Modern physicists are agreed that visible motion, heat, electricity, magnetism, and radiance (radiant light and heat) are forms of actual energy which are correlated and capable of mutual conversion. Any one form may, by suitable processes, be transformed, wholly or in part, to an equivalent amount of any other form of energy. So much is generally accepted by scientific men.
But in regard to the primary force or forces in which these forms of energy have their origin, there is not the same agreement among physicists. Some regard gravitation, cohesion, and chemical affinity as the three primary forces of nature; while others suggest that the last two are related with and probably derived from the first.
There is also a respectable school of physicists who teach that atomic attractions and repulsions are the universal cosmic forces which originate all molecular and mechanical motions. Then, again, each of these forms of force have their special advocates. On the one side it is affirmed, as an important generalization, that all primary force is attractive; "there is no such thing in nature as a primary repulsive force."[268] Universal attraction is the one world-forming and world-conserving energy. On the other side it is contended that gravitation is not a primary, but a secondary and derivative force, and that the grand primal force is a universal force of repulsion.[269]
It is beyond our province to discuss the merits of these conflicting theories. Our position is that no purely physical hypothesis is adequate to account for the conservation of the universe, and therefore it is of little consequence to our argument which of the above theories may find most favor with scientific men. The tendency of modern scientific thought is toward the conception of "one primordial form of matter, and but one primary form of force," as the simplest basis upon which a physical theory of inanimate nature can be erected. The ultimate nature of this one primary force is a question for pure metaphysics. From the stand-point of physical science it can only be thought "as a pull or a push in a straight line."[270] Universal attraction or universal repulsion must be the ultimate dynamical conception for the pure physicist.
1. Let us consider the first hypothesis. It is claimed that gravitation, or universal attraction, is the great conserving and sustaining principle of the universe. A stone falls to the earth, a round body rolls along a plane inclined toward the horizon; a liquid mass, as a brook or a large river, flows on the sloping surface which forms its bed. All these phenomena are the varied manifestation of a universal tendency in all bodies to fall one toward the other. In virtue of this tendency the great orbs which hang suspended in space gravitate toward one another; the moon and the earth fall toward each other, and they both gravitate toward the sun. All the planets of our solar system continually act one on the other, and on the immense sphere which shines at their common focus. By its enormous mass, the sun keeps all of them in their orbits. If we ask why one body falls toward another which is more than ninety millions of miles off, in preference to moving in any other direction, the answer given is that, "Every particle of matter in the universe attracts every other particle with a force whose direction is that of the line joining the two, and whose magnitude is directly as the product of their masses, and inversely as the square of their distance from each other." This force of attraction is the universal bond which holds the universe together, and sustains its physical life.
To the superficial thinker, the language of the Newtonian philosophy appears to sanction the materialistic notion that gravitation and attraction are active powers essential to and inherent in matter. Such, however, was by no means the doctrine of Newton, and he was careful to guard his readers against any such misapprehension of his meaning. "The words attraction, repulsion, or tendencies of whatever kind toward a centre, I use indifferently and without distinction for each other, considering these forces not physically but metaphysically. Wherefore let not the reader suppose that by words of this kind I any where mean a species or mode of action, or cause, or physical reason; or that I really and in a physical sense assign forces to centres (which are only mathematical points), even though I may say that centres attract, or that forces belong to centres."[271]
The history of scientific opinion on the point before us furnishes a striking illustration of the manner in which language reacts on the ideas which it is intended to express, and thus men fall into the habit of talking nonsense without knowing it. The conception of atoms having the property of exerting various forces across a void space seemed to follow as a matter of course from the discovery of the law of gravitation, and from the language in which it is expressed. After Newton a school arose which taught that atoms have the property of exerting force at a distance, and that this property must be inherent in the atoms, just as Lucretius taught that hardness and elasticity were original indefeasible properties of the primordial elements, the "semina rerum," or seeds of things. But Newton did not teach this; he stated a fact, but did not devise an hypothesis; he attempted no explanation of the law of gravitation.
"The law of gravitation considered as a result is beautifully simple; in a few words it expresses a fact from which most numerous and complex results may be deduced by mere reasoning—results found invariably to agree with the records of observation; but this same law of gravitation looked upon as an axiom or first principle is so astonishingly far removed from all ordinary experience as to be almost incredible. What! every particle in the whole universe is actively attracting every other particle [that is, every particle in the universe with the same force, without any expenditure of force], through void, without the aid of any communication by means of matter, or otherwise—each particle, unchecked by distance, unimpeded by obstacles, throws this miraculous influence to infinite distance without the employment of any means![272] No particle interferes with its neighbor, but all these wonderful influences are co-existent in every point in space! The result is apparent at each particle, but the condition of intermediate space is exactly the same as though no such influence were being transmitted across it! Earth attracts Sirius across space, and yet the space between is as if neither Earth nor Sirius existed! Can these things be? We think not; and Newton himself did not affirm this."[273] On the contrary, he earnestly rejects any such hypothesis. "It is inconceivable that inanimate brute matter should, without the mediation of something else which is not material, operate upon and affect other matter without mutual contact, as it must do if gravitation, in the sense of Epicurus, be essential to and inherent in matter.... That gravitation should be innate, inherent, and essential to matter, so that one body may act upon another at a distance, through a vacuum, without the mediation of any thing else, by and through which their action and force may be conveyed from one to another, is to me so great an absurdity that I believe no man who has in philosophical matters a competent faculty of thinking can ever fall into it. Gravity must be caused by an agent acting constantly according to certain laws."[274]