-NRLF 
 
 B M ESE 
 
GIFT OF 
 
 Kancroft 
 LIBRARY 
 
SCIENCE FOR THE PEOPLE. 
 
 No. I. 
 
 THE UNITY OF NATURAL PHENOMENA. 
 
 BY EMILE SAIGEY. 
 
 c c 'e ' c ^ ' c c c c e o ^ C c < c *' * N P R E S S : 
 
 THE SCIENCE AND PHENOMENA OF HE A T. 
 
 From the French of AMEDEE GUILLEMIN. 
 
 THE PHENOMENA OF PLANT LIFE AND SEX- 
 UALITY OF NATURE. 
 
 BY L. HARTLEY GRINDON. 
 
 THE PHENOMENA OF SOUND. 
 
THE UNITY 
 
 OF 
 
 NATURAL PHENOMENA. 
 
 A POPULAR INTRODUCTION TO THE 
 STUDY OF 
 
 THE FORCES OF NATURE. 
 
 FROM THE FRENCH OF 
 
 M. EMILE SAIGEY. 
 // 
 
 WITH 
 
 AN INTRODUCTION AND NOTES 
 
 BY 
 THOMAS FREEMAN MOSES, A. M., M. D., 
 
 PROFESSOR OF NATURAL SCIENCE IN 
 URBANA UNIVERSITY. 
 
 BOSTON : 
 
 ESTES AND LAURIAT. 
 -'S73- 
 
Entered, according to Act of Congress, in the year 1873, 
 
 BY ESTES AND LAURIAT, 
 In the Office of the Librarian of Congress, at Washington. 
 
 GIFT OF 
 
 flam: roil 
 LIBRARY 
 
 Stereotyped at the Boston Stereotype Foundry, 
 19 Spring Lane. 
 
CONTENTS. 
 
 PAGE 
 
 TRANSLATOR'S INTRODUCTION 9 
 
 AUTHOR'S INTRODUCTION -23 
 
 CHAPTER I. 
 THE GENERAL HYPOTHESIS. . . . . % 31 
 
 I. Atoms and Motion. Physical Phenomena may be reduced to a 
 
 single Principle, and considered as the Effects of Motion. 
 II. The Hypothesis of the Unity of Natural Phenomena in its Relation 
 
 to Science. 
 
 III. The Difficulty encountered by endeavoring to express New Ideas by 
 Means of Old Terms. 
 
 CHAPTER II. 
 SOUND AND LIGHT. . . . . . . . -55 
 
 I. Nature and Mechanical Equivalent of Sound. 
 
 II. The Nature of Light and of Interference. Universality of this last 
 Phenomenon. 
 
 III. Conclusions as to the Ether drawn from Luminous Phenomena. 
 
 IV. What does the study of Light teach us concerning the Molecular Con- 
 
 stitution of Bodies ? 
 
 CHAPTER III. 
 HEAT. 98 
 
 I. The Dynamic Theory and Mechanical Equivalent of Heat. 
 II. Changes of State produced by Heat furnish Information as to the 
 
 Constitution of Bodies. 
 III. Theory of Gases. 
 
 861372 
 
8 CONTENTS. 
 
 CHAPTER IV. 
 ELECTRICITY. .......... 126 
 
 I. It is necessary to determine the Electric Unit, a;id to find its Me- 
 chanical Equivalent. 
 
 II. The Electric Current apparently a Transport of Ethereal Matter. 
 III. Electricity and Light : their Mutual Relation. 
 
 CHAPTER V. 
 THE ATTRACTIVE FORCES. ....... 154 
 
 I. Points of Resemblance and of Dissimilarity presented by Gravity, 
 
 Cohesion, and Chemical Affinity. 
 
 II. Gravity may be considered as an Effect of the Motions of Ether and 
 of Ponderable Matter. 
 
 III. Historical Notions regarding the Idea of Universal Attraction. 
 
 IV. Hypotheses in regard to the Formation of Worlds and the Origin of 
 
 Gravity. 
 V. The Molecular Agencies, Cohesion, and Chemical Affinity. 
 
 CHAPTER VI. 
 LIVING BEINGS 205 
 
 I. Vital Activity consists in the Transformation, not in the Creation, of 
 
 Motions. 
 II. The Manner in which the Laws of Thermo-dynamics are Verified in 
 
 the Case of Animated Beings. 
 III. Muscular Contraction and Innervation. 
 
 CHAPTER VII. 
 CONCLUSION. . 232 
 
TRANSLATOR'S INTRODUCTION. 
 
 SINCE the discovery of the laws of gravity, more 
 than two hundred years ago, no scientific achievement 
 has been so fruitful in results as the determination of 
 the mechanical equivalent of heat. In 1842 Dr. May- 
 er, of Heilbronn, in Swabia, demonstrated that the 
 blow of a hammer weighing four hundred and twenty- 
 four kilograms upon an anvil, with the velocity it has 
 acquired by falling through a distance of one metre, 
 produces an elevation of temperature equal to one 
 degree centigrade. This experiment has wrought a 
 marvellous change in all our conceptions of Matter 
 and Force, and still gives promise of results which 
 the imagination fails to grasp. It is now considered 
 as demonstrated that heat, electricity, light, magnet- 
 ism, chemical attraction, muscular energy, and me- 
 chanical work, all are but exhibitions of one and the 
 same power acting through matter. The molecules 
 of matter, variously stirred by this all-pervading force, 
 
 9 
 
IO UNITY OF NATURAL PHENOMENA. 
 
 : ar^ throwm fejto^ weaves, which strike against our senses, 
 the motion hus< communicated to our nerves im- 
 s c a v s "heat, sound, or light, according to the 
 rapidity and breadth of the undulations. If an undu- 
 lation of one sort is interfered with, another immedi- 
 ately succeeds of exactly the same strength. Light 
 ransforms itself into chemical action, this into heat, 
 md heat into motion. Finally, all these modes of 
 motion are not only mutually convertible, but they 
 may also be turned into mechanical work. The 
 amount of work which a fixed quantity of each can 
 do is termed its "mechanical equivalent." In other 
 words, force is a constant energy, never increasing or 
 diminishing in absolute value. It is like a stream, 
 now flowing through uninterrupted channels, silently 
 and imperceptibly, now thrown into gentle undulations 
 as it comes into contact with the subtle forms of mat- 
 ter, and now giving striking displays of its power as it 
 meets with greater resistance. Light, sound, heat, the 
 invisible flow of the terrestrial magnetic currents, as 
 well as the aurora and zodiacal light, the flow of the 
 sap in plants, and the circulation of the blood in ani- 
 mals, are all exhibitions of this single Force. 
 
 From the unity of Force the induction was easy 
 and natural to the unity of Matter. The whole ten- 
 dency of modern physical science is to do away with 
 
TRANSLATOR'S INTRODUCTION. u 
 
 the notion of an indefinite number of primary ele- 
 ments, and to substitute instead a smaller and smaller 
 number of primitive forms of matter. We must ad- 
 mit, however, that this notion is not strictly a modern 
 one, for the idea that matter is identical, and only its 
 forms various, is as old as Aristotle. The results of 
 experimental investigation at first, and for a long time, 
 cast discredit upon this ancient doctrine, but modern 
 chemical and physical research serve only to confirm 
 it. Thus a great cycle of human thought is . com- 
 pleted. 
 
 What, in brief, is the theory of modern science in 
 regard to matter? 
 
 By changes in the mode of aggregation of the 
 atoms, which changes depend chiefly upon the degree 
 and kind of motion with which they are endowed, 
 matter appears to us under certain definite forms-, as 
 water, air, iron, etc. The analysis of many sub- 
 stances, once supposed to be compound, into simple 
 ones, leads naturally to the conclusion that such an 
 analysis is possible in the case of many substances 
 now placed in the category of elements. That won- 
 derful class of elements, the metals, would appear to 
 be the most stable of all ; but who can foresee the 
 result of spectroscopic investigation upon these, even ? 
 That there exists among them some common elemen- 
 
12 UNITY OF NATURAL PHENOMENA. 
 
 tal principle, of which each metal is a variety, is an 
 idea already broached. A comparison of their char- 
 acteristic spectrum lines may afford startling results, 
 surpassing the dream of the alchemists. Recent ap- 
 plication's of spectrum analysis to the nebulae and fixed 
 stars, in connection with the theory of the evolution 
 of suns and planets, in accordance with the nebular 
 hypothesis, are strikingly suggestive. The spectra of 
 some nebulae give evidence of but two elements, nitro- 
 gen and hydrogen, and a classification of the fixed 
 stars and nebulae may be so arranged as to exhibit a 
 gradual increase in complexity of structure.* In a 
 word, variety of form and unity of substance, the evo- 
 lution of the complex from the simple, of the hetero- 
 geneous from the homogeneous, are the fundamental 
 principles of modern philosophy. 
 
 The author of the following Essay assumes that 
 there is but one material substance, and that this sub- 
 stance is the Ether. This ether is the primitive and 
 most subtle element ; it is the tissue out of which the 
 entire universe is wrought a kind of mineral proto- 
 plasm, in fact. This theory is clearly and compactly 
 stated in the Author's General Hypothesis. The sub- 
 ject of the ether is engrossing so much attention at 
 
 * See an interesting article in Popular Science Monthly, Janu- 
 ary, 1873, entitled Evolution and the Spectroscope.. 
 
TRANSLATOR'S INTRODUCTION. 13 
 
 the present time, it may not be out of place here to 
 mention some of the views entertained in regard to it 
 by former and contemporary writers. 
 
 The revival of the doctrine of the ether, and its 
 support by many of the most eminent physicists of 
 the present day, is a remarkable event in the history 
 of science. There is indeed nothing new under the 
 sun. The ether dates back to the early days of scien- 
 tific research, and first occurs as a subdivision of the 
 air, one of the four elements of Empedocles-. It is 
 described as filling all the interplanetary spaces, and 
 also as penetrating and enveloping the particles com- 
 posing the heavenly bodies, impregnating them with 
 certain effluvia and influences. The Greek -poet Or- 
 pheus hails it as "the bright, life-giving element" 
 flowing around the sun, moon, and stars, and calls it 
 the blastema, or universal germ of things. In 1664 
 the undulatory theory of light demanded the hypothe- 
 sis of a homogeneous medium for the propagation of 
 the light waves. This theory received its first defi- 
 nite statement from Huyghens, in 1690. Light was 
 judged to be the vibration of an ether. Even New- 
 ton, while holding to his emission theory of light, 
 acknowledged the probability of the existence of such 
 a medium,* and speaks of the universal ether as the 
 
 *' Whewell, History Inductive Sciences, vol. ii. p. 89. 4 
 
14 UNITY OF NATURAL PHENOMENA. 
 
 "sensorium of God." Newton's authority in 'science 
 served to hold the new doctrine of light in abeyance 
 for more than a hundred years ; it was not until the 
 beginning of the nineteenth century that it was placed 
 beyond a doubt by the labors of Dr. Thomas Young. 
 Near the middle of the eighteenth . century, while the 
 Newtonian theory was yet in the ascendency, Sweden- 
 borg published his Principia, in which he maintains 
 the existence of an ether as one of five elementary 
 forms of matter. These are the solar vortex, which is 
 the cause of gravity ; second, the magnetic element ; 
 third, the ether ; fourth, the atmosphere ; and lastly, 
 the aqueous vapor. The ether is the third of these 
 elements in the order of succession, proceeding from 
 the sun to the planets. Its vibrations and undulations 
 are the cause of light and heat. Motion, diffused from 
 a centre through a contiguous meaium of ether parti- 
 cles, produced light and also heat. Our earth is sur- 
 rounded with an ethereal aura, the position of which is 
 intermediate between the solar vortex and our own 
 atmosphere, thus serving to keep up the contiguity of 
 expanse between the sun and our earth. It is in the 
 state of ether that matter has become " sufficiently 
 gross to be perceptible to our senses by its effects." 
 Swedenborg, in the calm and philosophic spirit char- 
 acteristic of him, took no pains to enforce these doc- 
 
TRANSLATOR'S INTRODUCTION. 15 
 
 trines upon the attention of his own age. This indif- 
 ference was shared by his contemporaries at a time 
 when the scientific world was distracted by the con- 
 flicting theories of light. In 1671 Leibnitz published 
 a new physical hypothesis, in which he deduced the 
 causes of most physical phenomena from a single uni- 
 versal motion. The particles of the earth are endowed 
 with separate motions, which give rise to shocks, from 
 which results an agitation of the ether radiating in all 
 directions. 
 
 Passing down to our own times, we find various the- 
 ories in regard to the ether. That of Hartman is in 
 substance as follows : There are two kinds of atoms, 
 ethereal and corporeal ; bodies are made of spherically- 
 shaped atoms, which act equally in all directions, the 
 activity proceeding from the centre. Between the 
 molecules of these bodies are scattered the atoms of 
 the* ether, and they also fill interstellar space. The 
 corporeal atoms have a tendency to converge towards 
 a single point, but are kept apart by the ether atoms. 
 Between the corporeal and the ethereal atoms a mutu- 
 al repulsion exists. Another writer, Spiller,* calls the 
 " world-ether the soul of the universe." By such an 
 eminent authority as Tyndall, the ether is recognized 
 
 * See Article on Philip Spiller's Aetherism, in the New York 
 Evening Post, January 24, 1873. 
 
1 6 UNITY OF NATURAL PHENOMENA. 
 
 as the interstellar medium filling all space. "The 
 luminiferous ether fills stellar space, makes the uni- 
 verse a whole, and renders the intercommunication of 
 light and energy between star and star possible. But 
 the subtle substance penetrates farther ; it surrounds 
 the very atoms of solar and liquid substances." And 
 again, " The intellect knows no difference between 
 great and small ; it is just as easy to conceive of a 
 vibrating atom as of a vibrating cannon-ball ; and there 
 is no more difficulty in conceiving of this Ether, as it 
 is called, which fills space, than in imagining all space 
 to be filled with jelly. You must imagine the atoms 
 vibrating, and their vibrations you must figure as com- 
 municated to the ether in which they swing, being 
 propagated through it in waves ; these waves enter 
 the pupil, cross the ball of the eye, and break upon 
 the retina at the back part, of the eye. The act, re- 
 member, is as real and as truly mechanical as -the 
 breaking of the sea-waves upon the shore. Their 
 motions are communicated to the retina, transmitted 
 thence along the optic nerve to the brain, and there 
 announce themselves to consciousness as light." 
 
 The essential difference between the foregoing theo- 
 ries and that of M. Saigey may be stated thus : In 
 the former the ether is regarded as filling all the spaces 
 between the stars and planets and the atoms of bodies. 
 
TRANSLATOR'S INTRODUCTION. 17 
 
 M. Saigey believes it to be all this and more. He 
 
 makes it also the constitutive element of the atoms 
 
 
 
 themselves. " The atom and motion, behold the uni- 
 verse ! " Such is his enthusiastic language. Ether in 
 a state of motion fills all space. The ethereal atoms 
 form societies, which are the molecules of bodies. A 
 body is a collection of these societies of molecules. 
 Between these atoms, molecules, and bodies, exchanges 
 of motion take place, which constitute heat, light, 
 chemical affinity, and gravity. These exchanges de- 
 pend upon the relations of mass and velocity. 
 
 Granting these premises, the remaining point in M. 
 Saigey's hypothesis that the laws which govern the 
 interaction of this primitive force and matter are none 
 other than the laws of mechanics is but a logical 
 deduction. These laws depend upon geometrical rela- 
 tions, and may be mathematically expressed. But it 
 must be confessed that the ether itself, as he defines it, 
 is a hypothetical substance, and its existence lacks 
 " mechanical confirmation." Still, this is not an in- 
 superable objection, for all scientific induction must 
 start with, and be based upon, hypothesis. The ether 
 is a scientific necessity. To illustrate : In the mental 
 conception which we form of the relations of bodies in 
 space, and of the parts of a body among themselves, 
 which relations we symbolize in geometrical figures, 
 2 
 
1 8 UNITY OF NATURAL PHENOMENA. 
 
 we are obliged to have recourse to a hypothetical 
 point. In geometry solids are regarded as generated 
 by the motion of surfaces, surfaces by the motion of 
 lines, and lines of points. Beyond this we are not 
 able to go. By an analogous train of. reasoning, M. 
 Saigey builds up the universe out of the ethereal atom 
 by the aid of motion. Masses are made of compound 
 particles ; the particles are aggregations of molecules, 
 and the molecules may be resolved into atoms. Be- 
 hind this veil of atoms lies the Infinite. Matter is a 
 series of orderly changes from the immaterial, becom- 
 ing more and more gross until recognized by the 
 senses.* Matter is, "at bottom, essentially mystical 
 and transcendental." f 
 
 From the organic forms of matter M. Saigey passes 
 to living beings, to plants and anim'als; and these are 
 likewise brought into his hypothesis. The primitive 
 cell of the plant, as well as the embryonic germ of the 
 animal, are formed out of the materials of the inorganic 
 world. In both a series of motions succeeds each other, 
 according to a fixed order. This series of motions 
 possesses a special character, it is true ; but their 
 transformations obey the laws of molecular mechanics. 
 The origin of force, the nature of life, human person- 
 
 * Swedenborg, Principia, vol. i. chap. ii. 
 f Fragments of Science, p. 415. 
 
TRANSLATOR'S INTRODUCTION. 19 
 
 ality questions naturally suggested in this connec- 
 tion are considered by M. Saigey .as entirely foreign 
 to his argument, and are, therefore, left untouched. 
 All scientific men have not shared the forbearance 
 of our author. Emotions and ideas display outward 
 phenomena which are subject to the laws of thermo- 
 dynamics. Without question these phenomena are 
 legitimate subject of scientific inquiry. Some time 
 ago a series of experiments was published, showing 
 that the operations of the mental and emotional facul- 
 ties are accompanied by a change of temperature in 
 the brain. The greatness of an idea and the strength 
 of an emotion may *peradventure be measured, and 
 their mechanical equivalent determined. Is it a logi- 
 cal inference from this that mind is a quality of nerve 
 tissue ? and does science lay us under any obligations 
 to accept the dogmas of a school of philosophy which 
 teaches that growth, development, and human progress 
 are results of self-determining processes, inherent in 
 the very nature of things ? There is, as there should 
 be, a spirit of fearless inquiry abroad, which will not 
 stop at prescribed bounds. Unhappily it is accom- 
 panied by a spirit of irreverence, for which a New- 
 ton or a Kepler would have blushed. The scientific 
 method of arriving at truth may be the most exact and 
 satisfactory one, but it is not the only one. The fun- 
 
2O UNITY OF NATURAL PHENOMENA. 
 
 damental facts of revelation, confirmed by the methods 
 of a spiritual science, rest upon proofs as sure and 
 convincing, to say the least, as any established by 
 natural science. Between these two methods the 
 utmost harmony may and ought to exist. In a recent 
 article in the Contemporary Review, Dr. Carpenter 
 expresses his opinion that science points to the origi- 
 nation of all power in Mind ; and, further, that there 
 are satisfactory grounds for the belief that the phe- 
 nomena of the material 'universe are the expressions 
 of a Mind and Will, of which man's is the finite proto- 
 type. To admit this will be to admit the existence of 
 the supernatural, always working in and through the 
 
 
 
 natural. Force ceases to be a blind attribute of mat- 
 ter, and becomes a living, active principle, spiritual in 
 its character. 
 
 I cannot forbear quoting here the sublime language 
 of one who may rightfully be called the greatest of 
 living naturalists, one who, as there is recent ground 
 for believing, has thus far withstood the current of the 
 Darwinian theories with a firmness and stability like 
 that of his native Alps. 
 
 " The combination in time and space of all these 
 thoughtful conceptions (just recapitulated), exhibits 
 not only thought, it shows premeditation, power, wis- 
 dom, greatness, prescience, omniscience, providence. 
 
TRANSLATORS INTRODUCTION, 21 
 
 In one word, all these facts in their natural connec- 
 tion proclaim aloud the one God, whom man may 
 know, adore, and love ; and Natural History must, in 
 good time, become the analysis of the thoughts of the 
 Creator of the universe, as manifested in the animal 
 and vegetable kingdoms." * 
 
 THOS. FREEMAN MOSES. 
 
 URBANA UNIVERSITY, March 2, 1873. 
 
 * Agassiz, Essay on Classification, p. 135. 
 
AUTHOR'S INTRODUCTION. 
 
 THAT heat and mechanical force are mutually equiv- 
 alent is a fact familiar to all who have interested them- 
 selves in the progress of science. Everywhere we 
 see heat converting itself into force, and force into 
 heat. In the steam engine, for example, the heat 
 disengaged by the combustion of the coal is turned 
 into the labor produced by the shaft of the engine. 
 On the other hand, if you turn a wheel in a body of 
 water, the water becomes warm ; if you rub together 
 two blocks of ice, the ice melts. All around .us, in the 
 work of every-day life, we see a certain quantity of 
 heat disappearing at the same time that a certain 
 amount of force is produced; and the converse is 
 equally well known from the most familiar facts. Sim- 
 ple as this notion appears to us, now that it has be- 
 come a part of our current ideas, its discovery is, 
 nevertheless, the principal achievement of modern 
 science. 
 
 23 
 
24 UNITY OF NATURAL PHENOMENA. 
 
 The works of Mr. Joule, the eminent philosopher 
 of Manchester, those of Mr. Jules-Robert Meyer, of 
 Heilbronn, of Hirn, the Colmar engineer, after hav- 
 ing determined the conditions of the convertibility 
 existing between heat and mechanical power, have 
 fully brought to light ' the principle itself, as well as 
 the reason of this convertibility. By force is meant 
 the displacement of a body. Now, heat, as every one 
 will at this day admit, is a molecular movement, a dis- 
 placement of molecules : is it not perfectly natural, then, 
 that these two phenomena should replace each other, 
 according to a fixed relation, and that between these 
 two kinds of motion there should exist a ready con- 
 vertibility governed by the common laws of me- 
 chanics ? 
 
 From the moment of the introduction of this pre- 
 cise and well-defined idea into science, every branch 
 of physics has undergone, in some degree, a renova- 
 tion. Upon many questions the new theory has 
 thrown a direct light ; upon others it has furnished 
 many luminous hints, and been the incentive to use- 
 ful research. Around undisputed facts brought to 
 light by the study of heat, other facts, less well es- 
 tablished, have ranged themselves, then ingenious 
 conjectures, and from this impulse of ideas has 
 sprung up a new conception of nature, which has 
 
AUTHORS INTRODUCTION. 2$ 
 
 already engaged the attention of many scientific 
 minds. 
 
 In turning our attention to this new way of view- 
 ing natural phenomena, we find it, at the outset, a 
 difficult one to define. 
 
 The unity of physical forces, such is the general 
 formula embracing the various considerations, of 
 which we shall attempt a rapid review. 
 
 In the system before us all the forces of nature are 
 traceable to the same principle, and are mutually con- 
 vertible under certain fixed laws, which are none other 
 tha.n the laws of mechanics. Such is a rude and gen- 
 eral statement of the new theory, a statement accept- 
 ed by different scientific minds, not without certain 
 restrictions. Those even who are almost agreed as to 
 the principle itself differ when it becomes necessary 
 to deduce .certain consequences on the subject of the 
 condition of matter and the constitution of the globe. 
 It is here that we meet with our first difficulty. We 
 shall not presume to set forth, in subjects so impor- 
 tant, a collection of opinions which are merely person- 
 al ; nor, on the other hand, may we venture to say, that 
 even among the partisans of this new theory has 
 there been harmony sufficient to establish a true 
 scientific system. 
 
 In undertaking the study of the equivalence of heat 
 
26 UNITY OF NATURAL PHENOMENA. 
 
 and motion we readily find some useful guides. Com- 
 plete treatises upon this subject already exist, and he 
 who wishes to examine its leading points in' v a precise 
 and substantial form may refer to the excellent lectures 
 of Mr. Verdet, published in the Memoirs of the Chemi- 
 cal Society of Paris, under the title of Expose de la 
 Theorie Mechanique de la Chaleur. In examining the 
 unity of the physical forces, however, we have no such 
 aid as the above. It would, therefore, seem opportune 
 at this time to give some definite outline to ideas 
 which have hitherto remained vague and ill defined. 
 We have already made an attempt in this direction 
 in a number of articles recently published in the Revue 
 des deux Mondes, and which form the substance of the 
 Essay now presented to the public. It will be grati- 
 fying to us if our attempt shall call forth new light 
 upon the correlation of natural phenomena, and if 
 our Essay shall hasten the publication of some im- 
 portant work on this subject. 
 
 In 1864, Father Secchi, director of the observatory 
 of the Roman College, published an interesting vol- 
 ume, L'Unita delle forze fisiche, saggio di filosofia 
 naturale. Father Secchi cordially adopted the idea 
 that the physical forces are all traceable to one and 
 the same principle. The study of astronomical phe- 
 nomena has itself furnished the grounds for this opin- 
 
AUTHOR'S INTRODUCTION. 27 
 
 ion. In reflecting upon the force of gravity, which 
 gives motion to the heavenly bodies, he was not ac- 
 customed to regard it as an elementary principle, but 
 traced it back to a still more general law, of which 
 this force is only a consequence. ' His work contains 
 novel and original suggestions upon this subject. At 
 the same time this book has the character of a compen- 
 dium of physical science. It reviews in a summary 
 manner the facts which to-day compose the resources 
 of science ; only accidentally and at intervals does he 
 touch upon the general principles which these facts 
 suggest. We do not find here any theory fully set 
 forth, by which the forces of nature may be traced 
 back to their unity. 
 
 We might mention a still older work, that of M. 
 de Boucheporn, published in 1853, under the title of 
 Principe general de la Philosophic naturelle. It is a 
 work compiled with care and zeal one of those 
 books in which a man condenses the thought of a life- 
 time. M. de Boucheporn seizes with boldness upon 
 the synthesis of natural phenomena. He recoils be- 
 fore no difficulties, he meets face to face every obsta- 
 cle. Herein lies the merit, as well as the defect, 
 of his work. M. de Boucheporn trusts himself too 
 hastily and too entirely to what are mere guesses. It 
 is wonderfuj to observe how a conjecture becomes a 
 
28 UNITY OF NATURAL PHENOMENA. 
 
 certainty with him as soon as he can make use of it 
 in accounting for certain facts. It is equally a won- 
 der to see how supple facts become in his hands, and 
 how readily they adapt themselves to the- demonstra- 
 tions which he exacts of them. Let us add that at 
 the time when M. de Boucheporn published the Prin- 
 cipe general de la PhilosopJiie naturelle, the new theory 
 of heat had not assumed a definite position in science; 
 if was but just being evolved, and its results were but 
 poorly comprehended, and the author, while he did 
 not ignore them, yet derived but little benefit there- 
 from. His book, too, while it remains full of interest 
 in what relates to astronomy, has lost much of its 
 value in the part treating of the laws of physics, prop- 
 erly so called. At the same time the moment we set 
 out from the facts revealed by the study of heat, the 
 general theory which we wish to develop can hardly 
 be presented as otherwise than hypothesis. As we 
 just now remarked, a serious difficulty presents itself 
 when we try to bring under a precise definition that 
 new conception of nature to which recent investiga- 
 tions have given rise. In what language shall we pre- 
 sent it in order that it may not appear rash to some, 
 to others chimerical, and to many useless ? Within 
 what bounds must we retain it, that we may not seem 
 to overstep the facts ? May we be permitted to adopt 
 
AUTHOR'S INTRODUCTION. 29 
 
 the following plan, not indeed as the wisest, but as 
 likely to throw the most light upon the subject ? 
 
 We shall begin by setting forth in its entirety, and 
 in all its simplicity, this grand hypothesis which we 
 have designated under the name of the Unity of Phys- 
 ical Forces, and we shall try to demonstrate its im- 
 mediate consequences. We shall not occupy ourselves 
 at the outset with bringing forward proofs to establish 
 this opinion. 
 
 It is only, in conclusion, that we shall endeavor to 
 indicate on what grounds the hypothesis rests, and 
 the restrictions and modifications will gradually pre- 
 sent themselves. In this presentation of proofs it can^ 
 easily be seen how much belongs to experience, how 
 much to imagination ; what can be believed without 
 scruple, and what must be the subject of. doubt until 
 further information. 
 
 With this understanding at the outset, we beg leave 
 to sketch our hypothesis in all its force, without being 
 obliged to weaken it, as we proceed, with restrictive 
 suggestions. 
 
THE 
 
 UNITY OF NATURAL PHENOMENA, 
 
 CHAPTER I. 
 THE GENERAL HYPOTHESIS. 
 
 I. 
 
 Atoms and Motion. Physical Phenomena may be 
 reduced to a single Principle, and considered as the 
 Effects of Motion. 
 
 THAT matter exists throughout the universe in an 
 unvarying quantity is now an undisputed fact, and one 
 beyond the reach of controversy. Never is it created 
 anew, never destroyed ; it simply passes through 
 transformations. The progress made by chemistry 
 in the beginning of this century set forth this truth 
 in all its clearness, and made it, in a manner, palpable. 
 
 What are, in fact, the properties of matter ? First, 
 impenetrability, as implied in its definition, a portion 
 
 31 
 
32 UNITY OF NATURAL PHENOMENA. 
 
 of matter being that which occupies a share of space 
 to the exclusion of any other portion ; second, inertia, 
 it being the principal result of human experiment, and 
 the foundation, indeed, of mechanics, that matter is 
 set in motion only when it receives an impression, and 
 loses its motion only in communicating it. 
 
 We can say the same of motion that we just now 
 said of matter it is neither created nor destroyed; 
 'its quantity is invariable. With motion, as with mat- 
 ter, it is only a question of transformation. 
 
 Here the idea of Force claims our attention. What 
 is force in the language of physics and mechanics ? 
 It is a cause of motion ; but what idea does this con- 
 vey to us ? The cause of a motion is another motion. 
 We will dispense, then, if it is possible, with this idea 
 of force, or rather, for it is necessary to employ cus- 
 tomary terms in order to be intelligible, we will un- 
 derstand by force whatever causes one motion to give 
 place to another motion. 
 
 If now, leaving these abstract considerations and 
 entering the domain of facts, we ask what are the 
 physical phenomena which appeal to our senses, heat, 
 light, electricity, magnetism, we find it demonstrated 
 that heat is one kind of motion, and that light is an- 
 other, and we are made to perceive that it is the same 
 with electricity and magnetism. There is nothing, 
 
THE GENERAL HYPOTHESIS. 33 
 
 then, which need surprise us if one of these motions 
 should engender the other ; that heat should be trans- 
 formed into electricity, and electricity into light. 
 When the solar rays draw up water from the surface 
 of rivers and lakes, when clouds are formed, when 
 these clouds become charged with electricity and the 
 lightning flashes, and when the watery vapor falls in 
 rain to the ground, we see under these various ap- 
 pearances only a series of successive motions. Not 
 only do we find, at the end of the phenomenon, the 
 whole quantity of water employed in it, but the mind 
 follows easily its various modifications from its first 
 motion. Now, it will be understood that these trans- 
 formations take place according to certain fixed rela- 
 tions. If the various kinds of motion be measured 
 according to certain established units, all these units 
 are reducible to a common scale ; a unit of heat cor- 
 responds always to 425 kilogrammetres,* to 425 
 units of mechanical power ; there is an analogous 
 relation between the electric unit and the heat unit, 
 and so on. 
 
 If, now, we touch upon another order of facts, if we 
 consider another group of forces, the cohesion which 
 
 * The kilogrammetre is the power represented by a kilogramme 
 (2^ pounds nearly) elevated to the height of a metre (about 40 
 inches). 
 
 3 
 
34 UNITY OF NATURAL PHENOMENA. 
 
 keeps bodies either in a solid or liquid state, the 
 chemical affinity which attracts molecules of differ- 
 ent kinds, that force of gravity, in a word, by virtue of 
 which bodies tend to move towards each other, the 
 new theory enables us to perceive still farther how the 
 play of all these forces is reducible to certain communi- 
 cations of motion. Here is,.for example, a piece of lead, 
 whose molecules adhere in such a manner as to form 
 a solid block. I know that in heating them, in com- 
 municating to them a certain kind of motion, I shall 
 destroy the cohesion by virtue of which the block re- 
 mains solid, and I shall subject it to a different cohe- 
 sion, which belongs to the liquid state. Heating it still 
 more, and thus augmenting the amount of communi- 
 cated motion, I shall destroy even this kind of cohe- 
 sion, and reduce the metal to vapor. Hence may we 
 not suppose that the cohesion which held together 
 the molecules of the lead was a motion relatively of 
 those molecules ? Whatever we destroy by a motion 
 must he itself a motion. Cohesion, we say, proceeds 
 from a relative motion. Do we not see it in some 
 cases result simply from a common velocity imparted 
 to neighboring molecules. 
 
 When a jet of water, for example, escapes from an 
 orifice under strong pressure, does it not assume a 
 solid form, and is there not a kind of cohesion result- 
 
THE GENERAL HYPOTHESIS. 35 
 
 ing from the fact that all its molecules in the same 
 plane move onward with an equal velocity ? The 
 familiar example we have cited will not be regarded 
 as a demonstration of facts. At present we do not 
 endeavor to crowd together phenomena ; we shall 
 strive only to show in what manner the new theory 
 presents itself to our view. We would not discuss its 
 revelations. We seek only to present them to view. 
 
 In regard to chemical affinity, we shall only be al- 
 lowed to say a word here, for in many respects its 
 action is similar to that of cohesion, and somewhat 
 analogous to that of gravity. When, under certain 
 conditions, particles of oxygen and carbon meet, they 
 are precipitated upon each other after the manner of 
 heavy bodies, and when they are combined to form 
 the oxide of carbon or carbonic acid, the fixed state 
 which they enter upon may be compared to that of 
 planetary bodies which revolve about each other. 
 
 What, then, is gravity ? What is that mysterious 
 force which causes two bodies to attract each other 
 in the direct ratio of their masses and the inverse 
 ratio of their distance ? Two bodies attract each 
 other ! Then matter is not inert ! Would there not 
 appear to be a real contradiction between the two 
 terms, matter and inertia ? 
 
36 UNITY OF NATURAL PHENOMENA. 
 
 The question is one well worthy our close attention 
 and examination. 
 
 Here are two molecules of matter. Is it a sound 
 notion to imagine them as setting out voluntarily from 
 a state of repose for the purpose of approaching each 
 other again ? Strictly speaking, I can conceive of 
 this being the case, and if all the particles 'of matter 
 attract each other by virtue of a hidden force which 
 resides in them, I can comprehend without difficulty 
 the vast amount of motion diffused throughout the 
 universe ; but from the very moment that I cease to 
 speak of matter as inert, I am obliged, on the other 
 hand, to say that it is active, since I acknowledge that 
 it encloses a principle of action. Here we find our- 
 selves face to face with an immense difficulty; and it 
 will be said, doubtless, that every scientific man since 
 Newton has been obliged to solve it, since it is impos- 
 sible to leave at the very foundation of Science two 
 contradictory assertions. In fact, minds habituated 
 to scientific pursuits know that it is necessary to seek 
 outside of bodies the cause by which they tend to- 
 wards each other ; they know that in enunciating the 
 law of universal gravitation, they regard results, and 
 not causes, and only mean that things take place as if 
 bodies were attracted in direct ratio of their mass, 
 and inverse ratio of their distance. Such is the res- 
 
THE GENERAL HYPOTHESIS. 37 
 
 ervation which all sensible minds have made, or ought 
 to have made, more or less explicitly. 
 
 Now, what light is this new theory going to throw 
 upon the principle of gravity ? Here is the answer. 
 A substance to which the name of ether has been 
 given is diffused throughout the entire universe. It 
 envelops bodies, and penetrates into their interstices. 
 The existence of this substance is deduced from a 
 series of proofs, among which luminous phenomena 
 hold the first rank. Ether is composed of atoms 
 which impinge upon each other and upon neighboring 
 bodies. It forms, in this way, a universal medium, 
 which exerts a constant pressure upon the molecules 
 of ordinary matter. The new theory accounts for the 
 reactions which are produced between the ethereal 
 atoms and the material molecules ; it proves that these 
 reactions are such that the material molecules must 
 tend towards each other precisely according to the con- 
 ditions which the law of gravity also observes. We 
 shall attempt farther on to give an idea of this ingen- 
 ious demonstration. For the present we leave aside all 
 the proofs, and only declare results. Every one will 
 understand the importance of the point to which we 
 have just arrived. It becomes evident that bodies do 
 not owe their gravity to an intrinsic force, but to the 
 pressure of the medium in which they are immersed. 
 
38 UNITY OF NATURAL PHENOMENA. 
 
 The motion of heavy bodies would no longer appear 
 to us other than as a transformation of the ethereal 
 motions, and gravity, henceforth, enters into that ma- 
 jestic unity to which we have conducted all physical 
 forces. 
 
 Thus heat, light, electricity, magnetism, cohesion, 
 chemical affinity, gravity, are all resolved into the 
 idea of motion. All these motions may be converted 
 into each other according to fixed relations, some of 
 which are known, but by far the greatest number of 
 which is, as yet, undetermined. 
 
 Let us see if the idea of matter will not henceforth 
 be rendered more simple and clear. At the founda- 
 tion of our system we have the atom of ether. But 
 is there, we supposed it just now for the sake of 
 making ourselves more easily understood, is there 
 really an ether, and an ordinary matter, differing from 
 ether in its essence ? To speak more clearly, are 
 there two kinds of matter? We can hardly conceive 
 it, now that we have resolved everything into motion. 
 In what respect would these two kinds of matter dif- 
 fer ? Would not the one be subject to the same laws 
 of motion as the other. Can there be two systems of 
 mechanics ? Certainly not ; since there is but one 
 law for motion, there can be but a single essence for 
 matter, and the molecules of ordinary matter must 
 
THE GENERAL HYPOTHESIS. 39 
 
 appear to us as aggregates of ethereal atoms. It is 
 under this form that we shall represent the elemen- 
 tary particles of simple bodies, of iron, lead, oxygen, 
 carbon. The molecules of these bodies do not differ 
 in their substance, but simply in the interior arrange- 
 ment of the ethereal atoms which compose them. 
 
 Because iron, lead, oxygen, carbon, chemically unite 
 in different combinations, must we suspect in them 
 some difference in substance ? Upon what would this 
 difference depend, since chemical affinity itself pre- 
 sents to us only the idea of motion ? 
 
 II. 
 
 The Hypothesis of the Unity of Natural Phenomena 
 in its Relation to Science. 
 
 FROM the point now reached, we may consider, in 
 all its bearings, the hypothesis whose principal fea- 
 tures we have just traced. If it be admitted in its 
 entire force, natural phenomena present themselves 
 under a form so simple as to surprise the mind at 
 once with wonder and awe. The physical world is 
 composed of atoms of one kind. By virtue of a mo- 
 tion received and communicated to each other by 
 these atoms, they become so grouped and inter- 
 
4O UNITY OF NATURAL PHENOMENA. 
 
 mingled as to form simple molecules, compound mole- 
 cules, gaseous, liquid, and solid bodies. It is to one 
 and the same cause, namely, ^to motions received and 
 converted into others, that. we must attribute mo- 
 lecular aggregations in the realm of the infinitely small, 
 and in that of the infinitely large the gravitation of 
 the heavenly bodies. It is this motion of a fixed na- 
 ture which, either in bodies or outside of them, con- 
 stitutes the phenomenon we call heat ; it is the same 
 motion which, under another peculiar form, consti- 
 tutes light, under another electricity, and so on. 
 
 The atom and motion ! Behold the universe ! 
 
 Upon this basis will the mathematician be able to 
 construct his calculations. While applying his equa- 
 tions to a medium composed of uniform atoms, and 
 seeking all the motions which may be produced, and 
 all the combinations that may spring from these mo- 
 tions, he will come again to the recognized phenomena 
 of physical science, the laws of the planetary circu- 
 lation, of the propagation of sound, of luminous undu- 
 lations. Entered upon this path, he will determine 
 by means of the analogies which such a study will sug- 
 gest, in addition to motions known and recognized, 
 motions which appear probable. He will here find 
 again, doubtless, the laws of matter studied already ; 
 he will here find, perhaps, properties towards which 
 
THE GENERAL HYPOTHESIS. 4! 
 
 the attention of man has never been directed. How 
 many important laws thus reign around us without 
 our even suspecting it ! How long men lived with- 
 out a suspicion of the electric phenomena whose action 
 encircles them ! What unexpected revelations may 
 spring up from this study of nature from a new point 
 of view ! 
 
 Here let us speak only of the obscurities which the 
 new hypothesis has already dispelled, and let us leave 
 to the future the task of justifying the hopes to which 
 it has given rise. By means of the bond which it es- 
 tablishes between all natural phenomena, our minds 
 are accustomed to seek, in every fact, through the 
 transformations which formerly obscured our vision, 
 its immediate origin and its direct result. 
 
 When we see a steam engine raise a weight, or 
 overcome a resistance, we think at once of the coal 
 burning in the furnace, whose combustion effects the 
 work of the machinery. But where does the coal get 
 this power which we know how to utilize ? It is the 
 product of a long-continued work of the sun, stored up 
 in fossil vegetables. Thus all facts are brought, as it 
 were, to a common standard, and we become accus- 
 tomed to look always for a due proportion between 
 cause and effect. 
 
 To give a familiar form to thought, may we here 
 
42 UNITY OF NATURAL PHENOMENA. 
 
 cite an anecdote ? We will borrow it from Father 
 Secchi, who relates it in his work upon the unity of 
 physical forces. There was, in 1855, at the Universal 
 Exhibition in Paris, a huge bell, of enormous weight ; 
 it was held up by a system of props so ingenious that a 
 single man was able to keep it in motion ; only the 
 tongue had been removed, without doubt out of regard 
 for the ears of visitors. The man who exhibited the 
 bell easily kept it swinging, and the spectators admired 
 the facility with which he made this formidable engine 
 move. An ecclesiastic, an educated and intelligent 
 man, we may suspect that it was Father Secchi 
 himself, approached the exhibitor, and said to 
 him, 
 
 " Your system of supports is very well contrived ; 
 it permits you to set this huge mass in motion with 
 extreme facility ; but would it be the same if the bell 
 had its clapper, and if it struck ? " 
 
 Those standing by doubtless did not understand the 
 thought of the jocose ecclesiastic. 
 
 The fact really was, if the bell had been forced to 
 give out sounds, that is to say, to make the air vibrate 
 strongly, he would have failed with his best efforts to 
 find the necessary strength to produce such a vibra- 
 tion. However perfect had been the mechanism of 
 the support, this strength would have been borrowed 
 
THE GENERAL HYPOTHESIS. 43 
 
 from the arm which pulled the rope. When a bell 
 vibrates, it is the labor of the bell-ringer which is 
 turned into sound. To remove the tongue that is to 
 say, to prevent the sound is to render the task easy 
 to the ringer. 
 
 As is the cause, so is the effect. We find ourselves 
 placed at this point of view whenever we attempt to 
 recall, briefly, the main facts upon which rests the 
 unity of physical forces. Before entering upon the 
 examination of this, we desire to reply to two ques- 
 tions which arise about the hypothesis we are devel- 
 oping. 
 
 Is this hypothesis a useful one ? 
 
 Is this hypothesis really new ? 
 
 Firstly, is it useful ? The great advancements 
 made by modern, science- are due to experiment and 
 observation. Non fingo hypotheses, wrote Newton as 
 preface to his works. Nullius in verba, proclaimed 
 the escutcheon of the" Royal Society of London. Pro- 
 vando e riprovando, was likewise the inscription upon 
 the shield of the Florentine Academy, founded by 
 Galileo. Certain it is that modern physical science 
 has done away with examining phenomena themselves, 
 independently of their supposed causes, by subjecting 
 them always to exact instrumental measurements. We 
 can to-day appreciate with the utmost exactness the 
 
44 UNITY OF NATURAL PHENOMENA. 
 
 thousandth part of a millimetre (a millimetre .039 
 inch), the ten thousandth part of a second, and we are 
 not likely to give over the researches which such means 
 of investigation enable us to undertake. Surely the 
 experimental method, which has given birth to such 
 brilliant results, is not likely to perish, and it is always 
 of the vernier of the physicist, the scales of the chem- 
 ist, the scalpel of the physician, the telescope of the 
 astronomer, that we must ask for sure information in 
 regard to Nature. 
 
 Is this to say, nevertheless, that, while this, incessant 
 work of research is pushed in all directions, we may 
 not attempt to group together facts already discovered, 
 in such a* manner as to rise to more and more general 
 laws ? Vain would be the attempt to combat this ten- 
 dency of the human mind. It is easy to say that one 
 must concern himself only with proven facts, and 
 leave the rest to dreamers ; but it is not so easy to 
 keep to this programme. Every one is irresistibly led 
 to form for himself an idea of the universe as a whole, 
 be it correct or not. Among men who make real 
 progress in science, those even who appear most ab- 
 sorbed in searching after particular facts, those who 
 confine themselves to the patient investigation of par- 
 ticular phenomena, certainly have their general theo- 
 ries, which they forbear, perhaps, to communicate to 
 
THE GENERAL HYPOTHESIS. 45 
 
 the public, but which guide them in their labors, 
 which induce them to attack one question rather than 
 another, which, true or false, suggest new ideas to 
 them, and classify their difficulties. 
 
 Far above all the theories which have thus guided 
 men of science, now rises this grand conception of the 
 unity of the physical forces. It is only an hypothesis, 
 but it offers itself with guaranties sufficiently sure to 
 necessitate a kind of revision of science as a whole. 
 It will illuminate with new light facts already known, 
 trace out a new path for researches in questions hith- 
 erto beset with difficulty and doubts, and point out in 
 what direction it is necessary first to question Nature. 
 Were the hypothesis false, experience would know 
 how to profit by it. 
 
 But, it will be said,' is it not to be feared that, led 
 away by this seductive theory, many observers will 
 come to pay but little attention to facts, and strive to 
 force them into the outline they have sketched in ad- 
 vance, and thus unwittingly falsify the results of their 
 experiments before presenting them to the public ? 
 This will doubtless happen it has already happened ; 
 but it is not a very serious evil, 'for science is well 
 enough armed against such a danger, and erroneous 
 assertions cannot long resist its control. 
 
 But, it will be again objected, scientific men are not 
 
46 UNITY OF NATURAL PHENOMENA. 
 
 the only ones concerned in the matter. Your hypoth- 
 esis lays hold upon philosophy. Not only does it 
 comprise all physical science, but it also enters the 
 domain of metaphysics. Philosophers will doubtless 
 adopt it, believing that they hold in their grasp a 
 scientific truth, and they will find, perhaps, that they 
 embrace only a chimera ! What reply do you make 
 to that ? The metaphysicians must have a care to 
 keep themselves well informed. 
 
 Now, is this veritably a new hypothesis, which pre- 
 sents the physical world to us as composed of uniform 
 atoms and diverse motions ? 
 
 Properly speaking, there are but few ideas which 
 can be brought forward as entirely new. If we con- 
 fine ourselves to mere definitions and the surface of 
 things, we shall discover the theory of the unity of the 
 physical forces in the remotest antiquity. The phi- 
 losophers of ancient Greece did not have a single fact, 
 scientifically demonstrated, at their disposal, so to 
 speak, and in this state of things they formed the 
 most simple hypothesis concerning Nature. They had 
 a clear field, and nothing interfered with their em- 
 piricism ; so they went straight to the most general 
 conceptions, and every one in his own way made unity 
 out of the grand total. Thales of Miletus, six hun- 
 dred years before our era, began by declaring that 
 
THE GENERAL HYPOTHESIS. 47 
 
 water was the principle of all things. Fifty years 
 later his countryman, Anaximene, beheld in air " the 
 uniform and primitive element." The Eleatic school, 
 in Magna Grecia, sought elsewhere the universal prin- 
 ciple. " Nothing proceeds from nothing, and nothing 
 can change," said Xenophanes ; " everything possess- 
 es the same nature ; " nevertheless he demanded, in 
 order to account for the multiplicity of variable sub- 
 stances, two elements, water and the earth. About 
 the year 500, Heraclitus adopted fire as the unique 
 principle and universal agent. " The world is neither 
 the work of gods nor of men ; it is an ever-living fire, 
 enkindling itself and extinguishing itself according to 
 a certain order." Here, then, are four elements suc- 
 cessively proclaimed water, air, earth, fire ; and 
 by a kind of eclecticism, they came to be admitted, all 
 four at once,, into the composition of the universe. 
 Aristotle accepted these four elements, and for long 
 ages after him they served as the basis for every sys- 
 tem of nature. During the eighteenth century the 
 four elements were still admitted, on the eve of the 
 great works which have founded modern chemistry. 
 Pursuing this general progress of ideas, we en- 
 counter the Atomic theory itself in very ancient 
 times. Leucippus, an Eleatic, who lived five hundred 
 years before our era, conceived the universe as formed 
 
4o UNITY OF NATURAL PHENOMENA. 
 
 of a vacuum, and a real substance, the last division 
 of which was the atom. " The round atoms," he said, 
 "have the property of motion. Jt is the combining 
 and separating of these which give birth to things and 
 destroy them. All physical phenomena are deter- 
 mined by the order and position of the atoms, and 
 only take place by virtue of necessity." Democritus 
 of Abdera, a Disciple of Leucippus, developed his 
 doctrine. He attributed to atoms, similar to each 
 other, original properties, impenetrability, and a sort 
 of weight. For him " every active influence or every 
 passive impulse is a motion following contact." 
 
 He distinguished impulsive (7zak[i6$) and the move- 
 ment of reaction (avtivvnia), whence results the circu- 
 lar or whirling motion (divrf). In this consists the law 
 of necessity (avctym]) pointed out by Leucippus. Epi- 
 curus, the Athenian, adopted the views of Democritus, 
 and made a kind of atomic theory. He gave to the 
 atoms a hooked form, and supposed them endowed 
 with an oblique motion in relation to each other, in 
 order that they might be able to grasp each other, and 
 form bodies. Such is the system of which Lucretius 
 sang in his magnificent poem of Nature. 
 
 But, need we repeat it ? the conceptions of these 
 philosophers, of these poets, were purely Utopian. 
 Formed outside of facts, they brought no light into 
 
THE GENERAL HYPOTHESIS. 49 
 
 the domain of physical science ; their authors could 
 see in them only what they themselves had put there, 
 that is, the caprice of their imagination ; thus they did 
 not possess for them the meaning which they now 
 have for us. In their eyes they were but simple for- 
 mulae, which they little cared to confront with the 
 facts of Nature, and which served only as preambles 
 to their systems of philosophy. 
 
 What we say of the ancients wholly applies to the 
 middle ages, to the period of the Renaissance, to the 
 primitive works of modern times. The physical sys- 
 tem of Descartes has scarcely more value than that 
 of Epicurus ; the same fantasy, the same vortices, 
 the same hooked atoms. 
 
 The great men who, at the time even of Descartes, 
 inaugurated the remodelling of the sciences, concerned 
 themselves only with facts, and left aside hypotheses. 
 Such was Kepler, such was Galileo. When the sec- 
 ond generation of great savans came, the generation 
 of Newton, of Leibnitz, of Huyghens, so rich was the 
 store of precise information that a general hypothesis 
 was well nigh impossible. Science was divided into 
 several branches. In each one of them they made 
 one or more particular hypotheses ; but it was a long 
 time before they thought to include in one general 
 4 
 
5O UNITY OF NATURAL PHENOMENA. 
 
 formula the numerous and exact phenomena which a 
 diligent and laborious search was bringing to light. 
 
 If, now, from the mere examination of these phe- 
 nomena a general formula arises, if a system springs 
 up spontaneously from the study of observed facts, we 
 may pronounce it veritably new, even if its formula 
 should be ancient, if even there might be found in 
 Democritus an almost complete enunciation of it. 
 The originality of the hypothesis brought forward at 
 this time consists in this, that it has the support of a 
 considerable number of facts ; that it has its birth in 
 these facts. It borrows its worth from the facts it 
 embraces ; it becomes, in a certain manner, a fact it- 
 self. 
 
 III. 
 
 The Difficulty encountered by endeavoring to express 
 . new Ideas by Means of Old Terms. 
 
 THE theory we are investigating will only appear 
 in its true light when we have examined some of the 
 phenomena upon which it rests, and pointed out the 
 new aspect which it gives to certain portions of 
 science. 
 
 We have no intention, as may be thought, of giving 
 a course of lectures on Physics. We shall only be 
 
THE GENERAL HYPOTHESIS. 51 
 
 able to touch upon certain points, to throw out a few 
 hints. 
 
 Do not ask of us a general picture of Nature, when 
 we only seek to sketch a few details of it ; through 
 these partial openings may doubtless be seen what 
 would be the scope of the work which we do not 
 dream of undertaking. 
 
 Furthermore, we shall adopt, in our cursory survey of 
 natural phenomena, the same order we have pursued 
 in our summary exposition of the system. We shall 
 speak first of what relates to light, heat, electricity ; 
 we shall next come to that other group of actions, 
 chemical affinity, cohesion, gravity, the principle of 
 which current prejudices more especially locate in the 
 very bosom of the molecules. 
 
 Heat ! Electricity ! Cohesion ! Gravity ! we say. 
 These words even bring us to make a declaration, the 
 advantage of which will accrue to us during the whole 
 course of this essay. 
 
 In every branch of physical science, we just now said, 
 particular hypotheses have been made. They have 
 influenced the language which has been adopted in the 
 various branches of science. In many cases the names 
 given to the phenomena, the classification of them 
 even, are in disagreement with the new theory. 
 
 What are we to do under these circumstances? 
 
52 UNITY OF NATURAL PHENOMENA. 
 
 * 
 
 Undoubtedly for a new situation there is needed a new 
 language. But shall we create here this new language 
 complete in all parts ? We have many other difficul- 
 ties to encounter. 
 
 Shall we have recourse to paraphrases in order to 
 avoid words which seem to contradict the idea we are 
 unfolding ? We should run a great risk of not being 
 understood. 
 
 We shall continue, then, to call all things by their 
 customary names ; if, however, this nomenclature 
 be found in disagreement with . our fundamental 
 idea, we beg that the accident may be kindly attrib- 
 uted to the state of transition through which physical 
 science is now passing. In former times the electri- 
 cians admitted the existence of a positive and a nega- 
 tive fluid. They recognized, henceforth, in a current 
 a positive pole and a negative pole ; we shall do as 
 they have done, without its leading to material conse- 
 quences. When a body is heated without being per- 
 mitted to expand, it absorbs, in order to acquire a 
 certain degree of temperature, a determined quantity 
 of heat, and if it is heated and also allowed to expand, 
 it requires, to arrive to the same temperature, a greater 
 quantity of heat. Physicists have given to the excess 
 of heat demanded in the second case the name of la- 
 tent heat of dilatation. We may continue to call it 
 
THE GENERAL HYPOTHESIS. 53 
 
 latent, while seeing clearly that it is used up' in pro- 
 ducing the mechanical work of 'dilatation. 
 
 In studying the molecular actions, there have al- 
 ways been recognized attractive forces and repulsive 
 forces ; we may do the same, prejudging nothing as to 
 the existence of these forces. 
 
 As to the word Force itself, we preserve it for want 
 of a better in our vocabulary. Every tirfle a motion 
 appears to us as the continuation or the transmuta- 
 tion of another motion, we can do without the idea of 
 force, and we ought tQ reserve this notion for motions, 
 the origin of which remains entirely concealed from 
 us. We shall continue, nevertheless, as we have done 
 in the preceding pages, to employ the word force in 
 its customary sense. We shall speak, without scruple, 
 of the force of gravity which causes a stone to fall, and 
 of the force of cohesion which maintains a body in 
 the solid state, at the same time supposing that the 
 fall of the stone and the solidity of the body are due 
 only to movements of the surrounding medium. 
 
 To speak truly, the inconvenience we here point 
 out is not a new one, and these difficulties of lan- 
 guage are well known in physics. As in each of the 
 
 
 
 parts, of this science different hypotheses have been 
 successively made for the purpose of grouping and 
 co-ordinating the phenomena, so physicists have 
 
54 UNITY OF NATURAL PHENOMENA. 
 
 learned, in a certain measure, to withdraw themselves 
 from the empire of words, to deal abstractly with the 
 ideas which a common signification awakens ; they 
 know how to see facts beneath the conventional pic- 
 ture of them which words give. 
 
 Nevertheless, the explanation we have just entered 
 upon is not useless ; it will justify the want of har- 
 mony that will often be found, without doubt, between 
 the names given to phenomena and our mode of ap- 
 preciating them. 
 
SOUND AND LIGHT. 55 
 
 CHAPTER II. 
 
 SOUND AND LIGHT. 
 
 I. 
 
 Nature and Mechanical Equivalent of Sound. 
 
 IT has been known for a long time that sound is 
 the effect of the vibration of bodies, propagated either 
 through the air or some other medium. Acoustic 
 phenomena are, so to speak, visible to the naked eye ; 
 their nature was likewise known at an early period. 
 If a plate of copper be firmly secured by one of its 
 sides, and a bow drawn across, the free edge, the eye 
 perceives the vibration of the plate. Again, if the 
 parchment of a drum, upon which fine sand has been 
 scattered, be exposed to agitated air, the disturbance 
 of the sand betrays that of the air, and its grains, 
 driven from the parts that are most shaken, are seen 
 to collect along the lines when the air and parchment 
 are in a state of rest. The rapidity of the propaga- 
 tion of sound is itself easily appreciable by the senses. 
 Everybody knows that if a cannon be fired off at a 
 
$6 UNITY OF NATURAL PHENOMENA. 
 
 distance, the light is seen much sooner than the report 
 is heard ; so that it can be easily ascertained how 
 many seconds sound requires to pass through a given 
 interval of space. Open to direct experiment, the 
 principles of acoustics have long been viewed in 
 their true light, and it has not been necessary to im- 
 agine, for their explanation and orderly arrangement, 
 either a special fluid or a particular force. Sound is 
 seen to be a vibratory movement, produced by a cer- 
 tain impulse, and propagated through a medium. 
 There has not been introduced into physics either a 
 sonorous fluid or a sonorous force. 
 
 We can say, then, but a few words concerning 
 sound. Let us observe, however, that the study of 
 sonorous vibrations, considered in its relations to the 
 history of science, possesses a peculiar interest for us. 
 These are the first vibratory movements which were 
 well understood, and on the day in which they were 
 exactly defined was laid one of the firmest foundations 
 of the new physics. The facts revealed by this study 
 aided in a powerful manner those great minds which 
 established the theory of light. Between sonorous 
 and luminous vibrations many analogies have been 
 sought for and found. Great dissimilarities have also 
 been encountered. Here is one of the most important, 
 which we mention here, although it is soon v to be re- 
 
SOUND AND LIGHT. 57 
 
 ferred to again. The sonorous vibration takes place in 
 the direction of the propagation of the sound ; each 
 molecule of the air that has been set in motion 
 executes a to-and-fro movement along the same line 
 in which the sound is propagated. On the other hand, 
 the luminous vibration takes place in a direction per- 
 pendicular to the ray of light. The points of resem- 
 blance and dissimilarity revealed by the study of the 
 motions of light and sound afford us at the outset a 
 primary view of the problems which the new physics 
 encounter, and of the methods it is able to employ for 
 solving them. 
 
 We have still another instance of the kind of inves- 
 tigation demanded by this new science when we pro- 
 pound a question apropos of sound that will succes- 
 sively arise in regard to all physical phenomena. We 
 have said that these various phenomena are susceptible 
 of being transformed into each other, and we are thus 
 led to look for a common measure in the dynamic 
 effect which they represent. What is the dynamic 
 effect of a sound ? or, to employ a term introduced into 
 the language of science by the study of heat, what is 
 the mechanical equivalent of sound ? Let us take a 
 bell, and strike it with a hammer ; we shall be able to 
 calculate exactly the mechanical work due to the 
 stroke of the hammer. This will be a certain num- 
 
58 UNITY OF NATURAL PHENOMENA. 
 
 ber of kilogrammetres.* The bell will vibrate, and 
 we shall be able to measure the amplitude of its vibra- 
 tions by means of a luminous ray reflected upon a 
 small mirror attached to the bell ; this is a method 
 often employed in acoustics to amplify the oscillations, 
 and to render them visible. If we make in this man- 
 ner a series of experiments, and if we compare the 
 figures which express* the shocks with those which 
 express the amplitude of the oscillations, we shall be 
 able to condense the result of this examination into a 
 formula which will give us an idea of the sonorous 
 effect of different blows. 
 
 But shall we in this manner obtain a true mechani- 
 cal equivalent of sound ? Shall we be able to say 
 that the unit of sound is equivalent to so many kilo- 
 grammetres ? To -do this, it would be necessary to 
 begin by determining the value of a unit of sound. 
 We distinguish in sound several properties ; it has 
 
 * The kilogrammetre is the work represented by a kilogramme 
 raised to the height of a meter. The English equivalent of this 
 term is "foot-pound." The quantity of heat necessary to raise 
 one pound of water one degree Fahrenheit in temperature, is 
 competent to raise a weight of seven hundred and seventy-two 
 pounds a foot high. By the French method of reckoning, the 
 quantity of heat necessary to raise a kilogramme (2.2 pounds) 
 of water one degree Centigrade in temperature, is sufficient 
 to raise a weight of four hundred and twenty-five kilogrammes 
 to the height of a meter (39.37 inches). Translator. 
 
SOUND AND LIGHT. 59 
 
 pitch, which depends upon the number of vibrations, 
 intensity, which depends upon their amplitude, quality, 
 which depends upon more complex conditions. What 
 phenomenon shall be selected for comparing different 
 sounds with each other, while keeping account of all 
 their effects ? Hitherto such a question does not 
 seem to have demanded consideration. Interest has 
 been felt alone in the number of the vibrations upon 
 which musical theories depend. 
 
 To tell the truth, we do not see that there is practi- ' 
 cally any special utility in selecting a sound unit which 
 shall correspond to the conditions we have just indi- 
 cated. We shall not insist, then, upon this point, and 
 we have only mentioned it in order to show one of the 
 new aspects presented by physical studies. 
 
 It has always been one of the chief difficulties of 
 the exact sciences to determine suitably the units by 
 which phenomena must be compared. This selection 
 
 j)f units possesses now an especial importance from 
 
 * 
 the new standpoint at which physicists are placed. 
 
 We are thus confronted here with a capital question, 
 which demands some elucidation. If we have only 
 touched upon it with regard to sound, it is because we 
 await an opportunity to treat it with more profit, for 
 we must necessarily encounter it at nearly every step 
 of the way. 
 
60 UNITY OF NATURAL PHENOMENA. 
 
 II. 
 
 The Nature of Light and of Interference. Universal- 
 ity of this last Phenomenon. 
 
 ACOUSTICS teaches us that sound is a vibratory 
 motion, be it of air, water, or of any other material 
 medium analogous to these. In examining optical 
 phenomena, we shall presently behold ether appearing 
 as the agent of the luminous wave, and this conception 
 of ether will soon become, as it were, the bond of all 
 the ideas which pertain to the unity of physical 
 forces. 
 
 Let us take a prism composed of two plates of 
 glass, separated by sulphide of carbon, and place it in 
 the path of a beam of solar rays, and receive the im- 
 age of this beam upon a screen. This image, as is 
 known, is called a spectrum ; the screen will show us 
 luminous rays of different colors, unequally refracted, 
 by their passage through the prismatic mass of the 
 sulphide of carbon. The red rays are least deflected, 
 and are consequently found towards the edge of the 
 prism ; then, proceeding from the edge to the base, 
 come orange, yellow, green, blue, indigo, violet. 
 
 If, now, we examine the spectrum with attention, 
 the phenomenon will not be simply a luminous one. 
 
SOUND AND LIGHT. 6l 
 
 It will acquaint us with the calorific and chemical 
 properties of the solar beam. Let us receive the 
 spectrum upon a plate pierced with a narrow slit, 
 through which the rays can act upon a thermo-electric 
 pile, and let us move the slit through the whole ex- 
 tent of the spectrum, beginning with the violet por- 
 tion. So long as we remain in the violet, the -indigo, 
 the blue, and even the green, the needle of the thermo- 
 scopic apparatus will be deflected but slightly. It will 
 indicate a heat, increasing according as the slit crosses 
 the yellow, next the orange, then the red ; but let us 
 pass beyond the red, and enter the dark part of the 
 spectrum ; we shall here find the maximum of heat* 
 
 Thus there is, beyond the visible image of the solar 
 beam, a warm spectrum which we cannot see. If the 
 rays, which are refracted on one side of the spectrum 
 beyond the red, have an especial aptitude for produ- 
 cing heat, those which are refracted upon the other side, 
 beyond the violet, have an especial aptitude for excit- 
 ing chemical action. These chemical rays may be 
 rendered visible by a contrivance well known in the 
 
 * From an examination of the distribution of heat in the spec- 
 trum of the electric light, it appears that the maximum of inten- 
 sity is in the dark region, beyond the red; and further, that if all 
 the visible rays were conveyed to a focus, its heat would be only 
 one ninth of that produced at the dark focus of the invisible rays. 
 Translator* 
 
62 UNITY OF NATURAL PHENOMENA. 
 
 laboratory. Take a sheet of paper, the lower part of 
 which is moistened with a solution of sulphate of 
 quinine, while the upper part remains dry. Let the 
 image of the solar ray fall upon this sheet, the spec- 
 trum preserves at the top of the sheet its ordinary ap- 
 pearance, while in the moistened portion a brilliant 
 phosphorescence appears beyond the violet rays. 
 
 Thus the spectrum extends beyond its visible por- 
 tion in two directions, to the right and to the left, and 
 analysis can distinguish in it, beyond the luminous rays, 
 calorific and chemical rays, the latter more particularly 
 deviated towards the violet portion, the former more 
 especially refracted towards the red portion. 
 
 All forms of light thus far known exhibit three 
 kinds of rays. Their phenomena vary, it is well 
 known, in a certain degree with the means of obser- 
 vation. And in the first place, simply to use a prism, 
 produces a spectrum in a manner merely conven- 
 tional ; the prism disperses differently the rays of 
 different refrangibility ; it leaves the red rays more 
 crowded together ; on the contrary, it gives more 
 breadth to the violet portion. We may, by other 
 means, obtain a spectrum in which the different rays 
 better preserve their relative value. The nature of 
 the prism also changes the relation between the lumi- 
 nous, calorific, and chemical rays. If a solar ray be 
 
SOUND AND LIGHT. 63 
 
 received upon a prism of water, the maximum of heat 
 will appear in the yellow, upon 'a prism, of common 
 glass in the red, upon a prism of flint glass beyond 
 the red, upon one of rock salt far beyond the red in the 
 entirely dark portion. It would likewise be necessary 
 to take into account the nature of the luminous source. 
 But passing over these details, we were desirous only 
 of showing how, in every emission of light, there is 
 found, in addition to luminous action, properly so 
 called, calorific and chemical action. We succeed in 
 dividing these three actions, but not without difficulty, 
 to such a degree do they appear to be commingled. 
 Let us not, then, forget this ready-made synthesis, 
 which is offered to us at the outset. If, after having 
 studied separately heat, light, and affinity, we arrive at 
 the law which unites these phenomena, let us remem- 
 ber that we found them united, and that we have sepa- 
 rated them in order the better to examine them. For 
 the present we must continue our analysis, leaving 
 aside heat as well as chemical action, and occupying 
 ourselves only with light. 
 
 What is light ? This subject has given full play to 
 the imagination of the earlier physicists. Some lo- 
 cated in the eye a visual force ; this force projected 
 rays which came in contact with objects. Others 
 supposed, on the other hand, that objects emitted all 
 
64 UNITY OF NATURAL PHENOMENA. 
 
 around them an infinite number of little images, which 
 entered the' eyes of men and of animals. It was 
 hardly possible to discuss seriously the nature of light 
 before the structure of the eye was known, and the 
 image of objects had been seen formed upon the retina, 
 as upon the end of a camera obscurd. The retina thus 
 impressed transmits the sensation to the optic nerve. 
 But how is the retina thus impressed ? how is the 
 image formed there ? 
 
 Newton supposed that luminous bodies shoot out 
 little corpuscles, the shock of which excites the retina. 
 This is the famous theory of emission, which gave rise, 
 towards the close of the seventeenth century, to such 
 hot disputes. Newton had established, while making 
 use of his hypothesis, the principal laws of optics, 
 those of reflection and those of refraction. Neverthe- 
 less difficulties continued to exist. Other optical phe- 
 nomena, more complicated, polarization and double 
 refraction, could not be explained by the Newtonian 
 theory. Questions were put to Newton, to which his 
 hypothesis gave no answer : " Where does the light 
 go when it is extinguished ? Whither go the cor- 
 puscles which are constantly leaving the sources of 
 light ? " 
 
 Descartes advanced the idea that a subtile matter 
 fills the planetary spaces. This conjecture, by which 
 
SOUND AND LIGHT. 65 
 
 he had vainly attempted to explain astronomical phe-v v 
 nomena, was eagerly seized upon, and applied to light. 
 Malebranche was among the first to suspect that light 
 is produced by the undulations of an ether, and that 
 the differences in the length of the waves are the 
 causes of the different colors. Huyghens adopted this 
 system, and subjected the deductions to mathematical 
 calculation. Thus admitted into science under a hy- 
 pothetical title, the existence of the ether became more 
 and more probable in proportion as experiment justi- 
 fied the conclusions drawn from this principle. 
 
 Nevertheless, Newton supported with- energy the 
 theory of emission, and accumulated in its defence 
 proofs, a great many of which would appear very 
 whimsical to-day. Euler supported Huyghens, and 
 beheld, in a kind of classification of the phenomena 
 which affect our senses, an argument in favor of undu- 
 lation. " In order to perceive an object by touch," 
 said he, " it is necessary that we be in contact with 
 the object itself. With regard to odors, we know that 
 they are produced, by material particles which issue 
 from the volatile body. As regards hearing, there is 
 nothing detached from the sounding body. The dis- 
 tance at which our senses recognize the presence of 
 objects is nothing in the case of touch, small in the 
 case of smelling, somewhat great in the case of hear- 
 5 
 
66 UNITY OF NATURAL PHENOMENA. 
 
 ing, while in the case of sight this distance becomes 
 considerable. Pursuing this progression, we must 
 believe that the sense of sight perceives in the same 
 manner as that of hearing, and not in the same 
 manner as the sense of smell ; it must be supposed 
 that luminous bodies vibrate like sonorous bodies, in- 
 stead of emitting particles like volatile substances." 
 
 During the debate there were brought forward some 
 curious facts, observed near the middle of the seven- 
 teenth century, by Father Grimaldi, a Bolognese monk, 
 who left behind him a very original treatise upon op- 
 tics (De Lumine, Coloribus, et Iride : Bologne, 1665). If 
 a shutter be pierced with a very small hole, and the 
 luminous cone which passes through the orifice be 
 examined, it is observed that the cone is much less 
 acute than would be expected, considering only the 
 rectilinear transmission of the rays. The experiment 
 becomes still more striking if there be interposed in 
 the path of the luminous ray a second shutter pierced 
 with a new hole, when it is readily apparent that the 
 rays of the second cone are more divergent than those 
 of the first. If a fine thread is introduced into the 
 luminous cone, and its shadow projected upon a 
 screen, the shadow appears surrounded by three col- 
 ored fringes, and there are also seen in this shadow one 
 or more luminous rays. Let the image of the orifice 
 
SOUND AND LIGHT. 6/ 
 
 in the shutter be received upon a screen, and a white 
 
 ** 
 
 circle is seen surrounded by a dark ring, next a white 
 ring, more brilliant than the central portion, then a 
 second dark ring, and finally another very faint white 
 ring. If in the shutter with which the experiment is 
 made, two very small holes are pierced at a distance 
 from each of one or two millimetres, and the two 
 images received upon a screen in such a manner that 
 they overlap each other, it is found that -in the len- 
 ticular segment formed by the overlapping of the im- 
 ages, the circles are more obscure than in the part 
 where they are separated. Thus it appears that by 
 adding light to light darkness is produced. 
 
 These curious facts, minutely described by Father 
 Grimaldi, appear to us quite decisive, now that we 
 grasp their real meaning. It seems to us that they 
 should have caused the system of undulations to have 
 triumphed forthwith ; but even those who could ap- 
 preciate their value in the seventeenth century, were 
 far from drawing from them all their consequences. 
 These experiments at least served to feed the dispute. * 
 Corpuscles, said Huyghens, coming directly from the 
 sun, and passing through a small aperture, would form, 
 in escaping from the holes, a straight cylinder, and 
 not a cone. The conical form is proof of a motion 
 which is propagated in a line lateral to the luminous 
 
68 UNITY OF NATURAL PHENOMENA. 
 
 ray. Newton retorted, " if light is a motion, it would 
 not remain confined in a narrow cone ; it should spread 
 itself in every direction, and scatter itself in a circu- 
 lar manner around each point of disturbance." 
 
 "Without doubt," replied Huyghens, "at every point 
 of the luminous ray spherical undulations go out in 
 the direction lateral to this ray, and extend into all the 
 surrounding space ; but they are not often enough 
 repeated to produce the sensation of light. They do 
 not yield to a force as strong as do those which occur 
 in the same direction as the ray, and they destroy 
 each other in their confusion." * 
 
 The first scientist who saw all that could be in- 
 ferred from these experiments of Grimaldi was Thom- 
 as' Young, that sagacious traveller, who developed sev- 
 eral branches of physical science, and who discovered 
 the key to the Egyptian hieroglyphics. The re- 
 searches of Young were continued by Arago and 
 Fresnel, and more recently by M. M. Fizeau and 
 Foucault. The labors of all of these have given a com- 
 plete explanation of the fringes of light pointed out 
 
 * In accounting for the fact that light is not diffused beyond 
 the rectilinear space when it passes through an aperture, Huy- 
 ghens says, " Although the partial waves produced by the parti- 
 cles comprised in the aperture do diffuse themselves beyond the 
 rectilinear space, these waves do not concur anywhere except in 
 front of the aperture." Translator. 
 
SOUND AND LIGHT. 69 
 
 by Grimaldi, and the theory of interferences which 
 they have founded is one of the most glorious achieve- 
 ments of modern thought. 
 
 The principle of interferences is difficult to grasp. 
 A ray of light, according to what we have just said, is 
 the propagation of a motion in which the atoms of 
 the ether oscillate around their point of equilibrium. 
 They are then endowed with a certain velocity in one 
 direction during the first half of this undulation, and 
 with the same velocity in the opposite direction dur- 
 ing the second half. Let us suppose, now, that we 
 can arrange two rays issuing from the same surface, 
 and that by any contrivance whatever one of the two 
 has been retarded behind the other a half undulation, 
 if two rays be placed upon each other at the point 
 of superposition, the atoms of either will remain im- 
 movable, since they will be equally solicited to motion 
 in both directions ; there will then be at this point 
 an absence of luminous motion, or darkness. There 
 will be an increase of light when the amount of retar- 
 dation is two demi-undulations, darkness when it is 
 three, and so on. 
 
 By means of experiments based upon this principle, 
 it has been possible to measure the length and the 
 duration of the waves which correspond to the differ- 
 ent colors of the spectrum. The wave increases in 
 
7O UNITY OF NATURAL PHENOMENA. 
 
 length and in duration from the red to the violet ; its 
 length, expressed in millimetres, is 0.000738, at the 
 extreme red ; 0.0005 5 3, at the middle of the yellow ; 
 0.000369 at the extreme violet.* 
 
 It has been proved, moreover, by particular methods, 
 that the same law of decrease extends to the invisible 
 portions of the spectrum ; the calorific vibrations be- 
 yond the red are slower and longer. The longest 
 wave of obscure heat which it has been possible to 
 measure up to this time is 0.001830 millimetres. As 
 regards the duration of the waves, a general idea may 
 
 be obtained from knowing that the vibration of the 
 
 * 
 
 yellow ray lasts 530 trillionths of a second. It is, 
 moreover, a recognized fact that the eye cannot per- 
 ceive a sensation of light unless it continue at least 
 several hundredths of a second ; there must be then 
 several billions of waves to produce the sensation of 
 light. 
 
 We here see confirmed by experiment the reason- 
 ing which we just now put in the mouth of Huyghens, 
 
 * According to Tjndall, the length of a wave of mean red 
 light is about the 39,oooth of an inch ; that of mean violet light, 
 the 57, 5ooth of an inch. Taking the velocity of light at 185,000 
 miles per second, as determined by Foucault, we have the num- 
 ber of waves of red light which enter the eye each second as 
 458,142,400,000,000. The number of waves which enter the eye to 
 cause the sensation of violet color is about one third more than 
 this, being upwards of 700 trillions. Translator. 
 
SOUND AND LIGHT. 7 1 
 
 and the limits within which the waves once departed 
 from the line of luminous shock, are no longer fre- 
 quent enough to produce light. 
 
 It may be understood, without dwelling further upon 
 this point, how important the study of interferences 
 becomes in the new physics. The interest pertaining 
 to it does not remain restricted within the limits of 
 optics, it extends to every branch of science. When- 
 ever a vibratory motion exists we may expect to find 
 the phenomena of interferences. 
 
 Acoustics, for example, has its own, which easily 
 admit of proof. Let us take a plate of copper, sup- 
 ported horizontally upon an upright stand, and having 
 scattered fine sand over it, let us draw a bow rapidly 
 over one of its edges. The surface is divided into 
 eight triangles ; the adjacent triangles vibrate in an 
 opposite direction, while those which do not touch 
 each other vibrate in a similar direction. The fine 
 sand gives evidence of this state of things by arran- 
 ging itself along the lines which cross the surface. 
 There is here a state of repose, because there is an 
 equal tendency to two opposing motions. These con- 
 trary impulses, which go out from the different parts 
 of the metal plate in order to cross each other in the 
 surrounding air, must produce therein true phenomena 
 of interferences, for sometimes they mutually strength- 
 
72 UNITY OF NATURAL PHENOMENA. 
 
 en each other, and sometimes they oppose each other. 
 This is demonstrated by a very simple instrument a 
 tube, one end of which forms a funnel over which a 
 membrane is stretched, while the other extremity ter- 
 minates in two branches, forming an angle with each 
 other. Having now the ear placed against the funnel, 
 the two terminal branches of the tube are passed over 
 the surface of the plate of copper, when it is easily 
 observed that the sound is very weak when they are 
 near the centre of two contiguous triangles, and that 
 it grows louder, on the contrary, when they touch two 
 triangles vibrating in the same direction. 
 
 We are now acquainted with sonorous interferences 
 and luminous interferences, but above all we must 
 expect to see these phenomena of universal occurrence 
 in physics. They will appear necessarily under the 
 most varied forms, according to the mode of motion 
 which produces them, and according to the nature of 
 the organ whose office it is to perceive them. In 
 every case, the researches which will be made in this 
 direction will be powerfully aided by the magnificent 
 works that have marked the study of luminous inter- 
 ferences. 
 
SOUND AND LIGHT. 73 
 
 III. 
 
 Conclusions as to the Ether drawn from Luminous 
 Phenomena. 
 
 WE must now consider more closely this idea of the 
 Ether, to which we have been led by the phenomena 
 of light ; we must clearly define it, and remove it from 
 the controversies to which it has given rise. 
 
 What is the ether ? Is it really imponderable ? and 
 in that case what does this property mean ? In what 
 does it differ from ordinary matter ? in what does it 
 resemble it ? What are its relations with it ? Is it 
 not a little strange to introduce here, at the very time 
 when we are banishing from science a host of con- 
 ventional entities and abstract forces, the idea of a 
 medium, which is, so to speak, immaterial ? 
 
 We shall have replied to this last question when we 
 shall have shown that the ether, according to our con- 
 ception of it, does not possess the fantastic properties 
 that are sometimes ascribed to it. 
 
 We conceive of a simple gas, oxygen for example, 
 as an assemblage of elementary molecules, animated 
 with motion, which strike against each other, from 
 which result the expansive force of the gas, and the 
 pressure that it exercises upon the bodies in which it 
 
74 UNITY OF NATURAL PHENOMENA. 
 
 is contained. This idea will become clearer when we 
 seek to inform ourselves concerning the interior con- 
 stitution of bodies, profiting by the ideas generally 
 adopted regarding the nature of heat ; but for the 
 present we can accept it as a kind of primitive con- 
 ception, with which the mind may be satisfied while 
 waiting for the testimony of science. It is under this 
 simple form that we conceive of ether, and we add, 
 that its elements are atoms ; that is to say, that they 
 cannot be divided. If the objection be offered that it 
 is difficult to comprehend that they are really indi- 
 visible, we reply, that it is sufficient for us to con- 
 ceive that they comport themselves as such, for no 
 one has the pretension to penetrate either the in- 
 finitely small or the infinitely great. The atoms 
 of the ether are endowed with motion, which they 
 communicate to each other, and to surrounding 
 bodies. 
 
 Are these, then, immaterial ? Certainly not. Two 
 properties belong to matter impenetrability and in- 
 ertia. The -ethereal atoms are impenetrable at the 
 outset ; they are so by definition. They are also in- 
 ert ; they have received the motion with which they 
 are endowed, and they lose it only in communicating it. 
 Nothing distinguishes ether, then, from matter ; and 
 when we just now presented it, in the preceding lines, 
 
SOUND AND LIGHT. 75 
 
 as awakening the idea of a medium, so to speak, im- 
 
 *i 
 material, we made, be it understood, a pure concession 
 
 to certain usages of language. Our ether is material, 
 just as oxygen is. 
 
 But it is imponderable ! Yes, and we here confront 
 a very delicate explanation. We should make our- 
 selves better understood if we should here show, in 
 some detail, under what aspect universal attraction ap- 
 pears in the new order of ideas upon which we are 
 entered ; but this is a point of view which we shall 
 unfold only in the course of this work. Under what- 
 ever form the interior state of an ordinary molecule 
 may be conceived, whether it be regarded as a primi- 
 tive substance, or be viewed as a reunion of ethereal 
 atoms associated together according to certain laws, it 
 must be admitted that this molecule possesses a mass 
 much larger than each of the atoms of the ether. 
 This granted, if two molecules are in the presence of 
 each other, the surrounding ether impinging upon 
 them both in every direction, there will result from 
 this very situation a disposition to approach, which is 
 known by the name of attraction or gravity. Let us 
 remain satisfied for the present with this summary ex- 
 planation, which will be completed in the sequel. It 
 is sufficient now to make it evident how ether is im- 
 ponderable ; if two molecules tend to approach each 
 
76 UNITY OF NATURAL PHENOMENA. 
 
 other, it is because their presence interrupts the uni- 
 formity of the ethereal impulsions in just such a man- 
 ner that they are necessarily forced one against the 
 other. Nothing of a similar character will be discov- 
 ered when we examine ether itself in its own motion ; 
 it moves in all directions, and there would appear to be 
 'nothing to force it in one direction rather than in an- 
 other. Thus this fluid produces attraction in matter 
 without itself being subjected to it ; it confers gravity 
 upon bodies, and itself is imponderable. 
 
 If, then, we wish to distinguish ether from pondera- 
 ble matter, it will be necessary, in order to employ an 
 accurate term, to call it imponderable matter. Granted 
 that the current phrase be ether on the one hand and 
 matter on the other, we shall still continue to use the 
 term, as we have done already, for the sake of brevity ; 
 we shall have at least shown what is expressed by 
 these words, and we shall have proved that the impon- 
 derability of ether must be admitted, without meaning 
 thereby to confer upon this fluid a title of immaterial- 
 ity. Let us add that there would be a real advantage 
 in getting rid of the term ether, which incurs the risk 
 of always possessing more or less of mysticism. 
 
 We have represented the ether as an assemblage of 
 atoms which strike each other, and rebound in all di- 
 rections. Here a capital objection presents itself, and 
 
SOUND AND LIGHT. 77 
 
 we must attack it. How do these atoms rebound ? 
 
 V 
 
 Are they elastic ? The notion of an atom and of 
 elasticity are incompatible. We can understand the 
 elasticity of a compound molecule ; the different parts 
 of the molecule, pressed upon by an external force, 
 are displaced while being compressed, then regain 
 their position while returning the impulsion which 
 they have received. This mechamism supposes a void 
 in the interior of the molecule ; but the atom is im- 
 penetrable, indivisible ; it does not enclose a void. 
 There is here a serious difficulty. Huyghens, it is 
 necessary to state, ascribed to the atoms of the ether 
 an elastic force. What did he understand by this ? 
 Did he then regard them as compound corpuscles ? 
 The difficulty had only changed place. Fortunately 
 mechanics have succeeded in solving this problem, 
 and the beautiful researches of Poinsot upon revolving 
 bodies explain how the ethereal atoms may rebound 
 from each other without being elastic.* It is suffi- 
 cient for understanding this effect to suppose that 
 
 * " It is just as easy to conceive of a vibrating atom as to con- 
 ceive of a vibrating cannon-ball; and there is no more difficulty 
 in conceiving of this Ether, as it is called, which fills space, than 
 in imagining all space to be filled with jelly. You must imagine 
 the atoms vibrating, and their vibrations you must figure as 
 communicated to the ether in which they swing, being propa- 
 gated through it in waves ; these waves enter the pupil, cross 
 
78 UNITY OF NATURAL PHENOMENA. 
 
 they possess, in addition to their motion of translation, 
 a rotatory motion. 
 
 From the theorem formulated by Poinsot, it results 
 that a hard and inelastic body may, if it revolves, be 
 turned aside by any obstacle, precisely like a body pos- 
 sessed of elasticity ; more than this, there is often, 
 after the blow, a greater velocity than before, because 
 a part of the rotation is converted into a motion of 
 translation. In general, when a revolving body strikes 
 against an obstacle, it cannot lose its two motions at 
 once ; at farthest, it may do so only in certain theo- 
 retical instances, which need not be considered here. 
 If the shock pass through the centre of gravity of the 
 body, it will assist its onward progress, but not its 
 rotation ; if it be eccentric, it will stop the rotation, 
 but not the forward movement. The two motions will 
 likewise be partly transformed, one into the other, in 
 such a manner as to produce more varied phenomena. 
 
 The game of billiards has made some of these ef- 
 fects familiar. It is here seen how the rotation of a 
 ball operates to modify both its direction and velocity 
 when a blow is received. In the illustration cited, 
 elasticity combines with rotatory motion ; but it is 
 
 the ball of the eye, and break upon the retina at the back of the 
 eye. The act is as real and as truly mechanical as the breaking of 
 the sea-waves upon the shore." Heat as a Mode of Motion, p. 268. 
 
SOUND AND LIGHT. 79 
 
 necessary to abstract the latter phenomenon, and^ to 
 give it separate consideration, in order to conceive 
 how the ethereal atoms may rebound without being 
 elastic. 
 
 Let us enter a little farther into the idea of these 
 motions ; we shall see the hypothesis of the rotation 
 of ethereal atoms explain, in a certain degree at least, 
 a phenomenon of capital importance, and one we have 
 already mentioned. 
 
 The undulation of light, we have said, is propagated 
 in a direction at right angles to the luminous ray, and 
 we have observed that in this respect it differs from 
 the sonorous undulation which takes place in the 
 same direction as the propagation of the sound. 
 There is nothing to astonish us in the manner in 
 which the luminous wave is propagated, and we find 
 many examples of it in nature. If a stone be thrown 
 into the water, we see the water rise in waves which 
 are perpendicular to the direction of its fall. In this 
 case it is evident that the liquid disturbed by the stone 
 moves in the direction in which it meets with the least 
 resistance.* A similar reason was assigned by Fresnel 
 
 * "In the case of sound, the vibrations of the air-particles 
 are executed in the direction in which the sound travels. They 
 are therefore called longitudinal vibrations. In the case of light, 
 on the contrary, the vibrations are transversal, that is to say, 
 
8O UNITY OF NATURAL PHENOMENA. 
 
 for the luminous motion. " I think," said he, " that 
 the shock is communicated to the ether longitudinally, 
 that is to say, in the direction of the ray, but the ether 
 possesses such a nature that it can only respond to the 
 impulse by a lateral vibration." This vague explana- 
 tion becomes strikingly definite if we suppose the 
 ethereal atoms to turn upon themselves. 
 
 We know from mechanics, that if a revolving body 
 receive a shock perpendicularly to the axis of rotation, 
 the centre of gravity of the body is carried at right 
 angles to the direction of the shock. Strike a whirl- 
 ing top, it will jump to one side. There is a well- 
 ? known experiment upon this subject. A top is placed 
 on a horizontal plane, and while it sleeps, if the plane 
 be inclined from south to north, immediately the top 
 moves from east to west ; if the plane be inclined 
 from east to west, it moves from south to north. 
 Thus the component gravity causes the top to move 
 in a direction at right angles to that component. The 
 phenomenon does not take place, of course, unless the 
 top is whirling, and there is no such result while it is 
 in a state of repose. 
 
 the individual particles of ether move to and fro across, the di- 
 rection in which the light is propagated. In this respect waves 
 of light resemble ordinary water-waves more than waves of 
 sound." Tyndall, Fragments of Science, p. 284. 
 
SOUND AND LIGHT. 8 1 
 
 Placed at this point of view, we understand without 
 difficulty how the rotation of the ethereal atoms ac- 
 counts for their lateral displacement during the lumi- 
 nous impulse ; their transverse vibration will appear 
 not only possible, but even necessary. 
 
 This explanation we borrow from the books of 
 Father Secchi upon the Unity of the Physical Forces. 
 The inference which the learned abb6 has drawn from 
 the rotation of the ethereal atoms, is not the least in- 
 teresting feature of his work. While we are dwelling 
 with some detail upon the transverse motion of light, 
 we cannot resist the desire of presenting, in opposi- 
 tion to the hints thrown out by Father Secchi, the 
 views which M. de Boucheporn offers upon the same 
 subject in his Principe general de la Philosophie natii- 
 relle. This digression will interrupt the orderly pres- 
 entation of our ideas ; but we shall at least acquaint 
 our readers, by a brilliant example, with those bold 
 conjectures characteristic of M. de Boucheporn, which 
 he knew how, with infinite art, to verify in fact. See- 
 ing his point of departure and the good to which he 
 arrives, one is persuaded, but not convinced. 
 
 M. de Boucheporn attributes the transverse undula- 
 tion to the friction of the ether against the revolving 
 surface of the sun. This hypothesis furnishes him at 
 once with the explanation of the phenomena of colors, 
 6 
 
82 UNITY OF NATURAL PHENOMENA. 
 
 and he finds its confirmation in the examination of the 
 length of the waves which characterize the principal 
 tints of the spectrum. Let us follow him in his argu- 
 ment. 
 
 If it is the sun's rotation which impels the ethereal 
 atoms in a direction tangent to its motion, this effect 
 should be produced in a very different manner at the 
 different points of the solar meridian ; it would neces- 
 sarily decrease in energy from the equator to the poles. 
 At the equator the friction is in full force, while at the 
 poles it is nothing. Between the equator and the pole 
 its energy decreases according to the radii of the paral- 
 lels, or as the cosines of the latitudes. M. de Boucheporn, 
 thereupon, supposes the differences in the lengths of 
 the waves, that is to say, the differences in the colors, 
 correspond to the impulses given at the different par- 
 allels. What will be the parallels which characterize 
 the different colors ? M. de Boucheporn immediately 
 searches for those which offer remarkable peculiarities, 
 those whose trigonometrical lines, the sines and cosines, 
 have the most definite relations with unity. He finds 
 eight, and he assigns to each of them one of the tints 
 of the spectrum, the red being placed at the equator. 
 In this manner he draws up the following table, in 
 which \h.Q.cosines of the latitudes selected will be found 
 opposite to the tints which are attributed to them : 
 
SOUND AND LIGHT. 83 
 
 Cosines of the 
 
 Solar Latitudes. 
 t 
 
 Violet, . . . . . . . 0.33 
 
 Indigo, 0.50 
 
 Blue, 0.60 
 
 Green, 0.70 
 
 Yellow^ . . . . . . .0.80 
 
 Orange yellow, f Fresnel had taken these two"! O.8/ 
 Orange red, \instead of the orange alone. J m O.Q3 
 
 Red, i. oo 
 
 It is next to be ascertained whether these numeri- 
 cal values are proportional to the length of the waves, 
 the determination of which has been made by Fresnel 
 with such admirable precision. M. de Boucheporn 
 observes here in his hypothesis, that the experimental 
 values of Fresnel represent the sum of two effects ; 
 the progressive motion of the sun exercises a friction 
 as well as its rotation. The first of these two effects 
 may be eliminated by cancelling a constant number 
 in the values given by Fresnel, and then these values 
 take the following form, the length of the red wave 
 being taken for unity : 
 
 Lengths of Undulation. 
 
 Violet, 0.396 
 
 Indigo, . . . . . . . 0.518 
 
 Blue, .... 0.600 
 
84 UNITY OF NATURAL PHENOMENA. 
 
 Lengths of. Undulation. 
 
 Green, 0.696 
 
 Yellow, 0.800 
 
 Orange yellow, . . . . . 0.865 
 
 Orange red, , 0.932 
 
 Red, . . . . . . . i.ooo 
 
 If the two numerical series which precede be com- 
 pared with each other, there will be found between 
 them the most perfect harmony that could be asked 
 for experimental calculations. 
 
 The views of M. de Boucheporn present a striking 
 aspect, if we consider the three principal colors of the 
 solar spectrum, the blue, the yellow, and the red, which 
 are alone able to make white light without the aid of 
 the intermediate tints. For these three fundamental 
 colors, the values expressed in the two series are ex- 
 actly as the numbers 3, 4, and 5 ; and not only do these 
 numbers present a very simple relation to each other, 
 but they are also the only ones which satisfy, in a 
 simple manner, another characteristic condition : the 
 square of one of them is equal to the sum of the 
 squares of the other two : 9 +16 =25. This is what 
 M. de Boucheporn calls the law of the three squares. 
 It takes a prominent part in his theories, and we 
 cannot help appreciating its importance. The motions 
 which strike our senses are the better grouped the 
 
SOUND AND LIGHT. 85 
 
 more simple are the numbers which express them ; at 
 the same time the intensity of our sensations is in a 
 direct ratio with the squares of these numbers. Our 
 senses, then, are called upon to judge of the double 
 condition which is fulfilled when these numbers and 
 their squares present such very simple relations. We 
 can here see, with M. de Boucheporn, one of the har- 
 monies of Nature. 
 
 The general explanation that has just been given 
 upon the subject of the transversal motion of light 
 will doubtless be regarded as but a brilliant flight of 
 fancy, and our chief object in mentioning it has been 
 to justify the judgment we passed at? the beginning of 
 this essay upon M. de Boucheporn's book. On the 
 other hand, the hypothesis of the rotation of atoms 
 which we have borrowed from Father Secchi, is a 
 very plausible and suggestive one, and it would be 
 necessary to be on one's guard against putting the 
 two conceptions upon the same footing. 
 
 Whatever may be the reason assigned for the trans- 
 versal motion of the luminous wave, the fact itself is 
 certain. It has been fully proved by the phenomena 
 of polarization. 
 
 When a ray of a single color, a red ray, for example, 
 is reflected by a plate of glass at an angle of thirty- 
 six degrees, it acquires from this single circumstance 
 
86 UNITY OF NATURAL PHENOMENA. 
 
 peculiar properties.* If to this reflected ray a second 
 mirror of glass be presented at the same angle of 
 thirty-six degrees, and the mirror is made to turn in 
 every direction around the point of incidence, it is 
 observed that the ray is no longer reflected with the 
 same intensity in all directions. There is a plane in 
 which the reflection is greatest, and one in which it 
 is almost nothing. The maximum takes place in the 
 plane parallel to the plane of reflection upon the first 
 mirror, and is consequently called the plane of polari- 
 zation ; the minimum occurs in the plane which 
 makes a right angle with this. If, instead of selecting 
 a ray of a particular color, just as now mentioned, we 
 
 * When a luminous beam impinges at the proper angle on a 
 plane glass surface, it is polarized by reflection. It is polarized, 
 in part, by all oblique reflections, but at one particular angle the 
 reflected light is perfectly polarized. An exceedingly beautiful 
 and simple law, discovered by Sir David Brewster, enables us 
 readily to find the polarizing angle of any substance whose re- 
 fractive index is known. This law was discovered experimental- 
 ly by Brewster, but the Wave Theory of light renders a Complete 
 reason for the law. A geometrical image of it is thus given : 
 When a beam of light impinges obliquely upon a plate of glass, 
 it is in part reflected and in part refracted. At one particular 
 incidence the reflected and the refracted portions of the beam are 
 at right angles to each other. The angle of incidence is then the 
 polarizing angle. It varies with the refractive index of the sub- 
 stance, being for water 52^, for glass 57^, and for diamond 68. 
 Fragments of Science, p. 257. 
 
SOUND AND LIGHT. 87 
 
 employ white light, we obtain similar results, only a 
 little less distinct, because the angle of incidence un- 
 der which they are produced varies a little for the 
 different colors. 
 
 What is, then, this modification which the ray un- 
 dergoes when placed in the conditions we have men- 
 tioned ? Why does it no longer behave like an ordi- 
 nary ray ? The transverse vibration furnishes us with 
 the reason. Before the luminous beam falls upon the 
 first reflector, the waves are propagated around it 
 transversally in all directions. They diverge around 
 this axis as the spokes of a wheel go out from the 
 hub. At the moment of its falling upon the mirror, 
 the glass absorbs one portion of the waves and re- 
 flects the other. What are chiefly the ones it sends 
 back ? Those which are parallel to its surface, and 
 therefore which can less easily penetrate it. If we go 
 to extremes in order to render the phenomena more 
 intelligible, we shall consider the reflected ray as no 
 longer containing other than parallel waves between 
 them and the surface of the first mirror. The ray, 
 then, is said to be polarized, and this term, though 
 invented by Newton for an altogether different hy- 
 pothesis, explains sufficiently well the fact. What will 
 now happen when these waves, conducted in a single 
 direction, shall fall upon a second surface of glass ? 
 
88 UNITY OF NATURAL PHENOMENA. 
 
 They will be wholly reflected at the moment that the 
 mirror is parallel to them, and they will, on the other 
 hand, be absorbed more and more in proportion as 
 this mirror is made to revolve. Such is, in its essence, 
 the phenomenon of polarization, and it appears that it 
 may be explained without difficulty, by taking for the 
 starting-point the transversal undulation. 
 
 Fresnel has even shown that if two rays, polarized 
 at right angles, happen to coincide, they give no sign 
 of interference, even when there is between them the 
 difference of a half undulation. This will be under- 
 stood by recurring to the fundamental notion of inter- 
 ferences, and it will not be a matter of astonishment 
 that vibrations, when they are perpendicular to each 
 other, do not destroy each other, as happens in other 
 cases. 
 
 If we could push this investigation a little farther, 
 we should be able to show how the data assumed in 
 the matter of luminous motion have, one after another, 
 received striking confirmation. The principles being 
 laid down, mathematical analysis has developed their 
 consequences, and observation has justified these re- 
 sults. In pursuing this twofold task Fresnel made 
 for himself a glorious name. His calculations, his 
 experiments, are alike memorable ; one hardly knows 
 what most to admire, the high intelligence with which 
 
SOUND AND LIGHT. 89 
 
 he has presented the facts, or the practical skill with 
 which he has verified them. In no other portion of 
 science has man so nearly arrived at the secrets of 
 nature, or submitted its fundamental phenomena to 
 such exact measurements. 
 
 IV. 
 
 What does the Study of Light teach us concerning the 
 Molecular Constitution of Bodies ? 
 
 IN pointing out some of the laws of optics, we have 
 endeavored to give form and substance to the notion 
 of ether. It has been seen how the motions of this 
 fluid have been analyzed and measured. It has been 
 seen that the ether, notwithstanding its imponderabili- 
 ty, possesses the properties of matter. Henceforward, 
 if we resume the thread which is to guide us, if we 
 place ourselves at the stand-point from which all phys- 
 ical phenomena appear as exchanges of motions, we 
 are led to ask if it has been possible to precisely de- 
 fine the conditions under which the atoms of the ether 
 exchange their motions with the ponderable mole- 
 cules. Diffused throughout the stellar spaces, enclos- 
 ing among its particles the celestial globes, the ether 
 thus penetrates into the deepest recesses of all bodies, 
 
QO UNITY OF NATURAL PHENOMENA. 
 
 and bathes their ultimate molecules. Thus there is 
 no phenomenon in which it does not play a part, either 
 the principal, or, at least, a secondary one. If, then, 
 we could know the mass and velocity of the ethereal 
 atoms, and the mass and the velocity of- the ponder- 
 able molecules, we should possess, in some sort, the 
 key to the physical sciences. He, at least, who should 
 discover some bond of union between these ; who 
 should be able to grasp, in* some degree, their rela- 
 tion ; such a one would open up a fruitful source of 
 discoveries. 
 
 Is there need to say it ? No such discovery has 
 been made up to the present time. We establish by 
 the results the reciprocal action of ether and of ordi- 
 nary matter, we see an incandescent body produce 
 light, we see this light converted into chemical action ; 
 but in no instance do we know how to reduce the 
 phenomenon to its mechanical elements,.and to seize 
 the moment itself of the exchange of motion. 
 
 In regard to the distances themselves of the atoms 
 from each other and of the molecules from each other, 
 we have only rough and contradictory estimates. It 
 is generally supposed that the spaces left between the 
 ponderable molecules are enormous in proportion to 
 their dimensions. Thomas Young did not hesitate to 
 affirm that the molecules of water are placed in rela- 
 
SOUND AND LIGHT. 9 1 
 
 tion to each other as if a hundred men were equally 
 distributed over the whole area of England, that is 
 to say, distant from each other thirty English miles. 
 Nevertheless, crystallographers are far from believing 
 in such considerable spaces. As far as ether is con- 
 cerned, Cauchy has deduced, from very delicate calcu- 
 lations, that the distance of the atoms approximates 
 to the two hundredth part of the red wave : according 
 to this, there would be three hundred thousand atoms 
 in the length of a millimetre. M. de Boucheporn, on 
 his side, believes he can affirm that the ethereal atoms 
 are crowded together to such a degree that the sum 
 of the empty spaces is reduced to the twentieth part 
 of the sum of those filled by the atoms. In conclu- 
 sion, these problems remain intact, and their solution 
 has not even been approached. 
 
 It would seem that the light which passes through 
 bodies ought to give us information in regard to the 
 molecular spaces. 
 
 There are transparent substances, the molecules of 
 which permit the luminous waves to pass freely, with- 
 out losing any part of their motion. Among trans- 
 parent bodies, a certain number are colored ; they 
 arrest or absorb the waves of certain colors only. 
 Thus a solution of sulphate of copper allows the blue 
 waves to pass, and stops the red rays ; if there be 
 
92 UNITY OF NATURAL PHENOMENA. 
 
 projected upon the screen through this solution a 
 spectrum, the red end of the spectrum is entirely cut 
 off. A piece of red glass, on the other hand, owes its 
 color to the fact that its substance may be freely 
 traversed by the red waves, while the shorter waves 
 become extinguished in it ; if it be placed in the path 
 of a luminous beam, the spectrum is reduced to a 
 band of lively red. If there be placed, at the same 
 time, in the path of the beam the solution of sulphate 
 of copper and the piece of red glass, these two trans- 
 parent bodies extinguish all the rays at once, and 
 produce a complete opacity. Some other body, a 
 solution of permanganate of potash, for example, will 
 extinguish at the same time the red and the blue rays, 
 and give passage only to the yellow, which constitute 
 the central portion of the spectrum. 
 
 Different bodies, then, exercise in relation to lumi- 
 nous waves a sort of power of election, extinguishing 
 some, permitting others to pass. Here it is the long- 
 est, there the shortest, which are arrested ; in other 
 cases both the longest and the shortest are stopped 
 at the same time, while those only of medium length 
 can obtain a passage. Whence comes this differ- 
 ence ? What law presides over this sort of select 
 choosing on the part of the luminous rays ? No 
 doubt it arises from the form of the molecules, and 
 
SOUND AND LIGHT. 93 
 
 the nature of their motions. We are hardly able to 
 say more about it. The difficult molecular* motions 
 seem to have rhythmical periods peculiar to each, in 
 virtue of which they come into harmony with those 
 of the ethereal atoms. 
 
 That there should thus be in molecular motions a 
 kind of rhythm, from which results the selection of 
 colors, has been demonstrated in a general manner by 
 the study of the spectrum ; but there may be men- 
 tioned in this connection several curious and charac- 
 teristic facts, which have been pointed out in the last 
 few years. 
 
 Project upon a screen the spectrum of a solid body 
 strongly heated. So long as the body remains incan- 
 descent only, so long as its molecules are not freed 
 from the bonds of cohesion, the spectrum remains 
 continuous ; there are seen neither dark nor bright 
 lines ; the waves of all the colors and of all the inter- 
 mediate shades are produced at the same time. If 
 the body be heated still more, it passes the point of 
 incandescence, and enters a state of combustion ; then 
 the molecules become free, at least for a moment. 
 Then, also, the bright and dark lines appear in the 
 spectrum. The waves are consequently re-enforced in 
 some places, and weakened in others ; they are gov- 
 erned by a new law. 
 
94 UNITY OF NATURAL PHENOMENA. 
 
 That it is the molecules themselves of the heated 
 body which, in their state of freedom, impress upon 
 the waves these peculiar modifications, cannot be 
 doubted, for every substance gives in this manner 
 lines so clear and definite that their appearance alone 
 is sufficient to distinguish it. 
 
 Acoustics furnishes us, upon the subject of these 
 phenomena, with analogies which present themselves 
 spontaneously to the mind ; that which takes place 
 when the body is incandescent may be compared to 
 the noises which result from waves mixed up and of 
 every length ; the effects produced by the free mole- 
 cules remind us of the harmonious sounds emitted 
 by strings, the vibration of which is not impeded by 
 any obstacle. 
 
 Here is also a new fact recently discovered. 
 
 We have just seen that every substance in burning 
 gives its own lines. When, for example, sodium is 
 burned, a very bright line is seen in the yellow por- 
 tion of the spectrum, in a clearly marked locality. 
 (Line D, of Fraunhofer.) If, now, instead of burning 
 sodium, we interpose the vapor of sodium in the path 
 of the ray which should give a continuous spectrum, 
 the phenomenon is completely reversed ; at the exact 
 point where just now there was a bright line, a dark 
 line appears. Thus the vapor of sodium, when it acts 
 
SOUND AND LIGHT. 95 
 
 as a screen, absorbs exactly those rays which it emits 
 
 " i 
 when it acts as the luminous source. 
 
 This fact observed in the case of the vapors of 
 iodine, of strontium, and of iron, has become gener- 
 alized ; it is now known under the name of the re- 
 versement of the spectrum ; it shows that bodies 
 tend at the same time to absorb and to emit the 
 same waves. 
 
 Shall we be astonished at this double tendency, 
 viewed from our present stand-point, and shall we 
 not recognize in it the necessary consequence of the 
 principles which explain, in our opinion, all physical 
 science ? From the moment that certain ethereal 
 motions have a special facility of becoming converted 
 into certain molecular motions, these latter must also 
 easily undergo the opposite conversion. The recipro- 
 city of the motive forces insures to us that of thd 
 phenomena. 
 
 If the natural bond of union of all the facts we 
 have mentioned be sought for, it will be seen that, 
 taken together, those facts come to the support of that 
 grand law we have endeavored to expound, and which 
 we have designated under the name of the Unity of 
 the Physical Forces ; but there will at the same time 
 be observed the defects in the method we are obliged 
 to employ. Were we dealing with a system ready- 
 
96 UNITY OF NATURAL PHENOMENA. 
 
 made, we might unfold it step by step, and pass from 
 one part to another without a gap. Far from this, 
 we have to do with a system seen only in glimpses, 
 scarcely sketched, even, the elements of which are so 
 incomplete as to be found insufficient, it may be. 
 Such being the case, what remains for us to do, if not 
 to show some parts brightly illuminated, while leav- 
 ing in the shade whatever is obscure ? From those 
 scattered lights, these fugitive glimmerings, must re- 
 sult the conception of the whole. 
 
 The digression we have just made with reference to 
 
 acoustics and optics has exhibited to us the branches 
 
 
 of science in which the phenomena of motion have 
 
 been best studied that motion which we now see un- 
 derlying all things. Sonorous motions, luminous mo- 
 tions, have been verified, measured, and scrutinized 
 in all their modes ; but, on the other hand, their me- 
 chanical effects have scarcely been glanced at. The 
 study of heat will afford a contrary result ; heat-mo- 
 tions are at the most suspected only, and continue to 
 be but little known as regards their real nature ; but 
 their mechanical effects have been demonstrated by 
 splendid experiments, and measured with the utmost 
 precision. 
 
 Sound and light on one side, heat on the other : 
 here are two subjects of study, as yet incompletely 
 
SOUND AND LIGHT. 9/ 
 
 explored ; but these two studies complement each 
 other, and as soon as we approach t*hem we see shi- 
 ning brightly forth that idea by virtue of which Nature 
 appears to us but as a system of mutually exchanging 
 motions. 
 
 It is from comparisons of this sort that our thesis 
 derives its principal strength. This is what our read- 
 ers must never lose sight of while we continue to 
 confront the system of the unity of the physical forces 
 with facts which experience has acquainted us with. 
 7 
 
98 UNITY OF NATURAL PHENOMEN-A. 
 
 CHAPTER III. 
 HEAT. 
 
 I. 
 
 The Dynamic Theory and Mechanical Equivalent of 
 Heat. 
 
 THE theory which reduces the physical world to 
 matter and motion, presents an external so attractive 
 as to provoke a sort of mistrust. Accustomed as we 
 are to complicated appearances, we are astonished at 
 this pretentious unity. We ask, with uneasiness, if we 
 are not the dupes of a desire to simplify everything. 
 Is not this hypothesis, which gives us, in a manner, an 
 insight into the whole plan of Nature, a deceitful mi- 
 rage? Are we not deluded by fallacious generaliza- 
 tions ? Are we not enticed into falsifying the phe- 
 nomena for the sake of forcing them to enter into the 
 frame of a preconceived theory ? To these questions 
 a reply can be made only by an examination of the 
 facts, and for this purpose we have taken a brief sur- 
 vey of the different provinces of physics. 
 
HEAT. 99 
 
 The natural sequence of this worl^ brings us now 
 to investigate heat, and we thus find ourselves led to 
 discoveries which have served as the origin of the 
 theory of the unity of the physical forces. 
 
 Here our task becomes easy, perhaps, but, it must 
 also be added, somewhat thankless. The equiva- 
 lence of heat and mechanical work has beeri for some 
 years past not a new idea ; books, public instruction, 
 and lectures, have spread it abroad ; the popular mind 
 has been zealously informed of it. We have not to 
 fear, then, that the subject will be a strange one to our 
 readers, or fear, rather, lest they have heard too much 
 of it, and that they regard it as commonplace. We 
 shall then only recall, very briefly, the principles of 
 thermo-dynamics, and we shall strive more especially 
 to throw light upon the consequences that may be 
 drawn from them as regards the constitution of 
 bodies. 
 
 Let us mention, in the first place, a book which pre- 
 sents in a distinct and agreeable form all the essential 
 data of the new theory of heat. It contains twelve 
 lectures, delivered by Mr. John Tyndall, at the Royal 
 Institution, London, upon Heat, considered as a Mode 
 of Motion. The course was delivered in 1862 ; the book 
 appeared, in France, translated by the Abb6 Moigne, 
 in 1864, an d it was immediately appreciated at its true 
 
IOO UNITY OF NATURAL PHENOMENA. 
 
 value by all persons who are interested in the general 
 advancement of science. It is impossible to give to 
 lessons in physics a greater charm, and, at the same 
 time clearness, than has been done by Mr. Tyndall 
 in the work we name. The book has preserved the 
 form of oral instruction ; we follow the words and 
 gestures of the professor ; we assist in the details, 
 even in the very mishaps of the experiments. It must 
 not be thought, however, that we have before us an 
 impromptu effort reproduced by the art of the stenog- 
 rapher. Much skill lies concealed under this appar- 
 ently easy method of procedure. Mr. Tyndall skil- 
 fully calculates all his. results ; the accidents in his 
 experiments occur only when well foreseen ; they are 
 fortunate mishaps that take place only when he wishes 
 to seize the attention of his public, whether listeners 
 or readers, and abruptly direct their thoughts to some 
 striking anomaly. 
 
 The experiments of*Mr. Tyndall are, moreover, con- 
 ceived with much skill and ability. For a long time 
 he has been a master in the art of lecturing before 
 a numerous audience. He has contrived ingenious 
 instruments to magnify the results of experiments. 
 He was one of the first to employ the electric light 
 for projecting on screens the enlarged images of the 
 most delicate phenomena. The dramatic effects which 
 
HEAT. IOI 
 
 jj, j * j j > J / 
 
 have made the success of Mr. TyiisBali$\ lectures, a'p-; 
 pear adroitly preserved in his book. -1J ,* - ; r';'- ^ ^ 
 
 As to the substance of these lectures, the professor 
 treats his subject bit by bit ; he takes his time to pro- 
 duce in the minds of his pupils those ideas which the 
 regenerated study of heat awakens. 
 
 " Remember," said he to them, " we are entering a 
 jungle, and must not expect to find our way clear. 
 We are striking into the branches in a random fashion 
 at first ; but we shall thus become acquainted with the 
 general character of our work, and, with due persis- 
 tence, shall, I trust, cut through all entanglement at 
 last." When he has sketched the principle of a new 
 conception, " Do not be disheartened," he makes haste 
 to say, "if this reasoning shoujd not appear quite 
 clear to you. We are now in comparative darkness, 
 but as we proceed light will gradually appear, and 
 irradiate retrospectively our present gloom." And, in 
 another place, "Whenever a difficult expedition is 
 undertaken in the Alps, the experienced mountaineer 
 commences the day at a slow space, so that when the 
 real hour of trial arrives, he may find himself hard- 
 ened instead of exhausted by his previous work. We, 
 to-day, are about to enter on a difficult ascent, and I 
 propose that we commence it in the same spirit ; not 
 with the flush of enthusiasm, which the necessity of 
 
IO2 UNITY OF NATURAL PHENOMENA. 
 
 "labor extinguishes, but with patient and determined 
 "?ic3rts vvhfichj will tfot recoil should a difficulty arise." 
 The professor conforms in every particular to his ex- 
 cellent programme, and employs a good deal of skill 
 in preparing his pupils for the abstract notions he 
 wishes to impart to them. 
 
 Nevertheless we cannot refrain from adding that the 
 conclusions of the book remain vague and unsatis- 
 factory. 
 
 We are acquainted with the history of the works, 
 and of the discoveries which successively modified 
 and defined the idea of heat. 
 
 As in the case of light, two theories for a long time 
 confronted each other : that which made of heat a 
 material substance, and that which saw in it only a 
 mode of motion. The material nature of caloric con- 
 tinued to be admitted much later than that of light. 
 During the latter years of the eighteenth century, 
 Lavoisier and Laplace, in presenting to the Academy 
 of Sciences a memoir compiled in common, upon the 
 subject of heat, seemed to hold the balance equally 
 between the two opinions. Their language was, "We 
 shall not decide between the two foregoing hypothe- 
 ses ; several phenomena would seem to favor the lat- 
 ter (that of motion), as, for example, the heat pro- 
 duced by the rubbing together of two solid bodies ; 
 
HEAT. IO3 
 
 but there are others which are more, simply explained 
 by the first (that of its material nature) ; perhaps both 
 obtain at once." 
 
 In reality they abandoned the idea of motion with- 
 out having made any use of it, and returned to the 
 theory of the material nature of heat. Laplace, espe- 
 cially, after the termination of his connection with 
 Lavoisier, became a confirmed supporter of this last 
 theory, which was thus strengthened by a weighty 
 authority. 
 
 A little later, during the last years of this century, 
 Rumford, an original, almost paradoxical mind, reso- 
 lutely pronounced himself against the materiality of 
 heat. " If heat," he said, " is a matter lodged in the 
 pores of different substances, it may be forced out, as 
 water is squeezed from a sponge, and the same body 
 will not be able to emit it indefinitely." Having thus 
 brought the question to a decisive experiment, he 
 caused an iron bar to turn upon another similar bar 
 in the midst of a liquid, and he showed that there was 
 a disengagement of heat as long as the iron bar was 
 turning. 
 
 The experiments of Rumford had not as much 
 .celebrity as they deserved. Thomas Young alone 
 seemed to understand their bearing. In a treatise 
 upon physics, published in 1807, he exhibited the 
 
104 UNITY OF NATURAL PHENOMENA. 
 
 labors of Rumford, and compared them with his own 
 discoveries in regard to heat ; but the old ideas upon 
 caloric continued to hold their sway in men's minds. 
 
 Steam engines came, and all the questions pertain- 
 ing to heat again became the order of the day. At 
 this time the materiality of heat was so little disputed 
 that Sadi Carnot took it as the basis of his celebrated 
 Reflections on the Motive Power of Fire (1824). It 
 is known how, starting from this erroneous doctrine, 
 Sadi Carnot, and his celebrated commentator, Clapey- 
 ron, revived the thermo-dynamic theory. They called 
 attention to the causes which enabled an engine burn- 
 ing charcoal in its furnace to produce work at its shaft. 
 They had this good fortune, that their arguments 
 their formulas, even could be disengaged from the 
 fundamental error which contaminated them, and serve 
 as a basis for the new theory of heat. 
 
 In 1839 M- Seguin published an Essay on the Influ- 
 ence of Railroads. In it heat was considered as a 
 motion, and the author gave very appropriate hints 
 concerning the transformation of this motion into 
 work ; but this subject was but touched upon in the 
 book of M. Seguin, who had particularly in view 
 questions of social economy. 
 
 * 
 
 Between the years 1840 and 1850 were produced 
 the remarkable works of M. M. Mayer and Joule. 
 
HEAT. IO5 
 
 Starting from very different premises, and placed at 
 totally different stand-points, tl\e one in Germany, 
 the other in England, they came at the same time 
 to clearly demonstrate the equivalent of heat and 
 mechanical work, and they determined the ratio of 
 this equivalence. 
 
 Immense result ! It was like a shining beacon 
 lighted up in the midst of the darkness which en- 
 shrouded physical science when this definite fact was 
 made known. A unit of heat is equivalent to four 
 hundred and twenty-five kilogrammetres, or, in other 
 words, the quantity of heat which is required for ele- 
 vating one degree the temperature of a kilogramme 
 of water, can also do the work which consists in ele- 
 vating four hundred and twenty-five kilogrammes to 
 the height of one metre. 
 
 This discovery has for fifteen years vastly increased 
 the field of vision for science. There springs from it, 
 as it were, a new philosophy of nature. A mental 
 revolution is taking place, of which we see only the 
 commencement, and we are endeavoring to sketch the 
 beginnings of this change. 
 
 All uncertainty in regard to the nature itself of heat 
 came to an end as soon as its mechanical equivalent 
 had been determined. What is it that could be trans- 
 
IO6 UNITY OF NATURAL PHENOMENA. 
 
 formed into motion in so regular a manner if it be not 
 another, motion ? 
 
 Doubtless there would not be discovered at once, 
 either in the action of steam engines, or any -other 
 phenomena, the precise mode of the transformation ; 
 but the principle of it was grasped by the mind with 
 conviction. The motion itself was not seen, but its 
 effects were both perceived and measured. 
 
 Heat is a motion ; but of what kind ? 
 
 Some physicists conceived at first that it might be 
 due to the longitudinal vibrations of the ether. They 
 knew that ether, by its transversal vibrations, pro- 
 duced light. With regard to the longitudinal action, 
 that which is produced -in the direction of the ethereal 
 ray, no especial property was known, and they dis- 
 posed of it by attributing to it the calorific effects. 
 This conjecture, which did not rest upon any well- 
 grounded fact, has gathered around it but a very small 
 number of supporters, and has scarcely been consid- 
 ered more than a work of the imagination. 
 
 According to present notions, heat is a motion of 
 the molecules themselves of bodies. All material 
 molecules are endued w r ith this motion ; they strike, 
 without cessation, against each other, and thus main- 
 tain or change their state. By means of their shocks 
 the molecules of bodies cause us to experience the 
 
HEAT. ID/ 
 
 sensation of heat, and from the intensity of these 
 shocks we determine degrees of temperature. This 
 perpetual vibration of the molecules itself constitutes 
 the phenomenon of heat ; but.it may naturally convert 
 itself into different effects. It can, when circum- 
 stances are favorable, agitate the ether, and produce 
 light ; it can agitate the air, and produce sounds ; it 
 may concentrate itself in order to move masses, and 
 produce what has very properly been called mechani- 
 cal work. 
 
 Properly speaking, the different effects we have just 
 mentioned heat, light, sonorous shock, mechanical 
 work, and other effects of the same class, which we 
 do not mention at this time, are but the different 
 manifestations of the same cause. The motion with 
 which each molecule is endowed at a given moment 
 constitutes for it a sort of intrinsic energy. In me- 
 chanics we are able to appreciate and measure the 
 energy with which a moving body is endowed. The 
 product of the mass of a body into the square of its 
 velocity expresses what is called the vis viva, or living 
 force. This product has not, properly speaking, any 
 physical representation, and it offers to the mind at 
 first a conception rather abstract ; but it assumes a 
 capital importance from this circumstance, that it is 
 equivalent to double the work that the body can pro- 
 
IO8 UNITY OF NATURAL PHENOMENA. 
 
 duce in losing all its velocity ; it gives then the meas- 
 ure of the dynamic effect which the body in motion 
 contains within it. We may now declare, making use 
 of this idea, that all particles of matter possess, at a 
 given instant, a certain amount of living force. They 
 may lose a portion of it in doing some work, that 
 is, by displacing a mass, but then the vis viva which 
 they lose becomes stored up in the work performed, 
 and it is renewed when this work is undone. 
 
 Let us consider a steam engine, and neglecting all 
 the losses of force or work which belong to the me- 
 chanism itself, let us think only of the theoretical or 
 ideal action of the engine. The steam expands, for- 
 cing out the piston ; each molecule of vapor thus loses 
 a certain quantity of vis viva. These accumulated 
 losses produce a revolution of the shaft, which is en- 
 gaged, for instance, in elevating a weight. At the end 
 of the operation all the vis viva which the steam has 
 lost is virtually found in the elevated weight. If I cut 
 the cord which sustains this weight, it will fall, and re- 
 produce in its fall all the vis viva which has been ex- 
 pended in order to raise it. It will appear in the form 
 of heat at the instant when the body strikes the 
 ground, and if this could be collected and restored to 
 the steam, the latter would be replaced in the condi- 
 
HEAT. 
 
 tion in which it was found at the beginning of the 
 operation. , 
 
 What we have indicated in this rough example, is 
 constantly taking place in all nature. To bring the 
 vis viva, or living force, into the condition of work, 
 and then to reproduce it, in this lies the whole activity 
 of Nature. 
 
 II. 
 
 Changes of State produced by Heat furnish Informa- 
 tion as to the Constitution of Bodies. 
 
 ADMITTING an incessant agitation of the molecules, 
 it is easier to account for the phenomena which take 
 place in bodies when they pass from the gaseous to 
 the solid and liquid states. 
 
 As a general rule, all bodies are susceptible of these 
 three states : carbonic acid gas has been liquefied and 
 solidified ; water appears to us under the form of ice 
 and vapor ; we know how to fuse and volatilize the 
 metals. We do not always possess sufficient means 
 for making every body pass successively through these 
 three states ; but we may, nevertheless, affirm that we 
 should see them under all three forms, if our re- 
 searches could include a sufficiently extended scale 
 of temperatures. 
 
IIO UNITY OF NATURAL PHENOMENA. 
 
 As a general rule, also, it may be said that heat must 
 be increasingly added to the same body to bring it 
 from the solid to the liquid state, then to the gaseous. 
 Thus heat triumphs over the bonds which bind to- 
 gether the molecules ; it combats those attractive 
 forces which manifest themselves in the interior of 
 bodies, and which have preserved until now so myste- 
 rious an aspect. 
 
 Plas it been possible, through the antagonism which 
 is displayed between heat and the attractive forces, to 
 isolate the calorific motion ? to separate it from the 
 phenomena which mask it ? to determine its special 
 mode and laws ? Unfortunately not. Nevertheless it 
 may be said that the study of gases has thrown much 
 light upon this question. 
 
 How must we conceive of the gaseous state ? To 
 begin with, it is characterized by a considerable dis- 
 tance between the molecules. Endued with a great 
 projectile velocity, these molecules hurl themselves 
 against each other, or against the bounds of the space 
 which confines them. Have they merely a projectile 
 motion ? They have necessarily a rotatory motion 
 also, for, if this motion did not exist at a given mo- 
 ment, it could not fail to be generated by the incessant 
 collisions of the different molecules. The eccentric 
 shocks, those which do not pass through the centres 
 
HEAT. Ill 
 
 of gravity, are, in fact, of such a nature as to produce 
 a rotation. This rotation combines with their elas- 
 ticity to cause the molecules to recoil from each other. 
 The former alone might produce this effect, if the 
 molecules, instead of being compound, were but sim- 
 ple atoms. A sort of medium state is thus established 
 in the gas. If the motion became weak at certain 
 points, it would at once be strengthened by the agita- 
 tion of the rest of the mass. Moreover, each mole- 
 cule recoils without definite direction, and may go in 
 all directions, to be successively projected into every 
 part of the entire mass. There is a state of complete 
 liberty. 
 
 Let us observe that the molecular distances are con- 
 siderable ; their velocities are also considerable. What 
 becomes, then, of that effect which must be produced 
 at the instant when two molecules approach and strike 
 each other, that effect which is attributed to the attrac- 
 tive forces, whatever they may be ? This effect is, so 
 to speak, annulled ; it lasts only a very short time 
 relatively, since the molecular distances are very great. 
 It is but a very transient effect, since the velocities are 
 enormous. It becomes so weak that it may be neg- 
 lected ; thus, in the gases, the attractive forces possess 
 no power. The calorific motion exists in them with- 
 out antagonism, and may be observed in its integrity. 
 
112 UNITY OF NATURAL PHENOMENA. 
 
 If we apply cold to a gas, if we cause it to lose a 
 portion of its vis viva, the energy and amplitude of 
 its oscillations will gradually diminish. A moment 
 will come when each molecule will be, as it were, im- 
 prisoned by its neighbors, and forced to oscillate along 
 a limited curve. The gas will have become a liquid. 
 
 From the very fact alone of the proximity of the 
 molecules the attractive forces have regained the su- 
 premacy, and have destroyed, in part, the mobility of the 
 system. Gravity, too weak before, now makes itself 
 felt, and the molecules are obliged to arrange them- 
 selves in such a manner as to present a surface paral- 
 lel to the horizon. 
 
 Along this surface they are detained in their new 
 position by one of their sides only ; upon the other 
 side their motions remain free, and they possess an 
 especial aptitude for returning to their former state, 
 and an evaporation takes place from the surface. 
 
 Moreover, in the remainder of the mass, the mole- 
 cules still enjoy a relative amount of liberty. They 
 are enclosed within restricted orbits, but their axes of 
 rotation continue to lie in all directions. They can 
 thus roll over each other to some extent. 
 
 Besides, the bonds which limit their movement 
 yield to the slightest effort, and the whole mass may 
 be mixed without difficulty. 
 
HEAT. 113 
 
 Let us continue the application of cold. The mole- 
 cules approach each other ; they enter, one may say, 
 within the sphere of each other's action, and they re- 
 main there ; their axes of rotation become upright, 
 and take a common direction ; the body has passed 
 into the solid state. 
 
 During these conditions the molecules still oscillate ; 
 but they can no longer, without outside assistance, de- 
 part from the circle in which they are kept by their 
 neighbors. 
 
 In describing the manner in which bodies change 
 their state, we have just brought forward the attrac- 
 tive forces. After the repeated declarations we have 
 already made, we might almost forbear to mention 
 that these forces are for us but the symbols under 
 which lie concealed the ordinary phenomena of 
 motion. 
 
 Before terminating this work, we shall be brought 
 to consider collectively these attractive forces, which 
 we admit this time by way of inventory. Nevertheless, 
 let us now make a rapid allusion to them, so as not to 
 leave them entirely in mystery. 
 
 It is a necessary consequence of the rotation of 
 
 molecules that they draw with them into their motion 
 
 a certain number of ethereal atoms. They are thus 
 
 wrapped in a sort of atmosphere whose radius may 
 
 8 
 
114 UNITY OF NATURAL PHENOMENA. 
 
 vary according to circumstances, and which nearly 
 represent that which we just now called the molecular 
 sphere of action. So long as these atmospheres do 
 not touch each other there is no action ; such is the 
 case with gases. If the molecules approach each 
 other, and the atmospheres slide over each other (this 
 is the case with liquids), action begins, an action 
 purely mechanical, due to the meeting of the ethereal 
 atoms. If, finally, the atmospheres penetrate more 
 deeply into each other, the effect is more strongly 
 pronounced ; the ethereal envelopes that are pene- 
 trated find themselves hindered in their progress, and 
 they behave so as to render parallel the rotations of 
 the different molecules respectively, as happens in 
 the case of solids. 
 
 With this sketch, let us pass hastily on, as we are 
 unwilling to delay, in order to speak of liquids and 
 solids, the constitution of which continues even now 
 very obscure. It is sufficient that we have shown how 
 the laws of this constitution are connected with those 
 laws, far better known, which govern the gaseous state. 
 Thanks to this solidarity, the gases offer us a con- 
 venient type for studying molecular motion, and we 
 may fix our attention upon them some moments 
 longer, sure of deriving from them information ap- 
 plicable to all the forms of matter. 
 
HEAT. 115 
 
 III. -, 
 
 Theory of Gases. 
 
 THE theory of gases, the principle of which was 
 just now pointed out, has been much studied of late 
 years, and it has given rise to a great number of re- 
 markable publications. It does not present itself, 
 however, as an entirely novel conception, for its fun- 
 damental notion could be found in the Hydrodynamics 
 of Bernouille, published in 1738 ; but, buried in the 
 work of Bernouille, it scarcely saw the light until the 
 last thirty years, and it has received its development 
 only through the quite recent works of M. Joule and 
 M. Clausius. 
 
 We cannot here follow those two physicists in the 
 analytical deductions, by means of which they have 
 defined, with wonderful accuracy, the theory of gases. 
 But we shall at least be able to show how the hy- 
 pothesis in regard to the constitution of gases, which 
 we have just sketched, accounts for the facts succes- 
 sively -revealed by experiment. 
 
 From the simple enunciation of this theory arise, as 
 a necessary consequence, several of those celebrated 
 laws which form the very foundation of physics. 
 
 It results primarily from our hypothesis, that the 
 
Il6 UNITY OF NATURAL PHENOMENA. 
 
 molecules of a gas may be considered as every instant 
 moving in a straight line, with a uniform velocity 
 common to the whole mass ; we have, in fact, elimi- 
 nated the perturbing phenomena existing at the mo- 
 ment of collision. Is it not henceforth evident that, if 
 the gas is confined in a receiver, the pressure exerted 
 upon its walls will be proportional to the number of 
 atoms in the gas, that is to say, to its density? 
 Equality of ratio between pressure and density, such, 
 we know, is Mariotte's law. 
 
 Now, at the same pressure and temperature, differ- 
 ent gases of the same volume contain the same num- 
 ber of molecules. This is a fact which chemists par- 
 .ticularly bear witness to, and it is deducible from our 
 hypothesis. Since the molecular actions, properly so 
 called, may be neglected, it is conceivable that the 
 molecules of the same gases, endowed with equal 
 liberty, will arrange themselves, other circumstances 
 being equal, at equal distances from each other, and in 
 the same volume of a gas the same number will be 
 found. A quart of hydrogen, a quart of oxygen, a 
 quart of nitrogen, thus contain a uniform number of 
 molecules. What will take place if two gases are 
 mixed ? The same principle applies to the mixture, 
 no special action resulting from the proximity of the 
 molecules, since the nature of the molecule appears to 
 
HEAT. II/ 
 
 have no influence upon the phenomenon. Atmospheric 
 air will behave in this respect, like pure oxygen or 
 pure nitrogen.. This is the law of gaseous mixtures 
 pointed out by Gay-Lussac. 
 
 Since the distance between the molecules remains 
 the same, whatever their mass, it is to be expected 
 that the same quantity of heat will be necessary in all 
 the gases, to raise the temperature of the elementary 
 molecule one degree. It will be objected, perhaps, that 
 the heaviest molecules will receive from this quantity 
 of heat a less velocity ; this is evident ; but it is also 
 evident that they require a less velocity for mani- 
 festing that effect which we call an elevation of one' 
 degree of temperature. We are then brought to this 
 result, that the temperature of the elementary mole- 
 cules of the different gases is elevated one degree by 
 a like quantity of heat, whatever may be their mass, 
 or, as chemists say, their atomic weight. Under this 
 form may be recognized a celebrated law, to which 
 Dulong and Petit have given their name. 
 
 Gay-Lussac, it is known, has established that the 
 coefficient of expansion is uniform for all gases. Now, 
 have we not here a natural result of the facts we have 
 just disclosed ? These molecules, which are all placed 
 at the same distance from each other, and which ab- 
 sorb the same quantity of heat in order to increase 
 
Il8 UNITY OF NATURAL PHENOMENA. 
 
 their temperature one degree, ought they not to sepa- 
 rate from each other equally under this increase of 
 temperature ? The experiments of Gay-Lussac have 
 shown that the coefficient of this uniform expansion 
 is 2T^ of the primitive volume.* 
 
 This examination might be continued ; but we have 
 said enough to show how, from our very definition 
 of gases, flow the laws which characterize the gaseous 
 state. 
 
 The laws of Mariotte, Gay-Lussac, Dulong, and 
 Petit have had a singular fate. Discovered at a 
 period when methods of experimenting were far from 
 the perfection to which they have since attained, they 
 were at first regarded as absolutely exact, and applica- 
 ble, in full force, to the different gases. When that 
 advance of improvement in the methods of experi- 
 menting, to which in France is attached the name of 
 M. Victor Regnault, took place, these laws, till now so 
 respected, were at fault in numerous cases ; they fell 
 into suspicion ; at least they came to be considered as 
 empirical formulae, which represented in only an ap- 
 proximate manner the general course of phenomena. 
 No theoretical conception, in fact, accounted for the 
 
 * Decimally expressed, this coefficient of expansion becomes 
 .00367, according to the Centigrade scale. 
 
HEAT. 119 
 
 numerous perturbations which the exact methods em- 
 ployed by scientific men gave evidence of. But now 
 we see why gases obey but imperfectly Mariotte's law, 
 and those other laws we have just referred to. In 
 order to establish them, we have been obliged to sup- 
 pose that every molecule might be considered as con- 
 stantly endowed with a uniform and rectilinear mo- 
 tion, and we have regarded as insensible the duration 
 of the periods in which this motion was # interfered 
 with. If this duration becomes appreciable, while at 
 the same time remaining very slight, the arguments 
 which we gave cannot be repeated in their full force. 
 Here may be seen the source of much of the distrust 
 in the old laws : it may even be seen how the perfect 
 gaseous state is, to a certain extent, but an ideal which 
 is scarcely realized in fact. Hydrogen would seem to 
 attain to it entirely ; oxygen and nitrogen, and, as a 
 consequence, atmospheric air, come near to it ; but 
 carbonic acid is sensibly removed from it. As to va- 
 pors, they behave like gases only as they are very far 
 from the point of liquefaction. 
 
 There are, then, but very few perfect gases ; but 
 they furnish us with valuable information, in showing 
 us matter entirely disengaged from those attractive 
 forces which complicate molecular phenomena. 
 
 When we heat a cubic metre of air under a constant 
 
I2O UNITY OF NATURAL PHENOMENA. 
 
 pressure, all the living force which the gas receives is 
 employed in increasing its volume by ^y-g- for every 
 degree of temperature.* When, instead of leaving 
 the pressure constant, we prevent the gas from ex- 
 panding, when, while heating it, we force it to remain 
 enclosed in the space of a cubic metre, all the living 
 force acquired by the air is employed in increasing its 
 pressure ^-yg- for every degree. If the initial tem- 
 perature is that of melting ice, f at two hundred and 
 seventy-three degrees, the pressure of the air is 
 doubled. 
 
 The same law holds true below zero. If, instead 
 of heating the gas, we cool it, its pressure goes on 
 diminishing -^\-^ for every degree. If we could attain 
 to two hundred and seventy degrees, the gas would no 
 longer possess any pressure ; it would be but an inert 
 mass of molecules, deprived of all living force. This 
 is what has been called the absolute zero of tempera- 
 ture. There is here a sort of limitation to which it is 
 not possible to attain in practice, and at which all 
 molecular motion would cease. 
 
 We have been considering a definite mass of air, 
 
 * That is, -j^-g- for one degree Centigrade ; this would be the 
 same as ^ for one degree Fahrenheit. 
 
 f The temperature of melting ice is 32 Fahrenheit, and o 
 Centigrade. 
 
HEAT. 121 
 
 and we have supposed that we were heating it one 
 degree, while allowing it to expand in such a manner 
 that the pressure should remain constant ; we have 
 afterwards supposed that we were heating it one de- 
 gree, while preventing it from changing its volume. 
 Will there be necessary in each case, in order to 
 produce this same elevation of temperature, a like 
 quantity of heat ? Evidently not. Under a con- 
 stant volume the air has no outside work to accom- 
 plish. Under a constant pressure it must displace the 
 exterior obstacle which opposes its expansion ; it has 
 thus a real work to accomplish. In this second case 
 it must absorb an excess of heat which is exactly the 
 equivalent of the work done. The heat-capacity un- 
 der a constant volume, and the heat capacity under a 
 constant pressure, differ then in a notable manner. 
 For air, they are in the ratio of I to 1.421. The dif- 
 ference between these two quantities represents what 
 was formerly called the latent heat of expansion, and 
 what is now the precise equivalent of the work which, 
 the air must do in order to become expanded. 
 
 We may even observe that it was from this expan- 
 sion of the air, the numerical conditions of which have 
 for a long time been fixed, that Dr. Mayer sought 
 to obtain, in 1842, a primary determination of the 
 mechanical equivalent of heat. The number which 
 
122 UNITY OF NATURAL PHENOMENA. 
 
 M. Mayer deduced does not differ sensibly from that 
 which has been definitively adopted after a series of 
 experiments of every kind. 
 
 We have said that air accomplishes outside work 
 in expanding ; such is usually the case ; but it may, 
 under particular circumstances, expand without hav- 
 ing performed any work. Now it is the work which 
 absorbs the heat, and not the expansion itself; if there 
 is no work in the expansion, it is not indicated by the 
 absorption of heat. 
 
 This phenomenon has, moreover, been proved by a 
 celebrated experiment which M. Joule made in 1845. 
 M. Joule took two receivers of the same size, con- 
 nected by tube and stopcock ; into one he put air 
 under a pressure of twenty-two atmospheres, in the 
 other he made a vacuum, and he permitted the com- 
 pressed gas of the first receiver to expand in the 
 second, and a state of equilibrium soon ensued 'un- 
 der a uniform pressure of eleven atmospheres. In 
 order to arrive at this state, the gas had not had 
 any outside work to do, and M. Joule showed that 
 the temperature was the same at the beginning as 
 at the end of the experiment. Doubtless there were, 
 at certain moments, changes of temperature, but the 
 partial losses and gains compensated each other, and 
 at the final analysis the absence of work done was 
 
HEAT. 123 
 
 indicated by the absence of variation in the tem- 
 perature. , 
 
 The experiment of M. Joule has been repeated 
 by several scientific men, and in a noteworthy man- 
 ner by M. Victor Regnault. But it demands a high 
 degree of precision, and is not of a nature to be re- 
 produced in a lecture upon physics. Mr. Tyndall, 
 in his course at the Royal Institution, shows the 
 results of it by means of suitable and familiar ap- 
 paratus. 
 
 He first takes a box in which a certain quantity of 
 air is compressed, and he opens its stop-cock to let 
 the gas escape. Here the gas no longer finds a 
 vacuum before it ; it must, in order to expand, drive 
 away the external air ; it must perform a work ; and 
 it is only from itself that it derives the necessary 
 heat. There is, then, a lowering of temperature, and 
 Mr.. Tyndall renders this result visible by directing 
 the jet "upon the face of a very sensitive thermo-elec- 
 tric pile ; * the needle of the galvanometer indicates 
 the cooling of the gaseous jet Instead of the box of 
 compressed air, Mr. Tyndall next takes an ordinary 
 
 * Mr. Tyndall has a thermo-electric pile so sensitive that, 
 maintained at a temperature of about fifty or sixty degrees, it 
 indicates at a distance of twenty paces the heat which is given 
 out by a man's body. 
 
124 - UNITY OF NATURAL PHENOMENA. 
 
 pair of bellows, and in working them he directs the 
 jet upon the front of the pile. In this case the gas 
 itself has not to yield the heat necessary for pushing 
 away the external air ; the hand of the operator fur- 
 nishes the work directly ; it even furnishes it in excess ; 
 and the needle of the galvanometer, instead of indi- 
 cating a diminution,* marks an elevation of tempera- 
 ture. 
 
 The theory of -heat is becoming more complete 
 every day ; but even now it is sufficiently advanced 
 to present a respectable state of entirety. If it still 
 exhibits gaps and doubtful points, the principal out- 
 lines are at least clearly marked. The molecular 
 motions which constitute heat are not immediately 
 perceptible to our senses, but they may be said 'to 
 lack but little of it. They are even almost discerni- 
 ble, so well known and so exact are their mechani- 
 cal effects. When the living force passes from the 
 molecules to the mass of a body, and return's from 
 that mass to the molecules, thus appearing succes- 
 sively under the form of work and of heat, we can- 
 not observe these changes ; but the phenomena, a 
 little before and shortly after the transformation, are 
 so well determined, that we believe we see the- change 
 itself. 
 
 Thermo-dynamics is a field sufficiently explored ; 
 
HEAT. 125 
 
 one in which the mistakes of the way are not seri- 
 ous, and in which one is certain, having gone astray, 
 to regain his path. Investigating electrical phenome- 
 na, we are about to enter a region far more obscure 
 and dangerous. 
 
126 UNITY OF NATURAL PHENOMENA. 
 
 CHAPTER IV. 
 ELECTRICITY. 
 
 I. 
 
 // is necessary to determine the Electric Unit, and to 
 fina( its Mechanical Equivalent. 
 
 WHAT is electricity ? How shall we regard that 
 common conception which is based upon the play of 
 a positive and a negative fluid ? Are there, in reality, 
 two electric fluids ? Is there even one ? 
 
 We put these questions ; but a reference to the 
 premises already laid down will leave but little doubt 
 as to the kind of answers we shall give them. 
 
 And first, the duality of the fluids can no longer be 
 regarded other than as a figurative expression. We 
 may even ask ourselves if it ever had an appearance 
 of reality. It has all the characteristics of a fiction 
 of analysis ; it carries the mind at once into the 
 realms of mechanics. In mechanics, motions are 
 termed positive or negative, according to their taking 
 place in one direction or another ; so the hypothesis 
 
ELECTRICITY. I2/ 
 
 of a duality of fluids becomes resolved into a mathe* 
 matical conception. , 
 
 Is there even any special fluid to which we must 
 attribute electrical properties ? It would doubtless 
 seem proper to make here at the outset, some partial 
 reply, and not to decide this question without some 
 reserve ; but we need not again declare that we have 
 banished all idle prudence, and we do not hesitate to 
 assign, at once, to the electric fluid a place outside of 
 science, and send it to join the company of those de- 
 lusions of the past, the calorific and luminous fluids 
 and the many so-called entities of former times. 
 
 With regard to magnetism, we may leave it entirely 
 aside, since standard instruction has long ago traced 
 to one and the same principle both the magnetic and 
 the electrical phenomena; a permanent or a tem- 
 porary magnet may be regarded as the seat of a series 
 of little currents setting in the same direction. 
 
 The field being now cleared, the question presents 
 itself to us in this form : Is electricity a motion of the 
 ether ? Is it a motion of ponderable matter ? Is it 
 a motion of both ? In a word, what is the character 
 of this motion ? 
 
 Before broaching these questions, we desire to call 
 attention to two important and decisive points in the 
 study of electricity. 
 
128 UNITY OF NATURAL PHENOMENA. 
 
 * Electrical phenomena have been studied with great 
 care during the past few years ; a vast number of lit- 
 tle facts have been collected, which present, however, 
 only a confused appearance, being badly grouped, and 
 throwing but little light upon each other. This is 
 doubtless due in a measure to the nature of the sub- 
 ject ; but it is also attributable, in part, to those who 
 make the observations. One essential and primary 
 condition is wanting in the researches which have 
 been made here and there on the subject of electrici- 
 ty, namely, an agreement as to the unit to which all 
 these actions shall be referred. 
 
 We have already had occasion to mention the capi- 
 tal importance that attaches, in physics, to the choice 
 of units. Every phenomenon results from the coex- 
 istence of a certain number of correlated facts, and in 
 order to illustrate the relation of these facts, it is 
 necessary to represent each of them in its proper 
 quantity by a particular variable. Thus, if we try to 
 define the orbit of a planet about the sun, we must 
 take for the elements of our research, on the one hand, 
 the variable length of the radius vector which joins the 
 sun to the planet, and on the other, the constantly 
 changing inclination of this radius to the axis of the 
 perihelion ; observation will show forthwith the rela- 
 tion between these two quantities which constitutes the 
 
ELECTRICITY. I2Q 
 
 equation of the ellipse, and we may declare that the 
 planet runs in an elliptical orbit* in which the sun oc- 
 cupies one of the foci. It would not do, however, to 
 suppose that a phenomenon -would be equally easy to 
 define with any variables whatever that might be se- 
 lected ; on the contrary, this selection exerts a very 
 decisive influence upon the results obtained. With 
 certain variables you will arrive at only confused re- 
 sults, from which you can derive no profit, while with 
 others, we shall bring to light precise laws. 
 
 We might thus mention, in the history of physical 
 science, many an unfortunate selection which has re- 
 tarded important discoveries, likewise also many for- 
 tunate guesses. An example of the latter occurred 
 in the case of Kepler's first law, the second of which 
 we just now alluded to. When Kepler sought the law 
 of the motion of a planet in its orbit, he selected as 
 variables the time, and the areas described by the 
 radius vector. It would have been just as natural, 
 more so perhaps, to seek a relation between the time 
 and one of the variables mentioned above, namely, the 
 length of the radius, or its inclination to the line of 
 the apsides. Had Kepler pursued this course, he 
 would not have found any simple relation between the 
 numerical values resulting from his observations and 
 those of Tycho-Brahe ; the intimate connection of 
 
I3O UNITY OF NATURAL PHENOMENA. 
 
 these values would have been concealed under rela- 
 tions so complicated that it never could have been 
 demonstrated. On the contrary, thanks to the varia- 
 bles he had chosen, Kepler was readily able to ob- 
 serve that the numerical values representing the times, 
 and those representing the areas, formed two propor- 
 tional series. In this way became plainly evident that 
 great law of astronomy, which we express by saying 
 that the areas described by the planets are in pro- 
 portion to the times, or that planets describe equal 
 areas in equal times. 
 
 A fortunate selection of variables is, then, indeed 
 an essential condition of success, and constitutes al- 
 most the chief difficulty in all physical researches. 
 How much more important becomes this considera- 
 tion when we are treating, not of quantities which 
 serve to verify a particular law, but of those which 
 form the standard of a whole class of phenomena. 
 
 We now see the first step to be taken by electri- 
 cians. They must agree upon some common and con- 
 venient measure of electrical actions. Failing in this 
 agreement, they work each for himself, unable to .sys- 
 tematize their discoveries, and never arriving at a 
 mutual understanding ; there reigns among them a 
 confusion of tongues. 
 
ELECTRICITY. 131 
 
 Who will put an end to this ? Who will furnish a 
 basis of common agreement ? ' 
 
 Five years ago the British Association made lauda- 
 ble efforts in this direction. The British Association 
 is, as is well known, a private society, devoting itself, 
 in England, to the advancement of science, and whose 
 watchful attention is directed successively to all points 
 where there is urgent need of making investigations. 
 To aid the progress of the submarine telegraph, it ap- 
 pointed, in 1862, a commission, which examined the 
 whole question of the measure of electrical phenom- 
 ena, and proposed a solution strictly applicable, though 
 very complicated.* 
 
 * A previous commission had been instituted in 1861. Its spe- 
 cial object was to determine a scale of resistance by which to test 
 the value of submarine cables, manufactifred in English work- 
 shops, .with respect to their transmitting qualities. The labors 
 of the British Association have exerted no small influence on the 
 wonderful improvements whiph have been brought about in the 
 manufacture of cables in England, which have at last resulted in 
 establishing between Europe and America telegraphic communi- 
 cation. The commission of 1861 was succeeded, in 1862, by a 
 new commission, consisting of Messrs. Wheatstone, Thompson, 
 C. W. Siemens, and Charles Bright. This new commission has 
 not limited its work to the measurement of resistances ; it has 
 confronted the whole question of electric units, seekingto connect 
 them closely with the units employed in mechanics. Experi- 
 ments have been made at King's College to determine what de- 
 gree of precision is practically attainable through the application 
 
132 UNITY OF NATyRAL PHENOMENA. 
 
 In France this problem would not even seem to be 
 the order of the day. We have, it is true, an associa- 
 tion for the advancement of the physical science of 
 the globe ; but its members would seem to have 
 nothing left to desire, while they have the moon 
 pointed out to them every month at the obser- 
 vatory. 
 
 Meanwhile, be the question of electric units de- 
 cided on this side of the channel or the other, the 
 study of heat clearly indicates the character of the 
 solution which must eventually result. So long as 
 calorific effects were estimated merely by changes in 
 the thermometer, we remained on the outside of phe- 
 nomena, and knew nothing of their essential nature. 
 Temperature is only one of the rTeculiarities of heat. 
 I have a kilogramrne of water at one hundred degrees 
 Centigrade. If it evaporates freely in the air, it ab- 
 sorbs the enormous quantity of 536 heat units, and 
 the kilogramme of vapor which results is still at one 
 hundred degrees.* 
 
 of the theoretical views of the commission. The result of these 
 investigations is contained in a report drawn up by Mr. Fleeming 
 Jenken, and published by the commission in the form of an ap- 
 peal to the scientific world. 
 
 * Sometimes a very elegant experiment is made in the labora- 
 tory, showing that different bodies, while at the same tempera- 
 ture, contain very different quantities of heat. A cake of bees- 
 
ELECTRICITY. 133 
 
 Between the motions which take place in the inte- 
 rior of bodies and the variations they effect upon the 
 thermometric scale, there exist only indirect, and, so 
 to speak, accidental relations. The study of these re- 
 lations has never afforded more than vague and con- 
 fused information. Real progress began the day when 
 calorific phenomena were no longer referred to the de- 
 grees of the thermometer only, but to an intrinsic unit, 
 the heat unit ; that is to say, the entire quantity of 
 heat necessary to effect a certain definite result, and 
 one easy to appreciate. 
 
 Hitherto* the galvanometer has been almost exclu- 
 sively employed as the measure of electrical phe- 
 nomena. Now, we may remark in passing, that the 
 galvanometer is a far more imperfect instrument than 
 
 wax, about twelve millimetres in thickness, is suspended from a 
 support; next a vessel of boiling oil is taken, and balls of differ- 
 ent metals of the same size are plunged into it balls of iron, 
 copper, tin, lead, and bismuth, for example. These balls, hav- 
 ing all taken the same temperature, that of the boiling liquid, 
 are taken out of the oil, and placed all at once upon the cake of 
 wax. They sink into the wax, but with different degrees of 
 rapidity. The iron and the copper enter powerfully into the 
 melting mass; the tin more gently; the lead and bismuth re- 
 main behind. The iron ball passes through the wax, and falls 
 out first; the copper one follows it; the others remain in it, un- 
 able to pierce the cake, and stop there at different depths, ac- 
 cording to the laws of their calorific capacity. 
 
134 UNITY OF NATURAL PHENOMENA. 
 
 the thermometer even. The thermometer, at least, 
 indicates directly by its linear expansions that part of 
 the calorific matter it is required to exhibit. The gal- 
 vanometer, which also indicates but a part only of the 
 electrical effect, has the further disadvantage of exhib- 
 iting them only by the angular variation of a needle. 
 We are, then, obliged to compare angles, that is, to 
 estimate sines and tangents. Already excluded from 
 actual contact with the facts, the observer finds them 
 still further masked by trigonometrical functions. 
 
 There is, then, urgent need of penetrating to the 
 very core of phenomena. In all our succeeding inves- 
 tigations, we must take for our fundamental idea the 
 electric unit, that is, the amount of electricity required 
 to produce a fixed result. 
 
 What shall be the effect henceforward selected for 
 our type ? Here is a question admitting of discussion. 
 Suppose, just to fix our ideas, we select the voltaic 
 decomposition of a kilogramme of water. The electric 
 unit thus determined, we shall be compelled to express, 
 by means of this fundamental unit, the various electri- 
 cal phenomena that have hitherto been characterized 
 only by special circumstances, by the intensity of the 
 current, or by the amount of heat developed. Instead 
 of halting at partial effects, we shall approach the facts 
 as a whole. Out of the mass of incoherent observa- 
 
ELECTRICITY. 135 
 
 tions now presented by the science of electricity there 
 will arise a sort of natural selection ; isolated laws will 
 be gathered into groups, and their inner meaning 
 made manifest. 
 
 To choose the electric unit, this is the first step in 
 advance to be made by electricians ; the second is to 
 ascertain the mechanical equivalent of electricity ; to 
 find out how many kilogrammetres are equivalent to 
 an electric unit. 
 
 We now see, by a characteristic example, the utility 
 of a hypothesis capable of comprehending natural 
 phenomena as a whole, and of tracing them back to a 
 single principle. By it the natural philosopher may 
 be guided in the imperfectly known regions he ex- 
 plores ; by it instructed in the path he must pursue 
 through the labyrinths of particular facts. 
 
 Let us observe, however, that in order to take the 
 two steps we have mentioned, it is not necessary to 
 get a preliminary view of the nature itself of elec- 
 tricity. If we look into the history of heat, we shall 
 see that the notion of a heat unit was not at all pecu- 
 liar to those who regarded heat as a motion. It might 
 even be remarked that this unity has a suspicious 
 look, and that it savors a little of the doctrine of the 
 materiality of caloric. The equivalence of heat and 
 mechanical work has also been established outside 
 
136 UNITY OF NATURAL PHENOMENA. 
 
 of all theory. This notion of equivalence is a pru- 
 dent and eclectic one ; it involves no preconceived idea 
 as to the facts one is comparing ; they are equivalent, 
 nothing more. When one is sure that he is com- 
 paring together two motions, the words equivalent 
 and equivalence become, so to speak, inadequate, and 
 he has the right to resort to more energetic terms. 
 
 First, to decide upon the electric unit, and then to 
 determine its mechanical equivalent, such are the two 
 points to which the efforts of electricians must first be 
 directed, and the ones we have desired to bring to view. 
 Having given these general suggestions, it remains for 
 us to show what experience teaches us at present 
 with regard to the conditions which characterize the 
 electric motion. 
 
 II. 
 
 The Electric Current apparently a Transport of Ethereal 
 Matter. 
 
 THE preliminaries just laid down show clearly 
 enough that we are far from possessing a general 
 theory in regard to electrical phenomena. 
 
 We were not in want of experimental data. Ob- 
 servers have placed at our disposal a large numbe/ of 
 facts, too many, almost, since the special laws estab- 
 
ELECTRICITY. 137 
 
 lished by them are not referable to a few principal 
 groups ; they exhibit only orie phase of each phe- 
 nomenon, and have, for the most part, but an obscure 
 and commonplace significance. Nevertheless, from a 
 general view of these confused observations, we con- 
 clude that electric motion is a real transfer of mat- 
 ter ; the word current, employed in ordinary language, 
 would thus correspond to the real nature of the phe- 
 nomena. 
 
 A decisive consideration may be brought to the 
 support of this opinion. If the two poles of a bat-* 
 tery are connected by a conductor of variable dimen- 
 sions, the intensity of the current, as measured by 
 its effects on the galvanometer, is the same in every 
 part of this conductor ; wherever it becomes thinner, 
 there the current is more rapid, so that each segment 
 gives passage in the same period of time to the same 
 amount of electricity. This peculiarity is easily made 
 visible by its calorific or luminous effects. We know 
 that if a very fine wire is interposed in the passage of 
 a current it grows red, and becomes heated even so 
 far as to melt. We are also acquainted with the ex- 
 periments made with Geissler's tubes. These are 
 glass tubes, in which the air is rarefied, and which are 
 laid in the course of the current, in order that the elec- 
 tricity may traverse them in the form of a luminous 
 
138 UNITY OF NATURAL PHENOMENA. 
 
 spray. Now, if we take a Geissler's tube, varying in 
 size in its several sections, we may easily demonstrate 
 that the spray becomes the more luminous the nar- 
 rower the tube. In the fact that the motion increases 
 in proportion as the calibre of the tube diminishes, 
 we recognize a fundamental law of the flow of liquids ; 
 a law known since the time of Leonardo di Vinci. 
 This fact alone excludes the idea that electricity may 
 be the result of simple vibrations. It does not ap- 
 pear, in fact, in any of the vibratory motions we 
 *are acquainted with, whether longitudinal, like those 
 of sound, or transversal, like those of light. 
 
 When these latter motions encounter an obstacle 
 which contracts the medium in which they are dis- 
 played, they are reflected into the body of the medium ; 
 but they do not hurry forward into the open space 
 before them ; it is those fluids which are endowed 
 with a motion of transportation that are thus accel- 
 erated in narrow passages. When an iron rod is 
 heated, we do not observe the temperature to be any 
 higher in those parts where the rod is thin. The case 
 is otherwise when the elevation of temperature is 
 caused by electricity, since, as was just now stated, 
 very fine wires placed in the circuit of an ordinary 
 conductor may be heated to a red heat and melted . 
 
 We find, then, at the outset, by means of a funda- 
 
ELECTRICITY. 139 
 
 mental fact, that the .electric motion is similar to the 
 flowing of a fluid. This analogy may be traced 
 through all the particulars revealed by experiment. 
 
 The science of telegraphy, especially, has furnished 
 us numerous hints in this direction. A telegraphic 
 wire is like a tube which is to be filled ; the bat- 
 tery is like a reservoir which fills the tube more or 
 less easily, according to the degree in which it is 
 itself filled. Be the wire charged, or half charged, 
 when the end which communicates with the battery 
 is placed in the ground, a part of the charge flows 
 back. The case is similar with a liquid flowing from 
 a tube open at both ends. Nothing of the kind would 
 have taken place in the case of a vibratory motion ; 
 such a motion, when the cause producing it has ceased, 
 does not take a backward direction, but continues 
 precisely in that in which it began. 
 
 By such examples we are guided in considering the 
 duration of the propagation of a current, that is, the 
 time necessary for the current to attain a uniform 
 state throughout the whole extent of the wire ; and 
 here again we are reminded of the transport of a*fluid, 
 since the time increases nearly in proportion to the 
 square of the length of the wire. This time varies in- 
 versely as the diameter of the wire, and this fact alone 
 shows 'that we have not to do with a vibration ; a vi- 
 
I4O UNITY OF NATURAL PHENOMENA. 
 
 bratory motion, in fact, assumes its uniform condition 
 as rapidly in a large tube as in a small one. This may 
 be verified in the case of sound. 
 
 But what is this fluid, the transportation of which 
 constitutes the electric current ? Is it, perchance, 
 ponderable matter itself, reduced to a state of vapor, 
 or at least brought to a condition of tenuity, which 
 imparts to it the properties of fluids ? Certainly not. 
 For, in the first place, we have no reason to suppose 
 that the passage of a fluid through a wire augments 
 its weight ; besides, if the electric flux were a trans- 
 port of ponderable matter, if the matter itself of the 
 conductors were transported, it ought to be perceptible 
 when two unlike wires are joined to each other ; when 
 the current, after passing through a copper wire, for 
 example, passes into an iron one, the copper ought to 
 leave some traces of its passage through masses of the 
 iron, and vice versa. Observation has not disclosed 
 any fact of the kind, unless it be at the very point of 
 junction. Even here, we must admit, the transporta- 
 tion of matter is a mere accident, an accessory phe- 
 nemegon, a purely local circumstance, that may be neg- . 
 lected without hesitation. 
 
 Are not our conclusions, then, self-established ? 
 This fluid, which is carried along a conductor, is 
 nothing else than the imponderable matter we are 
 
ELECTRICITY. 14! 
 
 acquainted with under the name of ether. The elec- 
 tric motion of the ether is, moreover, in no sense a vi- 
 bratory one ; it is a veritable flux, an actual transport. 
 
 We cannot but be confirmed in these opinions if we 
 make a further hasty examination of some of the pecu- 
 liarities presented by these currents. 
 
 The electric spark has been thoroughly investigated. 
 It offers an interesting subject for study. Physicists 
 have always hoped to find here, under a striking form, 
 some direct information concerning the nature of elec- 
 tricity. They have especially studied the spark pro- 
 ceeding from static machines ; but their conclusions 
 might legitimately be extended to that produced by 
 currents. 
 
 It is necessary to state that the study of the spark 
 has long been productive of deceptive arguments, and 
 especially has it been of service to the theory of two 
 fluids. On beholding the spark, compact and brilliant 
 at both poles, but larger and dimmer at its centre, one 
 was sure that he saw the two different fluids in the 
 very act of combination. There was the positive fluid 
 proceeding from one pole in the form of a fan, and the 
 negative fluid escaping from the other in the form of a 
 star. To us the brilliancy of the two poles seemed to 
 proceed from the agitation produced in them by the 
 electric flow ; but the flow may likewise produce this 
 
142 UNITY OF NATURAL PHENOMENA. 
 
 effect when escaping from one side and entering the 
 other. 
 
 Nevertheless, to prove that a fluid passed out of 
 both sides at the same time, an experiment was made, 
 which f seemed decisive. The spark was made to 
 pierce several sheets of paper, and it was shown that 
 the edges of the aperture were turned, some towards 
 the negative pole, some towards the positive pole, 
 a fallacious result, and one from which no conclusion 
 can be drawn as to the direction of the electric cur- 
 rent at either pole. In many cases a body forcibly 
 punctured exhibits edges turned in a contrary direc- 
 tion to that of the pressure. It would look, therefore, 
 as if the penetrating body has, in the second stage of 
 the perforation, experienced a rebound. The symme- 
 try of the ridge raised on both faces of the sheets 
 pierced by the spark affords, therefore, no proof of the 
 passage of a double fluid. 
 
 On the contrary, the recent advancements in spec- 
 troscopy go to prove the unity of the motion. It has 
 been proved that the spectrum of the spark depends 
 upon the nature of the metal composing the positive 
 pole, since it remains unchanged when the nature of 
 the other pole is altered ; the metallic particles carried 
 along by the current show, then, that the transfer 
 takes place in a single direction. 
 
ELECTRICITY. 143 . 
 
 Another important fact is, that the spark is strati- 
 fied. It is seen in layers ranged one upon the other. 
 It would appear, in fact, as if the electric flow was not 
 a continuous one. We have here developed a phe- 
 nomenon similar to that which is produced when we 
 see smoke issuing from a chimney in successive puffs. 
 When a flowing body, meets an obstacle, it produces, 
 in the effort to overcome that obstacle, certain onward 
 movements which arrange themselves over each other. 
 May not also the stratification of the electric spark be 
 like the transfer of metallic particles a purely local 
 accident ? May it not indicate a state of things exist- 
 ing throughout the whole extent of 'the conductor ? 
 We might assert, then, that the transportation of the 
 ether is effected by successive waves ; but these waves, 
 which accompany a movement of transportation, must 
 not at all be confounded with vibratory waves, of 
 which light and sound are examples. 
 
 It will be seen that we make here an important 
 reservation. We have thus far admitted that the 
 ether is. really carried from one end to the other of 
 the conductor ; that each atom employed in the circuit 
 runs through its entire length. It is possible, on the 
 other hand, that each atom is permitted only a partial 
 range, and that the current is produced somehow by a 
 
 * 
 
 series of relays more or less near together. We leave 
 
144 UNITY OF NATURAL PHENOMENA. 
 
 the door wide open for such a supposition, which is in 
 no way incompatible with what we have above stated ; 
 but, for simplicity's sake, we shall continue to speak as 
 if we were treating of a fluid in motion, the particles 
 of which all pass through the entire circuit. 
 
 One other question presents itself in the study of 
 the spark, - a question of high importance, and one 
 which can be only partially answered. Is the spark 
 produced in a perfect vacuum ? In other words, can 
 the electric stream, even though it be itself nothing 
 but a motion of ether, exist outside of ponderable 
 matter ? The importance of this question is manifest, 
 and it finds no answer in the general phenomena of 
 nature. The sun sends us light ; we receive electricity 
 from it, not directly but mediately. The frequent ex- 
 periments made to discover whether the electric spark 
 can pass through an absolute void, have been subject 
 to much dispute. How shall we procure an absolute 
 void ? We try to empty a tube of all ponderable mat- 
 ter ; we fill it several times with carbonic acid, which 
 we expel by means of an air-pump, and finally we use 
 potash to absorb the remains of the acid ; but are 
 there not vapors escaping from the joints, from the 
 valves of the machine, and from the potash itself? 
 How get rid of this source of error ? Therefore, we 
 'repeat, nothing could be less satisfactory than the 
 
ELECTRICITY. 145 
 
 result of such an experiment. The attempts which 
 have been made, however, go to prove that the spark 
 does not pass through a vacuum, and from considera- 
 tions of quite another kind we are led to the same 
 conclusion. It would seem, then, that the electric 
 movement can only be produced in the midst of pon- 
 derable matter.* 
 
 Let us now direct our attention to those phenomena 
 in which currents have their rise, the two principal 
 ones being heat and chemical action. How shall we 
 conceive, in either case, of the beginning of a current ? 
 If two metallic bars, one of bismuth and one of anti- 
 mony, for instance, are soldered at one end, and we 
 heat the point of union, a current arises in the arc 
 connecting the two metals. Such is the principle of 
 the thermo-electric pile. Observe that we must have, 
 at the place where the heat is applied, different metals ; 
 a junction of different sections of the same conductor 
 would not suffice to produce a current ; we must have 
 
 * This statement seems to be confirmed by experiments made 
 with PlOcher's tubes, used in spectrum analysis. In a tube, re- 
 duced as nearly as possible to a vacuum, no spark is observed, 
 although the platinum wires on the ends of the tube are very 
 closely approximated. In a second tube, containing the slightest 
 possible quantity of hydrogen, the light of the passing spark is 
 clearly visible. (See Schellen's Spectrum Analysis, p. 26.) 
 Translator. 
 
 10 
 
146 UNITY OF NATURAL PHENOMENA. 
 
 molecules which are unlike ; and what does this sig- 
 nify ? Let us refer again to the hypothesis we have 
 constructed to explain how bodies pass from a gaseous 
 to a liquid and solid state. We were compelled to ad- 
 mit that every molecule carries along with it in its 
 rotation a sort of atmosphere of ether. When unlike 
 molecules are placed side by side, then there will be a 
 meeting of atmospheres of different, densities and ve- 
 locities ; and if their equilibrium becomes disturbed by 
 the application of heat, we may see how this circum- 
 stance would set free a certain number of ethereal 
 atoms. These atoms rush into the conductor as into 
 a channel, and there form the current. The more dis- 
 cordant are the atmospheres of the two metallic ele- 
 ments, the more intense will be the effect ; there will 
 be no effect when the atmospheres are all alike, that 
 is, when only one metal is employed. Chemical action 
 produces an analogous effect on a larger scale. When 
 two bodies combine, the molecular atmospheres are 
 powerfully disturbed ; a new distribution of the ether 
 is forcibly effected about the new molecules, and this 
 sudden change drives away more or less of the ethereal 
 atoms. Thus different batteries, the thermo-electric 
 
 pile as well as those based on a chemical combination, 
 
 % 
 
 exhibit at the very origin of the current the beginning 
 of a flow of ether. 
 
ELECTRICITY. 147 
 
 Beginning in the pile, this flow is continued in the 
 conductor, and if we regard 'the entire circuit thus 
 formed, we shall readily see that chemical action, 
 electricity, heat, mechanical work, are all produced 
 according to that law of mutual transformation to 
 which we are compelled to reduce all physical phe- 
 nomena. 
 
 The vis viva due to the action of the pile sets the 
 ether in motion ; this, circulating in the conductor, 
 develops in it heat, because it agitates in its passage 
 the ponderable molecules, and leaves them a part of 
 its vis viva. But instead of producing heat, it may 
 produce work of a different kind. 
 
 We shall find a ready example of this by placing in 
 the circuit a voltameter,* filled with water. The two 
 poles of the current, the two electrodes of platinum 
 being directed into the upper part of the liquid, the 
 water becomes warm, and soon boils ; then if the poles 
 be more deeply plunged into the vessel, the water be- 
 gins to resolve itself into its two elements, the tem- 
 perature of the liquid diminishes, and we observe very 
 soon the ordinary conditions of electrolytic decomposi- 
 
 * An instrument to measure the strength of an electric current, 
 consisting of a graduated tube, which receives and measures the 
 amount of gas generated by the current in a given time. 
 Translator. 
 
148 UNITY OF NATURAL PHENOMENA. 
 
 tion which are accompanied by a slight elevation of 
 temperature. 
 
 We see here an electrolytic and a calorific action 
 directly exchanging with each other. If the experi- 
 ment was so conducted as to yield precise measure- 
 ments, if we were able to free it from every source of 
 error, we should discover just what weight of water 
 may be heated one degree by the quantity of elec- 
 tricity which will decompose a given weight of that 
 water ; in other words, we should find the relation be- 
 tween the electric unit and the heat unit, and electric 
 currents would thus be reduced to the common meas- 
 ure of mechanical work, to the kilogrammetre.* 
 
 In the example we have given, the current produces 
 a chemical work ; it might also produce a mechanical 
 
 * Father Secchi has made some experiments; from which we 
 might conclude that the quantity of electricity which decomposes 
 0.106 gramme of water, would raise, by one degree, the tem- 
 perature of thirty-eight grammes of the same liquid. Taking for 
 our electric unit, as we have above suggested, the quantity of 
 electricity capable of decomposing a kilogramme of water, it will 
 be found that .an electric unit is equal to three hundred and sixty 
 heat units, or one hundred and fifty-three thousand kilogramme- 
 tres. If, for the sake of smaller numbers, we compare the elec- 
 tric unit with the gramme, it will then be equivalent to 0.36 heat 
 units, or one -hundred and fifty-three kilogrammetres. We give 
 this result, but we are not willing to certify that the experiment 
 from which it is derived can be regarded as covering all the con- 
 ditions of the problem. 
 
ELECTRICITY. 149 
 
 work, raise a weight, or 1,urn a shaft, M. Favre, in 
 his series of well-known experiments, has shown that 
 the heat developed in a current decreases precisely in 
 proportion to the work produced. The vis viva of the 
 electric current is in part consumed by the lifting of 
 the weight or the turning of the shaft, and the calorific 
 disturbance of the circuit is diminished in proportion. 
 We see electricity converted into work, instead of being 
 transformed into heat ; if this conversion could be com- 
 plete, if we were able to eliminate entirely from the 
 experiment the manifestation of heat, we might at 
 length determine exactly the ratio of equivalence be- 
 tween electricity and mechanical work ; we might ob- 
 serve directly the relation between the electric unit 
 and the kilogram metre. 
 
 But this is a purely theoretical conception ; if, as is 
 probable, the electric flow takes place only through 
 ponderable matter, it necessarily sets in motion its 
 molecules ; that is to say, there is no electricity with- 
 out heat. We must mention in this connection, that 
 the conductibility of different substances follows al- 
 most the same order, both for electricity and for heat. 
 If, for example, we regard the metals in this twofold 
 relation, not only do they arrange themselves in both 
 cases in the same order (silver, copper, gold, tin, iron, 
 lead, platinum, bismuth), but the same series of fig- 
 
I5O UNITY OF NATURAL PHENOMENA. 
 
 ures might represent exactly their double conductibili- 
 ty. The close connection existing between calorific 
 and electric phenomena, hardly permits us to hope that 
 the mechanical action of electricity may be isolated in 
 practice, or reached by direct observation. 
 
 III. 
 
 Electricity and Light ; their Mutual Relation. 
 
 IN the degree that we have examined the peculiari- 
 ties which mark the propagation of the currents, the 
 origin of the electro-motor forces, and the distribu- 
 tion of work in the conductors, we have become con- 
 firmed in the idea that electricity consists in a trans- 
 port of the ethereal fluid, of the same fluid that pro- 
 duces light. 
 
 This will, indeed, be to many minds a resemblance 
 hitherto unexpected. To compare light and electrici- 
 ty is quite a new idea, and yet we have just been 
 regarding both as different motions of the same fluid. 
 Between these two modes of motion a new bond 
 appears. 
 
 If we look at the generality of natural objects, we 
 shall notice that those which are transparent are 
 usually non-conductors ; permeable to light, they re- 
 
ELECTRICITY. 151 
 
 fuse passage to electricity., On the other hand, 
 conductors are generally opaque ; witness all the 
 metals. 
 
 The objection may, perhaps, be urged that water is 
 both transparent and a conductor ; gutta-percha, opaque 
 and a non-conductor ; but let us consider here extreme 
 cases only, neglecting those of an intermediate char- 
 acter. We see, then, two clearly marked general 
 groups, transparent bodies and conductors. These 
 are ill-chosen designations, since they convey no idea 
 of contrast ; but beneath the terms let us look for 
 the facts. Among bodies of the first class, the ether 
 moves transversely only ; on the other hand, it can 
 take a longitudinal motion only in bodies of the sec- 
 ond class. Difference of molecular aggregation cre- 
 ates, therefore, a difference of mobility with regard to 
 ether. This is all we can say ; but we may assert 
 that the two classes of bodies enclose not two differ- 
 ent fluids, but one and the same ether, susceptible of 
 a variety of motion. 
 
 To admit the existence of a luminous fluid belonging 
 to transparent bodies, and of an electric fluid peculiar 
 to conducting bodies, would lead to very strange re- 
 sults. 
 
 When lead combines with silica to make glass, we 
 
 * 
 
 should have to suppose that the electric fluid is driven 
 
152 UNITY OF NATURAL PHENOMENA. 
 
 away and replaced by the luminous fluid! The dia- 
 mond in becoming charcoal, is no longer transparent 
 and non-conductor ; it becomes opaque and a con- 
 ductor. Here, then, would be another change of 
 fluids. Such a thing cannot be conceived as possible. 
 We can, on the contrary, very readily imagine how a 
 single ethereal fluid, following the molecular arrange- 
 ment of bodies, may sometimes find its motion ob- 
 structed in one direction, sometimes in the other. 
 
 We will here add an argument furnished by the 
 velocities with which light and electricity are propa- 
 gated. The velocity of light is about one hundred and 
 eighty-five thousand miles a second. That of electrici- 
 ty has been determined with far less exactness; since it 
 depends upon the nature of the conductors, and a 
 variety of circumstances which observers have not 
 succeeded in eliminating. But taking a mean between 
 the widely different results yielded by experiments, we 
 shall not come far from the same rate of one hundred 
 and eighty-five* thousand miles a second. 
 
 In this resemblance may be seen a confirmation of 
 our hypothesis. It should not in the least surprise us 
 that the same number represents two velocities cor- 
 responding, in our view, with two motions of the same 
 fluid in the same direction ; the velocity of light is, in 
 
ELECTRICITY. 153 
 
 fact, that of the longitudinal impulse from which the 
 transversal motions result. 
 
 Such are but very general views of this subject, and 
 it can hardly be said that we see clearly the connec- 
 tion between the phenomena of light and of electricity. 
 Scarcely can we conceive of the conditions capable 
 of effecting this twofold faculty of motion in the 
 ether. 
 
 We are acquainted with the ingenious explanations 
 Father Secchi had recourse to in order to show how 
 the impulse sent along a luminous ray betrays itself 
 in transverse vibrations. Other hypotheses have been 
 proposed to show how these transversal vibrations can 
 be extinguished in conducting bodies to the advan- 
 tage of the longitudinal ones. But let us leave these 
 problems in their obscurity, it is quite fitting that 
 we should conclude our hasty view of the phenomena 
 of electricity with an avowal of the uncertainty of our 
 position. These phenomena still offer much that is 
 obscure, and it is only in keeping with the actual state 
 .of our knowledge that we dismiss them, leaving im- 
 portant questions still pending. 
 
154 UNITY OF NATURAL PHENOMENA. 
 
 CHAPTER V. 
 THE ATTRACTIVE FORCES. 
 
 I. 
 
 Points of Resemblance and of Dissimilarity presented 
 by Gravity ', Cohesion, and Chemical Affinity. 
 
 THE charcoal points of an electric lamp grow hot, 
 and become luminous, when a current passes through 
 them. A furnace fire gives out heat and work. A ray 
 of light falling upon a sensitive plate determines an 
 electro-motor action, which becomes motion in the 
 needle of a galvanometer, heat in the thermometric 
 index. We might multiply to infinity similar examples 
 in which light, heat, and electricity appear as converti- 
 ble phenomena, or reducible to the idea of mechanical 
 work. Work produces them, and they produce work. 
 They originate in motion, and they are resolvable into 
 motion. The public mind is accustomed so, at 
 least, it seems to us to regard in this manner the 
 effects of light, heat, and electricity ; but the point is 
 not so well settled in regard to the attractive forces, 
 
THE ATTRACTIVE FORCES. 155 
 
 gravity, cohesion, and affinity, which appear to reside 
 in the recesses of matter. Until now they preserve a 
 more mysterious aspect. It remains for us to see if 
 we shall be able to dissipate, in part, the obscurity 
 which surrounds them by applying the principle which 
 now enables us to throw light on all natural phe- 
 nomena. 
 
 In the progress of our work we have briefly indi- 
 cated, as opportunity offered, the considerations which 
 enable us to reduce these forces to the effects of mo- 
 tion. We have especially, then, to classify and de- 
 velop here the hypotheses previously broached. 
 
 In the first place, the attractive forces should not be 
 considered as inherent in matter. 
 
 When Newton proclaimed the law of universal 
 gravitation, he took good care to make his reservations 
 in regard to it. After having described the planetary 
 motions in his book, The Mathematical Principles of 
 Natural Philosophy, he adds, "I have thus far ex- 
 plained celestial phenomena and those of the sea by 
 the force of gravity ; but I have nowhere assigned the 
 cause of this gravitation. This force comes from some 
 cause which penetrates to the very centre of the sun 
 and planets, without losing any of its activity ; it acts 
 according to the quantity of matter, and its action ex- 
 tends in all directions to immense distances, always 
 
156 UNITY OF NATURAL PHENOMENA. 
 
 decreasing in the inverse ratio of the square of the 
 distances. I have not yet been able to deduce from the 
 phenomena the reason of these properties of gravity, and 
 I do not conceive of any hypothesis. ... It is enough 
 that gravity exists ; that it acts according to laws that 
 have just been exposed ; and that it can explain all the 
 motions of the heavenly bodies, and those of the sea." 
 Again, in the same book he says, " I mean by the 
 word Attraction, the endeavor which bodies make to 
 approach each other ; whether this endeavor results 
 from the action of bodies which mutually seek each 
 other, or which influence each other by means of 
 emanations, or whether it result from the action of 
 the ether, of the air, or any other medium, corporeal 
 or incorporeal, which urge towards each other, in 
 some way or other, all the bodies which float in 
 them." 
 
 Thus Newton left this question undecided ; but after 
 him it became gradually a custom to consider gravity 
 a kind of quality inherent in bodies. Many people 
 admit to-day, as a primary axiom, that matter is inert, 
 and, for the second, that it attracts according to such 
 and such laws. We have already said that it is neces- 
 sary to choose between these two contradictory ideas. 
 If the molecules are drawn towards each other by 
 virtue of a cause which is within themselves, how can 
 
THE ATTRACTIVE FORCES. I $? 
 
 you say that they are inert ? , They are active, on the 
 contrary, and all the structure which you have raised 
 upon the idea of inertia crumbles to its foundation. 
 
 What will be the case, then, if we pass from gravity 
 to chemical affinity! If the molecules exercise a 
 choice by virtue of an inherent principle, they have, 
 then, a primary, active principle of their own ; they 
 have wills, caprices ! Chemistry becomes the study 
 of the molecular passions. We shall find in it sym- 
 pathies and antipathies ; base instincts and noble sen- 
 timents ; lawful affections and culpable desires ; happy 
 marriages and ill-assorted unions ; half-concealed quar- 
 rels and open contests. Such are the idyls and the 
 dramas which chemistry presents to us if we place in 
 the molecules a repulsive and an attractive principle, 
 as formerly a spirit of good and a spirit of evil were 
 made to dwell in human souls. 
 
 It is a pure geometrical fiction to suppose that two 
 molecules act upon each other at a distance. In 
 reality we are only acquainted with actions which take 
 place by contact, by the communication of motion. 
 Between the molecules are the ethereal atoms ; shocks 
 are transmitted from one to the other ; the matter re- 
 mains inert, and is only excited to motion on the side 
 where it is struck. The repellent forces have already 
 disappeared before the idea of calorific motion ; the 
 
158 UNITY OF NATURAL PHENOMENA. 
 
 attractive forces must likewise be reduced to the 
 effects of impulse. 
 
 When we compare the three forces, which we find 
 grouped in the same family, gravity, cohesion, and 
 chemical affinity, we are at once struck with the 
 disproportion between them. 
 
 How much more powerful is cohesion than gravity ; 
 an iron wire will not break under its own weight until 
 it reach a length of five thousand metres. Enormous 
 masses of metal are needed to overcome by their 
 weight the cohesion which 'exists in a single section 
 of the wire. 
 
 What is more extraordinary still is, that when once 
 the adhesion is overcome, and the wire broken, the 
 closest contact of the disjointed parts reproduces no 
 trace of the primitive cohesion. Thus cohesion, in- 
 comparably more intense than weight, is sensible only 
 at extremely small distances ; weight, more feeble, on 
 the contrary, continues its action at infinite distances. 
 
 If any one is desirous of getting a comparative idea 
 of these different forces, he may have recourse to the 
 following hints. They are due to a learned physicist, 
 M. Dupr6, who for long years has devoted himself to 
 the study of molecular activities. 
 
 M. Duprd deduced from his experiments and calcu- 
 lations the force necessary to overcome the mutual 
 
THE ATTRACTIVE FORCES. 1 59 
 
 affinity of the elements of water ; to separate by force 
 oxygen and hydrogen over the surface of a square mil- 
 limetre. He found this force would be about sixteen 
 hundred and seventy-three kilogrammes. 
 
 To overcome the molecular adhesion of water, to 
 tear away one layer from its neighbor, a force would 
 be required of seventy kilogrammes to the square mil- 
 limetre.* 
 
 Finally, it is known that over this same surface 
 gravity exerts an action of only 10.33 grammes. 
 
 Comparing the three numbers which represent the 
 power, respectively, of affinity, cohesion, and gravity, 
 we can appreciate the enormous difference in their 
 values. 
 
 II. 
 
 Gravity may be considered as an Effect of the Motions 
 of Ether and of Ponderable Matter. 
 
 LET us attack, without further delay, the theoretical 
 considerations which may enlighten us as to the na- 
 ture of the attractive forces, and let us begin with 
 gravity. 
 
 Let us imagine the ether uniformly diffused through- 
 out space. Its atoms, endowed with motions of pro- 
 
 * That is, nearly fifty tons to the square inch ! Translator 
 
l6o UNITY OF NATURAL PHENOMENA. 
 
 gression and rotation, strike each other in the manner 
 already mentioned. 
 
 Let us now suppose that, at some point within, 
 there is a special and permanent disturbing cause, 
 as, for example, a molecule having weight, and itself 
 endowed with a vibratory motion. The shock goes 
 on, extending throughout the ethereal mass, and by 
 reason of the nature of this medium is propagated in 
 all directions. The atoms nearest to the heavy mole- 
 cule will receive violent shocks ; they will be power- 
 fully urged, and their ranks will grow thin in the 
 neighborhood of the centre of disturbance, and the 
 layer contiguous to the molecule will become less 
 dense than the rest of the medium. The motor action 
 continuing, this same effect becomes propagated from 
 layer to layer throughout space. As a final result, 
 the ether becomes arranged around the centre of dis- 
 turbance in concentric layers, the first of which, and 
 nearest to the molecule, will be least dense, and they 
 will go on indefinitely increasing in density. This 
 condition of things might be easily represented and the 
 figure traced ; the molecule at the centre, around it 
 spheres of atoms, wide apart at first, then nearer and 
 nearer to each other. Let us remark in passing, that 
 the difference in density of contiguous layers, like all 
 effects which are propagated by concentric spheres, is 
 
THE ATTRACTIVE FORCES. l6l 
 
 inversely proportional to the' surface of these spheres, 
 that is, to the square of their radii. 
 
 This established, suppose a second molecule to be 
 situated at any point of this system. It will encoun- 
 ter on the side towards the first molecular layers of 
 ether less dense than upon the opposite side ; pressed 
 upon by the ether in all directions, it will receive, not- 
 withstanding, fewer shocks on the side towards the 
 first molecule, and it will consequently tend to move 
 towards it. 
 
 Such would seem to be the cause of gravity. 
 
 The second molecule is pushed towards the first, 
 because it encounters ethereal layers of different den- 
 sities, and the energy of this action, for the reason we 
 have just now pointed out, is inversely proportional to 
 the square of the distance between the two molecules. 
 In this statement we recognize the law by which 
 gravity acts. 
 
 What we have just said concerning isolated mole- 
 cules, is also applicable to those grouped in a way to 
 form a body. Such a group will effect in the ether 
 that variation of density we have described ; it will 
 effect it with so much the more force as the molecules 
 are more numerous, or the mass of the body greater. 
 The stars, in fact, are only huge bodies, impelled by the 
 same cause that makes heavy substances fall to the 
 II 
 
1 62 UNITY OF NATURAL PHENOMENA. 
 
 surface of the earth. With the former, as with the lat- 
 ter, the attraction is only that tendency to approach, 
 
 the origin of which we just now referred to external 
 
 t 
 impulses. 
 
 Of course the brief and general outlines just given 
 do not constitute an exact demonstration. To throw 
 light upon a question of such high importance, it 
 would be necessary to pursue the phenomena into their 
 minutiae ; to exhibit in detail the^various rebounds 
 made by the ether, which result in its arrangement 
 about the molecules in layers of different density. It 
 would be necessary to anticipate the doubts generated 
 by such an exhibit, and to reply to the principal objec- 
 tions that might be offered. 
 
 For example, it might be asked why the effect we 
 describe is peculiar to the material molecules ; why it 
 is not produced, at least here and there, around the 
 ethereal atoms. The answer to this is easy. In the 
 midst of the ethereal mass, in the absence of any 
 molecule, everything is symmetrical with regard 
 to each atom ; the effect begins, if you please, around 
 each of the atoms ; it is as if it did not begin around 
 any of them, and the medium remains uniformly 
 dense ; to break its uniformity, there is needed a 
 centre of disturbance. 
 
 It will be asked, again, if it is not a very arbitrary 
 
THE ATTRACTIVE FORCES. 163 
 
 supposition to give to atoms^and especially to mole- 
 cules, the round form which seems necessary at the 
 start, in order to explain the regularity of the shocks 
 and the symmetry of their effects. Here again the 
 answer is easy. The theory of rotation teaches, in 
 fact, that the shocks do not depend upon the exterior 
 form of the bodies, and that we may always conceive 
 of a solid, of any form whatever, as being replaced by 
 an ellipsoidal globe. The round form is not then 
 really necessary, either to the molecules or even to the 
 atoms. 
 
 There would be many other objections to overcome ; 
 but it is easy to see that we could not here analyze all 
 the circumstances of the phenomenon. Our end is 
 attained if the general principle of the explanation 
 just given has been grasped, and if it is seen how the 
 motion of the ether may produce terrestrial as well as 
 sidereal attraction. 
 
 There remains a point, however, upon which we 
 cannot help saying a few words. 
 
 It may be observed that modern astronomy is con- 
 structed independently of the idea of the ether. It is 
 the physicists who, first through their studies upon 
 light, then through the inductions drawn from them, 
 have imposed upon science the idea of this universal 
 
164 UNITY OF NATURAL PHENOMENA. 
 
 fluid.* It may, accordingly, be asked if this idea will 
 not be found in disagreement with the astronomical 
 laws that have been established without it. Those 
 who are averse to admitting the existence of the ether, 
 do not fail to object that the progress of the stars 
 must be retarded by this fluid ; that the planets, by 
 reason of the resistance they encounter, must con- 
 stantly be approaching the sun, and that notwith- 
 standing astronomers find no symptom of such an 
 effect 
 
 The retardation exists, perhaps, without it being 
 possible to prove it, reply the partisans of the ether. 
 
 * " Whatever the eye perceives in the ether, the ear perceives 
 in the air; whatever the ether presents to our organs by means 
 of colors, the air presents to us by means of modulations and 
 sounds. Thus Nature is always the same, always similar to her- 
 self, both in light and in sound, in the eye and in the ear; the 
 only difference is, that in one she is quicker and more subtle, and 
 in the other slower and more gross, exhibiting herself to our 
 various senses by means of her various degrees and momenta, 
 and being as perceptible to sense in one medium as she is in an- 
 other. How admirable are the varied and sportive movements of 
 nature! How charming and delightful does she render herself 
 solely by her varieties in the motions of her elements, being as 
 beautiful in the ether by the play of her colors as she is harmo- 
 nious in the air by the modulations of her sounds ! What grati- 
 fications does she afford to us in the diversified operations of her 
 living machinery! " Sivedenborg, Principia, vol. ii. p. 309. 
 
 For a full and interesting account of the.luminiferous ether, 
 the reader is referred to the above-mentioned work. 
 
THE ATTRACTIVE FORCES. 1 6$ 
 
 If we enter the domain of facets, we are convinced that 
 this retardation can only be very feeble, on account of 
 the tenuity of the fluid which produces it. Calcula- 
 tions have been established, according to which the 
 resistance of the ether would shorten, by three meters 
 a year, the distance of the earth from the sun ; the 
 duration of the year would .thus be shortened one 
 second in six thousand years. The state of our as- 
 tronomical observations does not allow the singling 
 out of such a consequence from the midst of the per- 
 turbations of the terrestrial orbit already known. 
 
 Failing of any decisive facts in the planetary mo- 
 tions, the controversy is thrown back upon the com- 
 ets. If the ethereal resistance is insensible for the 
 planets by reason of their great density, it must be 
 appreciable for the comets, which have, so to speak, no 
 mass, and which have been termed visible nothings* 
 
 * It is not very long since the extreme tenuity of cometary 
 matter has been established. Formerly the shock of these bodies 
 was always considered dangerous for the planets. It is to a shock 
 of this character that Buffon attributed the origin of our plane- 
 tary system ; a comet precipitating itself into the sun, had de- 
 tached fragments of matter from it, and hurled them into space. 
 Again, it is to shocks of this kind that various geologists attrib- 
 ute the terrestrial cataclysms; comets, coming in contact with 
 the earth, would have displaced the axis of rotation and deter- 
 mined the great deluges. Such opinions no longer exist. Com- 
 ets are regarded to-day as quite inoffensive heavenly bodies, in- 
 
1 66 UNITY OF NATURAL PHENOMENA. 
 
 Here a consideration comes in to obscure the problem. 
 The extreme lightness of the comets must render 
 them sensible to the resistance of a universal medium 
 undoubtedly ; but it exposes them also to perturba- 
 tions of other sorts. They are powerfully deviated 
 from their course when they pass in the neighborhood 
 of the planetary bodies. When Lexell's comet, in 
 1770, passed through the satellites of Jupiter, the time 
 of its revolution became abruptly shortened from fifty 
 years to five and a half years. How discern the in- 
 fluence of the ether in the midst of pertubations of 
 this kind ? Encke's comet, whose periodicity has 
 been known since 1818, has a revolution of very 
 short duration, about three and a quarter years ; its 
 orbit lies entirely within that of Jupiter. In compar- 
 ing its successive appearances since 1818, there has 
 been observed a gradual diminution in the time of its 
 revolution. It has been proved, moreover, that this 
 effect does not proceed from the perturbing influence 
 
 capable of disturbing the peace of the world. They have been 
 seen to pass close by planets without causing any disorder in 
 them. Twice has LexelFs comet been seen to rush through the 
 satellites of Jupiter, without producing any derangement in them. 
 According to the recent calculations of M. Faye, the nucleus of 
 comets, which is the most compact portion, is scarcely nine times 
 more dense than the air which remains in our pneumatic ma- 
 chines after we have made the vacuum as complete as possible; 
 as to the density of the tail, it would be ten billion times less. 
 
THE ATTRACTIVE FORCES. l6/ 
 
 of the planets. Certain astronomers have concluded 
 from this that it must be attributed to the resistance 
 of a medium, and have there seen the first astronomi- 
 cal demonstration of the existence of the ether ; but 
 this conclusion, drawn from a solitary example, in the 
 midst of, the uncertainty still hanging over the greater 
 part of the particulars of cometary motion, cannot be 
 regarded as very binding. 
 
 Thus astronomical observations furnish no charac- 
 teristic fact upon the subject of the resistance of a 
 medium, and no conclusion in regard to it is to be 
 drawn either from the course of the planets or that of 
 the comets. But we have now to ask ourselves if the 
 explanation just given upon the subject of the origin 
 of attraction does not illuminate the problem with 
 an entirely new light ? 
 
 Mathematical analysis refers to two forces as the 
 causes which produce the curvilinear motion of the 
 stars. One, the initial force of impulsion or ac- 
 quired velocity, tends to direct them in a straight line, 
 while gravity incessantly deviates them from this 
 course. This is that dynamic equilibrium, established 
 by astronomers independently of any notion of the 
 ether, which seemed to be compromised when the 
 physicists admitted the existence of a universal medi- 
 
1 68 UNITY OF NATURAL PHENOMENA. 
 
 um ; the ether must needs derange this balancing of 
 two forces instituted without its aid. 
 
 If now we recognize the ether as the origin of at 
 least one of the two forces, the question changes its 
 aspect. It may no longer be said that it has remained 
 foreign to the establishment of the equilibrium of the 
 heavenly bodies ; on the contrary, we find that we 
 have unwittingly forced it to take part in this equi- 
 librium. Henceforward . let us no longer speak of a 
 new resistance introduced by the ether ! Its mode of 
 resisting the celestial motions is precisely to deter- 
 mine the attraction, and so influence the course of 
 the stars. We say that the ether produces gravity ; 
 that it urges the heavenly bodies in a certain direc- 
 tion ; by so doing we have accounted for all the ac- 
 tions which it exercises, for the shocks it gives upon 
 all sides. It would be a double task to introduce a 
 second time into our calculations, under the form of 
 resistance to motion, the shocks which the stars re- 
 ceive from the direction in which they move. 
 
 If this is so, if it is true to say that the ether can- 
 not be considered at the same time as a cause of side- 
 real motion, and an obstacle to this motion, we need 
 no longer be surprised that astronomy finds in no part 
 of the heavens the mark of a resisting medium. 
 
'THE ATTRACTIVE FORCES. 169 
 
 III. 
 
 Historical Notions regarding the Idea of Universal 
 Attraction. 
 
 IT is possible, then, to bring within the compass of 
 our hypothesis the cause which produces the gravity of 
 bodies ; but this, we cannot disguise it, is one of the 
 most difficult points we have to treat. Such is the 
 power of habit over our minds, that the origin of attrac- 
 tion would seem to us unattainable. To connect this 
 conception with a more general idea seems a chimeri- 
 cal undertaking. In order to support the demon- 
 stration we have attempted in regard to this, it will 
 not be without use to give a slight sketch of the way 
 in which this grand idea of universal attraction had 
 its birth, and how it has been developed.' By indica- 
 ting the role which it has played in the history of our 
 sciences, we shall better mark the place which it 
 should hold in the physical science of our day. In 
 beholding how the human mind has attained to a law 
 so high, it will seem to us possible for it to go higher 
 yet, and that gravity, in order to have explained so 
 many things, cannot itself be inexplicable. 
 
 Modern astronomy begins with the book of the 
 Revolutions of the Heavenly Bodies, which Coper- 
 
I7O UNITY OF NATURAL PHENOMENA. 
 
 nicus published in 1543. Copernicus, overturning the 
 doctrine of Ptolemy, placed the sun in the centre of 
 the universe. Around this body he made revolve the 
 six planets then known, Mercury, Venus, the Earth, 
 Mars, Jupiter, and Saturn, and he endowed them also 
 with a motion of rotation upon their axes. 
 
 Although dedicated to Pope Paul III., the book of 
 the Revolutions of the Heavenly Bodies was con- 
 demned, as contrary to the text of the Scriptures. 
 
 Whether he desired to escape the censures of the 
 Roman court, or whether he had the ambition to at- 
 tach his name to a system which was peculiar to him- 
 self, Tycho-Brahe adopted an eclectic hypothesis. He 
 deprived the earth of its double motion, and made the 
 moon and the sun revolve around it, conformably to 
 the doctrine of Ptolemy ; but he admitted at the same 
 time the revolution of Mercury, Venus, Mars, Jupiter, 
 and Saturn around the sun. In spite of this whimsi- 
 cal theory, Tycho-Brahe is one of the founders of the 
 science of the heavens. Assisted by his disciples and 
 numerous colaborers in the little astronomical city 
 which he had founded, he searched the heavens in 
 all directions, and accumulated upon the subject of 
 the planetary motions a prodigious quantity of ob- 
 servations, which served as a basis for the labors of 
 Kepler. 
 
THE ATTRACTIVE FORCES. I/I 
 
 The three great laws to which Kepler has given his 
 name are well known. 
 
 Copernicus and Tycho-Brahe had preserved the 
 faith of the ancients, who regarded the course of the 
 planets as circular. It was upon this opinion that the 
 attention of Kepler was first brought to bear. Com- 
 paring the observations of Tycho upon the motions of 
 the planet Mars with those which he had himself 
 made, he convinced himself that the orbit of this star 
 was not circular ; after having vainly tried several 
 hypotheses, he finally discovered that he could satis- 
 fy the result of his calculations by supposing that 
 the orbit of Mars was an ellipse, one focus of which 
 was occupied by the sun. At the same time he found 
 that the areas described around the focus by the 
 radius vector are equal in equal times. Such are 
 the two first laws pointed out by Kepler. After hav- 
 ing verified them upon several planets, he published 
 them in 1609, in a memoir entitled, De motibus stellcz 
 Martis. 
 
 The third law is, that the squares of the times of 
 the planetary revolutions are proportional to the 
 cubes of the long axes of the orbits. It is this which 
 cost the highest efforts of Kepler's persevering genius. 
 The manner in which he announces this in his trea- 
 tise, Harmonicas Mundi, partakes of the enthusiasm 
 
172 UNITY OF NATURAL PHENOMENA. 
 
 which such a discovery caused him. " After having 
 found," he says, " the true dimensions of the orbits 
 through the observations of Brahe, by a long-contin- 
 ued and laborious effort, I have at length discovered 
 the proportion of the periodic times to the extent of 
 these orbits. And if you wish to know the exact date 
 (of this discovery), it was the 8th of March, this very 
 year, 1618, that, first conceived in my mind, then un- 
 skilfully attempted in figures, hence rejected as false, 
 afterwards reproduced the i$th of May, with a new 
 energy, it surmounted the darkness of my intelligence, 
 and so fully confirmed was I in it by my labor of sev- 
 enteen years upon the observations of Brahe, and from 
 my own researches, that I first believed that I was 
 dreaming. . . . But there is no longer any doubt ; 
 it is a very sure and very exact proposition, that the 
 ratio between the periodic times of two planets is 
 precisely sesquialter to the ratio of the mean dis- 
 tance."* 
 
 Thus Kepler had determined, in three truly great 
 laws, the orbits of the planets and the conditions of 
 their motion. He was so near the principle from 
 which these laws are derived, that it may be asked if 
 
 * Half the long axis of a planetary orbit is often called the 
 mean distance. It is, in fact, the mean between the greatest and 
 least distance of the planet from the sun. 
 
THE ATTRACTIVE FORCES. 1 73 
 
 he did not foresee it. End6wed with an ardent im- 
 agination, he naturally sought the cause of these mo- 
 tions, the nature of which he had discovered ; but in 
 this relation his works show us hardly more than the 
 exuberance of ancient astrological fancies. The old 
 Pythagorean theories, the mysterious properties of 
 numbers, here play a singular part ; and one is aston- 
 ished at the odd dreams which are mingled with the 
 most serious calculations.* 
 
 He had, nevertheless, his theory upon solar attrac- 
 tion. He gave to the sun a movement of rotation 
 upon an axis perpendicular to the ecliptic, thus fore- 
 seeing a- truth which experience was only to prove 
 somewhat later ; immaterial forces emanating from 
 this luminary in the plane of its equator, endowed 
 with an activity decreasing in proportion to the dis- 
 tances, caused each planet to participate in this circular 
 motion. The planet, carried along by this transcen- 
 dent effluence, followed the rotation of the sun, and at 
 the same time, by a sort of instinct or magnetism, it 
 
 * Yet it was this mystical part of Kepler' s opinions, this be- 
 lief in the mysterious properties of numbers, that led to a con- 
 viction, on his part, of a physical connection between the differ- 
 ent parts of the universe, and finally to the discovery of those 
 numerical and geometrical laws which govern them. Trans- 
 lator, 
 
UNITY OF NATURAL PHENOMENA. 
 
 alternately approached and receded from the central 
 luminary, sometimes rising above the solar equator, 
 and sometimes sinking below it. 
 
 At the same time that Kepler was determining the 
 constitution of planetary motion, Galileo discovered 
 the law of the acceleration of bodies which fall freely 
 to the ground, or which glide over inclined planes ; he 
 established the general properties of a uniformly ac- 
 celerated motion. 
 
 The laws of gravity at the surface of the earth con- 
 stitute the fundamental principles of mechanics. Ere 
 long Huyghens perfected the theory of the pendulum, 
 and gave, through his Theory of Central Forces in the 
 Circle, brilliant suggestions concerning centrifugal 
 force. 
 
 Such are the principal elements from which Newton 
 derived the grand discovery of universal attraction. 
 The methods of calculation had also just been en- 
 riched by some remarkable inventions. Descartes had 
 founded the analytical geometry, and Fermat had 
 just laid the principles of the infinitesimal calculus. 
 Thus the labors of a half century, fruitful in great 
 geometricians and great astronomers, concurred in 
 bringing together the materials which Newton was 
 able to employ. 
 
 Tradition relates that Newton, while in retirement 
 
THE ATTRACTIVE FORCES. 1/5 
 
 in the country, during the yeaV 1666, saw an apple fall 
 from a tree. Thereupon directing his thoughts to the 
 system of the universe, he conceived the idea that the 
 force which attracted bodies towards the surface of the 
 earth was the one which made the moon turn around 
 the earth, and the planets around the sun. 
 
 Kepler's laws furnished him with admirable data, 
 from which he drew the consequences resulting from 
 their analysis. From the law of the proportion be- 
 tween the areas and the times, he concluded that every 
 planet is submitted to an attraction constantly directed 
 towards the sun. From the elliptical motion, he con- 
 cluded that for the same planet the tendency towards 
 the sun varies from one point to another of the orbit 
 in the inverse ratio of the squares of the distances. 
 He had, then, the means of comparing the gravitation 
 of any one planet towards the sun in any two points 
 of its orbit ; but this was not sufficient ; it was neces- 
 sary, besides, to know how to compare the gravitation 
 of two different planets, for it might be that, passing 
 from one planet to the other, there would be a change 
 in the amount of attraction. The third law of Kep- 
 ler, the proportion between the squares of the times 
 and the cubes of the mean distances, permitted New- 
 ton to complete his theory, and to refer all these at- 
 tractions to one. This law signified, in fact, that all 
 
UNITY OF NATURAL PHENOMENA. 
 
 the planets, of the same mass and at equal distances, 
 would be equally attracted by the sun. The same 
 equality of gravity exists in all the systems of satellites, 
 and Newton assured himself of it in the case of the 
 moon, as well as in that of the satellites of Jupiter. 
 
 It was with the lunar attraction that he began the 
 verification of his theory. The question was to deter- 
 mine whether the force which incessantly deviates the 
 moon towards the earth be identical with terrestrial 
 gravity. In this case, the action of these forces re- 
 ferred to the centre of the earth would have to be in 
 the ratio of the earth's radius, taken for unity, to the 
 square of the distance separating the two heavenly 
 bodies. Newton undertook this verification, starting 
 with the experiments of Galileo upon heavy bodies ; 
 but there existed then only an inexact measurement 
 of the earth's radius, and the great geometrician saw 
 the result of his calculation in disagreement with his 
 hypothesis. Thereupon, persuaded that unknown 
 forces were joined to the moon's gravity, he gave up 
 for a time his ideas. Some years later, the Academy 
 of Sciences having just effected the measurement in 
 France of a degree of the meridian, and a new meas- 
 ure of the earth's radius having resulted from this 
 work, Newton recommenced his researches, and this 
 time he found that the moon was retained in its orbit 
 
THE ATTRACTIVE FORCES. 
 
 by the sole power of gravity' The sight of this re- 
 sult, of which he had despaired, caused him, so say his 
 biographers, so lively an excitement that he could not 
 verify his calculation, and was obliged to trust the 
 care of it to a friend. 
 
 Thus one and the same law, a law unique and grand, 
 explained all the motions of bodies on the surface of 
 the planets, and those of the stars in space. The 
 principal developments of this law were collected in 
 the immortal treatise, the Mathematical Principles, 
 which Newton published towards the close of the 
 year 1687. 
 
 Having reached a principle which embraced the 
 universe entire, Newton himself made brilliant appli- 
 cations of it. He proved that the earth in its rotation 
 must become flattened at the poles, and he determined 
 the amount of variation in the length of the degrees 
 of the meridian. He saw that the attractions of the 
 sun and moon give rise to, and maintain in the sea, 
 those oscillations which constitute its ebb and flow. 
 He demonstrated, finally, the mode in which the 
 spheroidicity of the earth at the equator and the 
 inclination of the polar axis to the ecliptic determine 
 the phenomenon of the precession of the equinoxes. 
 He recognized, in a general way, even with exactness 
 upon some points, the perturbations which affect the 
 
 12 
 
178 UNITY OF NATURAL PHENOMENA. 
 
 planetary system. If a single planet be considered as 
 gravitating towards the centre of the sun, it must obey 
 strictly the laws of Kepler ; but this is no longer the 
 case when it concerns the attraction of several of the 
 heavenly bodies towards each other, if instead of 
 two bodies there be three. The conditions are then 
 changed and complicated, even so far as to become 
 amenable to analysis only with great difficulty. New- 
 ton assigned the meaning, and sometimes the numeri- 
 cal value, of several of the planetary perturbations, thus 
 tracing in their germ the methods which were in our 
 day to make it possible for mathematical calculation 
 to find the planet Neptune at the extremity of the 
 solar system. He recognized those disturbing phe- 
 nomena which affect the elements of the planetary 
 orbits, and which astronomy divides into two cate- 
 gories, the secular inequalities at very long intervals, 
 and the periodic inequalities, the period of which is 
 only some years. 
 
 But when he saw that the planetary ellipses succes- 
 sively approached and receded from the circular form ; 
 that their orbits did not always remain equally inclined 
 to a fixed plane ; that they cut the ecliptic according 
 to lines which changed their position in space, a dis- 
 couraging thought entered his spirit. It seemed to him 
 that the feeble 'values of all these variations, accumu- 
 
THE ATTRACTIVE FORCES. 
 
 lating in the course of centuries, must overturn the 
 system of the universe. He declared that this system 
 did not possess the lasting elements of preservation, 
 and that it would be necessary for a transcendent 
 power to intervene from time to time to repair the 
 disorder. 
 
 Leibnitz eagerly opposed such an opinion, and ridi- 
 culed this faith in an intermittent miracle. Newton 
 retorted with railleries upon the subject of the doctrine 
 of the pre-established harmony, which was, it must be 
 confessed, one of the oddest conceptions of meta- 
 physics. The quarrel soon became bitter, and was 
 complicated with the sharp dispute in which were seen 
 these two great minds contending for the invention of 
 the differential calculus. 
 
 Newton had traced out a sublime draught of the 
 theory of sidereal motion ; but it was merely a draught. 
 It was necessary for mathematical analysis to accom- 
 plish prodigies ; it was necessary for Euler, Clairaut, 
 D'Alembert, Lagrange, and Laplace to amass their 
 efforts, that the sketch might become a completed 
 design. 
 
 Clairaut first gave a complete and satisfactory solu- 
 tion of the problem of the three bodies, which consists 
 in determining the course of a planet subjected to the 
 combined attractions of two other heavenly bodies. 
 
l8o UNITY OF NATURAL PHENOMENA. 
 
 There continued to be an uneasiness in regard to 
 the astronomical perturbations whose periodicity was 
 unknown. It was Laplace who first discovered in 
 them an evidence sufficient to reassure us as to the 
 conservation of the planetary system. In the midst 
 of perturbations of every kind which observation made 
 known, there was a quantity which remained constant, 
 or which at least was only subject to but slight period- 
 ical variations. This was the major axis of each orbit, 
 upon which depends, according to the third law of 
 Kepler, the time of revolution of each planet. The 
 solar system became, as it were, confirmed, and it was 
 seen that it only oscillated about a mean condition, 
 from which it never departed save by very small 
 quantities. 
 
 Scarcely had this result been obtained when it 
 seemed to be compromised. Constant inequalities 
 were pointed out in the journeys of Jupiter and Saturn. 
 Comparing ancient observations with modern, it was 
 found that the motion of Jupiter was constantly accel- 
 erating, and that of Saturn was subject to a gradual 
 retardation. The theoretical consequence of these 
 facts was of a character to strike attention. It was 
 necessary to conclude from them that Jupiter would 
 gradually approach the sun, and finally cast himself 
 into it. Saturn, on the contrary, was destined to be- 
 
THE ATTRACTIVE FORCES. iSl 
 
 come farther and farther removed from the centre of 
 our system, and to plunge forever into the -darkness of 
 the space which our telescopes do not reach. The 
 Academy of Sciences was disturbed at these possible 
 results ; it summoned to this question the labors of the 
 geometricians. Euler and Lagrange descended into the 
 arena, without solving the difficulty ; it was again the 
 sagacious analysis of Laplace which demonstrated, in 
 the reciprocal perturbations of Jupiter and Saturn, the 
 reason of the anomalies pointed out by observers, and 
 which explained them by an inequality of long period, 
 the development of which demands more than nine 
 hundred years. 
 
 There were observed, moreover, inequalities whose 
 period was still longer ; those which depend upon the 
 precession of the equinoxes have a duration of ten 
 hundred and sixty centuries ; the eccentricity of the 
 earth's orbit goes on diminishing from the most remote 
 ages, obeying a period whose duration is reckoned 
 neither by hundreds nor by thousands of years, a 
 duration of time, in which the history of astronomical 
 observations, that even of the human race, figure but 
 as a point. 
 
 We have now followed the Newtonian idea up to the 
 time when it accounted for all the celestial phenomena ; 
 but it must not be thought that this idea was accepted 
 
1 82 UNITY OF NATURAL PHENOMENA. 
 
 at once by all minds. Its beginnings were marked by 
 the liveliest disputes. The quarrel between Cartesian- 
 ism and Newtonianism filled all the first half of the 
 eighteenth century. 
 
 The system of physics of Descartes yielded but 
 slowly before that of Newton, and in the domain of 
 facts itself the supremacy between the two doctrines 
 remained for a long time undecided. Not- the synthe- 
 sis only, which Newton had drawn from Kepler's laws, 
 but those very laws themselves were for a long time dis- 
 puted. At the close of the seventeenth century, Dom- 
 inique Cassini proposed to substitute for the ellipses of 
 Kepler a curve, which seemed to be more accurately 
 adapted to the sidereal motions ; this curve has taken 
 the name of the Cassinoid* One of the first conse- 
 quences of Newton's theory, the flattening of the earth 
 towards the poles, was denied. The sons of Cassini, 
 heirs of the paternal traditions, proved by the measure- 
 ment of an .arc of the meridian that the earth was a 
 spheroid, elongated in the direction of its axis. This 
 opinion prevailed in our Academy of Sciences up to 
 the time when an expedition was organized for deter- 
 
 * In the ellipse, the sum of the radii drawn from a point of the 
 curve to the two foci is constant. In the Cassinoid, a curve of the 
 fourth degree, it is the product of the two radii, which is con- 
 stant. 
 
THE ATTRACTIVE FORCES. 183 
 
 mining the comparative lengths of a degree near the 
 pole and near the equator. Bouguer and La Con- 
 damine set out in 1735 for Peru ; Maupertius and 
 Clairaut betook themselves to Lapland. The hy- 
 pothesis of Newton came out of this trial victoriously, 
 and towards 1 744 the greater number of savants, in- 
 cluding the two Cassini themselves, recognized the 
 errors in the experiment or reasoning which had made 
 them take the earth for an elongated spheroid. Then 
 it was that Descartes' system of physics was finally 
 overturned, together with a great part of his meta- 
 physics, and the Newtonian idea, popularized by Vol- 
 taire, and afterwards by the Encyclopedists, remained 
 triumphant. 
 
 But, if we are going to the bottom of things, what 
 was the point especially discussed between the Carte- 
 sians and the disciples of Newton ? 
 
 Descartes set out with this principle, that the whole 
 universe is to be explained by motion, and we, at least; 
 shall not make this a reproach against him ; it is upon 
 this very stand-point that contemporary science places 
 herself, and the majority of our physicists, whether 
 they will or no, are found to be Cartesians in this 
 respect. But Descartes, his principle laid down, with- 
 out facts, without observations, without experiments, 
 without proof of any sort, by a pure conception of his 
 
184 UNITY OF NATURAL PHENOMENA. 
 
 mind, had created a system of the universe ; the uni- 
 verse, was full of matter, absolutely full, without any 
 void ; vast circular currents existed throughout this 
 matter, and carried along with them the planets, as the 
 current of a river carries vessels along. The disciples 
 of Descartes abated nothing from the idea of .their 
 master, and they heaped up laborious explanations in 
 order to show how the vortices could be propagated 
 in a space absolutely full ; how the particles of matter 
 could glide over each other without any interstitial 
 vacuum. 
 
 To confront this doctrine, Newton brought forward 
 the law of universal gravitation ; this law contained in 
 itself an enormous mass of facts. Not only did it 
 explain all that were already known, but it fore- 
 shadowed new ones, and experiment justified these 
 foreshadowings. The Newtonians then felt them- 
 selves upon very solid ground. In their enthusiasm 
 'they left the domain of facts, and came to look upon 
 gravity as a mysterious cause, of an order superior to 
 all physical phenomena. Newton, as we have just now 
 shown, guarded himself from this excess, at least in 
 the beginning of his career, and in the book of Princi- 
 ples. Perhaps he was less reserved in this respect in 
 his old age. As to his " followers, they had an evident 
 
THE ATTRACTIVE FORCES. 185 
 
 i 
 
 tendency to believe themselves in possession of a su- 
 pernatural principle. 
 
 It was against this tendency that the Cartesian 
 school reacted ; it rejected the hidden cause presented 
 to it ; but it rejected, at the same time, both the cause 
 and the effects demonstrated by experiment. It closed 
 its eyes in order not to behold the new astronomical 
 system, and it obstinately persisted in its fanciful sys- 
 tem of physics. It fell into ridicule, and the hypothe- 
 sis of vortices, of which Fontenello was the last 
 defender, drew down in its fall the whole Cartesian 
 doctrine. 
 
 Thus one often sees, in the conflict of human ideas, 
 when two great doctrines are violently opposed to 
 each other, one of them succumb entirely, and the 
 conquerors effacing, without distinction, all that the 
 conquered had inscribed upon their banner. 
 
 As for us, who now regard this . historical debate 
 through the softening influence of years, we see the 
 ground upon which the two hostile doctrines might be 
 reconciled. The gravitation of Newton,* and all the 
 facts which it embraces, appear to us in harmony with 
 the Cartesian principle, as consequences of the mo- 
 tions of matter. 
 
1 86 UNITY OF NATURAL PHENOMENA. 
 
 IV. 
 
 Hypotheses in regard to the Formation of Worlds and 
 the Origin of Gravity. 
 
 THE Newtonian principle and the Cartesian princi- 
 ple, so long hostile, became united, and merged into 
 each other, in the general idea we are now able to form 
 concerning the system of the universe. 
 
 This general idea is formed in our minds when we 
 comprise in one general view the hypothesis of La- 
 place, concerning the birth of the solar system ; the 
 conjectures drawn by contemporaneous astronomy, 
 from the appearance of the nebulae, and the facts we 
 have developed above with regard to the function of 
 an ethereal substance. 
 
 Let us begin by recounting, in a few words, the 
 hypothesis of Laplace. 
 
 Our planetary system was at first only a nebula ; its 
 limits extended far beyond the present orbits of our 
 planets, and ,it became successively condensed in the 
 course of ages. 
 
 Laplace sketches, in grand outline, the history of 
 this gradual condensation. A solar nucleus first 
 forms in the nebula. This nascent sun is a gaseous 
 mass, endowed with a motion of rotation, which it 
 
THE ATTRACTIVE FORCES. l8/ 
 
 *4 
 
 shares with an immense atmosphere. By the general 
 cooling of the system this atmosphere leaves suc- 
 cessively, in the plane of its equator, lenticular zones, 
 from which spring the planets. Sometimes these 
 zones preserve the circular form, of which the rings 
 of Saturn afford us examples. "Most frequently they 
 separate into several parts. The fragments may re- 
 main ununited, as we observe in the system of minor 
 planets, situated between Mars and Jupiter. They 
 may also and this is oftenest the case be united 
 together into a single mass. 
 
 The planets thus formed, are originally gaseous 
 masses, which continue to turn about the sun ; they 
 turn also upon themselves, because, in the original 
 ring, the molecules farthest removed from the solar 
 centre had a greater velocity than the rest. By this 
 rotation each of them takes the form of a spheroid, 
 flattened at the poles, and very soon, in these minia- 
 ture worlds, is begun anew the phenomenon just now 
 explained. The* planetary atmosphere gives off rings, 
 from which spring the satellites. 
 
 The nuclei of the planets and those of the satellites 
 become hard at the surface, the atmospheres become 
 denser between the nuclei, and the immense expanse, 
 which was first filled by the nebula, is no longer occu- 
 
1 88 UNITY OF NATURAL PHENOMENA. 
 
 pied, save by a few celestial globes, which move regu- 
 larly around their common centre. 
 
 The author of the Mechanique Celeste has only put 
 forth this pretentious hypothesis with reserve ; he 
 has modestly placed it in a note at the end of his Ex- 
 position of the System of the Universe. It has not 
 failed to assume a high importance, for it is the only 
 conception which accounts for the principal planetary 
 phenomena. It explains why all the planets circulate 
 about the sun almost in the same plane ; why this 
 plane of general circulation is exactly that of the solar 
 equator ; why the planets describe ellipses which near- 
 ly resemble circles ; why their motions of progression 
 and of rotation take place in the same direction ; why 
 all the circumstances observed in the journey of the 
 planets around the sun are reproduced in the circula- 
 tion of the satellites around their planets.* 
 
 * Is there need of recalling here the brilliant experiment to 
 which a Belgian physicist, M. Plateau, has attached his name, and 
 which reproduces the principal phases of thtse creations of the 
 heavenly bodies? In a vase there is placed a mixture of water 
 and of alcohol, in the centre of which is put a drop of oil. Into 
 this drop is introduced a needle, to which a regular motion of ro- 
 tation is imparted. The oily sphere turns on its axis, and be- 
 comes flattened at the poles. Soon, from the expanded equation- 
 al regions, if the experiment is well conducted, there escapes a 
 sort of ring, which breaks into globules, each of which begins 
 to turn about the central mass. One may thus construct a uni- 
 
THE ATTRACTIVE FORCES. 189 
 
 4 
 
 The hypothesis of Laplace leads us, then, from the 
 origin of the sun to the complete development of our 
 solar system ; but let us now conceive of a phase 
 anterior to this, and attempt to portray its history. 
 
 Let us go back to a period, in the succession of 
 ages, when no system as yet existed. 
 
 The ether alone filled all space with its atoms in 
 motion. 
 
 If this medium is strictly homogeneous in all its 
 parts, a uniform vibration will continue without end ; 
 but if, among these primitive atoms, there exists, at 
 certain points, some dissimilarity, the preponderating 
 atoms immediately become centres of aggregation. 
 They approach each other in the manner we have de- 
 scribed. A sort of selection is thus effected through- 
 out the universal mass ; the ether becomes more and 
 more homogeneous in proportion as the dissimilar ele- 
 ments become united at certain centres. Thus, there 
 is formed in the midst of the ether, now become more 
 
 verse in a glass of water. The reader will observe, on a little 
 reflection, that this experiment is not a fair illustration of La- 
 place's theory. The oily ring is thrown off by the centrifugal 
 force of the revolving globule, while in the nebular hypothesis 
 the rings, out of which the planets are formed, are successively 
 abandoned by the cooling and contraction of the nebular mass. 
 Translator. 
 
IQO UNITY OF NATURAL PHENOMENA. 
 
 and more purified, a cosmical essence universally ex- 
 tended, the subtle germ of ponderable matter. 
 
 It is, in fact, gravity that has just taken its origin in 
 the phenomenon we have sketched, and it becomes 
 more clearly pronounced in proportion as the molecu- 
 lar groups are better defined, and the ether reduced to 
 a state of atomic uniformity. 
 
 Here, then, is space occupied by a sort of embryonic 
 network, the interstices of which are filled by the 
 ethereal atoms. The motion of attraction thus begun 
 never ceases. 
 
 At the same time that the ether was tending towards 
 a uniform condition, the rudimentary molecules were 
 absorbing all the unlike elements ; thus they have 
 been unequally pressed upon in different directions ; 
 the motions of progression and rotation follow as a 
 natural consequence. 
 
 Variety is also characteristic of the cosmical net- 
 work, from the very nature of its origin ; it becomes 
 torn then into irregular shreds here and there, where 
 the effects of concentration are manifested. 
 
 We here reach a point where telescopic observation 
 
 comes to the aid of pure speculation. The farther 
 
 ' artronomers penetrate into the depth of the heavens, 
 
 the greater is the number of these cosmical bodies 
 
 which they discover, some of which are resolvable into 
 
THE ATTRACTIVE FORCES. IQI 
 
 stars, while others preserve trie appearance of irredu- 
 cible nebulosities. Do these latter owe this appear- 
 ance to distance only ? and are we to believe that by 
 the aid of stronger lenses they could be decomposed 
 into luminous points ? Opinion may vary in regard to 
 this in such or such particular case, with reference to 
 such or such especial nebula ; but the aggregate of 
 observations leads to the belief that these agglomera- 
 tions are worlds in various stages of formation. In 
 some, the cosmical matter would be still diffused ; in 
 others, the solar nuclei would be more or less formed ; 
 in others, again, the suns would have already generated 
 their corteges of satellites. 
 
 Thus we should have before us, more or less accessi- 
 ble to our telescopes, specimens of the various phases 
 through which worlds pass. 
 
 Greater importance will not attach to these sugges- 
 tions than they deserve. If Laplace deferred his hy- 
 pothesis to the end of one of his books, where shall we 
 assign the place of the cosmical outline just sketched ? 
 We have attempted to carry back to the very origin of 
 cosmical formations that conception which represents 
 to us gravity as a consequence of the motions of ether. 
 The views we have given in this connection may seem 
 unjustifiable. They may be rejected without at the 
 same time weakening the considerations which bear 
 
UNITY OF NATURAL PHENOMENA. 
 
 upon the nature itself of gravity, such as we can 
 observe it to be in our own world, in the midst of 
 circumstances accessible to our analysis. 
 
 V. 
 
 The Molecular Agencies, Cohesion, and Chemical 
 Affinity. 
 
 WE must now return, by an abrupt transition, from 
 the motions of 'heavenly bodies to the molecular phe- 
 nomena ; from the immense spaces to which gravity 
 extends to the infinitely small distances in which co- 
 hesion and chemical affinity display themselves^ We 
 have already pointed out the enormous power of these 
 last two forces ; but the numerical results we have 
 mentioned give only a faint idea of it. It is known 
 that changes in cohesion, the freezing of water, for 
 example, and the solidifying of bismuth, may shatter 
 iron bottles of the thickness of several centimeters ; 
 we say nothing of the formidable effects produced by 
 the action of affinities in explosible mixtures ; the 
 simple reactions which form and maintain the ordinary 
 aggregations of matter have a power so great that 
 they have been called, in figurative language, giants in 
 disguise. 
 
THE ATTRACTIVE FORCES. 193 
 
 i 
 
 It seems at first thought that the heavenly bodies, in 
 their journey through space, must use up the largest 
 part of the vis viva, or living force, diffused throughout 
 the universe ; the contrary is true. The living force, 
 represented by the motions of the heavenly bodies, is 
 very weak compared with that concentrated in the 
 molecular activities. 
 
 Before making a new step in the examination of 
 these activities, it is important that we recur to the 
 idea itself of the molecule, and that we define its 
 meaning. The molecules of bodies reputed to be sim- 
 ple, such as oxygen, hydrogen, carbon, are they indi- 
 visible unities, veritable atoms, or are they aggrega- 
 tions ? 
 
 This latter hypothesis, we have already said, alone 
 seems admissible. 
 
 After the first labors which established the science 
 of chemistry, when analysis stopped before a certain 
 number of substances which it could not decompose, 
 one was led to look upon these substances as different 
 in their very nature. Such was the doctrine of Ber- 
 gelius. By this theory carbon, gold, and platinum are 
 bodies entirely heterogeneous, their atoms enjoying 
 special and peculiar properties. Nevertheless, the 
 notion of equivalents, introduced into chemistry at 
 the beginning of this century, naturally inclined men's 
 13 
 
194 UNITY OF NATURAL PHENOMENA. 
 
 minds towards a different doctrine. When it was seen 
 that simple bodies combine and replace each other in 
 their combinations in clearly defined proportions, the 
 equivalent quantities of different bodies came necessa- 
 rily to be regarded as different collections formed out 
 of one and the same substance. 
 
 Prout was the first to give definite shape to this 
 opinion. According to him, the equivalent weights of 
 simple bodies were the multiples of that of hydrogen. 
 It was very soon observed that this law could not be 
 maintained in the exact terms of its annunciation. 
 The precise determination of certain equivalents was 
 not in agreement with it. The first exceptions were 
 made to disappear by taking as unity the half equiva- 
 lent of hydrogen ; but new difficulties arose, and it 
 was necessary to have recouise to a more complicated 
 division. Front's law has thus lost, little by little, its 
 primitive originality. It remains, notwithstanding, 
 corrected by necessary restrictions, as an important 
 agreement in favor of the elementary unity of bodies. 
 
 We have already shown how the new physics goes 
 back to the very ethereal atoms for this elementary 
 unity. Between the molecules of oxygen, of hydrogen, 
 of carbon, of gold, of platinum, it conceives of no dif- 
 ference which bears upon the quality of matter ; it be- 
 holds only in these different bodies the properties 
 
THE ATTRACTIVE FORCES. 1 95 
 
 * 
 
 which result from motion. If this is true of these 
 bodies compared with each other, it is true also of the 
 same bodies compared with the ether. Between them 
 and the ether where could there be found a difference 
 which should have an influence upon their material 
 quality ? Thus every elementary molecule appears to 
 us as formed of ethereal atoms. Heat disorganizes 
 bodies ; it goes so far as to separate the hydrogen and 
 the oxygen, which form the vapor of water ; there 
 would be a final step to make ; by additionally heating 
 these same molecules, they might, doubtless, in the 
 end be driven apart, and be resolved into ethereal 
 atoms, either directly or by successive steps. 
 
 Here, then, is the manner in which the scale of 
 material aggregation presents itself to our eyes. In 
 the state of extreme tenuity, the ethereal atom ; then 
 comes the elementary molecule of bodies regarded as 
 simple ; these molecules combine, and from them re- 
 sult compound or chemical molecules. These last 
 unite in their turn, and thus form the particles of 
 bodies. 
 
 It may be conceived, at least in a general way, how 
 the aggregation of an elementary molecule may result 
 from the activity of a medium and the relative motions 
 of its parts. Without insisting upon this point, we 
 can represent this order of phenomena by means of 
 
UNITY OF NATURAL PHENOMENA. 
 
 several rough examples, a few far-fetched analogies. 
 It is in this manner that the pressure of the air main- 
 tains, in opposition to each other, the segments of a 
 hollow sphere. It is thus that a liquid jet often as- 
 sumes the appearance and the consistency of a solid 
 through the common motion of its parts. It is thus 
 that we often see eddies of wind or of dust pass over 
 long distances without losing their shape, because the 
 elements that compose them are endowed with the 
 same angular velocity. 
 
 It is equally true, if one examines these questions 
 closely, that the new physics, in the light it throws 
 upon them, only reveals to us a few fugitive outlines. 
 We should demand of it in vain to show by some 
 decisive examples how the various properties of mole- 
 cules arise from a combination of motions. This 
 diversity, which springs, so to speak, from the very 
 bosom of matter, always has been, and still remains 
 one of the strangest phenomena which man can 
 investigate. 
 
 Early science saw in bodies a kind of duality ; it 
 imagined, on one part, a matter deprived of qualities 
 of its own, but capable of receiving them all, and on 
 the other essences, which joined themselves to bodies 
 in order to constitute their properties ; it supposed 
 that these essences could be isolated by distillation, 
 
THE ATTRACTIVE FORCES. 
 k 
 
 and the alchemist strove to collect them, in order to 
 infuse them into matter at his will. 
 
 After the doctrine of essences, there prevailed the 
 idea of forms ; an aesthetic principle, concealed in the 
 interior of bodies, determined the moulds in which the 
 molecular diversity was produced. Let us remark 
 that this conception approaches that of motion. The 
 idea of motion does not ultimate itself without a cer- 
 tain idea of form ; geometry determines the curves, 
 and the ideal surfaces in which the motions are pro- 
 duced, and by which they are limited. 
 
 The new physics refers to motion the structure and 
 properties of molecules. It draws this conclusion from 
 the aggregate of laws which it has discovered ; it 
 believes itself authorized in this by what it knows 
 of several great natural phenomena ; by what it has 
 learned of light, heat, and electricity ; by the induc- 
 tions to which it has been brought regarding the na- 
 ture of universal attraction. But the future alone will 
 show whether it can reach the original conditions 
 which diversify the motions taking place in the hidden 
 depths of matter. 
 
 We must not occupy ourselves longer with the 
 metaphysics of molecules. The principal results we 
 have successively enunciated are independent of every 
 hypothesis concerning the constitution of molecules. 
 
UNITY OF NATURAL PHENOMENA. 
 
 We have been careful to reserve the term Atom for 
 the elements of the ether, and to apply the name of 
 molecule to those of ponderable matter ; but other- 
 wise, throughout the course of this work, if certain 
 incidental theories, which are easily separated from it, 
 be put aside, we may preserve the primitive notion 
 furnished by chemistry, and regard the elementary 
 molecules as little indivisible blocks, whose interior 
 construction possesses no influence over phenomena. 
 
 It has been seen how molecules, immersed in ether, 
 come to attract each other. We require a new princi- 
 ple to explain cohesion, and we find it in the hypothe- 
 sis of molecular rotation, from which Father Secchi 
 has drawn so many ingenious results. 
 
 In their rotation molecules must carry along with 
 them an atmosphere of ethereal atoms ; this is a fact 
 we have already made prominent when treating of the 
 change of state of bodies. The existence of these 
 atmospheres and here we must avoid a possible 
 confusion is entirely distinct from the phenomenon 
 which distributes the ether in layers of different den- 
 sity. This latter effect extends to infinity ; the former 
 takes place only in a very limited space, in the imme- 
 diate vicinity of the molecule. In this space the atoms 
 share directly in the molecular rotation ; outside of 
 it they are independent of it. It has been shown 
 
THE ATTRACTIVE FORCES. 1 99 
 
 above how these atmospheres behave when a body, 
 losing its heat, is brought from the gaseous state to the 
 liquid form, and from this to the solid condition. Let 
 us remark here, that this hypothesis explains why the 
 liquid and solid states take place all at once, at a given 
 moment, when the molecules have been brought within 
 a fixed distance of each other. So long as the atmos- 
 pheres do not touch, no trace of cohesion shows itself ; 
 when they meet, this force arises. We understand 
 also why the temperatures of melting and solidifying 
 are fixed for the same body ; these effects take place 
 at the precise moment when the atmospheres, varying 
 with the temperature, have attained the desired 
 diameter. 
 
 Now what is affinity ? Let us note the nature of 
 its action. It acts for a time, more or less precipitate- 
 ly, in order to disturb an equilibrium ; the bodies con- 
 cerned saturate each other ; then a new equilibrium 
 succeeds. This phenomenon may be explained by the 
 very hypothesis we have just made use of. 
 
 Between homogeneous molecules all the atmos- 
 pheres are alike, and there is no cause determining 
 one to modify the other. In this case cohesion is 
 produced. If, on the other hand, molecules of differ- 
 ent kinds confront each other, there is a variety in the 
 atmospheres ; these may penetrate each other, and 
 
2OO UNITY OF NATURAL PHENOMENA. 
 
 modify in this way the position of their respective 
 molecules. 
 
 In this way the principle of chemical affinity be- 
 comes known to us. 
 
 The more unequal are the atmospheres, the more 
 opportunity will there be for the equilibrium to be 
 destroyed, and the greater will be the energy of the 
 chemical action. They may differ, besides, not only 
 in their volume, but also in their velocities, and they 
 thus present several elements of variation. Tempera- 
 ture naturally influences the state of the atmospheres, 
 and thereby changes the conditions of affinity. It 
 may happen that two molecules, which have at a cer- 
 tain moment dissimilar atmospheres, and consequently 
 a very great affinity, shall come to have, if the temper- 
 ature changes, like atmospheres, and consequently a 
 moderate affinity. It may even result, if the tempera- 
 ture continues to vary, that the relative value of the 
 atmospheres will be reversed. Known anomalies 
 could thus be accounted for ; in this manner could 
 be explained why, at temperatures very near each 
 other, we sometimes see iron decomposing water, and 
 setting the hydrogen at liberty ; and sometimes, on 
 the contrary, the hydrogen decomposing the oxide of 
 iron in order to get possession of the oxygen. 
 
THE ATTRACTIVE FORCES. 2OI 
 
 The chemical molecule has then a general envelope, 
 but that is not to say that the elementary molecules 
 remain without their own minor atmospheres. It 
 must be observed, moreover, that these general atmos- 
 pheres are, so to speak, external phenomena, in which 
 we naturally find again the motions themselves of the 
 molecules. Between all these motions, those of the 
 molecules, those of the partial envelope, and those of 
 the general envelope, an equilibrium is established, 
 from which results the stability of the combination. 
 The compound will be so much the more stable as 
 this dynamic equilibrium shall have less chance of 
 being disturbed. If the elements are numerous, a 
 slight variation of temperature puts disorder into this 
 aggregation, and destroys its bonds of union. 
 
 This effect shows itself more clearly in proportion 
 as we go from the mineral kingdom, where a certain 
 simplicity reigns, to organic substances, the structure 
 of which is more complicated. It is said that a mole- 
 cule of albumen contains nine hundred elementary 
 molecules. It is conceivable that compounds so com- 
 plex may be easily destroyed by variations of tempera- 
 ture. The complication is much greater yet in organ- 
 ized tissues. Thus vegetables are confined each to a 
 particular climate, and if animals are able to live in 
 regions more extended, it is because they carry in 
 
2O2 UNITY OF NATURAL PHENOMENA. 
 
 themselves a source of heat which renders the tem- 
 perature of each almost constant. 
 
 From the time of Lavoisier, the science of chemis- 
 try has been constructed upon the idea of masses ; 
 its relation to velocities remains wholly to be estab- 
 lished. Now masses and velocities form two series 
 of elements, which it is equally necessary to be ac- 
 quainted with in order to appreciate the living forces 
 with which molecules are endowed, and the varying 
 results which they may thus give rise to. 
 We cannot refrain from remarking, however, that 
 chemistry has made universal progress through the 
 consideration of masses only. The law of definite 
 proportions, the law of multiple proportions, even the 
 notion of chemical equivalence, which naturally re- 
 sulted from these two fundamental laws, are indepen- 
 dent of all idea of motion. By means of their rela- 
 tive weights simple bodies have been followed into 
 their elementary combinations, and the scale of their 
 saturation determined. Later, organic chemistry be- 
 comes founded, through the study primarily of fatty 
 substances, afterwards through the first analyses of 
 the alcohols and ethers ; the scales then prove insuffi- 
 cient for pursuing the complication of phenomena, and 
 yet the theories which chemistry gives rise to seem, 
 at first sight, to be applicable only to molecules in a 
 
THE ATTRACTIVE FORCES. 2O3 
 
 state of repose. The law of substitutions sums up 
 the progress of organic chemistry. This law does not 
 imply, so at least one might think, any idea of molec- 
 ular motion ; it may be understood as the mutual 
 replacement of partial groups among motionless groups 
 of atoms, if one is satisfied with a summary view of it. 
 But we no longer need show what incompleteness such 
 a manner of appreciating chemical phenomena would 
 possess. It would not be possible to compare molec- 
 ular structure to the superposition of the stones of a 
 building ; if it is to be represented by an appropriate 
 figure, we must liken it, on a limited scale, to solar vor- 
 tices which should penetrate each other, and whose 
 elements should take on in the encounter a new and 
 unstable equilibrium. 
 
 Furthermore, we are not here dealing with a merely 
 theoretical conception. If we return to the domain of 
 facts, we see that chemical action produces a work, a 
 result peculiar to masses, endowed with velocity. It 
 is chemical action ; it is the combustion of coal that 
 sets the most of our engines in motion. 
 
 Just now we do not know how to make a direct 
 i 
 
 measurement of chemical work ; we determine it only 
 through the medium of heat or of electricity ; by these 
 indirect means we already obtain an appreciation of it 
 sufficiently exact. We judge of chemical action by its 
 
204 UNITY OF NATURAL PHENOMENA. 
 
 exterior effects, and it is a result not to be despised. 
 To know it in itself, to penetrate its secret, to com- 
 prehend its interior play, we must determine the 
 velocities as well as the molecular masses. If we were 
 in possession of the terms of this twofold quality, we 
 should see disappear all that chemistry yet retains that 
 is whimsical and capricious ; we might explain the dif- 
 ferent combinations, and the material properties re- 
 sulting from them. 
 
 Then would be established molecular mechanics, 
 which would comprehend in their entirety, not only 
 chemical phenomena, but all the natural phenomena 
 we have successively treated of, those of gravity as well 
 as those of heat, those of electricity as well as those 
 of light ; a universal system of dynamics would em- 
 brace astronomy, physics, and chemistry. 
 
LIVING BEINGS. 20$ 
 
 CHAPTER VI. 
 
 
 
 LIVING BEINGS. 
 
 I. 
 
 Vital Activity consists in the Transformation, not in 
 the Creation of Motions. 
 
 WE have nearly exhausted the programme we had 
 marked out in advance ; we have directed our atten- 
 tion successively to the principal phenomena pertaining 
 to the physical sciences ; we have shown their mutual 
 relations, and pointed out their fundamental unity. 
 We could here terminate our examination, and con- 
 sider our task as accomplished ; the results we have 
 reached now appear in their general bearing. We 
 have not yet, however, paid any attention to living 
 beings, which also form a part of the physical uni- 
 verse. Should they also be comprehended in the phe- 
 nomenal unity upon which we have fixed our attention, 
 or should they be excluded from it ? Do they wholly 
 obey the laws, whose mutual connection has been 
 
2O6 UNITY OF NATURAL PHENOMENA. 
 
 shown, or, if they are independent of them in some 
 respects, what are their immunities ? 
 
 The simple enunciation of these questions calls to 
 mind the vast problems that have, from time to time, 
 
 agitated mankind ; so many theories concerning the 
 
 
 
 nature of life, so much effort laid out on human per- 
 sonality, so many discussions in regard to the princi- 
 ples of a higher essence ! Let it not be expected that 
 we shall attack these lofty questions. We may re- 
 serve them intact, and we need not venture upon the 
 field of transcendental speculation in order to show 
 how the great law under which we have brought 
 the operations of nature is verified also in organized 
 beings. 
 
 It seems, according to the labors of modern physi- 
 ology, that in the cell must be sought the principle of 
 vital activity. Vegetables, like animals, are composed 
 of cells. 
 
 Every vegetable is composed of an association of lit- 
 tle sacs or vesicles which assume, in crowding together, 
 the polyhedral form. Each one forms a closed organ 
 which has its own life, and which is, as it were, the 
 integral part of the vegetable. 
 
 The case is not otherwise with animals ; but the 
 more perfect the general organism, the greater the 
 variety observable in the cells. In the lower degrees 
 
LIVING BEINGS. 2O/ 
 
 of the animal scale, among the infusoria of the lowest 
 species, are found creatures of the simplest composi- 
 tion that it is possible to imagine. The cells, all of 
 them, entirely similar to each other, fill an envelope 
 furnished with vibratile cilia, by the aid of which the 
 animal moves. With the higher animals, with verte- 
 brates, with man, there are great differences between 
 the cells belonging to the tissues of different organs. 
 A nucleus, more or less complex, at the centre, a fine 
 membrane at the periphery, between the two a liquid, 
 either simple or compound, such are the constituent 
 principles of the cell, and they exhibit a sufficient 
 variety of elements to display very great dissimilarities 
 between the cells composing the different muscular 
 fibres, the various nervous filaments, the mucous and 
 serous membranes, &c. In the midst of this diversity, 
 each cell possesses, in the interior of the collective 
 being, a relative independence, a sort of autonomy. 
 Every family of these vesicles has its own govern- 
 ment, its food, poisons, diseases. 
 
 Moreover it is known, since the ingenious discover- 
 ies of Dutrochet, how these little sacs are nourished, 
 though entirely closed, separated even from each other 
 by the double partition which results from their back 
 to back arrangement ; it is known that they succeed 
 in absorbing the liquids outside of them, and in ex- 
 
2O8 UNITY OF NATURAL PHENOMENA. 
 
 pelling, in part, those with which they are filled. This 
 phenomenon of endo-exosmosis, together with capil- 
 lary attraction, is sufficient to account for the ascent 
 and descent of the sap in vegetables. It shows how 
 in animals the different cells may incessantly renew 
 their contents, and obtain, by an elective straining pro- 
 cess, all that is needed for their support. Not only 
 are these vesicles nourished, thanks to this mechan- 
 ism, but, by an action communicated from one to 
 the other, they succeed in drawing up liquids through 
 canals that have no opening, and in pouring them into 
 other canals equally closed, thus establishing through- 
 out the mass of the tissues a capillary circulation, the 
 principle of which for a long time escaped all re- 
 search. 
 
 Thus we find, at the origin of life, cells which form 
 the primary bases of organization.* It may be said 
 that they constitute, in the two organic kingdoms, in- 
 dividuals that may be compared with the atoms of the 
 mineral kingdom ; atom, individual, two terms bor- 
 
 * Concerning the character of the life of these ultimate cells, 
 see Physical Basis of Life, T. H. Huxley. The substance, pro- 
 toplasm, which is considered in this treatise as forming the 
 structural unit of all vegetable and animal life, 'contains only the 
 four elements, carbon, oxygen, hydrogen, and nitrogen. Trans- 
 lator. 
 
LIVING BEINGS. 2OQ 
 
 rowed from different languages to express the same 
 idea. 
 
 But do we know how the cells are produced ? and 
 has the secret of their formation been discovered ? 
 We have seen in the germ of vegetables a primary 
 cell nourished by the starch contained in the grain, 
 and converted by the process of germination into dex- 
 trine and sugar ; we have seen new cells arrange them- 
 selves beside the first one by a process of germination 
 or budding ; the soluble substances, elaborated in this 
 rudimentary life, thus come to constitute the primary 
 elements of vegetables. In the animal germ, in the 
 egg, we see a granular matter dividing itself into sev- 
 eral spheroidal segments, and each of these converted 
 into a vesicle by the coagulation of its superficial 
 layer ; then the vesicles attach themselves to each 
 other, multiply themselves by division by the forma- 
 tion of interior membranes, and finally constitute the 
 cellular tissue, out of which is to come the embryo. 
 It is in this tissue that are arranged, by an analogous 
 mechanism, the rudiments of organs, of a circulatory 
 apparatus, and a nervous system. 
 
 As to the elements themselves which compose liv- 
 ing organisms, vegetable or animal, it is here unneces- 
 sary to recall the fact that they are all borrowed in 
 their last analysis from the inorganic world. As we 
 14 
 
2IO UNITY OF NATURAL PHENOMENA. 
 
 ascend the animal scale, we find an increasing compli- 
 cation in this respect, notwithstanding there never 
 enters into living beings more than a very limited 
 number of simple bodies. The human body, the most 
 complex of all, comprises fourteen simple bodies 
 oxygen, hydrogen, azote, carbon, sulphur, phosphorus, 
 fluorine, chlorine, sodium, potassium, calcium, magne- 
 sium, silicon, and iron. However complicated may be 
 the architecture of the molecules, the entire man is 
 reducible to these fourteen elements. 
 
 If we now attempt to condense the primordial no- 
 tion of life, such as it results from these suggestions, 
 if we strive to reduce it to its essential principles, 
 what do we find ? 
 
 On the one side, the materials themselves of the in- 
 organic world. 
 
 On the other side, a series of motions, which suc- 
 ceed each other in a determined order. 
 
 The definite succession of these motions doubtless 
 exhibits a character entirely unique, but throughout 
 their successive transformations there will be found 
 nothing in them which jars with the laws of molecular 
 mechanics. 
 
 Do we mean that we have here all the elements of 
 life ? What is the cause which forms the first cellule, 
 which produces from it the development of the being, 
 
LIVING BEINGS. 211 
 
 which regulates and limits its evolution ? In- view 
 of the facts, it is too evident that we cannot reply to 
 this question. We have, then, only two courses to 
 pursue either to suspend our judgment, or to admit 
 a special cause, the principle of which is peculiar to 
 vital phenomena. 
 
 With the nature itself of this course we have not 
 to concern ourselves here ; and, since it is manifested 
 by means of motions, its name is to be found in the 
 language we speak, we must call it a force. 
 
 What do the preliminary considerations we have 
 just laid down teach us concerning the action of this 
 force ? Upon this point there must be a thorough 
 understanding. It determines motions, but it can only 
 produce them at the expense of anterior motions ; just 
 as it does not create the materials of organisms, but 
 merely shapes them by the aid of pre-existing elements, 
 so it does not create motions, but only transforms 
 them. It is thus that vital phenomena, without losing 
 their special character, enter entirely into the cate- 
 gory of material motions. If the vital force has a 
 peculiar activity, this activity consists in transforming, 
 not in creating. It furnishes us, then, with* a fresh 
 confirmation of the great law whose development we 
 are seeking throughout the entire universe. 
 
212 UNITY OF NATURAL PHENOMENA. 
 
 Such is the stand-point to which we shall constant- 
 ly be brought whenever we consider the phenomena 
 of life * 
 
 * The tendency of modern science is to include in the same 
 category all displays of force, whether exhibited in organized 
 beings or inorganic things. Heat, light, chemical affinity and 
 vitality are thus mutually correlated. The idea of a distinct 
 principle of vitality is, however, openly claimed by many, and 
 tacitly acknowledged by most scientific writers of the present 
 day. But the old boundary lines between the animal and vegeta- 
 ble kingdoms it has been necessary to abandon, since the micro- 
 scope has disclosed by its keen scrutiny that no such lines exist. 
 Even the power of voluntary motion, supposed to be the exclusive 
 privilege of animals, has its analogies, if not its exact counter- 
 part, in the spontaneous movements of many species of plants. 
 The distinction between the mineral kingdom and the other two, 
 between the inanimate on the one hand and the animate on the 
 other, is equally ill-founded, for the truth seems to be that all 
 created things possess life, each in the degree and kind corre- 
 sponding to its use* and destiny in the order of creation. Like- 
 wise the subdivisions of these kingdoms, with their systems of 
 classification, prove but temporary make-shifts, which need to be 
 constantly modified. The real fundamental distinctions which 
 undoubtedly exist between the three kingdoms of nature, appear 
 unattainable by science, whose province is to deal with effects, 
 and not with causes. Our author has thought best not to touch 
 upon these vexed questions ; but we must admit that his asser- 
 tion that vital phenomena, as being but manifestations in matter 
 of an interior active principle, can be included in the category 
 of material motions, is fully sustained by the results of modern 
 investigation. Translator. 
 
LIVING BEINGS. 
 
 II. 
 
 The Manner in which the Laws of Thermo-dynamics 
 are Verified in the Case of Animated Beings. 
 
 THE respiration of animals, the circulation of the 
 blood, nutrition, all contribute to a production of heat. 
 A production of heat is the grand result of all these 
 functions. Now direct observation has succeeded in 
 tracing this into its essential conditions, and in show- 
 ing how the heat is produced in accordance with the 
 principles of Thermo-dynamics. 
 
 First, let us regard the- state of repose ; let us con- 
 sider the condition of a man who performs no external 
 work. 
 
 Animal heat results from the slow oxidations which 
 take place within the organism. It may be added that 
 it is due almost entirely to the combinations of oxygen 
 with hydrogen and carbon. It is easy, then, by com- 
 paring the gases which enter and come from the lungs, 
 to calculate the number of heat-units which a man 
 produces in an hour. This is found to be an average 
 of one hundred and twenty heat-units, with a varia- 
 tion, according to the subject, of about a third of this 
 total amount. 
 
 What becomes of the heat-units thus produced ? 
 
214 UNITY OF NATURAL PHENOMENA. 
 
 The man must necessarily lose them as fast as devel- 
 oped, since the temperature of his body remains con- 
 stant.* He gives it forth, in fact, under several forms, 
 pulmonary and cutaneous evaporation, heating of 
 the expired air, radiation, contact with external objects. 
 If the amount of heat emitted from the body in these 
 various ways be directly measured, it will be found fo 
 equal that which is produced within the body ; and so 
 observation confirms the preconceptions of the theory. 
 Let us observe that, in the loss to be established 
 between man and the surrounding medium, we have 
 not reckoned the work which is accomplished in the 
 interior of the body. The heart, for example, operates 
 constantly after the manner of a force-pump ; it acts 
 incessantly with a force that may be estimated at the 
 seventy-fifth part of a single horse-power, and its 
 action thus represents the effect of nine heat-units per 
 
 * This temperature, as is known, is about thirty-seven degrees 
 Centigrade. Climates exert no influence in this respect; be- 
 tween the inhabitants of the warmest countries and those of the 
 coldest there is found hardly the difference of a degree. The 
 kind of food has itself no action upon human temperature. In 
 India it is likewise thirty-seven degrees with the native workmen, 
 who eat only rice and fish ; with the priests of Buddha, who live 
 on vegetables, and the soldiers, fed chiefly on meats. A varia- 
 tion of four or five degrees in the average temperature of the 
 human body constitutes a pathological condition which speedily 
 terminates in death. 
 
LIVING BEINGS. 215 
 
 hour. Many other interior motions take place that 
 might be estimated in the same way with more or less 
 exactness ; but the cycle of these phenomena being 
 accomplished entirely within the body, there is an 
 interior equivalence between the quantities of heat 
 and the work they represent, and they are not taken 
 into account in the exchange which takes place be- 
 tween man and the surrounding medium. 
 
 So much for the state of repose. Let us now con- 
 sider the man who executes movements, and who pro- 
 duces an external work. 
 
 The beautiful researches of M. Hirn have shown 
 that in the human body heat is transformed into work, 
 and work into heat, according to the numerical ratio 
 which we have already so often introduced ; a unit of 
 heat is. converted into four hundred and twenty-five 
 kilogrammetres, and reciprocally. 
 
 M. Hirn has selected for the object of his investiga- 
 tions the work which a man produces in raising his 
 own body. , When we ascend a slope, or when we de- 
 scend it, our muscular force and gravity are put in an- 
 tagonism. Practically this antagonism is complicated 
 with the horizontal reactions due to the friction neces- 
 sary for walking. M. Hirn, by an ingenious contriv- 
 ance, has succeeded in eliminating this source of com- 
 plication, leaving only the vertical forces to be taken 
 
2l6 UNITY OF NATURAL PHENOMENA. 
 
 into account. Imagine a man moving along the steps 
 of a movable wheel ; if the wheel be suitably turned, the 
 man, without really having to change his place, will real- 
 ize the artificial conditions of ascending, descending, and 
 level walking, in which vertical actions alone are put 
 into operation. The subject of his experiment would 
 produce an external work when displacing the centre 
 of gravity of his body in order to reach a higher step ; 
 if he descended, on the contrary, his weight would 
 operate as if he had received external work, and his 
 body would profit, in some sort, by a certain amount 
 of motor force ; if he walked without ascending or 
 descending, his centre of gravity would be alternately 
 raised and lowered in equal degrees ; there would be 
 an equivalent production and consumption of external 
 work. 
 
 The theory clearly indicated the calorific effects that 
 should be exhibited under these different circum- 
 stances, and they were produced in such a way as to 
 fully justify the inductions of the experifhenter. M. 
 Hirn had first established, by direct measurements, 
 that in a state of repose every gramme of oxygen ab- 
 sorbed invariably disengaged five heat-units ; next, ob- 
 serving the state of motion, he saw that this propor- 
 tion varied. If a man, weighing seventy-five kilo- 
 grammes, raised his weight four hundred and twenty- 
 
LIVING BEINGS. 
 
 five metres, each gramme of oxygen disengaged less 
 heat, and seventy-five heat-units, the exact representa- 
 tion of the work produced, disappeared. If the same 
 man descended four hundred and twenty-five metres, 
 each gramme of oxygen disengaged more than five 
 heat-units, and the descent thus left in the organism 
 seventy-five units, which, could not be attributed to 
 respiratory action. Moreo.ver, respiration continued 
 to yield five heat-units per gramme of oxygen in the 
 case of level walking. 
 
 These striking results have been confirmed by a 
 series of repeated experiments.* 
 
 At first thought, one may be astonished that walk- 
 ing on a level, as regards work, leads to no expendi- 
 ture, and that descent constitutes, in this respect, a 
 sort of gain, seeing that both of them even under 
 the conditions employed by M. Him demand cer- 
 tain efforts, and result in a certain amount of fatigue. 
 More than this, the case of ascent may even give rise 
 to an apparent objection. . How does it happen, it may 
 
 * It has been estimated that a man of average weight pro- 
 duces, in the climate of France, three thousand J:wo hundred and 
 fifty heat-units per day, or a sufficient amount of heat to raise 
 seven gallons of water to the boiling point. (See an article from 
 the French of Fernand Papiflon, in No. 10 of the Popular Science 
 Monthly, for interesting facts in regard to animal heat.) 
 Translator. 
 
2l8 UNITY OF NATURAL PHENOMENA. 
 
 be said, that the act of ascending "consumes heat when 
 the body is manifestly warmed in producing this work ? 
 It is important to do away with these apparent contra- 
 dictions, which would be of a character to leave in the 
 mind a vague distrust of the theory just unfolded. 
 
 Yes ; the work corresponding to the act of ascend- 
 ing consumes heat, but at the same time it accelerates 
 the respiratory action and circulation ; the volume of 
 inspired air increases, and the absorbing power of the 
 lungs is raised in a proportion often considerable. 
 The quantity of oxygen absorbed, consequently the 
 heat produced, increases even to five fold. M. Hirn 
 has established these facts by putting himself into the 
 apparatus which he employed for his experiments. 
 
 For an ascent of four hundred and fifty metres per 
 hour, the number of pulsations of the heart was raised 
 from eighty to one hundred and forty ; the number of 
 respirations a minute went from eighteen to thirty ; 
 the body of respired air in an hour was augmented 
 from seven hundred litres to twenty-ldiree hundred. 
 As a result of this increasing activity in the respira- 
 tion and circulation, the experimenter consumed no 
 longer thirty grammes, as in a state of rest, but even 
 one hundred and thirty-two ^grammes of oxygen per 
 hour. Thus, in spite of the consumption produced by 
 
LIVING BEINGS. 2IQ 
 
 the work, an excess of heat is developed within the 
 body, and the individual becomes warm. 
 
 Considerations of a like character would do away 
 with the difficulty which we pointed out with regard to 
 level walking and descending. To speak only of the 
 first case, every step is divided into two periods ; in 
 the one, the weight of the body is raised, in the other 
 it is lowered. The first period consumes heat ; the 
 
 second restores an equal quantity. Regarded in this 
 
 
 light, the calorific equilibrium is not disturbed ; but 
 
 the organism, responding to the call of the muscles, 
 alternately contracting and elongating, develops an 
 excess of heat. This excess may be sufficient for an 
 interior work of the muscles from which fatigue may 
 arise, but which, according to an example already 
 given above, is not at all to be considered in the 
 exchange effected between man and the surrounding 
 medium. 
 
 The mechanical theory, of heat is then confirmed 
 and illustrated in the human motor, as in all others. 
 The man who, in M. Hirn's experiments, gave the best 
 dynamic results restored in work in the ratio of 
 twelve to one hundred of the heat produced ; this is 
 nearly the amount yielded by our most perfect 
 engines. 
 
 If we follow up this paralellism between the weight 
 
22O UNITY OF NATURAL PHENOMENA. 
 
 of the motor and the power it develops, we still find a 
 sort of equality between man and our engines ; but 
 animated nature presents us, in this respect, a class of 
 beings especially favored. These are the birds. 
 
 These wonderful motors evolve a force of one horse 
 (steam), while possessing a weight of only five or six 
 kilogrammes. Their physiological structure, together 
 with their relative lightness, gives them the means of 
 enduring the enormous work they are obliged to pro- 
 duce in order to sustain themselves in the atmosphere. 
 The bird is a centre of combustion of exceeding activ- 
 ity ; his whole body is, so to speak, but a lung ; the 
 air, powerfully solicited through the very play of the 
 wings, enters abundantly to vivify the blood, which the 
 heart impels with prodigious power through the ves- 
 sels. The torrent of the circulation thus furnishes the 
 muscles with enormous stores of heat, which they are 
 able to convert into work. Thus, while the tempera- 
 ture of man remains fixed at about ninety-eight de- 
 grees Fahrenheit, that of the bird reaches one hundred 
 and nine to one hundred and eleven degrees. It ex- 
 ceeds, consequently, the limits beyond which our 
 organs become unfitted for life. It has been proved 
 that the bird consumes, in a state of repose, a great 
 quantity of oxygen ; one would undoubtedly be aston- 
 ished if it were possible to ascertain how much it ab- 
 
LIVING BEINGS. 221 
 
 sorbs in rapid flight. Let us add that, in order to 
 endure this active combustion, the bird must be able 
 to repair promptly the losses it suffers. Its organs of 
 nutrition respond to this necessity. Its gizzard, hard 
 as horn, grinds without difficulty the most resisting 
 articles of food ; a liver of great size pours forrents of 
 bile over the material which comes from the gizzard, 
 and digestion is effected with surprising rapidity. So 
 the bird cannot go hungry. It is sometimes remarked 
 of a person who takes but little nourishment that he 
 eats like a bird. Jlris is a phrase to be taken with 
 reservation, and one that should undoubtedly be re- 
 moved from our vocabulary. The species which feed 
 on living prey make a very great slaughter ; those 
 which live on fruits and grains eat a little at a time 
 perhaps, but it is on the condition of finding the table 
 always spread. 
 
222 UNITY OF NATURAL PHENOMENA. 
 
 III. 
 
 Muscular Contraction and Innervation. 
 
 WE have just established in the case of man, and 
 incidentally in the case of birds, the conversion of heat 
 into work. We must again examine, a little more 
 closely, the circumstances which accompany this phe- 
 nomenon. 
 
 The muscles swell up, and become shortened, to 
 effect the motions of the bones Jo which they are 
 attached. 
 
 When, in physiological experiments, a muscle is 
 made to contract by an artificial irritation, pinching it, 
 for example, or communicating to it an electric shock, 
 there result jerks and violent contractions, which do 
 not resemble the regular motions which the will calls 
 into action ; but if a continued series of irritations be 
 kept up, the muscle * is observed to contract in a 
 
 * Muscles are composed of fibres, which are contractile in a 
 a high degree, and which occur under two principal forms : the 
 smooth fibre belongs to the muscles which serve the purposes 
 of organic life, to that silent, and as it were, unconscious life, 
 which animates the various parts of the body; the striped fibre 
 belongs to the muscles of the life of relation, those which produce 
 the voluntary motions. Certain muscles, the heart, for example, 
 present a mixed composition. There would appear to be a dif- 
 
LIVING BEINGS. 223 
 
 permanent manner. Helmholtz, employing the inter- 
 rupted current of an induction coil, has shown that at 
 least twenty-two excitations per second are needed to 
 secure continued contraction. 
 
 The muscle thus contracted gives out a percepti- 
 ble, though very deep sound. Helmholtz was able- to 
 prove that the pitch of this sound corresponded to 
 the number of interruptions produced in the induction 
 coil. 
 
 There is, besides, a characteristic fact that accom- 
 panies muscular contraction, one that may be regarded 
 as its direct cause ; it is a powerful absorption of oxy- 
 gen. M. Matteucci proved this by comparing, in a 
 lime-water bath, the amount of carbonic acid given 
 out by muscles, when contracted, and when in a state 
 of rest. The oxidation of muscles is likewise directly 
 observed in the animal economy; it is known that 
 the venous blood, when it comes from muscles a long 
 time contracted, is completely deprived of oxygen, and 
 contains a large excess of carbonic acid. 
 
 Thus there is no doubt in regard to this. What 
 
 ference in the mobility of these two kinds of fibres. The striated 
 muscle, when irritated, contracts abruptly, and relaxes imme- 
 diately; the smooth fibre acts more slowly, and in a more pro- 
 longed manner. Physiology has chiefly studied the striped mus- 
 cles; it is these which possess the highest importance for us at 
 this time, since they are the instruments of voluntary motion. 
 
224 UNITY OF NATURAL PHENOMENA. 
 
 designates Contraction is an increase of energy in the 
 oxidation of the muscular tissues, a more active de- 
 composition of the hydrocarbonated materials by the 
 elements of the arterial blood. That chemical action 
 
 thus set up throughout the extent of the muscle 
 
 
 changes its form, that it shortens the muscle while 
 
 increasing it in size, is not a matter of astonishment 
 to us ; we often see a rope swell up and become tight 
 when it is wet, and produce in this way a considerable 
 traction. That the heat developed in muscular tissue 
 should be partially converted into work, is what we 
 also regard as an ordinary and common phenomenon. 
 M. Beclard has, moreover, made a series of ingenious 
 experiments upon this matter. He has studied, in its 
 calorific bearing, the same muscular contraction in the 
 case where it produces no external work, and in the 
 case where it does ; he thus observed, during a long 
 series of experiments, that the heat due to chemical 
 action was diminished by just that amount which was 
 transformed into work. 
 
 But let us not stop here ; let us endeavor to go back 
 to the origin of muscular action. 
 
 The nerves interpose to excite the .action of the 
 muscles. 
 
 The nervous system, if we regard it in its relations 
 to motion, may be represented in the following man- 
 
LIVING BEINGS. 22$ 
 
 ner. An external organ receives the sensations ; a 
 very slender tubular filament carries them to a nerve 
 cell which perceives them ; another cell, suited to 
 direct movements, communicates by means of a new 
 filament with the contractile apparatus that is to exe- 
 cute them ; finally, a nerve tube acts as a bond of 
 union between the cell which receives the impressions 
 and the motor cell. Such, reduced to its simplest 
 expression, is the general idea of nervous communi.ca- 
 tion. The act which is propagated from one extremi- 
 ty to the other of the system is termed a reflex act. 
 The elementary filaments, very thin and delicate, 
 since a great number of them possess a thickness of 
 scarcely a hundredth part of a millimeter, are bound 
 together and intertwined in such a way as to form 
 little cords ; the cells are also grouped together at 
 certain points, which bear the name of nervous cen- 
 tres. With vertebrates, with man, whom we have 
 especially in view, the greater number of these ner- 
 vous centres are united together in that long stem, 
 which constitutes the spinal marrow. Nevertheless 
 a certain number remain, which are scattered through- 
 out the body ; these are called nervous ganglions, and 
 taken together they are known under the name of the 
 great sympathetic system. A sort of hierarchy is thus 
 established in the reflex actions ; some concern the 
 
226 UNITY OF NATURAL PHENOMENA. 
 
 ganglions only, while others reach as far as the -spinal 
 marrow. Above this rises a still higher system. At 
 the origin of the encephalon are found the oval masses, 
 which preside over the respiratory movements and 
 the .contractions of the heart ; next comes the cere- 
 bellum, which co-ordinates the voluntary motions, then 
 the lobes of the brain, where the will and the intellect 
 reside. First the ganglions, afterwards the spinal mar- 
 row, make successively a sort of selection from among 
 the reflex acts, permitting only a certain number of 
 them to reach the higher regions of the system where 
 would seem to be concentrated the conscious govern- 
 ing power of the being. Thus may be reduced to a 
 few general outlines the infinite complication of this so 
 delicate network which ramifies throughout the whole 
 extent of the body. 
 
 How is nervous action propagated ? 
 
 Several years ago the works published by M. Du 
 Bois Reymond, and several German physiologists) 
 seemed to have solved this problem. A solution 
 which offered itself, under such an attractive exterior, 
 was accepted with eagerness. Innervation was an 
 electric current. A current was transmitted along the 
 sensitive nerve to end in the cell of sensation ; a 
 current quitted the motor cell in order to end in the 
 organ of motion ; whatever might have been the re- 
 
LIVING BEINGS. 22/ 
 
 actions effected within the cells, they assumed, hence- 
 forth, a manifest analogy with that which takes place 
 in a battery, or other electro-motor machine. 
 
 The excitement over this explanation cooled off. 
 Admitted at the outset, with insufficient proof, it was 
 finally rejected by many physiologists without suffi- 
 ciently good reasons. We do not find in the human 
 body the simple conditions presented by our electrical 
 apparatus. It is clear that a nerve cannot be entirely 
 analogous to an insulated conducting arc, since it is 
 itself, like everything around it, the seat of incessant 
 reactions. One was too readily discouraged by reason 
 of the confusion in the results yielded by experiments. 
 To invalidate the existence of nervous currents, reasons 
 are brought forward which do not appear to have great 
 weight. Electric currents, it is said, are transmitted 
 slowly in the nerves, having only a velocity of twenty- 
 four, or eve* of eighteen metres in a second ; they go 
 less rapidly in the nerves than in the muscles. It is 
 argued again that a nerve, when cut, however closely 
 the ends may be applied to each other, becomes unfit 
 for communication. These are matters which have 
 nothing of a decisive character. Whatever may be 
 said, we find ourselves still confronted with important 
 and highly significant facts. By causing electric cur- 
 rents to act upon a nerve, veritable currents produced 
 
228 UNITY OF NATURAL PHENOMENA. 
 
 by our machines, -*- we obtain the contraction of mus- 
 cles ; not an instantaneous contraction alone, but a 
 continued one. Let one take the hinder parts of a 
 frog, the two thighs attached to the lumbar nerves, 
 and these latter to a fragment of the spinal marrow, 
 and let a current be passed along one of the nerves, 
 and there will follow not only a direct excitation of 
 the corresponding limb, but also the reflex motion of 
 the other thigh. 
 
 It seems to us that these results, well known and 
 within the reach of common experience, furnish seri T 
 ous grounds for conviction. Now, if it be proved that 
 the stream which reaches the muscles is not to be 
 confounded with the electric current, that it is to be 
 regarded as possessing a special character, and to be 
 studied under a distinct name, there will yet be noth- 
 ing in this circumstance that can weaken the results 
 we present. Under cover of this declaration, we con- 
 tinue to speak of nervous action as of an electric cur- 
 rent. It will be possible, if so desired, to see in this 
 language merely a figurative representation of the phe- 
 nomena ; it will be sufficiently exact to justify the 
 results we wish to illustrate. 
 
 Thus the nerve excites the muscle. Does this mean 
 that the nerve possesses in itself all the energy which 
 is developed in the muscle ? No ; since the muscle 
 
LIVING BEINGS. 22Q 
 
 gets this fouce directly by its own oxidation. The 
 nerve merely excites chemical action ; it only sets go- 
 ing a piece of machinery. It is thus that a spark pro- 
 duces the explosion of a gaseous mixture ; it is thus 
 that a match effects the lighting of a fire ; it is thus, 
 by turning a stop-cock, that we let flow away all the 
 water accumulated in a reservoir. 
 
 One is naturally led to think that the work of the 
 nerve is exceedingly small compared to that of the 
 muscle. M. Matteucci has proved this a fact by direct 
 experiment. He hung a weight to the principal mus- 
 cle of the leg of a frog, and sent an electric current 
 through the nerve attached to this muscle. The con- 
 traction of the muscle raised the weight, and it was 
 easy to estimate the effort in foot-pounds. Likewise, 
 by a simple calculation, might be estimated the com- 
 bustion of the zinc produced in the battery during 
 the very brief period of excitation. M. Matteucci 
 thus found the work done by the muscle to be at least 
 twenty-seven thousand times greater than the chemi- 
 cal or calorific effect of the nervous excitation. 
 
 Let us go back yet farther, and approach the origin 
 of the motion. Small as the work of the nerve may 
 be, how is it accomplished ? To give rise to a cur- 
 rent in a nerve, it is enough that a circuit be formed 
 somewhere, either within or on the outside of the 
 
23O UNITY OF NATURAL PHENOMENA. 
 
 nerve cell, and this action itself is only, a very slight 
 portion of the action which the current can produce. 
 As to the mode in which such a circuit is formed, 
 nothing definite can be known. If it be a question of 
 voluntary motion, we say that the will intervenes. 
 
 But two things are to be noted. 
 
 First, the mechanical action attributed to the. will 
 becomes, from the preceding considerations, gradually 
 reduced to one so extremely minute that it seems to 
 disappear altogether. 
 
 Let us add, in the second place, that the will does 
 not create this work, however imperceptible it may be. 
 It can only be conceived of as a special agent of 
 transformation in motions infinitely small. The vol- 
 untary act, and for a still stronger reason the purely 
 reflex act, to whatever degree of tenuity it be re- 
 duced, does not proceed without a subtle modification 
 of the tissues in which it is effected, without I know 
 not what sort of delicate transformation of molecular 
 motions. 
 
 In ascending from muscular action to nervous ac- 
 tion, properly so called, and to the play of the will, we 
 have reached the limit where physical phenomena 
 give place to moral, and beyond this we have not to 
 pass. 
 
 Within the limits we have now reached we have 
 
LIVING BEINGS. 23! 
 
 been able to show how the principles to which we 
 have been led by the study of the inorganic ;world are 
 verified in the case of living beings. Our conception 
 of the physical universe would have been too incom- 
 plete had we been obliged to curtail from it all that 
 pertains to life. 
 
 We may now, without leaving behind us so formida- 
 ble a gap, resume the synthesis we have undertaken, 
 and endeavor to give it its final shape. 
 
232 UNITY OF NATURAL PHENOMENA. 
 
 CHAPTER VII.. 
 CONCLUSION. 
 
 CUVIER said, in his History of the Progress of the 
 Natural Sciences, " Once having quit the phenomena 
 of shock, we no longer possess any clear idea of the re- 
 lations of cause and effect. Everything is reduced to 
 the collecting of particular facts, and the searching out 
 of the general propositions which embrace the greatest 
 number of them. It is in this that all physical theo- 
 ries consist, and, to whatever degree of generality 
 each of them may have been reduced, they will still be 
 far from a conformity with the laws of shock, which 
 alone could change them into real explanations." 
 
 It cannot be said that physicists have already ful- 
 filled the programme marked out by these words. 
 And yet, if we cast a look behind over the road we 
 have traversed, and include in a general view all the 
 facts we have mentioned, we shall feel more and 
 more established in this idea, that all physical phe- 
 nomena consist in the exchange and transformation of 
 material motions. 
 
CONCLUSION. 233 
 
 Will it be said that our examination has not always 
 been rigid enough ? that we have sometimes asserted, 
 when we ought to have expressed a doubt ? that we 
 have not always laid sufficient stress upon the reserva- 
 tions we were led to make?- We shall not try to 
 shield ourselves from this censure, feeling too well 
 that we have deserved it. It would have been better, 
 perhaps, to have left more points in obscurity, and to 
 have limited ourselves to certain facts. May we be 
 pardoned for some suggestions too conjectural ? The 
 results acquired are considerable, and a few rash sup- 
 positions cannot compromise them. 
 
 These acquired results we could at need clothe with 
 the authority of an eminent man of science.- M. De 
 Senarmont, during the last year of the lectures which 
 he delivered with so much brilliancy at the Ecole Poly- 
 tecJinique, and which death came so early to interrupt, 
 thus summed up his views upon the progress of the 
 physical sciences : " Even lately each group of facts 
 acknowledged a special principle. Motion and rest 
 resulted from forces, ill-defined enough specifically, 
 but which it was agreed to term mechanical ; the phe- 
 nomena of heat, light, electricity, badly enough defined 
 themselves, were produced by so many particular 
 agents, fluids endowed with special activities. A 
 more searching examination has enabled us to dis- 
 
234 UNITY OF NATURAL PHENOMENA. 
 
 cover that this noticfn of a variety of specific and 
 heterogeneous agents has at bottom -only a solitary 
 and unique basis ; this is, that the perception of these 
 various kinds of phenomena is in general effected 
 through the different organs, and that in addressing 
 themselves more particularly tc each of our senses, 
 they necessarily excite particular sensations. The ap- 
 parent heterogeneity would then be less in the nature 
 itself of the physical agent than in the functions of 
 the physiological instrument which shapes the sensa- 
 tions ; so that in falsely attributing the dissimilarities 
 in the effect to the cause, we should have in reality 
 classified the intermediate phenomena through which 
 we have a knowledge of the modifications of matter, 
 rather than the essence itself of these modifications. 
 . . . All physical phenomena, whatever their nature, 
 appear at bottom to be only manifestations of one and 
 the same primordial agent. . . . This general result 
 of all modern discoveries can no longer be ignored, 
 however impossible it may still be to put into definite 
 shape their laws and their conditional details." 
 
 Such was the language of M. De Senarmont in a 
 course of academical instruction, in which no room 
 was allowed for any unsafe doctrine. 
 
 We are not bound to a like reserve. So we have 
 more explicitly stated the system which seems to sum 
 
CONCLUSION. 235 
 
 up the labors, and to express the general sentiment of 
 contemporary physical science. Ether in a state of 
 motion fills all space. The ethereal atoms, by their 
 aggregation, form molecules ; these last, bodies. Be- 
 tween these atoms, these molecules, these bodies, inter- 
 changes of motion take place, constituting what we 
 term heat, light, electricity, gravity, chemical affinity. 
 These interchanges depend upon the masses and ve- 
 locities which are concerned. The conception of the 
 physical universe is contained entirely in these prin- 
 ciples. Hitherto we have been able to get but a very 
 small number of the facts which this statement of 
 principles embraces, because we are unacquainted, 
 nearly always, both with the absolute and the relative 
 values of the masses and velocities which govern the 
 communication of motions. Practically we are satis- 
 fied with saying that heat, light, electricity, gravity, 
 affinity, are transformed into each other according to 
 the fixed relations of equivalence, and we assign them 
 a common measure, that of mechanical work. 
 
 Left thus on the outside of phenomena, we have but 
 a vague. notion of the circumstances that accompany 
 and determine transformations. There are, doubtless, 
 motions to which we are unable to give a name, and 
 which we are not qualified to perceive, although they 
 play their part in nature. 
 
236 UNITY OF NATURAL PHENOMENA. 
 
 Amid the variety and number of motions that would 
 seem possible, why are some produced and not 
 others ? 
 
 Is there among motions a sort of natural selection ? 
 
 We should possess the key of the transformations 
 taking place under our eyes if we could attain to that 
 measurement of the masses and velocities which, until 
 now, eludes us. In a steam .engine, for example, the 
 agitation which reigns in the heat of the furnace is 
 communicated to the tubes of the boiler, and from 
 these to the water itself ; the molecules of vaporized 
 water each expend a little of their living force upon 
 the piston, which moves under these accumulated 
 efforts, and sets in motion the shaft of the engine ; 
 but we perceive this series of changes only through a 
 veil. When a motion of a certain kind is replaced by 
 another of a different kind, the reason for this ex- 
 change usually escapes us ; and it is because of this 
 ignorance that we have recourse to the idea of force ; 
 we say that a force is exhibited, and produces such an 
 effect, because we are unable to grasp the anterior 
 motions from which this effect results. 
 
 The notion of physical force ought then to disap- 
 pear if the elements of molecular mechanics were 
 known. In the present state of our knowledge we 
 must, indeed, preserve it ; but we must also be on our 
 
CONCLUSION. 237 
 
 , t 
 
 guard against the errors it may entail. Let us' call 
 force every cause of motion, if it be desired ; but let 
 us not forget that this word most generally represents 
 only a provisional and conditional cause. The dread 
 of a vacuum has been a force in its time, that is, of a 
 vacuum to the extent of thirty-two feet. 
 
 If we recur with persistence to this consideration, 
 it is because it seems to us to be of capital impor- 
 tance, and we could not devote too great an effort to- 
 wards its illustration. It is the knotty point in the 
 system we have unfolded. And yet, among the phys- 
 icists even who have entered into the current of new 
 ideas, there is a school which persists in giving to 
 the physical forces an unaccountable individual ex- 
 istence. 
 
 M. Hirn, whose name in France is connected with 
 the determination of the mechanical equivalent .of 
 heat ; M. Hirn, whom we mention when we wish to 
 place a French name by the side of those of MM. 
 Joule and Mayer^; M. Hirn does not hesitate to re- 
 gard the physical forces as the constituent elements 
 of the universe. Under the title of intermediate prin- 
 ciples, he makes of them half transcendental essences, 
 which fill all space, and which have the property of 
 conferring motion jipon matter. He even makes an 
 enumeration of these principles, and finds four of 
 
238 UNITY OF NATURAL PHENOMENA. 
 
 
 
 them gravity-force, light-force, heat-force, and electric- 
 force. 
 
 What ! does matter here and there quit its state 
 of rest, and do new motions spring up at the will of 
 these forces ? This is not precisely what M. Hirn 
 means ; he knows too well it would be in contradiction 
 with the facts. Here is the theory he conceives. For 
 him each force is everywhere diffused. At the instant 
 the intensity of one increases in such a way as to pro- 
 duce a motion, the intensity of another force dimin- 
 ishes in a corresponding ratio. Now this diminution 
 of intensity in the second force itself corresponds to a 
 diminution of motion in matter. It is, evidently, a 
 sort of pre-established harmony. Doubtless we have 
 but to suppress these artificial intermediate agents, to 
 find ourselves face to face with motions themselves, 
 and the return is thus easy, when it is desired, from 
 M. Hirn's stand-point to that we just now occupied. 
 Why introduce, hereafter, between two motions which 
 beget each other, two semi-transcendental essences ? 
 Why have recourse to these intermediate principles ? 
 Why this mythology, this Olympus of forces ? 
 
 Why ? It is not very hard to give the reason, nor 
 will it be useless to do so. 
 
 These arbitrary conceptions are inspired in M. Hirn 
 by the disturbing influence of an easily alarmed spirit- 
 
CONCLUSION. 239 
 
 ualism. M. Hirn becomes distrustful when he sees a 
 doctrine which every day explains by the motions of 
 matter an ever-increasing number of facts. He dreads 
 the encro'achment He fears lest it may come to 
 reach the human soul ; lest it reduce to pure motions 
 the phenomena of will and of thought. It is for the 
 purpose of arresting it in its progress that he has re-' 
 course to gravity, heat, light, and electric forces. 
 These intermediate principles are the bulwarks he 
 raises to defend the soul-principle. Strange bulwarks 
 in tputh, and far from capable of such a defence ! 
 Must it be repeated, moreover, that the problems of 
 the soul are in no way concerned in the theories 
 against which M. Hirn endeavors to fortify himself? 
 In the midst of material transformations, causes active 
 in themselves may interfere, and we have pointed out 
 examples of them in indicating the nature and limits 
 of this interference. This is sufficient to leave the 
 
 
 
 field free for all the solutions of metaphysics. 
 
 Having shown how our hypothesis banishes the fal- 
 lacious entities with which physical science may be 
 encumbered, is there need of defending the theory 
 itself from the extravagant deductions that might be 
 drawn from it ? Is there need of indicating the point 
 of view from which an entirely healthy conception of 
 it is to be obtained ? 
 
24O UNITY OF NATURAL PHENOMENA. 
 
 Is admitting a scientific hypothesis equivalent to 
 believing one's self in possession of the realities of 
 things ? This would be, too easily forgetting so many 
 systems that have buried each other in their ruins. 
 It would be forgetting too easily that the physical 
 philosopher, lost in the infinity of time and space, 
 seizes only apparent relations, and does not even ar- 
 rive at a conception of the absolute ! What, then, 
 is it to group together into an hypothesis all our ideas 
 concerning nature? It is to afford us the means of 
 illustrating the things . we learn by comparing them 
 with each other, of establishing fruitful relationships 
 between facts, and so cause springs of discovery -to 
 burst forth. 
 
 What is of importance, correctly speaking, in such 
 an hypothesis, is not the picture it gives of nature, 
 but the plan it traces out for the explorer of physi- 
 cal science. 
 
 In this connection, the system we have exhibited is 
 admirably summed up in a solitary principle. It 
 evolves a luminous criterion, whose efficiency has al- 
 ready been revealed in scientific researches. 
 
 This precious symbol has a name in the language 
 of mechanics ; but before pronouncing it, let us hasten 
 to recall to mind what we have already said regarding 
 the difficulty one encounters of expressing new ideas 
 
CONCLUSION. 241 
 
 with old words. By a cruel irony of circumstances^ 
 we are about to fall in with a word we would like to 
 have escaped from at this time by reason of the am- 
 biguity it contains. Never have we more keenly felt 
 the need of employing a new expression, and if we 
 refrain from so doing, it is for the reason that our 
 declaration in this regard will doubtless stand us in 
 stead of a neologism. We conceive the presence 
 in the universe of an unchanging supply of ma- 
 terial atoms endowed with rapid motion, and group- 
 ing themselves into systems to form molecules and 
 bodies. Each of the atoms and systems possesses, in 
 proportion to its mass and velocity, what we have hith- 
 erto termed a living force, what we may now call, 
 if we desire to avoid this ambiguous term, an energy, 
 without gaining much by the change. Such are the 
 expressions against which we have desired to take pre- 
 cautions by a preliminary statement. We do not em- 
 ploy a new word ; but we have said enough to show 
 that under these usual designations, we are to see ab- 
 solutely only masses in motion. To say that energy 
 changes its locality, is merely to say that masses act 
 upon each other in mutually modifying their velocity. 
 
 Energy thus passes without limitation from one sys- 
 tem to another, thus giving origin to the variety of 
 natural phenomena. Sometimes it shows itself in a 
 16 
 
242 UNITY OF NATURAL PHENOMENA. 
 
 series of changes, in which its successive efforts may 
 be followed ; then it is said to preserve the active form> 
 Sometimes it hides itself in order to maintain, for a 
 longer or shorter period, an equilibrium, whose rupture 
 will regenerate it ; it is then said to pass into the po- 
 tential state. Active energy and potential energy 
 vary incessan-tly in their relative proportion, but their 
 sum remains constant. 
 
 Such is the principle usually designated under the 
 name of conservation of energy. 
 
 Doubtless, in order to verify entirely this constancy 
 of energy, it would be necessary to include the whole 
 universe. Energy may increase at certain periods, in 
 certain regions of space, and decrease in different re- 
 gions, though the ether would appear to be a sort of 
 regulator of this universal activity. Do the changes 
 which unceasingly take place between our terrestrial 
 globe and the sidereal medium become interpreted to 
 us by a loss, by a gain, by a periodic oscillation about 
 a mean condition ? How does our solar system com- 
 port itself with reference to other systems ? Such 
 are the vast problems to which the notion of universal 
 energy finds its application. 
 
 We do not mean that the principle of the coftserva- 
 tion of energy cannot be verified in the immediate 
 connection of ordinary phenomena. It establishes a 
 
CONCLUSION. 243 
 
 definite bond of union between all the facts which 
 surround us. The physical philosopher knows that 
 motions can pass from visible masses to invisible ones, 
 without ceasing to obey a law whose purport he 
 knows. If he is not- always fortunate enough to 
 gather the facts into complete cycles, where effects 
 and causes are linked into a chain whose ends meet, 
 at least he is no longer forced to regard phenomena 
 as isolated appearances. In the case of each one he 
 is able to ascend to its sources, or descend to its con- 
 sequences. He may fail in the application of his 
 method, he may represent to himself in a false light, 
 this or that family of facts, but the principle itself, 
 by virtue of which he seeks a fundamental unity be- 
 neath the infinite diversity of appearances, is to him 
 the most precious and the best assured conquest of 
 contemporary science. 
 
INDEX. 
 
 A. 
 
 ABSOLUTE zero of temperature, 120. 
 
 Acoustics, experiments in, 71. 
 
 Affinity, how explained by the new theory, 200. 
 
 Agassiz, Louis, recognizes the divine providence in nature, 20. 
 
 Anaximenes regards air -as the primitive element, 47. 
 
 Arago, his explanation of light interferences, 68. 
 
 Aristotle regards matter as identical, n. 
 
 Astronomy, history of, 169, et seq. 
 
 Atmospheres, ethereal, how they envelop the atoms, 114. 
 
 Atoms not elastic, 77. 
 
 Attraction and repulsion due to ethereal shocks, 157. 
 
 Attractive forces not inherent in matter, 155.* 
 
 B. 
 
 Beclard, his experiments upon muscular contraction, 224. 
 
 Bernouilles, his Hydronamics, 115. 
 
 Billiards, game of, illustrates the motions of translation and ro- 
 tation, 78. 
 
 Birds, amount of heat evolved in, 220. 
 
 Birds, temperature of, 220. 
 
 Birds, their rapid digestion, 221. 
 
 Boucheporn, M. de, his law of three squares, 84. 
 
 Boucheporn, M. de, his theory of the cause of colors, 82. 
 
 Boucheporn, M. de, his treatise upon a general principle in 
 natural philosophy, 27. 
 
 244 
 
INDEX. 245 
 
 
 Boucheporn, M. de, his views regarding the transversal motion 
 
 of the ether, Si. 
 
 Boucheporn, M. de, on the inter-atomic spaces, 91.. 
 British Association and the submarine telegraph, 131. 
 
 c. 
 
 Carbon, sulphide of, how the solar raj is affected by passing 
 through a prism of, 63. 
 
 Carnot, Sadi, on the motive power of fire, 104. 
 
 Carpenter, Dr.. refers the origin of all power to mind, 20. 
 
 Cartesians, their dispute with the Newtonians, 183. 
 
 Cassini, his curve of the sidereal motions, 182. 
 
 Cassinoid, the, 182. % 
 
 Cauchj, his calculation of the distances between atoms, 91. 
 
 Cell, a, the primary basis of organization, 208. 
 
 Chemical action determined by the molecular velocity and 
 mass, 204. 
 
 Chemical action is due to a change in the molecular atmos- 
 pheres, 200. 
 
 Chemical affinity not an inherent principle, 157. 
 
 Chemistry, history of, 202. 
 
 Clairant, his problem of the three bodies, 179. 
 
 Clapeyron, his thermo-dynamic theory, 104. 
 
 Clausius, his theory of gases, 115. 
 
 Climate, effect of, on human temperature, 214. 
 
 Cohesion, compared with gravity, 158. 
 
 Cohesion may result from the common velocity imparted to the 
 molecules of a body, 34. 
 
 Cohesion takes place <?nly when the ethereal atoms touch each 
 other, 199. 
 
 Comets, do they prove a resisting medium ? 166. 
 
 Copper, sulphate of, its action on the transmission of light, 92. 
 
 Cuvier limits the relations of cause and effect to the phenomena 
 of shock, 232. 
 
 D. 
 
 Democritus, his doctrine regarding atoms, 48. 
 Descartes, founder of analytical geometry, 174. 
 Descartes, similarity of his system to that of Epicurus, 49. 
 
246 INDEX. 
 
 Difficulty of expressing new ideas by means of old terms, 50. 
 Du Bois Reymond, his theory of innervation, 226. 
 Dulong and Petit, law of gases, nS. 
 
 Dupre, his experiments showing the force of cohesion, 158. 
 Duration of waves of light of the different colors, 70. 
 Dutrochet, discoveries of, 207. 
 
 E. 
 
 Eccentricity of the earth's orbit, periodicity of, 181. 
 
 Egyptian hieroglyphics, key to discovered by Thomas Young, 68. 
 
 Electric current, 136. 
 
 Electric fluid, a transport of ether, 141. 
 
 Electric motion does not take place in a vacyum, 145. 
 
 Electric motion similar to the flowing of a fluid, 139. 
 
 Electric spark shows that the fluid passes in one direction 
 only, 142. 
 
 Electric unit, determination of, 148. 
 
 Electricity, a duality of fluids discussed, 126. 
 
 Electricity, its mechanical action, 150. 
 
 Electricity, its relation to light, 150. 
 
 Electricity, not an entity, but a motion, 127. 
 
 Electricity, velocity of, 152. 
 
 Electricity, velocity of, through nerves, 227. 
 
 Elementary bodies, composed of ethereal atoms, 195. 
 
 Elementary bodies, variety of, due to motion, 195. 
 
 Empedocles, four elements of, 13. 
 
 Energy, active and potential, 242. 
 
 Energy, conservation of, 242. 
 
 Epicurus constructs an atomic theory, 48. 
 
 Essences, doctrine of, 197. 
 
 Ether constitutes the atoms of bodies, 74. 
 
 Ether harmonizes the doctrines of Newton and Descartes, 185. 
 
 Ether is imponderable, 75. 
 
 Ether is the cause of gravity, 96. 
 
 Ether the sole material substance, 12, 38. 
 
 Ether, objections to, 164. 
 
 Ether, objections to, answered, 168. 
 
 Ethereal atoms, their rotation the cause of their transverse vi- 
 bration, 81. 
 
 Euler. his arguments against the emission theory, 65. 
 
INDEX. 247 
 
 F. 
 
 Favre, experiments, 149. 
 
 Fermat, founder of infinitesimal calculus, 174. 
 
 Fizeau, M., his explanation of light fringes, 68. 
 
 Force, living, determination of, in mechanics, 108. 
 
 Force, that which causes one motion to give place to another. 32. 
 
 Force, unity of, 10. 
 
 Foucault, his explanation of the cause of light fringes, 68. 
 
 Fraunhofer, lines of, 94. 
 
 Fresnel, as a man of science, 88. 
 
 Fresnel, his theory of the vibration of the ether, 80. 
 
 G. 
 
 Galileo on the law of acceleration of falling bodies, 174. 
 
 Galvanometer, the, as a measure of electrical phenomena, 133. 
 
 Gases all contain same number of molecules at equal pressure 
 and density, 116. 
 
 Gases, co-efficient, of the expansion of, 118. 
 
 Gases, molecular arrangement of, when they pass into the liquid 
 and solid states, 112. 
 
 Gases, motion of their molecules, m. 
 
 Gases, why attraction does not exist among the molecules of, HI. 
 
 Gay-Lussac, his law of gaseous mixtures, 117. 
 
 Geisslers tubes, 138. 
 
 Germination, mode of, 209. 
 
 Gravity explained by the doctrine of an ether, 161. 
 
 Gravity, phenomena of, are caused by the pressure of the ether 
 upon the atoms of bodies, 37. 
 
 Gravity, unsoundness of the usual ideas regarding its nature, 36. 
 
 Grimaldi, phenomena of interference, his experiment to pro- 
 duce, 67. 
 
 H. 
 
 Hartmann, his theory of the ether, 15. 
 
 Heat and mechanical force mutually equivalent, 23. 
 
 Heat, animal, amount evolved in birds, 220. 
 
 Heat, animal, loss of, equal to that produced within the body, 214. 
 
248 INDEX. 
 
 Heat, animal, mechanical equivalent of, 215. 
 
 Heat, animal, the result of the oxidation of tissues, 213. 
 
 Heat is a motion, 106. 
 
 Heat, its material nature discussed, 103. 
 
 Heat, latent, of dilatation, 53, 121. 
 
 Heat, mechanical equivalent of, 33. 
 
 Heat, point of maximum intensity beyond the visible spec- 
 trum, 61. 
 
 Heat units, number, produced hourly in man, 213. 
 
 Heat units, number of, produced in the human body per day, 217. 
 
 Helmholtz, his experiments on muscular contraction, 223. 
 
 Heraclitus regards fire as the sole principle of the universe, 47. 
 
 Hirn, M., his notion of physical forces as the primordial es- 
 sences, 237. 
 
 Hirn, M., researches upon animal heat, 215. 
 
 Human body, composed of fourteen elements, 210. 
 
 Huyghens, his theory of central forces, 174. 
 
 Huyghens proves mathematically the undulatory theory of 
 light, 65. 
 
 I. 
 
 Interferences, luminous, doctrine of, explained, 69. 
 Interferences, sonorous, illustrated, 71. 
 
 J. 
 
 Jenkin, Fleeming, his report on submarine cables, 132. 
 
 Joule, M., his determination of the principle of the convertibility 
 of heat and mechanical force, 24. 
 
 Joule, M., his discovery of the mechanical equivalent of 
 heat, 104. 
 
 Joule, M., his experiment showing that the loss of heat in ex- 
 pansion is due to work, 122. 
 
 Joule, M., his theory of gases, 115. 
 
 Jupiter, perturbations of his orbit, 180. 
 
INDEX. 249 
 
 K. 
 
 Kepler, discovery of his first law, 129. 
 
 Kepler, his description of the discovery of his third law, 172. 
 
 Kepler, his mysticism, 173. 
 
 Kepler, his theory of solar attraction, 173. 
 
 Laplace considered heat material, 103. 
 Laplace, his nebular hypothesis, 186. 
 Laplace proves the stability of the solar system, 180. 
 Lavoisier, his memoir on heat, 102. 
 Leibnitz, his theory of the ether, 15. 
 Leucippus, his theory of the universe, 48. 
 Life a succession of motions. 210. 
 Light, duration of waves of the different colors, 70. 
 Light, its resolution by means of a prism of bi-sulphide of car- 
 bon, 60. 
 
 Light, its transmission through colored media, 92. 
 Light, Newton's theory of, 64. 
 Light, undulation of waves of, transversal, 79. 
 Light, velocity of, 70, 152. . 
 
 M. 
 
 Malebranche, one of the first to regard light as the undulations of 
 
 an ether, 65. 
 
 Matter, an aggregate of ethereal atoms, 39. 
 Matter, indestructibility of, 31. 
 Matter, unity of, n. 
 
 Matteucci, his calculation of the work done by muscles, 229. 
 Marriotte's law, 116. 
 Mayer, Dr., determines the mechanical equivalent of heat, 9, 
 
 24, 105. 
 
 Mechanique Celeste, the, 188. 
 Metals, derived from a single base, n. 
 
 Metals, conductibility of, 149. ' 
 
 Metals, their different calorific capacity, 132. 
 
25O INDEX. 
 
 Molecular spaces, their relation to the size of the molecules, 90. 
 Molecules, effect of their shape on the passage of the luminous 
 
 rays, 92. 
 
 Molecules, structure and properties of, due to motion, 197. 
 Motion, never created or destroyed, 32. 
 
 Muscles absorb oxygen, and give out carbonic acid in action, 223. 
 Muscles, effect of electricity on, 223. 
 
 Muscles, Energy of, proportioned to activity of oxidation, 224. 
 Muscles, structure and contraction of, 222. 
 
 N. 
 
 Nature, activity of, consists in turning vis viva into work, 109. 
 
 Nebular hypothesis, 186. 
 
 Nerves excite, but are not the cause of muscular energy, 228. 
 
 Nerves, structure and function of, 225. 
 
 Nerves, their mode of action, 226. 
 
 Nerves, their work compared with that of muscles, 229. 
 
 Nervous currents compared with electric currents, 227. 
 
 Newton, his belief in the existence of an ether, 13. 
 
 Newton, his emission theory of light, 64. 
 
 Newton, his* quarrel with Leibnitz, 179. 
 
 Newton, his views on the nature of gravity, 155. 
 
 Newton, manner in which he discovered law of gravity, 175. 
 
 o. 
 
 Orpheus, his reference to the ether, 13. 
 
 P. 
 
 Philosopher, the physical, has to do with apparent, not absolute 
 relations, 240. 
 
 Plateau, his experiment to prove the nebular hypothesis, 188. 
 
 Pltlcher's tubes, 145. 
 
 Poinsot shows how the ethereal atoms may rebound without be- 
 ing elastic, 77. 
 
 Polarization, mode of, described, 86. 
 
INDEX. 25 1 
 
 Polarization, its phenomena explained by the theory of transverse 
 
 vibrations, 87. 
 
 Popular Science Monthly, reference to, 12. 
 
 Potash, permanganate of, extinguishes the red and blue rays, 92. 
 Pressure, constant, its relation to heat capacity, 121. 
 Prism, how its nature affects to location of the maximum of 
 
 heat, 63. 
 
 Protoplasm, 208. 
 Prout, his law, 194. 
 
 Q- 
 
 Quinine, sulphate of, its action upon the spectrum, 62. 
 
 R. 
 
 Red, the'color, number of light vibrations necessary to, 70. 
 Regnault, his contribution to thermo-dynamics, 119.' 
 Resistance to submarine cables, 119. 
 
 Rock salt, prism composed of, its effect upon the solar beam, 63. 
 Rumford, Count, his experiment to show that heat is not mate- 
 rial, 103. 
 
 s. 
 
 Saigey, Emile, his idea of the ether as the constitutive element of 
 matter, 17. 
 
 Saigey, Emile, phenomena of organic life obedient to the laws of 
 mechanics, 18. 
 
 Saturn, perturbations of his orbit, "181. 
 
 Secchi, Father, concerning transverse motion of light, 81. 
 
 Secchi, Father, his treatise upon the unity of the physical 
 forces, 26. 
 
 Secular inequalities, 178. 
 
 Seguin, essay on railroads, 104. 
 
 Senarmont, M. De, regards physical phenomena as the manifesta- 
 tions of a single agent, 234. 
 
 Similarity of animal and vegetable structure, 206. 
 
 Soul, problems of the, independent of the laws of physics, 239. 
 
 Sound, analogies between it and light, 56. 
 
 Sound,^ mechanical equivalent of, difficult to determine, 58. 
 
 Sound,* the effect of vibrations, 55. 
 
252 INDEX. 
 
 Sound, undulation of waves of longitudinal, 79. 
 
 Spark, electric, stratification of, 193. 
 
 Spectrum, analogy between its colors and harmonious sounds, 94. 
 
 Spectrum, continuous, how produced, 93. 
 
 Spectrum, its invisible calorific and chemical portions, 62. 
 
 Spectrum, reversement of, 95. 
 
 Spiller, Philip, his theory of the ether, 15. 
 
 Substitution, law of, 203. 
 
 Swedenborg on the ether, 14. 
 
 Swedenborg on the luminiferous ether, 164. 
 
 T. 
 
 Temperature, absolute zero of, 120. 
 
 Temperature produces changes in the form of bodies, no. 
 
 Thales made water the principle of all things, 46. 
 
 Theories, their use in scientific investigations, 44. 
 
 Thermo-electric pile, 145. 
 
 Top, experiment with, illustrating lateral displacement, 80. 
 
 Translation of atoms, motion of, 78. 
 
 Transversal vibrations extinguished in conducting bodies, 153. 
 
 Tyndall, concerning vibration of atoms, 77. 
 
 Tyndall, estimate of his work, "Heat as a Mode of Motion," 99. 
 
 Tyndall, his experiment with compressed air, 123. 
 
 Tyndall, on the luminiferous ether, 15. 
 
 u. 
 
 Units, electric, necessity of determining, 130. 
 Units, their importance in physics, 128. 
 Unity of physical forces, an ancient doctrine, 46. 
 Universe, author's theory of, 189. 
 
 V. 
 
 Verdet, treatise on the mechanical theory of heat, 26. 
 Vis viva, definition of, in mechanics, 107. 
 
 Vis viva of the heavenly bodies compared with that of mole- 
 cules, 193. 
 
INDEX. 253 
 
 Violet color, number of vibrations necessary to cause, 7 
 Vitality considered as a material motion, 211. 
 Voltameter, the, 147. 
 Volume, constant, its relation to heat capacity, 121. 
 
 w. 
 
 Water, effect of passing a solar beam through a prism of, 63. 
 Whewell, History of the Inductive Sciences, 13. 
 Will, the, its relation to mechanical work, 230. 
 
 x. 
 
 Xenophanes, his doctrine regarding matter, 47. 
 
 Y. 
 
 Yellow, the color, number of vibrations necessary to'produce, 70. 
 Young, Dr. T., confirms undulatory theory of light, "14. 
 
ERRATA. 
 
 Page 18, line i$,for organic read inorganic. 
 
 " 41, last line, supply the word our before thought. 
 
 " 47, line 2, for Anaximine read Anaximines. 
 
 " 48, line i8,/or advantages read advantage. 
 
 " 80, line 18, after word component insert of. 
 
 " 81, line 21, for good read goal. 
 
 " 195, line 19, for agreement read argument. 
 
 The notes at the bottom of pages 78, 80, 86, 118, 120, 164, are by 
 
 the translator. 
 The note at the bottom of page 188 is by the author, as far as 
 
 " The reader will observe," etc. 
 
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