure. The general idea is simple; the details of the theory are profoundly difficult. " Imagine matter to consist of a crowd of separate particles with in- terspaces. Contraction and expansion are then merely a drawing in and a widening out of the crowd. Solution is merely a mingling of two crowds, and evaporation merely a dispersal from the out- skirts. The most evident properties of matter are then similar to what may be observed in any public meeting." * Among the many theories of matter, the following stand out prominently. Perfectly Hard Atoms. — (1) The idea which was expressed by Democritus and Lucretius, which re- ceived some measure of approbation from Newton, was that matter consists of perfectly hard atoms with void spaces between these. Newton used this theory in his interpretation of the propagation of sound. Centres of Force. — (2) A second view, which is associated with the name of Boscovich, replaces the perfectly hard atom by a centre of repulsive and at- tractive forces. " According to Boscovich an atom is an indivisible point, having position in space, capable of motion, and possessing mass. ... It has no parts or dimensions; it is a mere geometrical * J. J. Poynting. Address Section A, Rep. Brit. Ass. for 1899, p. 619. THE PROGRESS OF PHYSICS. 167 point without extension in space; it has not the property of impenetrability, for two atoms can, it is supposed, exist at the same point." * A similar view was held by Faraday. Heterogeneousness. — (3) In his Recent ^Advances (1876, p. 288), Prof. P. G. Tait described " a third notion — that the matter of any body, where it does not possess pores, like those, for instance, of a sponge (which obviously does not occupy the whole of the space which its outline fills), fills space continu- ously, but with extraordinary heterogeneousness." If the moon were built up of irregular stones and mortar, it would seem homogeneous to us (at a dis- tance of 250,000 miles), so the drop of water (re- moved as it were to a distance by its minuteness) may only be apparently homogeneous. Vortex Atoms. — (4) A more fertile theory, sug- gested in 1867, is that of Lord Kelvin — " that what we"fcall matter may really be only the rotating por- tions of something which fills the whole of space; that is to say, vortex-motion of an everywhere present fluid." f The beautiful circular vortex-rings which can be so readily made with tobacco or other smoke in air, and with a little ingenuity in water, have very inter- esting properties (first mathematically deduced by Helmholtz). Thus a vortex ring cannot be cut; " it simply moves away from or wriggles round the knife, and, in this sense, it is literally an atom." $ It moves through the air of the room as if it were an independ- ent solid body ; one will pass through another and al- low that other to pass through it; and it obviously has an extraordinary power of persistence. * Glazebrook. James Clerk Maxwell and Modern Physict, 1896, p. 108. t Recent Advances, p. 20. t Recent Advances, p. 297. L 168 PROGRESS OF SCIENCE IN THE CENTURY. But " a common vortex ring of air or water con- tains within itself the seeds of its own decease; it is composed of an imperfect fluid, possessing that is to say viscosity, and accordingly its life is short ; its peculiar energy being dissipated, its vortex motion declines, and as a ring it perishes. But imagine a ring built of some perfect fluid, of some medium devoid of viscosity, as the ether is; then it may be immortal; it can neither be produced nor annihi- lated by known means ; and it is just this property, combined with other properties of elasticity, rigidity, and the like, that led Lord Kelvin origi- nally to his brilliant and well-known hypothesis." Thus if the universe be filled with ether, and if that universal medium be a perfect fluid, " then, if any portions of it have vortex-motion communicated to them, they will remain forever stamped with that vortex-motion; they cannot part with it; it will re- main with them as a characteristic forever, or at least until the creative act which produces it shall take it away again. Thus this property of rotation may be the basis of all that appeals to our senses as matter" f The Atomic View of Nature. — Opinions differ as to the fittest way in which to express the facts known in regard to matter, but even those who believe, for instance, that " all matter is resolvable into an ag- gregate of electric charges of opposite sign," will admit their acceptance of the atomic view of nature, though all may not agree verbally with Prof. Oliver Lodge when he says " a lump of matter is as surely composed of atoms as a house is built of bricks." * Prof. Oliver Lodge. Modern Views of Matter. The Inter- national Monthly, I. (1900), p. 501. t Prof. Tait's Recent Advances, 1876, p. 294. THE PROGRESS OF PHYSICS. 169 " That is to say," he continues, " matter is not continuous and homogeneous, but is discontinuous; being composed of material particles, whatever they are, and non-material spaces. There is every reason to be certain that these spaces are full of a connecting medium, full of ether; there is no really void space." But while the atomic view is generally accepted, there is less unanimity as to the fittest conception of the atom. " No one now believes that an atom is simply a vortex ring of ether, and that the rest of the ether is stagnant fluid in which the vortex rings sail about. Any quantity of difficulties surround such an hypothesis as that. Its apparently attrac- tive simplicity is superficial. ^Nevertheless it is not to be supposed that every hydro-dynamical theory of the universe is thereby denied. It is quite con- ceivable that a single kind of fluid in different kinds of motion — some kinds of motion not yet imagined perhaps — may possibly be found capable of explain- ing all the facts of physics and chemistry." * "I hold," says Prof. Lodge, "that the ether is most certainly not atomic, not discontinuous ; it is an absolutely continuous medium, without breaks or gaps or spaces of any kind in it, — the universal con- nector,— permeating not only the rest of space, but permeating also the space occupied by the atoms themselves. The atom is something superposed upon, not substituted for, the ether, it is most likely a defi- nite modification of the ether, an individualisation, with a permanent existence and faculty of locomotion, which the ether alone does not possess. Matter is that which is susceptible of motion. Ether is that which * Modern Views of Matter, International Monthly, I. (1900), pp. 499 and 501. 170 PROGRESS OF SCIENCE IN THE CENTURY. is susceptible of strain. All energy appertains either to matter or to ether, and is continually passing from one to the other." * It is now time to turn to the actual progress of scientific discovery and to note a few of the steps which have led towards the modern views of matter, as above suggested. A. In Connection with the Kinetic Theory of Gases. — In his Hydrodynamica (1738), Daniel Bernouilli supposed a gas to consist of moving parti- cles, and argued that the pressure, if due to the im- pacts of these, must be proportional to the square of their velocity. In 1816 (published 1821), Herapath followed on the same tack, and in spite of fundamental errors (e.g., that the temperature of a gas is measured by the momentum of each of its particles), gave a theoretical justification of Boyle's law (that with con- stant temperature the product of pressure and volume is constant). In 1846, Waterston (whose work was overlooked until disinterred from the archives of the Royal So- ciety of London by Lord Rayleigh in 1892) showed that the temperature of a gas " is measured by the mean kinetic energy of a single molecule, and that in a mixture of gases the mean kinetic energy of each molecule is the same for each gas," f thereby furnishing the theoretical basis for the laws of Boyle, Gay-Lussac, and Avogadro. In 1848, Joule used Herapath's results as a basis for calculating the mean velocity of the molecules of a gas, and obtained from hydrogen at freezing point and atmospheric pressure the value of 6,055 feet * Loc. tit., pp. 499-500. fGlazebrook. James Clerk Maxwell, 1896, pp. 118-19. THE PROGRESS OF PHYSICS. 171 per second, or about six times the velocity of sound in air. In 1857, in his famous paper " On the Kind of Motion we call Heat," and in his second paper in 1859, Clausius greatly advanced the incipient kinetic theory, calculating, for instance, the average length of the path of a molecule in the interval between two " collisions," or near approaches to another molecule. In 1859 and 1860, Clerk Maxwell gave his " Illus- trations of the Dynamical Theory of Gases " in which he demonstrated " the laws of motion of an indefinite number of small, hard, and perfectly elastic spheres acting on one another only during impact." By the application of an ingenious statistical method and of general dynamical methods to molec- ular problems, Maxwell greatly advanced the theory of gases and the theory of matter. That he was helped by Boltzmann and Clausius and Kelvin and others goes without saying, but it seems legitimate to asso- ciate with his name the coming of age of the molec- ular theory of matter. It matters not a whit for our general purpose how many corrections may have to be made on his computation that the length of the mean free path of molecules of air is ^rr.Vinr of an inch, or that the number of collisions per second ex- perienced by each molecule is about eight thousand millions; the point is rather that he justified a. molecular or atomic conception, harmonising the laws of Boyle, Charles, and Avogadro, and suggesting fur- ther developments which are still prompting re- search. B. Cauchy's Suggestion of the Heterogeneity of Matter. — As a second illustration of the nature of the argument which has resulted in the modern 172 PROGRESS OF SCIENCE IN THE CENTURY. view or views of matter we may refer to the inves- tigations of the French mathematician, Cauchy, as to the motion of light in solid bodies and liquids. He showed " that if matter were homogeneous, there might be refraction, but there would be no dispersion. All kinds of light would travel with the same velocity in glass, just as they did in the air outside; and, therefore, the mere fact that the different kinds of light can be separated from one another in passing through a prism, gives, at least, a hint that the mat- ter of the prism is heterogeneous, is not infinitely more fine-grained than the length of a wave of any of the kinds of light which it enables us to sepa- rate in their courses." * This kind of argument — developed by Lord Kelvin — leads to the result that 400,000,000 in the inch is a rough approximation to the heterogeneity or grained structure of matter. C. Other Methods of Estimating the Heterogeneity of Matter. — In his Recent Advances in Physical Science Prof. P. G. Tait gave an account of two other methods ingeniously used by Lord Kelvin in forming an estimate of the grained structure of mat- ter. " The second method was founded upon con- siderations of the amount of heat which would be generated by electrical action between particles of different materials when they were combined to- gether. The third method was founded upon the forces employed in drawing out a film of liquid, — in fact (to take the simplest case), in blowing a soap- bubble." The various methods yielded approxi- mately the same result, " pointing consistently to something not very largely differing from the 500,- 000,000th part of an inch as being the distance be- tween the successive particles of matter in a liquid." * P. G. Tait. Recent Advances, 1876, p. 304. THE PROGRESS OF PHYSICS. 173 D. Argument from the Behaviour of Gases. — Clausius and Maxwell deduced theoretically the con- clusion that the length of the mean free path of a moving particle in a gas (i.e., the distance which it will pass through between every two successive colli- sions), divided by the diameter of any one particle, is equal to the ratio of the whole space occupied by the particles to about eight and a half times the bulk of the whole particles.* In various ways it was found possible to form an equation with approximate data, and the result comes out that the diameter of a par- ticle is not very different from ^T^Tnnr.innr of an inch. As a good-sized plum or a small orange is to the whole earth, so is the coarse-grained particle to a drop of water £ of an inch in diameter. The calculations of Joule and Clausius, Maxwell and Boltzmann lead to such statements as the follow- ing:— " Atoms are big things, the thousand millionth of an inch in diameter, and they cannot travel far without mutual collisions. They are constantly col- liding, even in a very good vacuum. In ordinary air every atom strikes another about six thousand million times a second, and it cannot travel even a microscopic distance without collision; its free path is microscopic, or on the average ultra-microscopic." f E. From Electrical Phenomena. — As Prof. Oliver Lodge says, " atoms are big things " — " the thou- sand millionth of an inch in diameter, and they can- not travel far without mutual collisions." Much too big and cumbrous these are to figure in an interpre- tation of the cathode rays, the Lenard rays, the Eontgen rays! For here we are brought face to * See Recent Advances, p. 316. t Oliver Lodge. Modern Views of Matter. International Monthly, I. (1900), p. 515. PROGRESS OF SCIENCE IN THE CENTURY. face with the astounding conception of fragments of atoms, of foundation-stones of atoms, of a unifica- tion of all matter in terms of corpuscles of which five hundred or so go to an atom of hydrogen. But the daring speculation carries us further — to doubt whether there is any matter at all, or rather whether inertia is not fundamentally electrical. Matter and Ether. — We have previously spoken of one of the aims of science as that of finding the common denominator of the fractions of reality which we know. For a time the word Matter was a conspicuous part of this common denominator, but the nineteenth century has left us ignorant of its real nature, and aware only of some of its many properties, and even of many of these properties how little we know. " Impenetrability," the text-books say, and yet Boscovich and Maxwell seem to regard it as conceivable that two atoms should occupy the same space. " Inertia," the text-books say, and yet how little we know of the meaning of this term, how doubtful Lodge seems to be whether there is any but electric nl inertia ! The common denominator would now read " the relations of matter, energy, and ether." But the fact is that the scientific conception of matter tends to be- come more and more monistic. Some years ago we thought of material atoms and molecules, floating in ether, like the crowds of minute organisms in the plankton of the ocean. But various attempts have been made, as Prof. Poynting puts it, "to get rid of the dualism " : — Boscovich's theory of point-cen- tres surrounded by an infinitely extending atmos- phere of force, Faraday's theory of point-centres with radiating lines of force, Lord Kelvin's theory of atoms as vortices or whirls in a perfect fluid ether, THE PROGRESS OF PHYSICS. 175 Larmor's theory of atoms as loci of strain in the ether, and so on. " So, as we watch the weaving of the garment of Xature, we resolve it in imagination into threads of ether spangled over with beads of matter. We look still closer, and the beads of mat- ter vanish; they are mere knots and loops in the threads of ether." * An Analogy. — An analogy which has often ap- pealed to our biological mind may possibly make the subject clearer. In Biology we are accustomed to speak of three big facts — organism, environment, and function. The environment includes the world of external influences; the organism is the living crea- ture which contains nothing sensible that is not also in the environment; function consists of action and reaction between organism and environment. We do not know the secret of the synthesis which has made it possible for the organism to be a persistent, though ever changeful, a unified and yet reproductive, whirl- pool in the stream of the environment. But there it is. Now it may be that molecules, atoms, corpuscles are persistent unities individualised in the stream or ocean of the ether, as the organism is in its environ- ment, the syntheses being secrets in both cases. And it may be that energy corresponds to function, — con- sisting of action and reaction between matter and ether. SUMMAEY. — That matter cannot be conceived as built up of perfectly hard atoms seems quite certain; that it has a heterogeneous structure seems equally certain; some modification of a theory of vortex- atoms would find acceptance as an interpretative * J. J. Poynting. Address, Section A, Rep. Brit. Ass. for 1899, p. 619." 176 PROGRESS OF SCIENCE IN THE CENTURY. idea; but it may be that what we call matter will turn out to be conceivable as loops and knots in the threads of ether. THEORY OF THE ETHER. Among the concepts which have come to stay in scientific thinking, that of the ether must now be in- cluded. It is as real as the concept of " atom " or " molecule," but hardly more so. Perhaps the most natural way of appreciating its validity is by con- sidering some of the facts which have made it seem to many a necessary hypothesis. Premonition of the Idea. — Long before the nine- teenth century, the scientific mind, e.g., Newton's, seemed to feel the need of supposing that there was " something " occupying space between the heavenly bodies. It does not seem very evident why the extent of distance should make much difference, but, for his- torical purposes at least, it is well to recall the im- pression made by the discovery or rather demonstra- tion of the fact that most of the heavenly bodies are at a literally immense — unmeasurable — distance from the earth. Light travels at a rate of about 186,000 miles in a second, and could flash nearly eight times round the earth in that time; but if a hypothetical inhabitant of the nearest star could by any means see the earth, he would see the events of three or four years ago. Now, as we are sure that light is not any kind of stuff or substance, but a form of energy or power, we may, in some measure, understand why to some minds it has seemed necessary to suppose that there is some sort of something linking that star to us. THE PROGRESS OF PHYSICS. 177 If light consists of waves, the question naturally arises: "Waves in what?" Especially when the study of polarisation and double refraction showed that the elastic properties of air or water which act as media for sound, will not work when applied to the interpretation of light-phenomena, the conception of the ether f oreed itself upon physicists. At first it seems to have been thought of as an ex- ceedingly rare form of matter pervading space and composed of discrete particles; and it was of course invested with the requisite elastic qualities. But gradually the conception became subtler. Identification of Luminiferous and Electro-mag- netic Ether. — The luminiferous ether was invented as a conception which fitted the facts known in re- gard to light. Similarly Faraday and Clerk Max- well postulated a special ether for electrical and magnetic phenomena. But when Clerk Maxwell made the further step of showing that one hypo- thetical medium would suffice for the interpretation of luminous, electric, and magnetic radiations, the case for the ether became much stronger. That the ether is a necessary conception in modern physics seems to be unanimously admitted by experts, but how exactly the ether is to be conceived of re- mains quite uncertain. For some imagine it as an elastic solid, others as a labile fluid, others as a vortex sponge (a phrase which we cannot pretend to explain), and others otherwise. The modern conception of the ether is that of an absolutely continuous medium, " without breaks or gaps or spaces of any kind in it," " a universal con- nector," permeating space whether otherwise occu- pied or not, susceptible of stress, but not of locomo 178 PROGRESS OF SCIENCE IN THE CENTURY. tion, probably full of vorticity, but in any case not a stagnant homogeneous fluid, the seat of waves which we call " light " and of others which we call " electro- magnetic phenomena," on the whole the most marvel- lous scientific concept which the mind of man has conceived ! Value of these Hypotheses. — We can well imagine a practical man saying that all this talk of atom and molecule and ether is unreal and unverifiable, and in a certain sense he is undoubtedly right. These molecular and ethereal hypotheses are human imagin- ings,— and nothing more; they are constructed in terms of one sense, that of sight; they are attempts to see that which is invisible, to invent a machinery of Nature since the real mechanism is beyond our ken; but it must be observed that these hypotheses are not vain imaginings, for they prove themselves yearly most effective tools of research, and that they are not random guesses, for they are constructed in harmony with known facts. CRAPTEE VI. ADVANCE OF ASTRONOMY. FEOM COPEENICUS TO NEWTON. ^Astronomy an Ancient Science. — Astronomy is usually ranked as the most ancient of the concrete sciences, and this at least is certain that evidence of astronomical observation is furnished by the posi- tion of buildings which are much older than all written records. Perhaps one of the first scientific discoveries to become clear and definite was the dis- covery of the year, with its fine demonstration-lesson of recurrent sequences. From that unknown date to the latest announcements from the observatories of Greenwich and Potsdam, Harvard and Lick, there extends a long procession of discoveries, sometimes almost monotonous in their continuity and sameness, but relieved at intervals by some great and novel achievement which has given a new meaning to the whole. That astronomy reached a stable position sooner than the ^ other sciences was partly because the sub- lime subject attracted men of genius who " attended their minds thereunto," and partly because a great part^of astronomy is concerned with simple relations of distance, mass, and motion. Three Main Chapters. — Balfour Stewart has summed up the long history of astronomy in three 180 PROGRESS OF SCIENCE IN THE CENTURY. main chapters. First it passed through an observ- ing-period lasting through thousands of years of nightly study by watchers in the plains of the East to its culmination in the discoveries of Copernicus and Keppler. It then passed into a stage of analysis and generalisation, when the genius of Newton rationalised a huge mass of facts in the formula of gravitation. " God said, Let Newton be, and there was light." It finally reached a stage of deduc- tion, which, from a knowledge of the positions and movements of the heavenly bodies, predicts their fu- ture courses. This might also be called the evolu- tionary period, since one of its dominant aims has been to show how the solar and other systems have come to be what they are. The Succession of Systems. — The Ptolemaic sys- tem— which placed the earth immovable in the centre of the universe — was superseded by the system of Copernicus (14T3--1543), which made the sun the immovable centre. This again was reformed by Keppler (1571--1630), who stated the famous laws or descriptive formulae of the movements of the planets in their orbits, but was impelled to call in the service of guiding spirits to account for them. Galileo Galilei (1564-1642) was the first to use for systematic study the telescope which the Dutchman, Hans Lippersheim, had invented, and in spite of his revelation of some of the wonders of the heavens — the broken surface of the moon, the countless stars of the Milky Way, the satellites of Jupiter, and the spots on the sun — was almost made a martyr for his dogged adherence to Copernican doctrine. But we must not do more than mention these great names, which are separated by a long interval from the nine- teenth century. ADVANCE OF ASTRONOMY. 181 The Gravitation Law. — It is necessary, however, to dwell for a little on what is perhaps the greatest of all scientific achievements — Newton's formulation of the Gravitation Law (1687), — the foundation of what has been called the astronomical view of nature. " Every particle of matter in the universe attracts every other particle with a force whose direction is that of the straight line joining the two, and whose magnitude is proportional directly as the product of their masses, and inversely as the square of their mutual distance " — this is the generalisation known as the Law of Gravitation.* Another way of phras- ing it may be quoted : — " The law of gravitation states that to each portion of matter we can assign a constant — its mass — such that there is an accelera- tion towards it of other matter proportional to that mass divided by its distance away. Or all bodies resemble each other in having this acceleration to- wards each other." f The fundamental concept is that of mutual acceleration. This formula applies with equal accuracy to a stone falling to the ground and to the motion of the earth round the sun. As far as we know, it is uni- versally true. It may not be possible to trace the logical processes of genius, but it should be noted that just as Keppler deduced his three laws from the observations of Tycho Brahe, so Keppler's laws formed a basis of deduction for Newton. SUMMARY. — The science of astronomy, most an- cient in its origin, may be said to have passed through three main phases — (a) of observation, (b) of analysis and generalisation, and (c) of deduc- * Cited from Chambers's Encyclopedia. t Prof. Poynting, Pres. Address, Section A., Rep. Brit. Ass. for 1899, p. 616. 182 PROGRESS OF SCIENCE IN THE CENTURY. tion; but activity continues on each of these lines, and it may be more accurate to say that the suc- cession of astronomical systems — Ptolemaic, Co- pernican, Kepplerian, Newtonian, etc., implies mainly a progress in the lucidity, validity, brevity, and universality of descriptive formulce. APPLICATIONS OF THE GKAVITATION-FOKMULA. A great part of astronomy is concerned with appli- cations of the gravitation-formula to the phenomena of the heavens; another department has to do with topographical relations, with mapping out positions and orbits ; while a third kind of enquiry deals with the physical and chemical nature of the celestial bodies. Laplace, Bradley, and Herschel may be named as representative great masters in these three departments, which have been — not very hap- pily— distinguished as " gravitational," " observa- tional," and " descriptive." Adopting this classifica- tion, Mr. Berry notes in his Short History of As- tronomy * that " gravitational astronomy and exact observational astronomy have made steady progress during the nineteenth century, but neither has been revolutionised, and the advances made have been to a great extent of such a nature as to be barely intelligible, still less interesting, to those who are not experts. . . . Descriptive astronomy, on the other hand, which can be regarded as being almost as much the creation of Herschel as gravitational as- tronomy is of Newton, has not only been greatly de- veloped on the lines laid down by its founder, but has received — chiefly through the invention of spec- trum analysis — extensions into regions not only un- thought of, but barely imaginable a century ago." * P. 355. ADVANCE OF ASTRONOMY. 183 In illustrating the century's confirmations and ex- tensions of the gravitational theory, account should be taken of re-estimates of the sun's distance, re- investigations of the movements of the moon and the planets, further elaboration of the theory of the tides, and so on. We have confined ourselves to a brief notice of the discovery of the minor planets, the discovery of Neptune, and the study of comets. Discovery of the Minor Planets. — Kant had sug- gested that the zone in which a planet moves might be regarded as the empty area from which its ma- terials had been derived, and that some definite re- lation should therefore be found between the masses of the planets and the intervals between them. In 1772 Titius pointed out that the distances from the sun of the six planets then known might be represented by a certain numerical series, except that there was nothing to correspond to the term succeed- ing the one which corresponds to the orbit of Mars. Johann Elert Bode, astronomer in Berlin, filled the gap with a hypothetical planet, and the search for it began. In 1801 Piazzi discovered Ceres, and with the help of Gauss's mathematical genius (used to pre- dict where the planet should be at certain dates) von Zach and Olbers were soon able to confirm Piazzi. In spite of Hegel's protest that the number of planets could not exceed the sacred number seven, a second (Pallas) was soon discovered (1802) by Olbers, and in 1807 four were known. Three of these " as- teroids," as Sir W. Herschel called them, corre- sponded approximately with the requirements of the series indicated by Titius and usually referred to as " Bode's Law," and the idea commended itself that these bodies were the remains of an exploded planet. As we now know, neither Bode's Law nor the no- li 184: PROGRESS OF SCIENCE IN THE CENTURY. tion of an exploded planet can be regarded as tenable, but both served a useful purpose in prompting re- search. They led to the recognition of the minor planets, now known to be very numerous (over five hundred) and the discovery must have served as a useful hint of the complexity of relations which fur- ther study of the heavens was to reveal. The story is of interest in illustration of a scientific prophecy which was rewarded even more richly than its basis deserved. In 1857 Clerk Maxwell proved the truth of what had been several times suggested — that the rings around Saturn could not be continuous solid bodies nor liquid zones, but that they behaved as if they were composed of a multitude of small solid bodies revolving independently around the planet, somewhat as the minor planets do around the sun. This has received corroboration from telescopic and spectro- scopic observations, and is one of the facts which lend countenance to the hypothesis of the meteoric constitution of the heavenly bodies: — that meteoric dust, shooting stars, meteor rings, Saturn's rings, comets, minor planets, nebulae, and so on, are all, as it were, terms in an evolution-series. Discovery of Neptune. — There are few chapters in the history of astronomy more familiar, and, at the same time, more instructive, than the story of the discovery of Neptune. It illustrates the method of science, — discovering an anomaly, tracing out the reason for it, and thereby corroborating a general con- clusion. In the first quarter of the century it was repeat- edly remarked that the real orbit of Uranus (which Herschel had removed from among the fixed stars to a place among the planets) was not (to the astro- ADVANCE OF ASTRONOMY. 185 nomical eye) in harmony with its theoretical path as deduced from the gravitation-formula. To explain these puzzling discrepancies of orbit, it was suggested by several astronomers that they must be due to the influence of some undetected exterior body. But precision was first given to this suggestion in 1845, when John Couch Adams succeeded in calculating out the probable mass and position of the hypothetical planet. In the same year Leverrier (b. 1811) began a similar quest by a different method ; in 1846 he de- termined the probable position of the supposed cause of the disturbance; in the same year he announced that it should be visible in a certain place. He wrote to Galle of the Berlin observatory, told him where to look, and Xeptune was discovered. In the same month (September, 1846) the discovery was con- firmed by Challis of Cambridge, who had been ad- vised by Adams. It is almost needless to remark on the importance of the discovery as a confirmation of the gravitational formula ; here, if anywhere, the exception proved the rule. It should be noted, how- ever, as S. C. Walker first showed, that Xeptune had been observed as a fixed star by Lalande in 1795, and furthermore that " the planet was found to have a different orbit from that assigned by the calculators. Their (hypothetical) planets were not identical, nor were they the (real) planet Xeptune. But they must ever have credit for the sagacity and ability with which, aiming at so indefinite a target, they so nearly struck the centre." * The prophetic recognition of the existence of Nep- tune and its verification may be taken as one of the *E. B. Kirk. Article, Astronomy, Chambers's Encyclo- padia, vol. I. p. 528. 186 PROGRESS OF SCIENCE IN THE CENTURY. ^finest illustrations of the stability of the gravitational theory. Comets. — Another series of confirmations of New- tonian laws is concerned with comets. For, although Newton had shown that their movements were in har- mony with his general formula, he had few data at his command, and a clearer demonstration was given by Halley, who, from a basis of calculations, accu- rately predicted the return of " Halley's comet " in 1758-9. The physician Olbers (d. 1840) introduced a sim- plification in the method of computing the paths of comets, and for half a century was one of the most assiduous and successful students of these periodic visitants. Among the many whom he helped and stimulated during his long life was Encke, a pupil of Gauss, one of those who have passed through the portal of mathematics to the study of the stars. Sixty-three years after Halley's prediction was veri- fied, Encke in 1822 had a similar success with a comet " of short period," which revolves round the sun in about three and a quarter years. More than 200 comets have been studied in the nineteenth century; and by means of the spectro- scope, applied to the study of comets by Donati in 1864 and by Huggins in 1868, it has been possible to advance a little beyond the computation of paths and periods, and to prove, for instance, that at least some comets are in part self-luminous, while others, especially those of short period, appear to owe most of their brilliance to light reflected from the sun. Professor Tait seems to have been the first to give definite expression to the idea (expounded by Lord Kelvin in 1871) that the light of comets, and of nebula? as well, may be due to flashes of ignited gas ADVANCE OF ASTRONOMY. 187 induced by the encounters amid the swarms of me- teoric stones. It is impressive to read how the comet of 1811 was assigned an orbit requiring 3065 years for its completion, such that " when it last visited our neighbourhood, Achilles may have gazed on its im- posing train as he lay on the sands all night bewail- ing the loss of Patroclus; and when it returns, it will perhaps be to shine upon the ruins of empires and civilisations still deep buried among the secrets of the coming time." It is impressive to note the measurements of some of the great comets whose highly rarefied emanations or " tails " may extend for several millions of miles, but the behaviour of the tail points to the conclusion that it is but " a stream of matter driven off from the comet in some way by the action of the sun," and the density must be small indeed, since the earth has passed through a tail at least twice in the century without the fact being known until afterwards. Indeed the progress of knowledge has robbed comets of some of their dignity, for since the middle of the century it has been generally recognised that, with the possible ex- ception of the bright central " nucleus," a comet is small in mass, and in a state of great tenuity, unable to affect the motion of the planet it approaches, and allowing the light of a star to pass even through its " head." Numerous interesting observations point to some close connection between comets and meteors or " shooting stars." Thus Biele's comet (with a period of sixty-seven years), which scared the popular im- agination in 1832, was first seen to become double, and was afterwards lost altogether, while on two sub- 188 PROGRESS OF SCIENCE IN THE CENTURY. sequent occasions (1872 and 1885), as the earth was crossing the path of the comet when it (if it had persisted) was nearly due at the same place, there was an unusually brilliant shower of meteors. Meteors may be fragments of a broken-up comet, or a comet may be a swarm of meteors. In the study of comets the accuracy of the gravi- tational formula has been beautifully illustrated, and, during the latter half of the century, considerable progress was made towards an understanding of their physical nature. THE STUDY OF THE STARS. Almost until the end of the eighteenth century, it was the general belief, even among astronomers, that the stars were fixed and unchanging. As Miss Clerke says, " their recognised function, in fact, was that of milestones on the great celestial highway traversed by the planets." Gradually, however, it became evident that this emphatically static image was far from being true. What Giordano Bruno had imagined, was confirmed by Halley in 1718, when he showed that Sirius, Aldebaran, Betelgeuse, and A returns had changed their positions in the sky since Ptolemy marked these out. Many similar facts came to light, and in the last quarter of the eigh- teenth century, sidereal astronomy included " three items of information — that the stars have motions, real or apparent; that they are immeasurably re- mote ; and that a few shine with a periodically varia- ble light." * William Eerschel. — It was about the beginning of * Agnes M. Clerke. A popular History of Astronomy During the Nineteenth Century, 1885, p. 13. ADVANCE OF ASTRONOMY. 189 the last quarter of the eighteenth century that Wil- liam Herschel (1738-1822) began to realise his ambition of obtaining " a knowledge of the construc- tion of the heavens," and rapidly passed from being " a star-gazing musician " to the post of royal astron- omer. He made clear, what had been suspected by some, that there were systems of stars, in some measure comparable to the planetary system, but varying greatly in the periods and forms of their revolutions. A double star had been usually regarded as an opti- cal phenomenon due to the fact that two stars which might be very far apart, happened to be nearly in the same line of sight from the earth; Herschel proved that many double stars were real binary combina- tions, " intimately held together by the bond of mu- tual attraction." In the apparent motions of the stars he distinguished one component due to a trans- lation of our planetary system towards a point in the constellation Hercules, and another component due to a real movement of the stars themselves. In his study of nebula? he was gradually forced to the conclusion that there were nebulosities which could not be resolved in stars, but consisted of a " shining fluid " or " self-luminous matter " diffused in space, and " more fit to produce a star by its condensation, than to depend on the star for its existence." This led him about 1791 to a theory of the nebular origin of stars, apparently in complete independence of thie nebular theory of Laplace (1796). Two main contributions, then, must ~be traced io Herschel, — that fie extended Neuionian methods to the study of the stars, and that he made the whole scientific picture of the heavens vividly kinetic. On the one hand, he extended the range of precise 190 PROGRESS OF SCIENCE IN THE CENTURY. measurement and calculation; on the other hand, he emphasised the idea of change or, one may almost say, of evolution. The heavens no longer seemed fixed and unchanging, when it was shown that new systems were being formed and that others were dying away. Herschel's work was continued at Konigsberg by Bessel; at the Russian observatory of Pulkowa by Struve, succeeded in 1858 by his son Otto; and by many other illustrious workers. In Britain the father found an intellectual heir in the son, John F. W. Herschel (1792-1871), whose Cape observa- tions (1834-38) did for the Southern heavens what had been done for the Northern. Published in 1847, they represent the state of sidereal astronomy at the middle of the century. " Not only was acquaint- ance with the individual members of the cosmos vastly extended, but their mutual relations, the laws governing their movements, their distances from the earth, masses, and intrinsic lustre, had begun to be successfully investigated." * Improvements in telescopes and other instruments aided in the progress of the sidereal astronomy to which Herschel had given so much impetus, and with' improved mechanical means was associated a re- formed method of observation. Friedrich Wilhelm Bessel (1784-1846), who made himself famous at the age of twenty by calculating an orbit for Halley's comet, did a gigantic piece of work by instituting '(1813, 1830) a uniform system of " reduction " (or, correction of observations) which lengthened out the period of exact astronomy by half a century. In other words he made a uniform correction of Brad- ley's Greenwich observations, making allowances for * A. M. Clerke. History, 1885, p. 65. ADVANCE OF ASTRONOMY. 191 precession, aberration, refraction, and instrumental errors. And the edition of Bradley's results was only a prelude to fresh catalogues of his own, exe- cuted between 1821 and 1833, and including about 62,000 stars. It is hardly necessary to say that Bradley's work was continued through the century by many illustrious astronomers. Measuring the Distance of a Star. — To the an- cients the stars remained altogether mysterious; they were points of fire set in the concave vault of the firmament and borne by it in daily revolution around the fixed earth. Keppler seems to have been the first to dare to deduce from the Copernican sys- tem the conclusion that the stars are extremely dis- tant suns, — so distant that most of them appear un- affected in direction throughout the year ; e. g., when viewed from opposite ends of the earth's orbit. If so distant and yet so clearly visible, they must be sunlike ; i. e., great sources of radiant energy. This conclusion was less hesitatingly accepted by Galilei. But while it came to be generally recognised that the stars were unthinkably distant suns, it was not till 1838 that the distance of any star was measured. In that year, Friedrich Wilhelm Bessel (1784- 1846), using Fraunhofer's heliometer, or " divided object-glass micrometer," was able to determine the parallax, and thus to deduce the distance of a small star in the constellation of the Swan (61 Cygni). Soon afterwards analogous results were published by Thomas Henderson for a Centauri (1839), and by Struve (1840) for Vega. The method of estimating the distance of a star is simple in theory. Suppose that the direction of a star is observed at a certain time with all possible accuracy; suppose that the same star is observed 192 PROGRESS OF SCIENCE IN THE CENTURY. six months later when the earth has travelled over one-half of its orbit, another direction-line may be observed; suppose the two direction-lines produced till they meet, the point of intersection must be the position of the star. Then we have a triangle whose base is the diameter of the earth's orbit, and a geo- metrical calculation enables us to determine the pro- portion that the sides bear to this. The method of determining parallax is theoreti- cally so simple that it could not but be known to Copernicus and his followers. Indeed for three hun- dred years before Bessel's success there were pains- taking attempts to apply it, attempts which invaria- bly ended in the disappointing result that the two direction-lines from opposite ends of the earth's orbit always seemed to be parallel. We know this to mean that the star observed was too distant, or that the instruments used were not precise enough, to show appreciable parallax. As we have noted, Bessel succeeded and the im- portance of the step thus taken is not affected by the fact that his estimate of the distance of 61 Cygni as 600,000 times that of the Sun is now reduced to 440,000. A few months after Bessel announced his dis- covery, Henderson of Edinburgh published his esti- mate of the distance of « Centauri, which is, so far as we know, the star nearest the solar system. Hen- derson calculated its distance at 180,000 times that of the Sun, this has now been extended to 270,000 times. Writing in 1885, Miss Clerke says: "The same work has since been steadily pursued, with the gen- eral result of showing that as regards their over- whelming majority, the stars are far too remote to ADVANCE OF ASTRONOMY. 193 show even the slightest trace of optical shifting from the revolution of the earth in its orbit. In about a score of cases, however, small parallaxes have been determined, some certainly (that is, within moderate limits of error), others more or less precariously." Dr. Fison notes that for forty years after Bessel's discovery the record is chiefly one of accumulation of experiences ; " and when in 1881 Dr. Gill and Dr. Elkin commenced a series of observations at the Cape of Good Hope, the parallaxes of not more than half a dozen stars had been detected with cer- tainty. Since that date, however, parallax hunters have been better rewarded, though up to the present time (1898) it is doubtful whether success has been achieved in more than fifty instances." f The distances of the stars whose remoteness is measurable are so enormous that they produce almost no impression on the ordinary mind. " It follows," said Bessel, " that the distance of 61 Cygni from the sun is 657,700 times the half diameter of the earth's orbit. The light from the star takes ten years to traverse this enormous dis- tance. It is so vast, that though it may be conceived, it cannot be visualised. All attempts to realise it, fail either because of the size of the unit of measure- ment or because of the number of repetitions of the unit. The distance which light traverses in a year is not more realisable than that traversed in ten years. Or if we choose a realisable unit, such as the distance of 200 miles which a locomotive (bicycle, we should say) travels in a day, it would require 68,000 millions of such daily journeys, or about 200 * A. M. Clerke. History, 1885, p. 48. t Recent Advances in Astronomy, 1898, p. 7. 194 PROGRESS OF SCIENCE IN THE CENTURY. millions of yearly journeys to reach the star in ques- tion." * It seems on the whole most convenient to use, as Bessel suggested, as a unit " the light journey of one year." The velocity of light is 186,300 miles a second, about six billion miles a year. " Light takes four years and four months to reach the earth from a Centauri, yet a Centauri lies some ten billions of miles nearer to us (so far as is yet known) than any other member of the sidereal system ! " f In other words, we see a Centauri, not as it is now, but as it was more than four years ago. Similarly, light takes more than six years to reach us from 61 Cygni. Given a determination of the parallax and distance of two stars in a system, and a knowledge of their period of revolution, it became possible to calculate their combined mass in terms of that of the sun ; and the process of weighing the stars began. Herschel's conclusion as to movement of the solar system as a whole, often doubted, was repeatedly confirmed ; the general direction was more carefully stated ; and even the rate has been guessed at. F. G. W. Strove (1793-1864) continued Herschel's study of double stars, and published in 1837 his monumen- tal Mensurce Micrometricce, which " will for ages serve as a standard of reference by which to detect change or confirm discovery." The distances of some of the nearer stars can lie calculated by the determination of annual parallax, a method first successfully employed by Bessel (1838), Henderson (1839), and Struve (1840); this is historically important as a confirmation of * Freely translated from Dannemann's Grundriss einer Oeschichte der Nalurwissenschaften, vol. 1, 1896, p. 825. t A. M. Clerke. History, 1885, p. 49. ADVANCE OF ASTRONOMY. 195 {he Copernican system and as a suggestion of the sunlike nature of the stars. Life of Stars. — If the view be accepted that the sun was once a diffused body of gas extending be- yond the present limits of the solar system, and that it has slowly shrunk, giving rise to the present phase of things, and if the stars be regarded as sunlike, we should expect to find in the immensity of the heavens some confirmatory evidence. In other words, we should expect to see stars a-making and others a-dy- ing. The former are now familiar to astronomers, and the existence of dead stars is generally admitted. Nebulce. — It is generally agreed that the faint clouds of light called nebulae, which occur scattered in the sky, are in many cases at least early stages of star-making, — embryo stars in an undifferentiated state. Two of these nebulae are visible to the un- aided eye on clear dark nights, namely, in the con- stellations of Orion and of Andromeda. In the seventeenth century, after Galilei had intro- duced the use of the telescope, many nebula? were de- tected, but they were generally passed over quickly as " diffusions of self-luminous matter," or " shining fluid," or " fire-mist," and so forth. Towards the end of the eighteenth century (1780) William Her- schel began his study of nebula?, and not only in- creased the list from 150 to 2,500 in about a score of years, but showed that many of them had a de- tailed structure. At first he regarded nebula? as clusters of stars, and stated the evolutionary idea that stars and clusters of stars arose from nebular condensations. Subsequently, however, he reverted to the older view in regard to many nebula?, includ- ing that of Orion. In the first half of the nineteenth, century it was Herschel's earlier view that prevailed ; 196 PROGRESS OF SCIENCE IN THE CENTURY. improved telescopes, such as that constructed by Lord Rosse at Parsonstown in Ireland, resolved one nebula after another into collections of stars. Indeed imag- ination far outstripped the evidence, and it was wide- ly supposed that nebulae were systems of suns, multi- ples, as it were, of the architectural unit which our solar system was believed to display. So far telescopic analysis had alone been possi- ble, but the next great step was taken in 1864, when Sir William Huggins applied the spectroscope to the study of a small but bright nebulae in the con- stellation of the Dragon. The spectrum (yielding no continuous band) was like that of a glowing gas, and therefore it was concluded that this nebula? was not a galaxy of stars, but a vast area of incandescent gas. In the next few years many others, including the Great Nebulae of Orion, were shown to be gaseous while others (yielding "continuous" spectra) seemed to be either star clusters or gases in process of con- densation. It is important to notice that the growth of ther- modynamics has led to a rejection of the old view that nebulous stuff was originally or is still " instinct with fire." The essay of Helmholtz in 1854 made it plain that this supposition is unnecessary, " since in the mutual gravitation of widely separated matter we have a store of potential energy sufficient to gener- ate the high temperature of the sun and stars. We can scarcely go wrong in attributing the light of the nebulae to the conversion of the gravitational energy of shrinkage into molecular motion.'' * " It is difficult not to see in the gaseous nebulae the stuff of which future stars will be made. Grant- ing that their substance is subject to the law of gmvir * Huggins. Rep, Brit. Ass. for 1891, p. 22. ADVANCE OF ASTRONOMY. 197 tat ion, it appears certain that in coming ages their glowing matter must, under its influence, be drawn towards centres of condensation; the smaller and more symmetrical of the nebulae possibly developing into single stars, but such majestic collections of cloudy structures as are revealed in Orion being more probably the origin of hosts of separate suns/' Dead Stars. — While some nebulae are plausibly interpreted as stars a-making, there are also phe- nomena which indicate stars dying or dead, or in other words, dark. It is obvious that the existence of a dark star cannot be demonstrated to the eye ; but it may be inferred (a) from the occurrence of the total or partial eclipse of a bright star, or (fc) from disturbances in the movement of a bright star such as the gravitational influence of a dark neighbour would explain. In both these ways the existence of dark stars has been indirectly proved. The regularity in the variations of the light of Algol — the best-known of the variable stars — was hypothetically interpreted by Goodriche (1782) as due to the revolutions of a dark companion star which caused partial eclipse; and the researches of E. C. Pickering of Harvard (1888) and of Yogel of Potsdam (1888-1891) have justified the hypothesis. " The possibility of an unseen system of stars per- meating the seen is beyond doubt" * Condensing Dr. Fison's account of the subject, we may sum up the possible history of a nebula. A diffused area of gases, perhaps comparatively cool, perhaps holding part of its contents in the form of solid or liquid particles; gravitational attraction brings about a spherical form; heat is lost by radi- ation and the parts of the area draw together; tem- *Fison. Recent Advances, p. 35. 198 PROGRESS OF SCIENCE IN THE CENTURY. perature rises and the nebula becomes more thor- oughly gaseous, if it was not so at the start; as the outer parts cool they condense into the clouds of a photosphere and the nebula becomes a sun; for a time, as shrinkage increases, the temperature rises; but the limits to this must be reached sooner or later and the sun, passing the zenith of its splendour, grad- ually sinks into dark coldness. " Fixed Stars." — One of the many instances of the characteristic nineteenth century transition from static to kinetic conceptions, may be found in the hesitancy with which astronomers now speak of " fixed stars." In many cases it has been shown that their positions relative to one another change in the course of years, and the displacement, though ap- parently very minute, indicates an enormous velocity of movement. " Sirius drifts over the face of the sky with such speed that in 1,400 years its position will be removed from its present one by a distance that would just be covered by the diameter of the full moon. ... To do this it must travel athwart the direction of vision with a speed of over ten miles per second, more than one-half of that of the Earth in its orbit; and this takes no account of any velocity the star may possess in the direction of the line of vision, a displacement in which direction would obviously not affect its position upon the face of the heavens." Similarly, to take the most rapid known dis- placement, a star in the Great Bear named Groom- bridge, 1830, would move in 257 years over the moon's diameter, and this at a distance of 2,300,000 times that of the sun implies a rate of 227 miles per second. JsTor should we forget here that the sun itself is travelling in a line directed towards the star Vega, * Fison, p. 46. ADVANCE OF ASTRONOMY. 199 at a rate which some estimate at 12-18 miles per second. There has been no justification of the hope of a century ago that some star (Sirius was suggested by Kant) or some point (in the Pleiades, according to Madler) would turn out to be the hub of the uni- verse, the centre to which all the heavenly bodies are related; the system or goal of the grandest of all movements is unknown. EXTENSION AND INTENSIFYING OF OBSERVATION. Extension of Observational Astronomy. — In as- tronomy, as in other sciences, a large part of the available intellectual energy has gone and must go to extend the area of observation, or to revise with intensified carefulness what has been already ob- served. It is difficult to give any account of this ungeneralised work, whose value is in the future rather than in the present. Numbering the stars is like cataloguing Eadiolarians or Diatoms, a means not an end; and a telescopic photograph of a corner of the Milky Way is like a similar picture of a micro- scopic section — interesting and marvellous, of course, for everything is — but not attaining full interest until it can be used as an item in some generalisa- tion. The Milky Way. — To take an instance : The Milky Way — the high road to Olympus — has been the sub- ject of imaginings since men first saw the stars. Its poetical interpretations are many, but as to its sci- entific interpretations there has been little progress since Galilei's telescope confirmed the conjecture of Pythagoras that the haze of the dimly luminous arch was " the combined shimmer of hosts of stars, each one too faint by itself to be distinguished by the unaided eye." N 200 PROGRESS OF SCIENCE IN THE CENTURY. Both the Herschels, Struve, Proctor, and others sought to explain the appearance of this majestic way of light as due to perspective effect or optical projection, but there seems to have been a complete acceptance of " the more simple and direct view, that the Milky Way is a definite and complicated structure, and that its bifurcation, its vacuities, its gaps, and its other irregularities, are definite physi- cal facts." * The great " Bonn Durchmusterung " compiled (1859-1862) under the supervision of Argelander, the more recent Harvard catalogue by Pickering, and Gould's list of the stars visible from the southern hemisphere, illustrate supreme patience and care- fulness, but as yet we remain unaware of any securely established or intelligible generalisations as to the stellar distribution. The Bonn list in- cludes 324,198 stars down to a certain (9.5) mag- nitude (estimated in terms of brightness), but mere number does not impress the imagination, especi- ally since the sight of the starlit sky suggests le- gions upon legions of luminaries visible to the un- aided eye, — a suggestion very far from the truth. The more impressive aspect is that which remains vague, of which, indeed, we have as yet only sug- gestions, that there is probably a system of the stars, — hidden from our gaze not only by distance, but by its inherent complexity. A quotation from one of the modern masters may serve to suggest the present tentative position: — " The heavens are richly but very irregularly in- wrought with stars. The brighter stars cluster into well-known groups upon a background formed of an enlacement of streams and convoluted windings and * Fison, Recent Advances, 1898, p. 85. ADVANCE OF ASTRONOMY. 201 intertwined spirals of fainter stars, which become richer and more intricate in the irregularly rifted zone of the Milky Way. " We, who form part of the emblazonry, can only see the design distorted and confused ; here crowded, there scattered, at another place superposed. The groupings due to our position are mixed up with those which are real. " Can we suppose that each luminous point has no other relation to those near it than the accidental neighbourship of grains of sand upon the shore, or of particles of wind-blown dust of the desert ? Surely every star from Sirius and Vega down to each grain of the light-dust of the Milky Way has its present place in the heavenly pattern from the slow evolving of its past. We see a system of systems, for the broad features of clusters and streams and spiral windings which mark the general design are reproduced in every part. The whole is in motion, each point shifting its position by miles every second, though from the august magnitude of their distances from us and from each other, it is only by the accumulated movements of years or of generations that some small changes of relative position reveal themselves. " The deciphering of this wonderfully intricate constitution of the heavens will be undoubtedly one of the chief astronomical works of the coming cen- tury." * One interesting result as to method should be noted, namely, the development of stellar photogra- phy. When even the trained eye, with the telescope to help, cannot detect, the photographic plate may reveal. The invention and improvement of the gela- * Sir William Huggins. President's Address, Rep. Brit. Ass. for 1891, pp. 35-36. 202 PROGRESS OF SCIENCE IN THE CENTURY. tine dry plate, which on sufficiently long exposure will register an image of a body whose luminosity falls far below the limit of visibility to our eyes, has meant a remarkable extension of our sense of sight. It has meant seeing the invisible ! Of some importance, too, has been the develop- ment of more exact methods of measuring star bright- ness (photometry), and the resulting classification (first suggested by Pogson in 1856) into definite de- grees of " magnitude." Thus a star of the sixth magnitude is one hundred times fainter than one of the first magnitude. Intensifying of Observation. — Inspection of the recent moon-maps and photographs, as seen, for in- stance, at the Paris Exposition (1900), will illus- trate what is meant by an intensifying of observa- tion. The Moon. — The large size of our satellite (2,160 miles in diameter), and its relative nearness to us (238,833 miles from the earth's centre), facilitated the careful study of its superficial characters, at least of that side which is alone presented to our view. The systematic and interpretative study of the moon's face practically began with the century, for it dates from Schroter's Solenotopographische Fragmente (1791-1802). Lohrmann and Schmidt, Beer and Madler, Nasmyth and Carpenter, Neison and Secchi, and many more have added detail to de- tail, so that it is safe to say there is no country mapped so nearly up to the present limits of possible precision. The heights of some of its mountain ranges have been computed from their shadows and the depths of some of its extinct craters have been sounded. We have certainly advanced far from the older view, which even Herschel did not entirely get ADVANCE OF ASTRONOMY. 203 rid of, that the moon might be habitable like the earth, and yet there seems no unanimous answer to the question: — Has the moon no atmosphere, or one of extreme tenuity ? We have got far from the belief of Schrb'ter, who imagined he had discovered a lunar city ; what were called seas are now said to be cov- ered with dry rock ; what are called rills are now said to be great clefts or gorges certainly waterless, but we remain in doubt as to the meaning of the broad white rays which diverge for hundreds of miles from some of the principal " ring-plains," and there are many who attribute to glaciation what others confidently interpret as due to volcanic action. Perhaps the most interesting observations are the few which point — though with insufficient security — to some slight changes on the moon's apparently changeless face. Similarly, there are maps of Mars now in circu- lation, which surpass in detail those available in re- gard to Africa a century ago. And though the pre- cision of these Martian maps may be fallacious the same is true of many of the early maps of Africa, and we cannot gainsay the impression of a greatly increased intensity of observation. To what is this due ? To more powerful telescopes, to the use of the spectroscope and polariscope, to the development of photography, and to an exact knowledge of the times (in " opposition " to the sun, i. e., nearest the earth) when Mars can be studied to best advantage. The study of Mars illustrates the growing intensity of observational study, while the imaginary super- structure reared by some on the supposed existence of an intricate system of canals illustrates the danger of outstripping the evidence. PHYSICAL AND CHEMICAL PKOBLEMS. Beginnings of Physical Astronomy. — In 1610, 204: PROGRESS OF SCIENCE IN THE CENTURY. Fabricius and Galilei discovered sun-spots, which are still of fascinating interest to astronomers. In early days, some regarded them as due to the transits of small planets across the sun's disc, others thought of them as clouds, others as masses of cindery slag in process of being sloughed off, and so on. In 1774, Prof. Alexander Wilson of Glasgow was able to give geometrical definiteness to the suggestion, which had been repeatedly made, that the spots were due to great excavations in the sun's substance. He also expounded the idea, which William Herschel elabo- rated, that the sun was like an earth within, but sur- rounded by an aurora of resplendent clouds. Some estimate of the state of knowledge in regard to the physical constitution of the sun may be got from Sir William Herschel's eloquent descriptions about the beginning of the nineteenth century. It was to him a sort of glorified earth, with hills and valleys, luxuriant vegetation, and a population, protected by a cloud-canopy from a radiant outer shell some thou- sands of miles in thickness. This " was nothing less than the definite introduction into astronomy of the paradoxical conception of the central fire and hearth of our system as a cold, dark, terrestrial mass, wrapt in a mantle of innocuous radiance — an earth, so to speak, within — a sun without." * Herschel's author- ity gave vitality to this conception, whose main util- ity was that it helped to definitise error — often the first step to its demolition. But it would be histor- ically unjust to ignore the fact that although Her- schel's main idea was quite erroneous, it was the peg to which a number of accurate observations were tem- porarily attached. * A. M. Clerke. History, 1885, p. 71. ADVANCE OF ASTRONOMY. 205 William Herschel's picture of the sun seems to have been generally accepted for about seven decades. His son, Sir John Herschel, while working at the Cape, was probably beginning to doubt its validity when he maintained that the sun's rotation was inti- mately concerned with the formation of sun-spots; and the attention which he, Baily, Airy, Arago, Struve, and others paid to the corona, chromosphere, and other luminous appendages of the sun observed during the eclipses of 1842 and 1857, led to further suspicions. The careful patience of an amateur — Heinrich Schwabe (d. 1875) — made the next step possible, for by the observations of a quarter of a century he showed, about 1850, that there was a periodicity in the appearance of sun-spots. But this, in itself in- teresting, acquired additional importance when the magnetic observations which the enthusiasm of Hum- boldt, Gauss, and others had secured in five conti- nents led Dr. John Lamont and Sir Edward Sabine (1852) independently to the conclusion (based on different sets of data), that there was a remarkable harmony between periods of disturbance in terrestrial magnetism and the periods of the sun-spots. The congruence was confirmed in the same year (1852) by Eudolph Wolf and by A. A. Gautier, and although Sir William Herschel's association of the price of bread, periods of sunny weather, and frequency of sun-spots was not borne out, the influence of fhe sun on the earth's magnetism was henceforth recognized as a fact. It is now generally believed that the sun is sur- rounded by a halo of incandescent clouds — the photo- sphere— outside of which there is a solar atmosphere composed of vapours of hydrogen, calcium, iron, and 206 PROGRESS OF SCIENCE IN THE CENTURY. other metals, besides a few non-metallic elements. The clouds of the photosphere may be due to fog-pre- cipitates from the cooling atmosphere, while depres- sions or gaps in the photosphere probably give rise to the phenomena of sun-spots. Herschel's idea of a solid core — cool and even habitable — gave place to the idea of an ocean of molten matter, but this, with fuller knowledge of the conditions of the various states of matter, has given place to the generally ac- cepted view that the sun is in the main or wholly gaseous. The Sun's Heat. — About 1836, Sir John Herschel at the Cape and Pouillet in France took a step which meant much to the progress of physical astronomy. It is hardly necessary to say that the step was one of measurement. They tried to measure how much of the sun's radiant energy is intercepted by the earth — a mere speck in the heavens (one part in two thousand millions!) Although their estimates were afterwards shown, by the work of Young, of Lang- ley (1880-81), of Janssen (1897), and others to be far under the mark, they were sufficient to indicate the magnitude of the flood of energy which pours forth from the hearth of our system. Herschel calculated that the heat received by the earth in a year (including that caught in .the atmos- phere) would suffice to melt a covering of ice 120 feet thick over the whole surface of our planet; Young's estimate leads to the result that " each square metre of the Sun's surface pours out enough heat to main- tain about half a dozen mighty Atlantic steamers at their utmost speed night and day, from year's end to year's end ; " * Langley remarks that " though there *Sir Robert Ball, The Story of the, Sun, 1893, p. 263. ADVANCE OF ASTRONOMY. 207 is coal enough in the State of Pennsylvania to sup- ply the wants of the United States for many centuries to come, yet the heat which could be generated by the combustion of all the coal in Pennsylvania would not be sufficient to supply the sun's radiation for the thousandth part of a single second." * From experiments on the intensity of the radi- ation emitted by an incandescent body, Le Chatelier has argued (1892) that the temperature of the sun cannot be less than 7,600°C., and probably much more. These and similar figures convey little mean- ing in themselves, but they are significant in rela- tion to the problem of how the supply of energy is sustained. Maintenance of Solar Energy. — Especially after the formulation of the doctrine of the conservation of energy (about 1843), the problem of the main- tenance of the sun's heat urgently claimed atten- tion. It soon became evident that it is impossible to think of the sun as like an enormous fire giving out heat by combustion. " Massive as the sun is, if its materials had consisted even of the very best materials for giving out heat by what we understand on the terrestrial surface as combustion, that enor- mous mass of some 400,000 miles in radius could have supplied us with only about 5000 years of the present radiation." f From what we know of the sun's age and the amount of its radiation, it is cer- tain that its heat cannot be mainly due to chemical processes at present known to us. Setting aside the chemical solution of what Sir John Herschel called " the great secret," we find two * Sir Robert Ball, The Story of the Sun, 1893, p. 265. tP. G. Tait, Recent Advances, 1876, p. 151. 208 PROGRESS OF SCIENCE IN THE CENTURY. other suggestions. About 1848, Mayer, who shared in stating the idea of the conservation of energy, brought forward a " meteoric hypothesis " according to which it was supposed that the meteorites swarm- ing around the sun engendered heat by impact with it, — thus furnishing a supply of heat many thousand times greater than if they underwent complete com- bustion. This view, also suggested by Waterston, was developed in 1853 by Sir William Thomson (Lord Kelvin) and was supported by Tyndall and Tait. The latter says : " We find, by calculations in which there is no possibility of large error, that this hypoth- esis is thoroughly competent to explain 100,000,000 of years' solar radiation at the present rate, perhaps more; and it is capable of showing us how it is that the sun, for thousands of years together, can part with energy at the enormous rate at which it does still part with it, and yet not apparently cool by perhaps any measurable quantity." * On the other hand, while the infall of meteorites and the heat they produce by impact may be re- garded as certain, it is urged by competent au- thorities that the " intra-planetary " supply is too scanty to be more than a makeshift, while Lord Kelvin himself excluded an " extra-planetary " supply on the ground that if it were true the year would be shorter now by six weeks than at the open- ing of the Christian era.f In 1854, Helmholtz gave the answer which is now generally accepted. If we start with the reason- able assumption of a once larger and less condensed sun, we can understand that as the sun shrank there was thereby accumulated a great thermal store * Recent Advances, 1876, pp. 153-54. f See Miss Clerke's History, p. 352. ADVANCE OF ASTRONOMY. 209 — the direct result of the condensation. Most of this has already been lost ; but as the cooling proceeds, further condensation of the interior (gases) ensues, and this implies further evolution of heat. Thus as the sun parts with heat it compensates for its loss by evolving more. In brief, gravitational energy is exchanged for radiant energy. Ho\v long it can continue to do so before ceasing to glow, before fad- ing away into a dark star, is really indeterminable in the present state of our knowledge of the sun's physical constitution, but some rough calculations have been made. Helmholtz estimated the rate of the sun's contraction at about 220 feet a year, and granted a lease of life for many millions of years to come. Whether the sun is at present becoming actually cooler we do not certainly know, but it is interesting to take note of Lane's theorem (1870), which, on the assumption that the sun is gaseous and behaves as a perfect gas (one whose relations of volume to pres- sure are indicated by Boyle's Law), seeks to show that the temperature must be increasing, not decreas- ing. As we cannot assert that the behaviour of gases in the sun's interior is such as Boyle's Law indicates, we cannot at present decide whether the sun has yet attained its maximum splendour or whether it has now begun to wane. Collisions and Impacts. — From what has been said it is evident that the picture of the sun's origin which astronomers incline to give, is that of a vast primitive nebula, with a great store of energy in the mutual gravitation of its parts. We have also noted the importance of the suggestion due to Helm- holtz— that cooling induced shrinkage, and that this in turn evolved more heat But another possible 210 PROGRESS OF SCIENCE IN THE CENTURY. factor in the production of the sun's heat has been suggested by several astronomers. Sir Robert Ball illustrates this by the story of the new star in Auriga, whose appearance was observed in February, 1892. Where a few days before the photographic plates had shown nothing, a bright star suddenly became apparent. " Everything we have learned about the matter suggests that the new star in Auriga during the time of its greatest bril- liance dispersed a lustre not inferior to that of our own sun. ... It became clear that the brightness of the new star in Auriga was the result of a collision which had taken place between two previously ob- scure bodies. Perhaps it would hardly be right to describe what happened as an actual collision. It is, however, perfectly clear from the evidence that two objects, whose relative velocities were some hundreds of miles to a second, came into such close proximity that by their mutual action a large part of their energy of movement was transformed into heat, and a terrific outburst of incandescent gases and vapours proclaimed far and wide throughout the universe the fact that such an encounter had taken place." * From the analogy of Nova Auriga — which is no isolated instance — it has been conjectured, by Lord Kelvin among others, that our sun may have arisen from the collision of two bodies which attracted each other until they became a single sun with an enormous store of heat derived from the crash of their impact. This speculation is of interest when we look forward to the time in the life of a sun or star, when further compression no longer compensates for the * Sir Robert Ball, loc.cit., p. 277. ADVANCE OF ASTRONOMY. 211 loss of heat by radiation. There seems then no possi- bility of the star recovering itself, unless through a collision with another. For it is possible that the heat produced by the impact might restore them to the primitive nebulous state. If the two colliding bodies were solid the result might be a shattering into fragments which would be projected with high velocities into space; but if the stars had not cooled enough to be solid, fragmentation would be less like- ly, and the collision might lead to rejuvenescence. The establishment of stellar physics practically dates from the application of the spectroscope to the investigation of the composition of the sun, the plan- ets, and the stars. The facts illustrate what has been repeatedly true in the history of science, that the application of a new instrument or method, may lead to development at a rate and in a direction which no one would have ventured to predict. SPECTRUM AXAJLYSIS. The spectroscope is a combination of prisms (or equivalent structures such as a " diffraction- grating") by means of which the various rays com- posing a particular kind of light can be separated out and arranged in a line, the differences of wave- length showing themselves as differences of colour. Thus the presence or absence of certain kinds of light can be seen at a glance. The use of the instru- ment in astronomy is based on the facts (1) that the quality of light is not affected by distance; (2) that each element when in a glowing state emits charac- teristic rays of light or has a definite discontinuous spectrum; and (3) on what is known as Kirchhoff's law of selective absorption. Thus the spectroscope 212 PROGRESS OF SCIENCE IN THE CENTURY. furnishes a means of showing that certain kinds of glowing matter — known to our terrestrial experience — also occur in sun and stars. But the recognition of the importance of this new organon came about very gradually. Gradual Discovery. — In 1672 Sir Isaac Newton made the simple but beautiful experiment (which Kepler had also tried less effectively) of using a prism to split up a ray of sunlight which entered a darkened room through a round hole bored in the shutter. He thereby produced a spectrum or image of the differently coloured constituents of light, due, as he showed, to the fact that these constituents (rays of different wave-length, as we now say) have differ- ent refrangibilities. This was the beginning of the analysis of sunlight, which was destined to have such a remarkable future. The historians tell us that a young Scotchman Thomas Melvil (d. 1753) began the study of the spectra of salts, and the spectroscope was certainly a chemist's instrument before its astronomical value was recognised. It may be recalled that several elements — caesium, rubidium, thallium, indium, gal- lium, and scandium were discovered by means of the spectroscope. In 1802, Wollaston replaced " the round hole in the shutter " by a fine slit parallel to the edge of the prisms, showed that there were gaps in the solar spectrum, and made the further im- portant step of contrasting the spectrum of sunlight with that of a candle flame. Mechanical improvements were soon introduced by Fraunhofer (1814) and Simms (1839). Fraun- hofer, independently of Wollaston, also mapped out a large number of the dark lines in the spectrum of sunlight, and called particular attention to the fact ADVANCE OF ASTRONOMY. 213 that two adjacent yellow lines in the spectrum of a candle flame (now known to be due to sodium) coin- cided with a pair of dark lines in the solar spectrum. Similarly Brewster showed that the potassium lines coincide with other Fraunhofer lines. In 1822 Sir John Herschel noted the bright lines of flames in which certain metallic salts are burnt, and in 1825, along with Talbot, he suggested the importance of using the spectroscope to detect the presence of minute quantities of certain substances in minerals. In 1826 Talbot almost reached the fun- damental conclusion that the presence of a certain line in the spectrum tells unerringly that a certain substance is glowing in the fire of the luminous body. Brewster followed on the same track, and William Swan noted the delicacy of the spectroscopic test in detecting the presence of various substances, such as common salt. As we have already hinted, gaps or dark lines in the solar spectrum mean that rays of a certain re- frangibility (which depends upon wave-length) are absent. It is plain that they may be absent from, the start or simply because they are absorbed in passing through the earth's atmosphere. Thus it was an important step when, in 1832, Sir David Brewster noted that some of the dark lines which Fraunhofer had mapped out on the solar spectrum, were intensified when the sun was near the horizon, that is to say when its rays have a longer path through the earth's atmosphere and are therefore more liable to absorption. Gaps thus due to absorption by the earth's atmosphere are called " telluric lines." The coincidence noted by Fraunhofer between two yellow lines on the sodium spectrum and a pair of dark (D) lines in the solar spectrum, was carefully 214 PROGRESS OF SCIENCE IN THE CENTURY. tested by Professor Miller; and Sir Gabriel Stokes suggested in 1850, as Angstrom did in 1853, that the double D line must be due to the absorptive action of sodium vapour in the sun's atmosphere. Interesting also in this connection was Swan's ex- planation that the appearance of the two yellow sodium lines in all sorts of flames was due to the almost universal distribution of common salt (sodium chloride) in the earth's atmosphere. In 1849 Foucault had shown, without seeing the importance of the fact, that the D lines were dark- ened when the sunlight was passed through an elec- tric arc which gave bright sodium lines in its spec- trum. It was reserved for Kirchhoff ten years later to show clearly what this meant. Thus spectrum analysis " has grown out of some apparently insignificant and disconnected observa- tions made by Marcgraf, Herschel, and others upon the light emitted by flames coloured by certain salts. The spectra of such flames were investigated by various physicists, among whom Talbot, Miller, and Swan deserve first mention; but it was only after Kirchhoff (in 1860) had made and proved the def- inite statement that every glowing vapour emits rays of the same degree of refrangibility that it absorbs, — that spectrum analysis became developed by Bun- sen and himself into one of the great branches of science." * Again we find an illustration of the historical fact that apparently trivial beginnings often lead to great issues, and should never be judged hastily. Bunsen and Kirchfioff. — These two investigators were the first to show conclusively that definite * E. von Meyer. History of Chemistry. Trans. 1891, p. 445. ADVANCE OF ASTRONOMY. 215 bright lines in the spectra of various flames are due to the presence of definite glowing vapours in these flames. In other words the presence of certain lines in the spectrum is a sure index of the presence of certain elements in the luminous body. In a famous experiment, Kirchhof? and Bunsen interposed the flame of a spirit lamp, on whose wick some salt had been sprinkled, in the line of the rays from a lime-light, and found that on what would have been a continuous spectrum there were two dark sodium lines — the phenomenon of " reversal." Yet when the salted flame of a Bunsen burner was sub- stituted for that of the spirit lamp, the " reversal " phenomenon did not occur, but a bright yellow pair of lines was superposed on the lime-light spectrum. Thence they inferred that to effect " reversal " the temperature of the vapour through which the light passes must be less than that of the radiating source — a conclusion afterwards developed by Balfour Stewart, and of great importance in the study of the eolar spectrum. For it led investigators to recog- nise that the appearance of dark lines in the spec- trum of the sun implies that the gases in the sun's atmosphere must be at a lower temperature than those in the photosphere behind. Kirchhoff's Law. — The experiment of the reversal of the lines was the concrete proof of what Kirchhoff had reached mathematically — the law of selective absorption — which was also approached by Ang- strom and Balfour Stewart. " The law states that the ratio between the emissive power and the absorptive power is the same for all sub- stances at the same temperature for rays of the same wave-length. From this it follows that all opaque sub- stances begin to glow at the same temperature — that is, 216 PROGRESS OF SCIENCE IN THE CENTURY. that they give out light of the same wave-length — and that incandescent substances only absorb such rays as they themselves emit. Since, however, incandescent gases possess maxima and minima of light intensity, while solid and liquid substances emit light of every kind when sufficiently heated, the former must also possess a selective absorptive power, and this is not the case in general with the latter. The Fraunhofer lines are thus explained as consequent upon absorptions by incandescent vapours."* Applications. — From the coincidence of the two yellow sodium lines in the spectrum of a candle flame with two of Fraunhofer's dark lines in the solar spectrum, Kirchhoff concluded that sodium was present in the sun's atmosphere; and the same kind of argument was used over and over again. The method is to find in the spectra of terrestrial elements bright lines which exactly coincide with the dark lines in the sun's spectrum. Thus Kirchhoff showed that besides sodium, the sun's atmosphere contained iron, calcium, magnesium, nickel, barium, copper, zinc, and chromium, while others such as gold and silver were similarly shown to be absent. In 1852 Angstrom added hydrogen and others to the list; in 1872-1876 Lockyer increased the number from 14 to 34; in 1887 Trowbridge and Hutchins demonstrated the presence of carbon; in 1891 Eow- land detected silicon. The absence of some elements, notably of oxygen, is as remarkable as the presence of others, but there is, as Lockyer and others have shown, some reason to suspect that elements may be present when they are apparently absent; that is to say they may exist under physical conditions which * Ladenburg. History of Cliemistry. Trans, by Dobbin, 1900, pp. 317 to 318. ADVANCE OF ASTRONOMY. 217 disguise or modify their spectrum, or they may per- haps be " dissociated " into more elementary forms of matter. In short, the date 1859 or 1860 marks the widen- ing of astronomy from being a science descriptive of movements to be also a science descriptive of the chemical constitution and changes of the heavenly bodies. Extension to the Stars. — There is no greater triumph of scientific analysis than that by which a minute beam of sunlight has been made to disclose the chemical constitution of the sun's atmosphere, and this, as we have seen, was the first general result of the application of the spectroscope to astronomy. But what can be done with sunlight can also be done in some measure with starlight, and the application of the spectroscope to the stars has been one of the characteristic features of the astronomical work of the second half of the nineteenth century. As early as 1814, Fraunhofer observed that the dark lines of stellar spectra, though sometimes agree- ing with those in the sun's spectrum, were oftener different, both in arrangement and intensity; but it was with Kirchhoff s researches that the spectro- scopic study of the stars began in earnest. About 1863 Sir William Huggins and Dr. Miller began the systematic study of stellar spectra, and the former extended his observations to nebulae, showing that some of these (with a spectrum of bright lines) are not star-clusters but areas of incandescent gas. As early as 1864, Huggins was able to identify some of the dark lines in the spectra of stars with those of known elements, such as hydrogen, iron, sodium, and calcium, — a kind of work which has since been vigorously prosecuted. 218 PROGRESS OF SCIENCE IN THE CENTURY. But while the use of the spectroscope revealed the presence of certain chemical elements in the stars, and distinguished gaseous from star-cluster nebulae, it led to an even more important achievement — the detection and measurement of the motion of certain stars in the line of sight. We cannot briefly explain the suggestion of Christian Doppler (1848) that " the colour of an object should be affected by the motion of the source, becoming more violet as the object approached, and inclining toward red as it receded from, the observer," * or the method of Fizeau (1848) by which the displacement of the dark lines in the spectrum was used as an index of approach or recession. These led to the work of Sir William Huggins who announced in 1868 that he had found spectroscopic evidence (a minute displace- ment of a dark hydrogen line) of the recession of Sirius and estimated the rate of this recession (from the sun) at 29^ miles per second. He extended the discovery to thirty other stars and confirmed the method by the spectroscopic study of Venus at different times — when the planet was known to be moving towards or away from the earth. It is interesting to notice that displacement of lines has also been detected in the observation of sun- spots, and has led to the conclusion that these are due to downrushes of gases. From 1870 onwards, the splendid work of Huggins was continued by Hermann Vogel, at Potsdam, who in 1887 availed himself of the valuable aid afforded by the dry gelatine plate and the microscopic ex- amination of its photographic record of the spectrum. The motions of approach and recession of many stars were thus calculated with great accuracy, and * Fison, Recent Advances, 1898, p. 200. ADVANCE OF ASTRONOMY. 219 this is only one of many results with which spectrum analysis has enriched astronomy. Thus we might refer to the remarkable argument from spectroscopy which led Pickering of Harvard in 1889 to infer that a certain star in Ursa was really double, or Vogler to confirm the suggestion that the variability of Algol was due to its being periodically eclipsed by a dark or nearly dark companion star. In short, besides chemical information, the spectroscope affords a means of determining celestial motions in the line of sight, and has detected binary which the telescope could never have revealed. Sir William Huggins writes : " In no science, perhaps, does the sober statement of the results which have been achieved appeal so strongly to the imagina- tion, and make so evident the almost boundless powers of the mind of man. By means of its light alone to analyse the chemical nature of a far dis- tant body ; to be able to reason about its present state in relation to the past and future ; to measure within an English mile or less per second the otherwise invis- ible motion which it may have towards us or from us ; to do more, to make even that which is darkness to our eyes light, and from vibrations which our organs of sight are powerless to perceive, to evolve a revelation in which we see mirrored some of the stages through which the stars may pass in the slow evolutional progress — surely^ the record of such achievements, however poor the form of words in which they may be described, is worthy to be re- garded as the scientific epic of the century. " * The extension of spectrum analysis to the stars has yielded information as to the chemical elements which occur in them, has distinguished gaseous neb- * President's address. Rep. Brit. Ass. for 1891, p. 4. 220 PROGRESS OF SCIENCE IN THE CENTURY. ulce from star-clusters, has afforded a method of measuring the motions of stars in the line of sight, and has led to many other results which afford fine historical illustration of the value of co-operation between sister-sciences. THE EVOLUTION-IDEA IN ASTRONOMY. The evolution-idea has asserted itself in astron- omy especially in connection with what is called the nebular hypothesis, — an attempt to give an account of the origin of a solar system. It is said to have arisen as a transcendental conception in the mind of Swedenborg; it was suggested on general grounds by Kant; it was formulated in mechanical terms by Laplace; and it has been the subject of much dis- cussion— on the whole unfavourable to its details, though confirmatory of the general idea. It was in 1755 that Immanuel Kant (1724-1804) published his General Natural History and Theory of the Heavens, more than a quarter of a century before his Critique of Pure Reason. Based, as its title indicates, on Newton's Principia, the essay pic- tures a possible mode of origin for the sun and the planets from a homogeneous distribution of vaporous particles in the space now occupied by the solar sys- tem. A more important step was taken in 1796 when Laplace presented his " Nebular Hypothesis." Start- ing from a vast fluid nebula in slow rotation, he supposed that as this cooled it contracted, that as it contracted its rate of rotation increased, that event- ually the " centrifugal force " of the great nebular sphere exceeded the centripetal gravitational attrac- tion, and a nebulous ring was separated off from the ADVANCE OF ASTRONOMY. 221 equatorial regions. This ring afterwards broke up, but its parts condensed to form the furthest planet. With further shrinkages and accelerations of the parent nebular mass, the various planets were thrown off in succession, themselves to repeat the process in forming rings like Saturn's, or satellites like those of Jupiter. One of the chief reasons which led Laplace to think out a possible unity of origin for the solar system, was that the planets and their satellites revolve and rotate in the same direction as that in which the sun rotates, — a coincidence of many (40 or more) motions which almost suggests a common origin. We now know that the satellites of Uranus and Neptune move in the opposite direction, and that there are other exceptions, e.g., that the inner Martian moon revolves in a shorter time than Mars, to the uniformity which Laplace proposed; on the other hand we know that there are many more in- stances of uniformity of motion than he was aware of. There are many other sets of facts which favour the general idea of the nebular hypothesis. Thus we have a rapidly increasing mass of information in regard to the nebulae which Herschel was the first to begin to study in earnest, some of which look like the primeval nebula which Laplace postulated, while others present appearances suggestive of systems in process of being made. The great Nebula in Andromeda, as photographed by Roberts, " suggests," as Huggins says, " a stage in a succes- sion of evolutional events not inconsistent with that which the nebular hypothesis requires." That the same substances occur (as the spectro- scope proves) in sun and planets is another fact which 222 PROGRESS OF SCIENCE IN THE CENTURY. would fit in well with the evolutionary theory, being suggestive of community of origin. Corroboration may also be found in Helmholtz's shrinkage theory (previously noted) of the origin and maintenance of solar energy, for it leads us back to a larger and less condensed sun, and thence to one larger still, until finally we approach some- thing like Laplace's primitive nebula. " We can reason back to the time when the sun was sufiiciently expanded to fill the whole space occupied by the solar system and was reduced to a great glowing nebula. Though man's life, the life of the race perhaps, is too short to give us direct evidence of any distinct stages of so august a process, still the probability is great that the nebular hypothesis, especially in the more precise form given to it by Roche, does repre- sent broadly, notwithstanding some difficulties, the succession of events through which the sun and plan- ets have passed." * " So little is, however, known of the behaviour of a body like Laplace's nebula when condensing and rotat- ing that it is hardly worth while to consider the details of the scheme, and that Laplace himself did not take his hypothesis nearly so seriously as many of its ex- pounders, may be inferred from the fact that he only published it in a popular book, and from his remarkable description of it as ' these conjectures on the formation of the stars and of the solar system, conjectures which I present with all the distrust which everything which is not a result of observation or of calculation ought to inspire.' " f Meteoritic Hypothesis. — We have already alluded to the speculation, which is now particularly asso- * Sir W. Hupgins. Rep. Brit. Ass. for 1891, p. 20. f Arthur Berry. Short History of Astronomy, 1898, p. 322. ADVANCE OF ASTRONOMY. 223 elated with the names of Faye and Sir J. Norman Lockyer, that crowds of discrete meteoric bodies drawn together into aggregates by gravitational at- traction, and evolving heat by collisions, may have given rise to nebula?, with further condensation to luminous stars, and eventually to dark planets, whose vitality is at an end unless a collision make it possi- ble for the evolutionary process to recommence. But this remains in the speculative phase. The possibility, however, must be borne in mind that some of the existing nebulae may have originated in the collisions of dark suns, and are thus the chil- dren, as it were, of a later generation. " During the short historic period, indeed, there is no record of such an event; still it would seem to be only through the collision of dark suns, of which the number must be increasing, that a temporary reju- venescence of the heavens is possible, and by such ebbings and Sowings of stellar life that the inevita- ble end to which evolution in its apparently uncom- pensated progress is carrying us can, even for a little, be delayed. . . . We cannot refuse to admit as possible such an origin for nebulse." Tidal Friction. — An interesting recent contribu- tion to the theory of the evolution of planetary sys- tems, and of satellites in particular, has been made by Mr. G. H. Darwin, in his papers on the influence of tidal friction, but the subject is too intricate for discussion within our limits. SUMMARY. — A cautious summary forms the last paragraph of Berry's Short History of Astronomy, and this we venture to quote: — " Speaking generally , we may say that the outcome of the nineteenth-cen- tury study of the problem of the early history of the * Sir W. Huggins. Rep. Brit. Ass. for 1891, p. 24. 224: PROGRESS OF SCIENCE IN THE CENTURY. solar system has been to discredit the details of La- place's hypothesis in a variety of ways, but to estab- lish on a firmer basis the general view that the solar system has been formed by some process of condensa- tion out of an earlier very diffused mass bearing a general resemblance to one of the nebulce which the telescope shows us, and that stars oilier than the sun are not unlikely to have been formed in a somewhat similar way; and, further, the theory of tidal friction supplements this general but vague theory, by giving a rational account of a process which seems to have been the predominant factor in the development of the system formed by our own earth and moon, and to have had at any rate an important influence in a number of other cases." CHAPTER VII. GROWTH or GEOLOGY.* CATACLYSMAL, UNIFOBMITABIAN, EVOLTTTIONABY. THESE are cumbrous words for the heading of a paragraph, and yet they are serviceable to sum up the three chief phases of geology during the nine- teenth century. For if it be borne in mind that phases of science do not end abruptly like the reigns of kings, but overlap and dovetail, the words cata- clysmal, uniformitarian, and evolutionary may serve with some usefulness to emphasise the changes of outlook in the geology of the period under discussion. Cataclysmal. — The nickname cataclysmal or catas- trophic applies to those who saw no way of explain- ing the features of the earth's face — its ridges, wrinkles, dents, and scars — without postulating con- vulsions and cataclysms, fires and flood, not only on a scale vastly greater than any analogous occurrences now to be observed on our, on the whole, very sedate earth, but even different in kind. Cuvier, and to some extent Buffon, may be named as champions of the catastrophic theory. Uniformitarian. — From this way of looking at things a recoil was inevitable when a growing appre- ciation of scientific method made it clear that in geological interpretation, as elsewhere, we must not * The history of geology relied on is Karl Alfred von Zittel's Geschichte der Geologic und PaWontologie, 1899; translated (1901) by Dr. Maria Ogilvie-Gordon. 226 PROGRESS OF SCIENCE IN THE CENTURY. invent hypothetical agencies; that we must exhaust the full potency of known and verifiable causes before we admit even the Tjeed of postulating others which are unknown and unverifiable. The uniformitarian view, well expressed by Hut- ton and Playfair, was right when it insisted that we must in our interpretation exhaust the possibilities of actually observable factors, but it was wrong if it assumed that these were necessarily all the factors, or that they had never changed in the rate or amount of their influence. In the hands of Lyell (1797-1875) the uniformi- tarian interpretation found its best expression, and at the same time, as many think, signed its own death- warrant. For in spite of the progress of physics and 'astronomy since the time of Hutton, Lyell deliber- ately shut out the light of the evolution-idea — the thought of a beginning and of an end to the earth which the theory of energy presses home. " He con- sistently refused to extend his gaze beyond the rocks beneath his feet, and was thus led to do a serious injury to our science ; he severed it from cosmogony, for which he entertained and expressed the most pro- found contempt, and from the mutilation thus in- flicted geology is only at length making a slow and painful recovery." * A reaction from extreme uniformitarianism was inevitable. It began to be felt that although " Lyell, in his great work, proved that the agents now in operation, working with the same activity as that which they exhibit at the present day, might produce the phenomena exhibited by the stratified rocks, *W. J. Sollas, Pres. Address, Sec. C, Rep. Brit. Ass., 1900; Nature, Sept. 13, 1900, p. 481. GROWTH OF GEOLOGY. 227 . . . that is not the same thing as proving that they did so produce them." * Such proof can only be afforded by a detailed study of the strata, more ex- tensive and intensive than even now exists. But as this detailed study has proceeded, it has become more and more clear not only that the earth has evolved from a very different primitive state to its present form, but furthermore that through the immense expanse of its history there have been nota- ble changes in the earth-sculpturing factors. The indisputable proof of great Ice-Ages and of enormous thrust-movements may serve to show that uniformi- tarianism recoiled too far from catastrophism. To try to explain the phenomena of glaciation without glaciers strained the uniformitarian theory to the breaking-point. Evolutionary. — The cataclysmal geology was un- scientific, for it invoked the aid of undemonstrable factors ; the uniformitarian geology was inconsistent, for while it sought to interpret the past in terms of the present, it rejected the evolution idea which sums up the whole history as a process of becoming; the modern evolutionary geology has inherited the strength of the uniformitarian school and has given this fresh virility by recognising that the history of the earth is a natural development in which at every stage the present is the child of the past and the par- ent of the future. The evolutionist school differs from the uniformitarian, (a) in admitting in its full- est sense the hypothesis that the earth has had a natural history from a nebular or molten mass down to the twentieth century, and (6) in admitting the likelihood that in the course of the evolution there * J. E. Marr, Address Section C, Rep. Brit. Ass., 1896. 228 PROGRESS OF SCIENCE IN THE CENTURY. may have been rhythms and changes in the action of the known factors." * SUMMARY. — " From, Steno onward the spirit of geology was catastrophic; from Hutton onward it grew increasingly uniformitarian; from the time of Darwin and Kelvin it has become evolutional." f " The Catastrophists had it all their own way until the Uniformitarians got the upper hand, only to be in turn displaced by the Evolutionists/' $ FOUNDATION STONES OF GEOLOGY. Even in the later decades of the eighteenth century geology as a distinct science did not exist, but its sure foundations were being laid. Thus Sir Archibald Geikie has rescued from undeserved oblivion (in Britain at least) the name of Jean fitienne Guettard (1715-1786) — "the first to construct, however im- perfectly, geological maps, the first to make known the existence of extinct volcanoes in Central France, and one of the first to see the value of organic re- mains as geological monuments, and to prepare de- tailed descriptions and figures of them. To him also are due some of the earliest luminous suggestions on the denudation of the land by the atmospheric and marine agents." ** Another illustrious pioneer was Nicholas Desmar- est (1725-1815), who amid the labours of a life devoted to fostering the industries of France, found time to map the volcanic rocks of Auvergne, to work out a theory of the volcanic origin of basalt, to trace * See J. E. Marr. Address Section C, Rep. Brit. Ass., 1896, p. 775. t Sollas, loc. cit. $ Geikie. Founders of Geology, 1897, p. 288. ** Sir Archibald Geikie. Founders of Geology, 1897, p. 46. GROWTH OF GEOLOGY. 229 with persistent patience the various effects of denu- dation on beds of lava, to propound the doctrine of the origin of valleys by the erosive action of the streams which flow in them, and in short, to lay, not one but several of the foundation-stones of modern geology. In Sir Archibald Geikie's fascinating account of the founders of geology, the next two names are Peter Simon Pallas (1741-1811) and Horace Benedict de Saussure (1740-1799). Pallas was in charge of a famous Russian expedition (1768-1774) ordered by the Empress Catherine II., primarily with the object of observing the Transit of Venus, but also with in- structions to make a complete regional survey of everything from mountains to man. Geologically, the expedition was signalised by the discovery of the widespread remains of mammoth, rhinoceros, and buffalo in the Siberian basins, and by Pallas's re- searches on the origin and history of mountains. Far beyond the limits of geology, the work of Pallas has an acknowledged importance. " The labours of De Saussure among the Alps mark an epoch, not only in the investigation of the history of the globe, but in the relations of civilised mankind to the mountains which diversify the sur- face of the land." He broke down a strange tradi- tional prejudice against the horror of the great hills and inspired the modern enthusiasm for mountain- eering; he began experiments in rock-making; he furnished a model of how mountain ranges should be studied and described ; and he seems to have been the first to adopt the terms Geology and Geologist* When theoretical critics came to Desmarest with objections, he used to say " Go and see " ; and if it * See Geikie, p. 88. 230 PROGRESS OF SCIENCE IN THE CENTURY. be true that any vindication of the necessity for an observational basis in science is now an anachronism, we should not forget the early struggles towards this essential virtue. Desmarest's conclusion as to the igneous origin of basalt may seem a small result for years of patience, but we have only to contrast it with the old idea that basaltic columns were petri- fied bamboo stems to see its historical importance. It may not be easy to cite any particular conclusion of De Saussure's which is now part of the frame- work of tektonic geology, but his lifework was none the less a vindication of the precept " Go and see." Nowadays, no one who is interested in the nature and origin of the sculptured earth around him can " go and see " without bearing with him the idea that the earth's crust is a great history-book, that the various layers and strata are pages recording particu- lar processes, and that there has been a " geological succession " still to be deciphered though he who runs may not read it. Yet this familiar and ele- mentary idea of a geological succession had a long history ! Werner. — Sir Archibald Geikie refers to Leh- mann, Fuchsel, and Werner as three observers who advanced the idea of geological succession during the latter half of the eighteenth century. Of the three, Werner was the most important. He tried to put minerals in order, as Linnaeus had done for plants; he was one of the first to expound the general idea of the sequence of geological formations ; and he was an influential teacher of great personal charm. Hutton. — In 1Y85, after years of travel and thought, James Hutton communicated to the Royal Society of Edinburgh the first outlines of his Theory of the Earth. GROWTH OF GEOLOGY. 231 For the main purpose of this volume, which is to illustrate the working of the scientific mood, the theory of the earth which Hutton suggested is full of significance. Significant, because its author had so clearly grasped the scientific method of seeking to appreciate the full force of known factors instead of invoking the aid of others whose reality is hypotheti- cal. Waters wear the stones, the solid earth melts away, the mountain is transplanted piece-meal to the sea, there is a ceaseless decay of continents; on the other hand, underground forces cause upheaval, con- solidated debris is once more brought to light, and molten masses are here and there thrust upward to form eruptive rock. What is, has been, and that through a vast antiquity of ages, so that " little causes, long continuing," have wrought great changes. The present is the child of the past and the parent of the future. In short it was the idea of development that Hutton had, perhaps subcon- sciously, in mind. The keynote of his work may be found in his sentence : " !N"o powers are to be em- ployed that are not natural to the globe, no action to be admitted of except those of which we know the principle, and no extraordinary events to be alleged to explain a common appearance." * Unlike Werner, Hutton started from observations not from preconceptions. He studied the present, and in the process now occurring found the key to the history of the past Among his conclusions we may note : — The aqueous origin of sedimentary rocks, the influence of subterranean force (essentially due to heat) in contorting strata, the theory of subterranean intrusions of molten matter forming veins or dykes * Theory of the Earth, Vol. II. p. 547. Quoted by Sir A, Geikie, Founders of Geology, p. 182. 232 PROGRESS OF SCIENCE IN THE CENTURY. of " whinstone " and the like, the idea of the meta- morphism of rocks under the influence of new condi- tions, and the doctrine of earth-sculpture by denuda- tion (through rain, rivers, glaciers, etc.). Neptunists and Plutonists. — The masterly and lucid Illustrations of the Huttonian Theory by Hutton's friend and disciple John Playfair, did much to help the new theory of the earth towards acceptance. But this was further delayed by the bitterness of the strange controversy which sprang up between Hutton's followers — nicknamed Plu- tonists— and those of Werner, who were similarly called Neptunists. Hutton had emphasised the im- portance of subterranean heat in consolidating and upheaving sedimentary deposits ; Werner had almost exclusively emphasised the agency of water, believ- ing that the rocks had arisen for the most part as precipitates in a primeval ocean. To one looking backward it does not seem an instructive controversy, and it is perhaps enough to say that the more stable doctrines of Hutton were those that survived. Hall. — The Neptunists had urged against the Plu- tonists that if basalt and the like had really arisen from molten masses, they ought to be found as glasses or slags. To this Sir James Hall retorted by ex- periment, showing that basalt could be fused and vitrified, and that if a portion of this basalt-glass was re-fused and allowed to cool very slowly, it resumed its familiar stony textures. From pounded chalk, fused under pressure, he obtained a substance resem- bling marble. In another direction he also experi- mented most suggestively, for he arranged a mechan- ical device for contorting layers of clay (by lateral compression under considerable vertical pressure), and showed that the foldings of strata could thus be GROWTH OF GEOLOGY. 233 imitated. These and other experiments may be justly regarded as the foundation of experimental geology. William Smith. — While the Xeptunists and Plu- tonists were bickering in Edinburgh — which has been a centre of geological activity through the cen- tury— the land-surveyor and engineer William Smith (1769-1839), was walking through the coun- ties of England, and working out his momentous conclusion that the stratified rocks occur in defi- nite sequence, and that each well-marked group can be recognised and tracked by its characteristic fossils. In 1815 he published his epoch-making Geo- logical Map of England, and this he followed up during the succeeding nine ' years by twenty-one county maps, in the execution of which he was helped by his nephew and pupil, John Phillips. This was the foundation of stratigraphical geology. In regard to the importance of William Smith's work, the verdict of one of the foremost living geolo- gists may be cited. " No single discovery," says Sir Archibald Geikie, " has ever had a more momen- tous and far-reaching influence on the progress of a science than that law of organic succession which Smith established. At first it served merely to de- termine the order of the stratified rocks of England. But it soon proved to possess a world-wide value, for it was found to furnish the key to the structure of the whole stratified Crust of the earth. It showed that within that crust lie the chronicles of a long history of plant and animal life upon this planet, it supplied the means of arranging the materials for this history in true chronological sequence, and it thus opened out a magnificent vista through a vast series of ages, each marked by its own distinctive 234 PROGRESS OF SCIENCE IN THE CENTURY. types of organic life, which in proportion to their an- tiquity, departed more and more from the aspect of the living world." * Along with the achievements of William Smith, we must place the researches of Cuvier and Brongni- art, and of others who early realised the value of fossils as indices in determining the sequence of strata. The idea of interpreting the history of the past in terms of changes observed in occurrence in the pres- ent; the conception of the sequence of strata; the recognition of the value of fossils as indices, are three of the foundation-stones of geology which were laid at the beginning of the nineteenth century. THE EVOLUTION-IDEA IN GEOLOGY. At various dates we find exceptional recognition of the Evolution-Idea as applied to the Earth. It fas- cinated a few long before Darwin brought it home to all. Thus Descartes propounded a scheme of the Earth's development from a globe of molten liquid, and Leibnitz's Protogcea (published long after his death, about the middle of the eighteenth century) contained a similar attempt. Buffon, too, starting with the bold idea that the Earth, like the planets, was detached from the mass of the sun by a cometary shock, sketched with a free hand the successive chapters of a problematical history in his Epochs of Nature (17T8). Even when uniformitarianism was in its full strength, — inquiring minds here and there were be- ginning to suspect that there was something to be said for the heresies of Buffon, Lamarck, Erasmua * Op. cit., 1892, pp. 9-10. GROWTH OF GEOLOGY. 235 Darwin, and other pioneers who spoke of a progres- sive evolution of plants and animals. The evolution- idea was whispered by many, and a few proclaimed it prematurely on the house-tops. The cosmological speculations of Kant and Laplace as to the possible evolution of suns and their sys- tems did not apparently much excite the geologists, but they must have raised some disquieting thoughts. Sir William Thomson's early insistence (1862- 1868) on the secular loss of heat from both earth and sun brought the question nearer home, for the con- clusion was inevitable that the present state of affairs could not have lasted forever. Without going back to a nebular mass we must at least think of a time when the earth was much hotter than now, when the waters of our ocean formed part of a hot atmosphere, and we may also look forward to a time when the earth will be much colder than now, and again without an ocean unless it be one of liquid air. In neither of these conditions could life, as we know it, exist. " Some- where between these two indefinite points of time in the evolution of our planet it is our privilege to live, to investigate, to speculate concerning the antecedent and future conditions of things." * This is the evo- lutionist attitude. It is interesting, however, to pause to notice a few of the lines of inquiry which led to the transition from Uniformitarian to what may be called Evolu- tionist geology. From the early works of Fourier (1820), Poisson (1835), and Hopkins (1839), down to the more mod- ern researches of Thomson and Tait and Helmholtz, there has been a prolonged attempt to map out the * Sir John Murray, Rep. Brit. Ass., 1899, p. 796. 236 PROGRESS OF SCIENCE IN THE CENTURY. great steps in the early history of the Earth before it became fit to be a home of life, and also to reach from physical and astronomical data some secure conclusion as to the present physical state of the Earth's interior. Chapters in the Ancient History of the Earth. — The Earth probably had its beginning as one of the many rings swirled off from the great nebular mass which gradually condensed into our sun ; but other origins are conceivable. In any case, it had a be- ginning as a rapidly rotating molten planet It solid- ified about the centre into a metallic nucleus, which was probably composed in great part of iron ; it was surrounded by a deep atmosphere, the larger part of which has been condensed into the waters of our present seas. Its molten ocean was profoundly dis- turbed by solar tides, for there was as yet no moon, and it was perhaps a particularly high tide which made the earth give birth to its satellite. " This event may be regarded as marking the first critical period, or catastrophe if we please, in the history of our planet. The career of our satellite, after its escape from the earth, is not known till it attained a distance of nine terrestrial radii; after this its progress can be clearly followed. At the eventful time of parturition the earth was rotating, with a period of from two to four hours, about an axis in- clined at some 11° or 12° to the ecliptic. The time which has elapsed since the moon occupied a position nine terrestrial radii distant from the earth is at least fifty-six to fifty-seven millions of years, but may have been much more." * " The outer envelope of the earth drawn off to form the moon was charged with steam and other *W. J. Sollas, Pres. Address, Sec. C, Hep. Brit. Ass., 1900; Nature, 13th Sept., 1900, p. 482. GROWTH OF GEOLOGY. 237 gases under a pressure of 5,000 Ibs. to the square inch; but as the satellite wandered away from the parent planet this pressure continuously diminished. Under these circumstances the moon would become as explosive as a charged bomb, steam would burst forth from numberless volcanoes, and while the face of the moon might thus have acquired its existing features, the ejected material might possibly have been shot so far away from its origin as to have ac- quired an independent orbit," * and some of the meteorites which now descend upon the earth may be returned portions of the early envelope. Soon after the birth of the moon, the earth became consolidated (with a surface temperature of about 1170°C.), and the moon may have been influential in determining high-pressure areas where the crust was depressed, and low-pressure areas where it was lowered. This, as Sollas says, was the second critical period in the history of the earth, the stage of the " consistentior status." Since this epoch, on Lord Kelvin's estimate, twenty to forty millions of years may have elapsed. Below the surface the temperature increased, as it still does; at a depth of twenty-five miles, it would be (according to Lord Kelvin's calculations) about 1430 °C., or 260°C. above the fusion point of the matter forming the crust. But the great pressure at this depth would counteract the heightened temper- ature, and still keep the crust solidified even at a depth of twenty-five miles. When, with continued cooling, the temperature of the surface fell to 370 °C., the steam in the atmos- phere would begin to liquefy, and this was the first step in the origin of the oceans. Supposing, as * Sollas, loc. cit. 238 PROGRESS OF SCIENCE IN THE CENTURY. Sollas suggests, a localisation of the water in primi- tive faint depressions (anti-cyclonic areas), and a corresponding reduction of pressure on the incipient continental areas, there might result an expansion of the underlying rock of these areas, " for a great change of volume occurs when the material of igneous rocks passes from the crystalline state to that of glass." In some such way, the ocean basins might be deepened and the continental areas raised. The hot water of the primeval ocean would act energetically on the silicates of the primitive crust ; it would begin to be " salt " with saline solutions and to precipitate deposits. Since the condensation of the oceans, Prof. Joly suggests a lapse of eighty to ninety mil- lions of years. To sum up dogmatically would be absurd, but it may be said that a nebular mass probably gave rise to a rapidly rotating molten planet; that after central solidification, this may have given birth to the moon; and that as cooling slowly continued, there followed the consolidation of the crust and the beginning of the distinction between ocean basins and continental areas. Through phases more or less like those outlined above the Earth has reached its present state. The vast nucleus or " centrosphere " is practically solid, the melting-point of the metals and metalloids being raised by the immense pressure. Outside the cen- tral mass there is " a shell of materials bordering upon fusion," that which Sir John Murray calls the " tektosphere." On this plastic shell there rests the heterogeneous and wrinkled crust or lithosphere, always slightly pulsating. Wrinkling of the Lithosphere. — How the crust or lithosphere has come to be elevated into continental GROWTH OF GEOLOGY. 239 areas, on an average three miles above the ocean floor and to be folded into mountain chains, is one of the most difficult of geological problems, but there are several factors on which the evolutionary geolo- gist relies. Perhaps the most important is the contraction of the centrosphere. But, before noting a few opinions of experts on this subject, it may be useful to recall that, stupendous as mountain-chains are, their height is minute when compared with the radius of the earth. Indeed, it has been pointed out that on an artificial globe a foot in di- ameter, they should not stand out more than the slight elevations which might result where the edges of the covering paper-slips overlap. "As the solid centrosphere slowly contracted from loss of heat, the primitive lithosphere, in accommodat- ing itself — through changes in the tektosphere — to the shrinking nucleus, would be buckled, warped, and thrown into ridges. . . . The compression of moun- tain chains has most probably been brought about in this manner, but the same cannot be said of the eleva- tion of plateaus, of mountain platforms, and of con- tinents." * " It was at first imagined that during the flow of time the interior of the earth lost so much heat, and suffered so much contraction in consequence, that the exterior in adapting itself to the shrunken body, was compelled to fit it like a wrinkled garment. This theory, indeed, enjoyed a happy existence till it fell into the hands of mathematicians, when it fared very badly, and now lies in a pitiable condition, neglected of its friends." f The mathematicians maintained * Sir John Murray, Rep. Brit. Ass., 1899, p. 797. tSollas, Rep. Brit. Ass., 1900. See Mature, Sept. 13, 1900, p. 487. 240 PROGRESS OF SCIENCE IN THE CENTURY. that the amount of contraction was altogether inade- quate to explain the wrinkling, but Prof. Sollas finds sufficient flaws in the data to warrant him in still maintaining the theory of contraction. " The con- traction of the interior of the earth, consequent on its loss of heat, causes the crust to fall upon it in folds, which rise over the continents and sink under the oceans, and the flexure of the area of sedimenta- tion is partly a consequence of this folding, partly of overloading." * Another factor may be chiefly alluded to. Since the floor of the ocean has a temperature about zero, and is some three miles below the continental level, surfaces of equal internal temperature will not be spherical, but will rise beneath the continents and sink beneath the ocean, and the effect will be to ren- der the continents mobile as regards the ocean floor ; or vice versa (Sollas). We have cited enough to illustrate a kind of in- quiry eminently characteristic of the end of the nineteenth century which the new century is certain to develop to more stable and precise results. The general result may ~be summed up in a sen- tence; the contraction of the interior probably ac- counts for much of the folding and crumpling of the exterior; other physical factors are and have been at work; and the transforming influences of water, of the atmosphere, and of life have been continuous and momentous since they first began to act. It must not be supposed that the evolution-idea in Geology has been restricted in application to the recondite problem of the Earth's early phases; the idea has influenced the whole science and is illus- trated in the modern treatment of river-development, or of coral reefs, or of details of scenery, and so on, * Sollas, Joe. tit* GROWTH OF GEOLOGY. 241 just as markedly as in connection with the big ques- tion of the history of the Earth as a whole. AGE OF THE EAKTH. In the early days of geological science, the preva- lent opinion seems to have been that the earth was about 6,000 years old. But this belief was for the most part an outcome of " wresting the Scriptures " from their proper use, and is quite irrelevant in scientific discussion. The Age of the Earth as Realised ~by Uniformi- tarians. — When James Hutton (1726-1797) began to show that the present supplies the key to the inter- pretation of the past, and saw " the ruins of an older world in the present structure of the globe," it be- came plain to inquiring minds that the earth must be old beyond all telling. William Smith's revelation of the succession of strata in England — the vision of age before age stretching back into an inconceivably distant past ; the founding of palaeontology by Cuvier and others, and the suggestion of successive faunas and floras leading us back and back to the mist of life's begin- nings; the publication of John Playfair's Illustra- tio?is of the Huttonian Theory (1802); and other great events led to an accentuation of the idea of an- tiquity. Indeed, Playf air went so far as to deny that either earth or cosmos furnished tangible hint of any beginning at all. Thus the earth, which had not long before been credited with a short duration of 6,000 years, was at the beginning of the century con- ceived of as a sort of inanimate Methuselah, " with- out beginning of days or end of years." Recognitions of Limits. — A reaction began in 1862, when Lord Kelvin (then Sir William Thomson) sent 242 PROGRESS OF SCIENCE IN THE CENTURY. his first shell into the camp of the geologists, which he has not since ceased to bombard. From that date the history has been this, — the physicists have calcu- lated out certain limits; the geologists have agreed that they do not require eternity, but yet much more than the physicists will grant them; there has been much criticism of data and calculations and some reconsideration on both sides; of late the biologists have also insisted on being heard. (a) Physical Arguments. — The chief arguments of the physicists as to the age of the earth are based (1) on the downward increase of terrestrial temper- ature, (2) on the retardation of the earth's angular velocity by tidal friction, and (3) on the limitation of the sun's age. Lord Kelvin began by declaring that the age of the earth must be more than twenty millions of years, and less than four hundred mil- lions ; but he subsequently cut down his maximum to the former minimum, and Professor Tait would not allow even half as much. In one of his last utter- ances on the subject, Lord Kelvin states " it was more than twenty and less than forty million years, and probably much nearer twenty than forty." * That the physicists are far from being agreed among themselves may be inferred from the frank statement of Professor George Darwin : " At pres- ent our knowledge of a definite limit to geological time has so little precision that we should do wrong to summarily reject any theories which appear to demand longer periods of time than those which now appear allowable." f (&) Geological Arguments: From the rate of deposition of rock-forming materials. — Ever since Hutton published his observations and reflections on * Pres. Address Victoria Institute for 1897. Phil. Mag., January, 1899. f Rep. Brit. Ass., 1896, p. 518. GROWTH OF GEOLOGY. 043 the decay of continents, it has been a recognised fact that there is a universal degradation of the dry land. The span of the longest human life is but a tick of the geological clock, and so we speak of the eternal hills. But there is no doubt in the mind of any observer that even the hills are slowly melting and crumbling away. " The hills are shadows, and they flow from form to form, and nothing stands." Rain and frost, lichens and burrowing animals, run- ning water and whistling wind, and other agencies contribute to the unceasing weathering and denuda- tion. There are, indeed, conservative agencies, but the wasting goes on steadily. The present land surface is being reduced in height, on an average of ^iVs to 33*00 foot per annum. But what is lost here is gained somewhere else, denudation and deposition must be almost equivalent in amount (though not in area, the latter being usually much smaller), and thus we can arrive at some estimate of the amount of wasting by measuring the amount of sediment deposited. " Actual measurement of the proportion of sediment in river water shows that while in some cases the lowering of the surface may be as much as Ts-jj- of a foot in a year, in others it falls as low as rejrff. In other words, the rate of deposition of new sedimentary formations, over an area of sea-floor equivalent to that which has yielded the sediment, may vary from one foot in 730 years to one foot in 6,800 years." * Now, a considerable part of the outer crust of the earth is made up of sedimentary rocks ; among these it is possible with considerable accuracy to distin- guish the deposits which were laid down at different * Sir Archibald Geikie, Pres. Address, Report Brit. As*, for 1892, p. 21. 2M PROGRESS OF SCIENCE IN THE CENTURY. and successive times (as proved in some cases de- cisively by their fossils and in other cases by other facts) ; and " on a reasonable computation, these stratified masses, where most fully developed, attain a thickness of not less than 100,000 feet." * There- fore, if we assume that the present rate of change is at all comparable to the past rate of change, we can form geologically some estimate of the antiquity of our earth. " If they were all laid down at the most rapid recorded rate of denudation, they would re- quire a period of seventy-three millions of years for their completion. If they were laid down at the slowest rate they would demand a period of not less than six hundred and eighty millions." f But how much experts may differ is here again illustrated, for Prof. Sollas says : — " The total maxi- mum thickness of the stratified rocks is 265,000 feet, and consequently if they accumulated at the rate of one foot in a century, as evidence seems to suggest, more than twenty-six millions of years must have elapsed during their formation." $ Against this line of argument various objections may be raised. It may be said that the rate of denudation and therefore of deposition may have been much more rapid a few million years ago than it now is, and the possibility cannot be denied. But some evidence should be forthcoming; and there is not much. In ancient sedimentary rocks we see ripple marks and sun-cracks and worm or mollusc tracks and it may even be the markings of desiccated jellyfishes, just as we see them on the beach to-day, and this certainly does not point to rapid deposition. * A. Geikie, op. cit., p. 21. t A. Geikie, op. cit., p. 21. JW. J. Sollas, Address Section C, Rep. Brit. Ass., 1900. Nature, Sept. 13, 1900, p 485. GROWTH OF GEOLOGY. 245 Moreover, we must recall the fact that the sedi- mentary rocks are in scores of cases interrupted in a manner which forces us to infer periods of up- heaval or subsidence or volcanic intrusion, — still further extending the demand for millions of years. In an exceedingly interesting paper, Goodchild * has tried to estimate the time required for the vari- ous sedimentary formations considered seriatim, and the time represented by great unconformities, and computes the total time since the commencement of the Cambrian period at over 700,000,000 years. But life was already ancient in the Cambrian times, and this leads, as Goodchild indicates, to an enor- mous increase of the seven hundred millions. Argument from the Saltness of the Sea. — Another interesting line of argument is that which has led Prof. Joly to conclude that eighty to ninety millions of years represent the maximum period of time since the oceans were formed. His argument is that since the salt sea was once fresh, and since the saltness is due to dissolved salts carried into the sea by rivers, an estimate of the annual amount brought down by the rivers will show how long it must have taken to give the sea its present salinity. Taking sodium alone, it is computed that the amount in the sea is at least ninety millions of times greater than the quan- tity which rivers pour in annually (about 160,000,- 000 tons). Joly's argument is clear and simple; everything depends, however, on the reliability of the data. (c) Biological Arguments. — 'Apart from domesti- cation and cultivation we know almost nothing in re- gard to the present rate of variation of living crea- tures, though researches like those of Prof. Weldon * Proc. Roy. Phys. Soc., Edinburgh, xiii., 1897, pp. 259-308. 246 PROGRESS OF SCIENCE IN THE CENTURY. on the crabs of Plymouth Harbour are beginning to remedy this discreditable ignorance. Until we have much information of this sort it is quite idle for one biologist to say that he thinks one hundred millions of years enough for the evolution of living creatures, and for another to declare himself contented with a grant of a quarter of that amount. We are certain that the evolution of backboned ani- mals, from Silurian Fishes to Man, has occupied " a period represented by a thickness of 34 miles of sedi- ment " ; and although we are familiar with long-lived types, like the tongue-shell, Lingula, which has per- sisted with " next to no perceptible change " from the Cambrian till to-day, we are also aware of races, like some of the extinct Reptiles, which have appeared, grown great, and disappeared within a relatively short time, as time goes. " To select Lingula, or other species equally sluggish, as the sole measure of the rate of biologic change would seem as strange a proceeding as to confound the swiftness of a river with the stagnation of the pools that lie beside its banks" (Sollas). The biological argument has been particularly dis- cussed by Professor Poulton,* with the general result that he feels it necessary to demand much more than even the geologist demands. The general fact of im- portance is that in the oldest fossil-containing rocks we find highly specialised animals which must have had a long history behind them; that in the Cam- brian, Ordovician, and Silurian almost all the great phyla or stocks of animals are already represented, and in many cases by forms which are anything but primitive. To the geologist's computation of the period required to account for the strata between the * Address Section D, Rep. Brit. Ass., 1896, pp. 808-828. GROWTH OF GEOLOGY. 247 Cambrian and those now forming, we are forced to make a large addition in order to account for the evolution of the rich Cambrian fauna. Under the Cambrian beds there is evidence of some 80,000 feet of stratified rock, in which there are no remains of organisms, but during which it seems al- most necessary to assume that the chief types of back- boneless animals and simple plants had their origin. The absence of fossils is most plausibly interpreted as mainly due to the absence of hard or preservable parts in the primitive forms ; and even the modest es- timate of twenty-six millions of years as the period, since the earth became fit to be a home of life, leaves a considerable number of millions for this pre- Cambrian period during which the unicellular crea- tures may have given origin to multicellular bodies, taking the form of polyps and worms, even of trilo- bites and molluscs. The suggestion has often been made that in early times, among simple creatures, the rate of progress may have been much more rapid than among the higher forms whose stages of evolu- tion are recorded in the rocks. But this is mere opinion. At the beginning of the nineteenth century there was an irrelevant belief that the habitable earth was some 6,000 years old. But the work of James Hut- ton alone was enough to convince the unprejudiced that the antiquity of the earth must be inconceivably great. The tendency of progressive geologists to draw without stint upon the bank of time, had to face a wholesome reminder from the physicists that their credit was not unlimited. The limitations imposed by the physicists have been vigorously rebelled against, and criticism has tended to show that they were too narrow and not altogether warrantable. The Q 248 PROGRESS OF SCIENCE IN THE CENTURY. data as to the rate of cooling of earth and sun, as to tidal retardation, as to the rate of sedimentation, as to the rate of evolutionary change in organisms, are in varying degrees only approximate, and the age of the earth remains a problem for the twentieth century. BEADING THE BOCK-EECOBD. We have now grown accustomed to the idea that the strata of the earth's crust form a great library of historical documents relating to the history of our world and its inhabitants, — a library never very com- plete, but, worse than that, disordered, half-burnt, flooded, and buried. There are two ways of reading history in this underground library. The nature of the rock, sand- stone or shale, limestone or chert, or otherwise — tells the experienced observer something about the physi- cal conditions of the time when the rock was formed ; and the relation of one stratum or set of strata to another makes it possible to determine the order of succession in time. Yet, on the whole, the decisive evidence as to the physical conditions of the distant age and as to the order of succession in time is afforded by the remains of plants and animals which the rocks contain. That fossils furnish the clue which makes it pos- sible to determine the historical order of sequence in the various strata that compose the earth's crust is a familiar fact now; but the realisation of it was a momentous event in the history of geology. And it may be noted that although the study of fossils had begun in the seventeenth century in the in- quiries of Stenson, Hooke, Woodward, and others, al- most no progress was made till the end of the eight- GROWTH OF GEOLOGY. 249 eenth when in 1795 Cuvier and Brongniart began their immortal researches on the remains of animals and plants in the Paris basin, and William Smith (1799) published his table of strata and their charac- teristic fossils. It mav thus be said that the utili- sation of fossils as aids in stratigraphical geology is only about a century old. But the whole progress of the century may be illustrated by the difference between Smith's general use of fossils and — say Lapworth's specific use of Graptolites in deter- mining the succession of closely approximated zones. Gradually the key which Smith has used to so much purpose came to be generally appreciated. Zittel notes the historical importance of the Out- lines of the Geology of England and Wales, bv W. D. Conybeare and W. Philips (1822) in whict the indispensable value of fossils was clearly recog- nised. Lyell, Deshayes, d'Omallius d'Halloy and Bronn are probably the most outstanding of the early geologists who vindicated the union of palaeon- tology and geology which has proved so profitable to both sciences. To follow the development of stratigraphical g