CHAPTER XII

THE LAW OF SUBSTANCE

The Fundamental Chemical Law of the Constancy of Matter--The Fundamental Physical Law of the Conservation of Energy--Combination of Both Laws in the Law of Substance--The Kinetic, Pyknotic, and Dualistic Ideas of Substance--Monism of Matter--Ponderable Matter--Atoms and Elements--Affinity of the Elements--The Soul of the Atom (Feeling and Inclination)--Existence and Character of Ether--Ether and Ponderable Matter--Force and Energy--Potential and Actual Force--Unity of Natural Forces--Supremacy of the Law of Substance

The supreme and all-pervading law of nature, the true and only cosmological law, is, in my opinion, _the law of substance_; its discovery and establishment is the greatest intellectual triumph of the nineteenth century, in the sense that all other known laws of nature are subordinate to it. Under the name of "law of substance" we embrace two supreme laws of different origin and age--the older is the chemical law of the "conservation of matter," and the younger is the physical law of the "conservation of energy."[23] It will be self-evident to many readers, and it is acknowledged by most of the scientific men of the day, that these two great laws are essentially inseparable. This fundamental thesis, however, is still much contested in some quarters, and we must proceed to furnish the proof of it. But we must first devote a few words to each of the two laws.

The law of the "_persistence_" or "_indestructibility of matter_," established by Lavoisier in 1789, may be formulated thus: The sum of matter, which fills infinite space, is unchangeable. A body has merely changed its form, when it seems to have disappeared. When coal burns, it is changed into carbonic-acid gas by combination with the oxygen of the atmosphere; when a piece of sugar melts in water, it merely passes from the solid to the fluid condition. In the same way, it is merely a question of change of form in the cases where a new body seems to be produced. A shower of rain is the moisture of the atmosphere cast down in the form of drops of water; when a piece of iron rusts, the surface layer of the metal has combined with water and with atmospheric oxygen, and formed a "rust," or oxyhydrate of iron. Nowhere in nature do we find an example of the production, or "creation," of new matter; nowhere does a particle of existing matter pass entirely away. This empirical truth is now the unquestionable foundation of chemistry; it may be directly verified at any moment by means of the balance. To the great French chemist Lavoisier belongs the high merit of first making this experiment with the balance. At the present day the scientist, who is occupied from one end of the year to the other with the study of natural phenomena, is so firmly convinced of the absolute "constancy" of matter that he is no longer able to imagine the contrary state of things.

We may formulate the "_law of the persistence of force_" or "_conservation of energy_" thus: The sum of force, which is at work in infinite space and produces all phenomena, is unchangeable. When the locomotive rushes along the line, the potential energy of the steam is transformed into the kinetic or actual energy of the mechanical movement; when we hear its shrill whistle, as it speeds along, the sound-waves of the vibrating atmosphere are conveyed through the tympanum and the three bones of the ear into the inner labyrinth, and thence transferred by the auditory nerve to the acoustic ganglionic cells which form the centre of hearing in the temporal lobe of the gray bed of the brain. The whole marvellous panorama of life that spreads over the surface of our globe is, in the last analysis, transformed sunlight. It is well known how the remarkable progress of technical science has made it possible for us to convert the different physical forces from one form to another; heat may be changed into molar movement, or movement of mass; this in turn into light or sound, and then into electricity, and so forth. Accurate measurement of the quantity of force which is used in this metamorphosis has shown that it is "constant" or unchanged. No particle of living energy is ever extinguished; no particle is ever created anew. Friedrich Mohr, of Bonn, was very near to the discovery of this great fact in 1837, but the discovery was actually made by the able Swabian physician, Robert Mayer, of Heilbronn, in 1842. Independently of Mayer, however, the principle was reached almost at the same time by the famous physiologist, Hermann Helmholtz; five years afterwards he pointed out its general application to, and fertility in, every branch of physics. We ought to say to-day that it rules also in the entire province of physiology--that is, of "organic physics"; but on that point we meet a strenuous opposition from the vitalistic biologists and the dualist and spiritualist philosophers. For these the peculiar "spiritual forces" of human nature are a group of "free" forces, not subject to the law of energy; the idea is closely connected with the dogma of the "freedom of the will." We have, however, already seen (p. 204) that the dogma is untenable. Modern physics draws a distinction between "force" and "energy," but our general observations so far have not needed a reference to it.

The conviction that these two great cosmic theorems, the chemical law of the persistence of matter and the physical law of the persistence of force, are fundamentally one, is of the utmost importance in our monistic system. The two theories are just as intimately united as their objects--matter and force or energy. Indeed, this fundamental unity of the two laws is self-evident to many monistic scientists and philosophers, since they merely relate to two different aspects of one and the same object, the _cosmos_. But, however natural the thought may be, it is still very far from being generally accepted. It is stoutly contested by the entire dualistic philosophy, vitalistic biology, and parallelistic psychology; even, in fact, by a few (inconsistent) monists, who think they find a check to it in "consciousness," in the higher mental activity of man, or in other phenomena of our "free mental life."

For my part, I am convinced of the profound importance of the unifying "law of substance," as an expression of the inseparable connection in reality of two laws which are only separated in conception. That they were not originally taken together and their unity recognized from the beginning is merely an accident of the date of their respective discoveries. The earlier and more accessible chemical law of the persistence of matter was detected by Lavoisier in 1789, and, after a general application of the balance, became the basis of exact chemistry. On the other hand, the more recondite law of the persistence of force was only discovered by Mayer in 1842, and only laid down as the basis of exact physics by Helmholtz. The unity of the two laws--still much disputed--is expressed by many scientists who are convinced of it in the formula: "Law of the persistence of matter and force." In order to have a briefer and more convenient expression for this fundamental thought, I proposed some time ago to call it the "law of substance" or the "fundamental cosmic law"; it might also be called the "universal law," or the "law of constancy," or the "axiom of the constancy of the universe." In the ultimate analysis it is found to be a necessary consequence of the principle of causality.[24]

The first thinker to introduce the purely monistic conception of substance into science and appreciate its profound importance was the great philosopher Baruch Spinoza; his chief work appeared shortly after his premature death in 1677, just one hundred years before Lavoisier gave empirical proof of the constancy of matter by means of the chemist's principal instrument, the balance. In his stately pantheistic system the notion of the _world_ (the universe, or the cosmos) is identical with the all-pervading notion of God; it is at one and the same time the purest and most rational _monism_ and the clearest and most abstract _monotheism_. This universal substance, this "divine nature of the world," shows us two different aspects of its being, or two fundamental attributes--matter (infinitely _extended_ substance) and spirit (the all-embracing energy of _thought_). All the changes which have since come over the idea of substance are reduced, on a logical analysis, to this supreme thought of Spinoza's; with Goethe I take it to be the loftiest, profoundest, and truest thought of all ages. Every single object in the world which comes within the sphere of our cognizance, all individual forms of existence, are but special transitory forms--_accidents_ or _modes_--of substance. These modes are material things when we regard them under the attribute of _extension_ (or "occupation of space"), but forces or ideas when we consider them under the attribute of _thought_ (or "energy"). To this profound thought of Spinoza our purified monism returns after a lapse of two hundred years; for us, too, matter (space-filling substance) and energy (moving force) are but two inseparable attributes of the one underlying substance.

Among the various modifications which the fundamental idea of substance has undergone in modern physics, in association with the prevalent atomism, we shall select only two of the most divergent theories for a brief discussion, the kinetic and the pyknotic. Both theories agree that we have succeeded in reducing all the different forces of nature to one common original force; gravity and chemical action, electricity and magnetism, light and heat, etc., are only different manifestations, forms, or _dynamodes_, of a single primitive force (_prodynamis_). This fundamental force is generally conceived as a vibratory motion of the smallest particles of matter--a vibration of atoms. The atoms themselves, according to the usual "kinetic theory of substance," are dead, separate particles of matter, which dance to and fro in empty space and act at a distance. The real founder and most distinguished representative of the kinetic theory is Newton, the famous discoverer of the law of gravitation. In his great work, the _Philosophiae Naturalis Principia Mathematica_ (1687), he showed that throughout the universe the same law of attraction controls the unvarying constancy of gravitation; the attraction of two particles being in direct proportion to their mass and in inverse proportion to the square of their distance. This universal force of gravity is at work in the fall of an apple and the tidal wave no less than in the course of the planets round the sun and the movements of all the heavenly bodies. Newton had the immortal merit of establishing the law of gravitation and embodying it in an indisputable mathematical formula. Yet this _dead mathematical formula_, on which most scientists lay great stress, as so frequently happens, gives us merely the _quantitative_ demonstration of the theory; it gives us no insight whatever into the _qualitative_ nature of the phenomena. The action at a distance without a medium, which Newton deduced from his law of gravitation, and which became one of the most serious and most dangerous dogmas of later physics, does not afford the slightest explanation of the real causes of attraction; indeed, it long obstructed our way to the real discovery of them. I cannot but suspect that his speculations on this mysterious action at a distance contributed not a little to the leading of the great English mathematician into the obscure labyrinth of mystic dreams and theistic superstition in which he passed the last thirty-four years of his life; we find him, at the end, giving metaphysical hypotheses on the predictions of Daniel and on the paradoxical fantasies of St. John.

In fundamental opposition to the theory of vibration, or the kinetic theory of substance, we have the modern "theory of condensation," or the pyknotic theory of substance. It is most ably established in the suggestive work of J. C. Vogt on _The Nature of Electricity and Magnetism on the Basis of a Simplified Conception of Substance_ (1891). Vogt assumes the primitive force of the world, the universal _prodynamis_, to be, not the vibration or oscillation of particles in empty space, but the condensation of a simple primitive substance, which fills the infinity of space in an unbroken continuity. Its sole inherent mechanical form of activity consists in a tendency to condensation or contraction, which produces infinitesimal centres of condensation; these may change their degree of thickness, and, therefore, their volume, but are constant as such. These minute parts of the universal substance, the centres of condensation, which might be called _pyknatoms_, correspond in general to the ultimate separate atoms of the kinetic theory; they differ, however, very considerably in that they are credited with sensation and inclination (or will-movement of the simplest form), _with souls_, in a certain sense--in harmony with the old theory of Empedocles of the "love and hatred of the elements." Moreover, these "atoms with souls" do not float in empty space, but in the continuous, extremely attenuated intermediate substance, which represents the uncondensed portion of the primitive matter. By means of certain "constellations, centres of perturbation, or systems of deformation," great masses of centres of condensation quickly unite in immense proportions, and so obtain a preponderance over the surrounding masses. By that process the primitive substance, which in its original state of quiescence had the same mean consistency throughout, divides or differentiates into two kinds. The centres of disturbance, which _positively_ exceed the mean consistency in virtue of the _pyknosis_ or condensation, form the ponderable matter of bodies; the finer, intermediate substance, which occupies the space between them, and _negatively_ falls below the mean consistency, forms the ether, or imponderable matter. As a consequence of this division into mass and ether there ensues a ceaseless struggle between the two antagonistic elements, and this struggle is the source of all physical processes. The positive ponderable matter, the element with the feeling of like or desire, is continually striving to complete the process of condensation, and thus collecting an enormous amount of _potential_ energy; the negative, imponderable matter, on the other hand, offers a perpetual and equal resistance to the further increase of its strain and of the feeling of dislike connected therewith, and thus gathers the utmost amount of _actual_ energy.

We cannot go any further here into the details of the brilliant theory of J. C. Vogt. The interested reader cannot do better than have recourse to the second volume of the above work for a clear, popular exposition of the difficult problem. I am myself too little informed in physics and mathematics to enter into a critical discussion of its lights and shades; still, I think that this pyknotic theory of substance will prove more acceptable to every biologist who is convinced of the unity of nature than the kinetic theory which prevails in physics to-day. A misunderstanding may easily arise from the fact that Vogt puts his process of condensation in explicit contradiction with the general phenomenon of motion; but it must be remembered that he is speaking of vibratory movement in the sense of the physicist. His hypothetical "condensation" is just as much determined by a movement of substance as is the hypothetical "vibration"; only the kind of movement and the relation of the moving elements are very different in the two hypotheses. Moreover, it is not the whole theory of vibration, but only an important section of it, that is contradicted by the theory of condensation.

Modern physics, for the most part, still firmly adheres to the older theory of vibration, to the idea of an _actio in distans_ and the eternal vibration of dead atoms in empty space; it rejects the pyknotic theory. Although Vogt's theory may be still far from perfect, and his original speculations may be marred by many errors, yet I think he has rendered a very good service in eliminating the untenable principles of the kinetic theory of substance. As to my own opinion--and that of many other scientists--I must lay down the following theses, which are involved in Vogt's pyknotic theory, as indispensable for a truly monistic view of substance, and one that covers the whole field of organic and inorganic nature:

I. The two fundamental forms of substance, ponderable matter and ether, are not dead and only moved by extrinsic force, but they are endowed with sensation and will (though, naturally, of the lowest grade); they experience an inclination for condensation, a dislike of strain; they strive after the one and struggle against the other.

II. There is no such thing as empty space; that part of space which is not occupied with ponderable atoms is filled with ether.

III. There is no such thing as an action at a distance through perfectly empty space; all action of bodies upon each other is either determined by immediate contact or is effected by the mediation of ether.

Both the theories of substance which we have just contrasted are _monistic_ in principle, since the opposition between the two conditions of substance--mass and ether--is not original; moreover, they involve a continuous immediate contact and reciprocal action of the two elements. It is otherwise with the _dualistic_ theories of substance which still obtain in the idealist and spiritualist philosophy, and which have the support of a powerful theology, in so far as theology indulges in such metaphysical speculations. These theories draw a distinction between two entirely different kinds of substance, material and immaterial. Material substance enters into the composition of the bodies which are the object of physics and chemistry; the law of the persistence of matter and force is confined to this world (apart from a belief in its "creation from nothing" and other miracles). Immaterial substance is found in the "spiritual world" to which the law does not extend; in this province the laws of physics and chemistry are either entirely inapplicable or they are subordinated to a "vital force," or a "free will," or a "divine omnipotence," or some other phantom which is beyond the ken of critical science. In truth, these profound errors need no further refutation to-day, for experience has never yet discovered for us a single immaterial substance, a single force which is not dependent on matter, or a single form of energy which is not exerted by material movement, whether it be of mass, or of ether, or of both. Even the most elaborate and most perfect forms of energy that we know--the psychic life of the higher animals, the thought and reason of man--depend on material processes, or changes in the neuroplasm of the ganglionic cells; they are inconceivable apart from such modifications. I have already shown (chap. xi.) that the physiological hypothesis of a special, immaterial "soul-substance" is untenable.

The study of ponderable matter is primarily the concern of chemistry. Few are ignorant of the astonishing theoretical progress which this science has made in the course of the century and the immense practical influence it has had on every aspect of modern life. We shall confine ourselves here to a few remarks on the more important questions which concern the nature of ponderable matter. It is well known that analytical chemistry has succeeded in resolving the immense variety of bodies in nature into a small number of simple elements--that is, simple bodies which are incapable of further analysis. The number of these elements is about seventy. Only fourteen of them are widely distributed on the earth and of much practical importance; the majority are rare elements (principally metals) of little practical moment. The affinity of these groups of elements, and the remarkable proportions of their atomic weights, which Lothar Meyer and Mendelejeff have proved in their _Periodic System of the Elements_, make it extremely probable that they are not _absolute species_ of ponderable matter--that is, not eternally unchangeable particles. The seventy elements have in that system been distributed into eight leading groups, and arranged in them according to their atomic weight, so that the elements which have a chemical affinity are formed into families. The relations of the various groups in such a natural system of the elements recall, on the one hand, similar relations of the innumerable compounds of carbon, and, again, the relations of parallel groups in the natural arrangement of the animal and plant species. Since in the latter cases the "affinity" of the related forms is based on descent from a common parent form, it seems very probable that the same holds good of the families and orders of the chemical elements. We may, therefore, conclude that the "empirical elements" we now know are not really simple, ultimate, and unchangeable forms of matter, but compounds of homogeneous, simple, primitive atoms, variously distributed as to number and grouping. The recent speculations of Gustav Wendt, Wilhelm Preyer, Sir W. Crookes, and others, have pointed out how we may conceive the evolution of the elements from a simple primitive material, the _prothyl_.

The modern atomistic theory, which is regarded as an indispensable instrument in chemistry to-day, must be carefully distinguished from the old philosophic atomism which was taught more than two thousand years ago by a group of distinguished thinkers of antiquity--Leucippus, Democritus, and Epicurus: it was considerably developed and modified later on by Descartes, Hobbes, Leibnitz, and other famous philosophers. But it was not until 1808 that modern atomism assumed a definite and acceptable form, and was furnished with an empirical basis by Dalton, who formulated the "law of simple and multiple proportions" in the formation of chemical combinations. He first determined the atomic weight of the different elements, and thus created the solid and exact foundation on which more recent chemical theories are based; these are all _atomistic_, in the sense that they assume the elements to be made up of homogeneous, infinitesimal, distinct particles, which are incapable of further analysis. That does not touch the question of the real nature of the atoms--their form, size, psychology, etc. These atomic qualities are merely hypothetical; while the _chemistry_ of the atoms, their "chemical affinity"--that is, the constant proportion in which they combine with the atoms of other elements--is empirical.[25]

The different relation of the various elements towards each other, which chemistry calls "affinity," is one of the most important properties of ponderable matter; it is manifested in the different relative quantities or proportions of their combination in the intensity of its consummation. Every shade of inclination, from complete indifference to the fiercest passion, is exemplified in the chemical relation of the various elements towards each other, just as we find in the psychology of man, and especially in the life of the sexes. Goethe, in his classical romance, _Affinities_, compared the relations of pairs of lovers with the phenomenon of the same name in the formation of chemical combinations. The irresistible passion that draws Edward to the sympathetic Ottilia, or Paris to Helen, and leaps over all bounds of reason and morality, is the same powerful "unconscious" attractive force which impels the living spermatozoon to force an entrance into the ovum in the fertilization of the egg of the animal or plant--the same impetuous movement which unites two atoms of hydrogen to one atom of oxygen for the formation of a molecule of water. This fundamental _unity of affinity in the whole of nature_, from the simplest chemical process to the most complicated love story, was recognized by the great Greek scientist, Empedocles, in the fifth century B.C., in his theory of "the love and hatred of the elements." It receives empirical confirmation from the interesting progress of cellular psychology, the great significance of which we have only learned to appreciate in the last thirty years. On those phenomena we base our conviction that even the _atom_ is not without a rudimentary form of sensation and will, or as it is better expressed, of feeling (_aesthesis_) and inclination (_tropesis_)--that is, a universal "soul" of the simplest character. The same must be said of the molecules which are composed of two or more atoms. Further combinations of different kinds of these molecules give rise to simple and, subsequently, complex chemical compounds, in the activity of which the same phenomena are repeated in a more complicated form.

The study of ether, or imponderable matter, pertains principally to physics. The existence of an extremely attenuated medium, filling the whole of space outside of ponderable matter, was known and applied to the elucidation of various phenomena (especially light) a long time ago; but it was not until the second half of the nineteenth century that we became more closely acquainted with this remarkable substance, in connection with our astonishing empirical discoveries in the province of electricity, with their experimental detection, their theoretical interpretation, and their practical application. The path was opened in particular by the famous researches of Heinrich Hertz, of Bonn, in 1888. The premature death of a brilliant young physicist of so much promise cannot be sufficiently deplored. Like the premature death of Spinoza, Raphael, Schubert, and many other great men, it is one of those brutal facts of human history which are enough of themselves to destroy the untenable myth of a "wise Providence" and an "All-loving Father in heaven."

The existence of ether (or cosmic ether) as a real element is a _positive fact_, and has been known as such for the last twelve years. We sometimes read even to-day that ether is a "pure hypothesis"; this erroneous assertion comes not only from uninformed philosophers and "popular" writers, but even from certain "prudent and exact physicists." But there would be just as much reason to deny the existence of ponderable matter. As a matter of fact, there are metaphysicians who accomplish even this feat, and whose highest wisdom lies in denying or calling into question the existence of an external universe; according to them only one real entity exists--their own precious personality, or, to be more correct, their immortal soul. Several modern physiologists have embraced this ultra-idealist view, which is to be found in Descartes, Berkeley, Fichte, and others. Their "psycho-monism" affirms: "One thing only exists, and that is my own mind." This audacious spiritualism seems to us to rest on an erroneous inference from Kant's correct critical theory, that we can know the outer world only in the phenomenal aspect which is accessible to our human organs of thought--the brain and the organs of sense. If by those means we can attain only an imperfect and limited knowledge of the material world, that is no reason for denying its existence altogether. In my opinion, the existence of ether is as certain as that of ponderable matter--as certain as my own existence, as I reflect and write on it. As we assure ourselves of the existence of ponderable matter by its mass and weight, by chemical and mechanical experiments, so we prove that of ether by the experiences and experiments of optics and electricity.

Although, however, the existence of ether is now regarded as a positive fact by nearly all physicists, and although many effects of this remarkable substance are familiar to us through an extensive experience, especially in the way of optical and electrical experiments, yet we are still far from being clear and confident as to its real character. The views of the most eminent physicists, who have made a special study of it, are extremely divergent; they frequently contradict each other on the most important points. One is, therefore, free to choose among the contradictory hypotheses according to one's knowledge and judgment. I will put in the following eight theses the view which has approved itself to me after mature reflection on the subject, though I am no expert in this department:

I. Ether fills the whole of space, in so far as it is not occupied by ponderable matter, as a _continuous substance_; it fully occupies the space between the atoms of ponderable matter.

II. Ether has probably no chemical quality, and is not composed of atoms. If it be supposed that it consists of minute homogeneous atoms (for instance, indivisible etheric particles of a uniform size), it must be further supposed that there is something else between these atoms, either "empty space" or a third, completely unknown medium, a purely hypothetical "interether"; the question as to the nature of this brings us back to the original difficulty, and so on _in infinitum_.

III. As the idea of an empty space and an action at a distance is scarcely possible in the present condition of our knowledge (at least it does not help to a clear monistic view), I postulate for ether a special structure which is not atomistic, like that of ponderable matter, and which may provisionally be called (without further determination) _etheric_ or _dynamic_ structure.

IV. The consistency of ether is also peculiar, on our hypothesis, and different from that of ponderable matter. It is neither gaseous, as some conceive, nor solid, as others suppose; the best idea of it can be formed by comparison with an extremely attenuated, elastic, and light jelly.

V. Ether may be called _imponderable_ matter in the sense that we have no means of determining its weight experimentally. If it really has weight, as is very probable, it must be so slight as to be far below the capacity of our most delicate balance. Some physicists have attempted to determine its weight by the energy of the light-waves, and have discovered that it is some fifteen trillion times lighter than atmospheric air; on that hypothesis a sphere of ether of the size of our earth would weigh at least two hundred and fifty pounds(?).

VI. The etheric consistency may probably (in accordance with the pyknotic theory) pass into the gaseous state under certain conditions by progressive condensation, just as a gas may be converted into a fluid, and ultimately into a solid, by lowering its temperature.

VII. Consequently, these three conditions of matter may be arranged (and it is a point of great importance in our monistic cosmogony) in a genetic, continuous order. We may distinguish five stages in it: (1) the etheric, (2) the gaseous, (3) the fluid, (4) the viscous (in the living protoplasm), and (5) the solid state.

VIII. Ether is boundless and immeasurable, like the space it occupies. It is in eternal motion; and this specific movement of ether (it is immaterial whether we conceive it as vibration, strain, condensation, etc.), in reciprocal action with mass-movement (or gravitation), is the ultimate cause of all phenomena.

"The great question of the nature of ether," as Hertz justly calls it, includes the question of its relation to ponderable matter; for these two forms of matter are not only always in the closest external contact, but also in eternal, dynamic, reciprocal action. We may divide the most general phenomena of nature, which are distinguished by physics as natural forces or "functions of matter," into two groups; the first of them may be regarded mainly (though not exclusively) as a function of ether, and the second a function of ponderable matter--as in the following scheme which I take from my _Monism_:

THE WORLD (NATURE, OR THE COSMOS)

---------------------------------+------------------------------------- ETHER--Imponderable. | MASS--Ponderable. ---------------------------------+------------------------------------- | 1. _Consistency_: | 1. _Consistency_: | Etheric (_i.e._, neither | Not etheric (but gaseous, fluid, gaseous nor fluid, nor solid). | or solid). | 2. _Structure_: | 2. _Structure_: | Not atomistic, not made up of | Atomistic, made up of infinitesimal, separate particles (atoms), but | distinct particles (atoms) continuous. | discontinuous. | 3. _Chief Functions_: | 3. _Chief Functions_: | Light, radiant heat, electricity,| Gravity, inertia, molecular heat, and magnetism. | and chemical affinity. ---------------------------------+-------------------------------------

The two groups of functions of matter, which we have opposed in this table, may, to some extent, be regarded as the outcome of the first "division of labor" in the development of matter, the "primary ergonomy of matter." But this distinction must not be supposed to involve an absolute separation of the two antithetic groups; they always retain their connection, and are in constant reciprocal action. It is well known that the optical and electrical phenomena of ether are closely connected with mechanical and chemical changes in ponderable elements; the radiant heat of ether may be directly converted into the mechanical heat of the mass; gravitation is impossible unless the ether effects the mutual attraction of the separated atoms, because we cannot admit the idea of an _actio in distans_. In like manner, the conversion of one form of energy into another, as indicated in the law of the persistence of force, illustrates the constant reciprocity of the two chief types of substance, ether and mass.

The great law of nature, which, under the title of the "law of substance," we put at the head of all physical considerations, was conceived as the law of "the persistence of force" by Robert Meyer, who first formulated it, and Helmholtz, who continued the work. Another German scientist, Friedrich Mohr, of Bonn, had clearly outlined it in its main features ten years earlier (1837). The old idea of _force_ was, after a time, differentiated by modern physics from that of _energy_, which was at first synonymous with it. Hence the law is now usually called the "law of the persistence of energy." However, this finer distinction need not enter into the general consideration, to which I must confine myself here, and into the question of the great principle of the "persistence of substance." The interested reader will find a very clear treatment of the question in Tyndall's excellent paper on "The Fundamental Law of Nature," in his _Fragments of Science_. It fully explains the broad significance of this profound cosmic law, and points out its application to the main problems of very different branches of science. We shall confine our attention to the important fact that the "principle of energy" and the correlative idea of the unity of natural forces, on the basis of a common origin, are now accepted by all competent physicists, and are regarded as the greatest advance of physics in the nineteenth century. We now know that heat, sound, light, chemical action, electricity, and magnetism are all modes of motion. We can, by a certain apparatus, convert any one of these forces into another, and prove by an accurate measurement that not a single particle of energy is lost in the process.

The sum-total of force or energy in the universe remains constant, no matter what changes take place around us; it is eternal and infinite, like the matter on which it is inseparably dependent. The whole drama of nature apparently consists in an alternation of movement and repose; yet the bodies at rest have an inalienable quantity of force, just as truly as those that are in motion. It is in this movement that the potential energy of the former is converted into the kinetic energy of the latter. "As the principle of the persistence of force takes into account repulsion as well as attraction, it affirms that the mechanical value of the potential energy and the kinetic energy in the material world is a constant quantity. To put it briefly, the force of the universe is divided into two parts, which may be mutually converted, according to a fixed relation of value. The diminution of the one involves the increase of the other; the total value remains unchanged in the universe." The potential energy and the actual, or kinetic, energy are being continually transformed from one condition to the other; but the infinite sum of force in the world at large never suffers the slightest curtailment.

Once modern physics had established the law of substance as far as the simpler relations of inorganic bodies are concerned, physiology took up the story, and proved its application to the entire province of the organic world. It showed that all the vital activities of the organism--without exception--are based on a constant "reciprocity of force" and a correlative change of material, or metabolism, just as much as the simplest processes in "lifeless" bodies. Not only the growth and the nutrition of plants and animals, but even their functions of sensation and movement, their sense-action and psychic life, depend on the conversion of potential into kinetic energy, and _vice versâ_. This supreme law dominates also those elaborate performances of the nervous system which we call, in the higher animals and man, "the action of the mind."

Our monistic view, that the great cosmic law applies throughout the whole of nature, is of the highest moment. For it not only involves, on its positive side, the essential unity of the cosmos and the causal connection of all phenomena that come within our cognizance, but it also, in a negative way, marks the highest intellectual progress, in that it definitely rules out the three central dogmas of metaphysics--God, freedom, and immortality. In assigning mechanical causes to phenomena everywhere, the law of substance comes into line with the universal law of causality.