D

URING the last fifteen years the views of scientific men with regard to the relation between the powers of the universe have been growing constantly clearer. The labours of Mayer and Helmholtz in Germany, and of Joule, Grove, Thomson, and Tyndall in England, have led gradually up to the differentiation of force and energy; while Dr. Balfour Stewart's little work, in the International Scientific Series,* has embodied the latest developments of thought upon the subject in a popular and comprehensible form. But even that valuable book has hardly carried out the differentiation to its furthest limits, or given a final definiteness to the conception of those antithetical notions with which it deals. The mistiness appears not to have lifted itself fully out of the mental horizon; the concept of the two great powers which divide the universe seems not to have been realized and assimilated in all their separation and antagonism. Perhaps a pair of definitions, given by two of our best-known authors, will make this more apparent than pages of criticism. Professor Tyndall says, "Let us employ, generally, the useful and appropriate term energy to denote the power of performing work."+ And so, too, Dr. Balfour Stewart, almost echoing his language, "Let us define by the term energy this power which the rifle-ball possesses of overcoming obstacles, or of doing work." These are the only definitions of energy given in either volume; and I think every reader will feel that there is in both a certain lack of con

*The Conservation of Energy." I may as well acknowledge here, once for all, my obligations to this volume, without which the present paper would probably never have been written.

+ Heat Considered as a Mode of Motion," 2nd edition, § 154, p. 140.

"The Conservation of Energy," p. 13, § 18.

ciseness and scientific rigour, due, not indeed to any want of those qualities in their authors' minds, (for it is hardly necessary for me to add my meed of admiration to the world-wide fame of those great thinkers), but to the present unsettled and transitional state of scientific opinion upon the subject. I trust, therefore, it may not be unbecoming for one who is not a physical specialist to jot down the aspect in which this question presents itself to his mind.

The conclusion to which all late speculations seem to point is this. There are two powers in the universe, of opposite nature to one another-force and energy. Of these, force is attractive, or aggregative; and energy repulsive, or disjunctive. Both are indestructible; or, in other words, the sum total of each in the kosmos is always a fixed quantity. But while force (or aggregative power) remains always inherent in, and inseparable from, each atom of ponderable matter; energy (or disjunctive power) is capable of being transmitted from one atom of matter to another, or from matter to that hypotheical imponderable substance which we call æther. As a result of these properties, it happens that the dynamical formula for the kosmos, in its existing phase, is this: force is aggregating ponderable matter, immediately round certain centres of unknown number (the stars), and ultimately round the common centre of all solar and sidereal systems; while energy is being dissipated through imponderable æther; this process being locally interrupted or retarded wherever the energy dissipated (as radiant heat and light) from one mass is intercepted by the surface of an adjacent mass, on which it initiates sundry changes, known as storms, ocean currents, chemical reaction, organic life, &c.

Such are, briefly stated, the main proposi

force to draw together the various elements of which it would be composed. It would, therefore, though more diffused, be as changeless as that endowed with force alone. But in the kosmos as actually existing, we find both these powers side by side, producing a constant re-arrangement of particles according to a certain definite plan. Force is perpetually aggregating matter round many subordinate centres, and ultimately (as I shall endeavour to prove further on) round a single galactic centre; while energy is being dissipated into surrounding space. How this re-arrangement takes place is our next consideration.

Energy being equivalent to separation, the law of the conservation of energy amounts to this: that the sum total of separation in the kosmos is always the same, whether that separation be between masses, molecules, atoms, or the positive and negative electrical factors. If two masses stand apart from one another, as the earth and the sun, or a ten kilogramme weight suspended by a string, and an iron plate beneath it, they possess potential molar energy, in virtue of such separation. If the earth fell into the sun, or the weight upon the iron plate, heat would be generated proportionate in a known ratio (J) to their masses and their previous separation; that is, molecular separation would take the place of molar. Now, intermediate between these two forms of energy would be the kinetic state; which is thus seen to be the mode through which one form of energy is transformed into another, and through which, as we shall see hereafter, all the energy now possessed by ponderable matter is being transformed to imponderable æther. The exact amount of one kind of energy, which is equivalent to a certain unit of another kind, is a question for mathematical physicists, and has been fully dealt with by Mayer, Joule, and Thom

son.

The point to which attention is here directed is this, that some kind of separation invariably replaces another.

The primordial form of all energy is potential energy of visible position, or, as it might be better termed, of passive separation. But energy in this form is essentially unstable. Force tends to draw together any two bodies, however placed; and unless it be counteracted by some form of kinetic energy, will do so immediately. If we throw up a stone into the air, when it has reached

the dead-point, it possesses energy of visible position but it does so only for an indivisible portion of time; force begins at once to draw it downward again. Only when another form of force, such as that of cohesion, balances gravitation, as in the case of a rock perched on a mountain-top, or a weight suspended by a string, is energy of visible position lasting and stable. Now, all sidereal bodies in the universe are similarly situated to the stone in the air: by virtue of their separation from one another, they possess energy of visible position. But if they possessed that kind of energy only, they would rush together at once, and the molar separation would be transformed into molecular; in other words, they would assume an enormously high temperature, and a greatly diffused state. Some other kind of energy, then, must be keeping them asunder. What is this? I think the consideration of the solar system will give us the solution. Here we see a large central mass, the sun, and a number of minor masses, the planets. These latter all possess energy of visible position relatively to the sun, in an ascertained ratio of their mass and their distance from it. Suppose they were at rest, and were to fall into it, then their energy of position would be transformed into heat, or molecular separation. But they are not at rest: they possess a kinetic energy, which prevents them from falling into the sun, and this is centrifugal power. Now the question arises, "How did they get this centrifugal power?" Obviously not, as some people seem to think, from their rotary motion. This is a curious hysteron proteron. If I twirl a ball on a string round my head, the ball possesses centrifugal power; but it got that energy from my hand which twirls it. The sun, however, does not twirl the planets. The energy makes the rotary motion, not the rotary motion the energy. Whence, then, did it come? The answer to this question leads us into the very heart of our subject. In order to arrive at it, we must glance at the history of the kosmos from its earliest manifestations.

stages is a question with which we have now nothing to do, though I may touch upon it in another connection further on. Taking the universe, then, in this its earliest ascertainable stage, all the energy it now possesses in various shapes was then potentially present in the diffused and separate state of its atoms, as energy of passive separation; while all the force it now possesses was then existent, as at present, in the tendency with which every atom was endowed to unite chemically and mechanically with every other. And in this higher generalization of the indestructibility of power, are united the two subordinate ones of conservation of energy and persistence of force. It will now, perhaps, be evident in what sense the primordial form of all energy is that of visible position. The atoms of the universe may be supposed to have originally stood off from one another in a state of mechanical, chemical, and electrical separation. But this state could only have existed for a second. Whether we regard the universe as having been thus created, or as so existing through the agency of previous and hitherto incomprehensible conditionsand I allow that we have here reached the utmost verge of our intellectual horizon-we cannot but suppose that the force with which it was endowed would at once begin to act upon its atoms. An aggregation would thus necessarily set in towards certain common centres, a point for whose mathematical bearings I can only now refer the reader to Laplace; and for whose wider philosophical reasons I must direct him to Mr. Herbert Spencer's chapter on the Instability of the Homogeneoust. If, now, we take one of these separate aggregations into which the differentiating kosmos would divide itself— say our own solar system-we shall see that as the atoms of which it was composed clashed against one another, under the influence of the force of gravitation, and chemical and electrical attraction, a large part of their potential energy would assume the form of heat that heat, in fact, the unradiated remnant of which is now being given off by the sun, and, more slowly, by the molten nucleus of our own planet. But a portion of the energy thus metamorphosed would remain as kinetic molar energy; which, as the

:

Système du Monde, vol. ii. chaap. x First Principles," 3rd edition, chap. xix.

mass gradually changed from the spiral to the spheroidal condition, would assume that definite direction which we know as centrifugal power. It is not necessary to trace the evolution of the planetary bodies through its various stages; it will be clear that each nebulous ring, as it was left behind by the retreating nucleus, owed its separation to the centrifugal energy thus generated; and this will be equally true, whether we accept the theory of Laplace in its naked form, or in the modified shape given to it by Mr. R. A. Proctor. Thus, then, both the existing heat of the sun and planets, and the centrifugal power which causes their various rotations, with those of their satellites, is derived from the original energy of passive separation, variously modified by surrounding conditions.

It will now, I think, be evident to the reader that neither masses nor molecules (and we may probably infer that the same is true of atoms and electrical factors) can retain the separate state, except by means of actual motion, which in the case of masses is rotary, and in the case of molecules is supposed to be vibratory (though there are certain reasons, too long for parenthetical insertion, which lead some physicists to suspect that it is rotary in all cases). Two masses or two molecules freely suspended in space will rush together at once and assume the closest possible union, unless actual motion, either of centrifugal power or of heat, prevent them. We shall see hereafter that both motions are tending perpetually towards extinction, relatively to the bodies in which they exist, through communication of their energy to the surrounding matter, or to the ætherial medium. For the present we must direct our attention to a still wider application of the foregoing principles.

What is thus true of our system as a whole, and of the various molecules composing it, will in all probability be equally true of the kosmos at large, towards which the solar system may be said to bear somewhat the same relation as that borne towards itself by such a system of atoms as we call a molecule of protein. The various sidereal bodies composing our galaxy possess energy of visible position in virtue of their separation from one another. But if they possessed that form of energy alone, they would rush together in the shortest period at which gravity could act, would transform

it all into kinetic molecular separation or heat, and would begin radiating off that heat (for reasons which we have presently to examine) into surrounding æther. That they do not do so can only be explained, it seems, by supposing that what is true of each solar system is also true of the galaxy as a whole : namely, that all the bodies composing it are prevented from falling together by a spiral or rotary motion similar to that which is set up in each aggregating nebula or solar system, and due to the same causes. This à priori hypothesis is rendered all the more probable by observations on proper motion, especially that of double and multiple stars, which show, à posteriori, that such motion is, in some cases at least, a vera causa, and one which we know otherwise to be quite adequate to counteract, while it lasts, the attraction of each mass upon the other.

But if energy and force are both indestructible, how comes it that matter is gradually settling round common centres, and ultimately round a single kosmical centre? The answer to this question is to be found in the yet shadowy relations between matter and the unknown something, æther. No atom has any tendency to be deprived of any portion of its force. But every atom tends always to change its potential energy into kinetic, and to part with a portion of its kinetic energy to every other atom with which it comes in contact. If two masses, call them A and B, possessing mutual energy of position, be left free to act, they will rush together; but their molar energy of passive separation will be replaced by molecular energy of actual motion or heat. However, this transformed energy will not all continue to exist within the limits of the now united body A B, to which it originally appertained. The molecules, rapidly vibrating, will come into contact with other molecules, either of adjacent material bodies, such as the atmosphere, or of the ætherial medium; and they will impart to these molecules a portion of their motion, which will thus be diffused on every side into surrounding space. The par ticles of the body A B, being deprived of that form of energy which kept them asunder, will draw closer and closer together, under the influence of force (called in this aspect cohesion); and if the process be not interrupted from without, by the integration of fresh energy, it will continue until the

body is deprived of all its energy, and occupies the minimum of space, if any,* into which its molecules can be packed.

Similarly with a centrifugal power. Any body revolving round another in virtue of this energy (say the earth round the sun) is constantly coming in contact with molecules of æther, to which it imparts a portion of its energy, which is thus radiated off into space. For each measurable unit of energy so lost, the earth approaches a proportionate unit nearer the sun, under the influence of gravitation; and if this process be not interrupted from without, it will continue to communicate its energy and to approach nearer and nearer, until it finally has lost all its centrifugal power, and glides (not falls) into the sun. It is true that, as has been suggested, this energy may be from time to time recruited by meteoric showers, and similar accelerating causes; still, as all these are finite in number, it will none the less happen that any body so situated will ultimately have communicated to the æther all its energy, and will gradually unite with its primary. Upon this point, again, more hereafter. Enough has been said to show that in this case, too, energy will ultimately be transferred from matter to æther.

As to chemical and electrical separation, the two remaining forms of energy, they, being of the potential class, are evidently of their very nature unstable and fugitive. They are usually produced by artificial means, and can only, for the most part, be artificially preserved, often with great difficulty. If nature be allowed to work freely, chemical elements in an uncombined state will almost invariably combine with other elements for which they have an affinity, and less stable compounds will be exchanged for more stable; while again the positive and negative electricities will always rush together, unless forcibly prevented. In either case, as they change from the potential to the kinetic state, the separation which before existed between the atoms of the electrical factors will re-appear as heat; which will as usual be radiated off into space. We thus see that every kind of energy tends to change from the potential to the kinetic

I add this saving clause for those who hold Boscovich's theory of points, or Sir William Thomson's vortex-rings, on either of which theories we must suppose that the matter would be crowded out of existence.