The ancient axiom that "Matter can not act where it is not any more than when it is not," was universally believed till Newton's time, and Newton himself regarded it as a self-evident truth. Some of his disciples asserted that gravitation must be considered as an essential property of matter, and they were under the necessity of assuming that atoms can exert a force upon one another across a void. This to Leibnitz was either miraculous or absurd; and in modern times the doctrine is rejected by the first physicists—by Faraday, Helmholtz, Thomson, Tait, and Maxwell.[275] Sir William Thomson, the Newton of modern physics, says emphatically, "I have no faith whatever in attractions and repulsions acting at a distance between centres of force according to various laws."[276] And Clerk Maxwell, in his lecture on "Action at a Distance,"[277] explains how Faraday, by his discovery of magnetic rotation of polarized light, and by his showing how lines of force arise in media, "rudely shook the theory of attraction and repulsion at a distance across a void."
If, now, "direct action at a distance" is rejected by scientific men as inconceivable and absurd, how can it be that the sun pulls the earth toward it, and holds the planets in their orbits? The verbal statement of the law of gravitation is no answer to this question. It expresses a fact, but it does not assign a cause. Gravitation is a phenomenon which demands an explanation, and some of the first scientists of the day are engaged in devising a theory which shall afford a rational answer to the question, What is the cause of gravity?[278]
The first and most fundamental presupposition for any physical hypothesis which seeks to explain the action of gravitation is that some medium of communication exists. This is suggested by every physical analogy. Sound is communicated through a medium. The influence which is exerted at a distance by heat, light, electricity, and magnetism is effected through media. The most plausible suggestion yet made is that "a single omnipresent fluid, ether, fills the universe," which by various forms or modes of motion transmits light, radiant heat, magnetism, and electricity.[279] May not gravitation, it is asked, be transmitted by the same fluid? may it not consist of or result from actual recurring impulses propagated in ethereal waves?
The hypothesis that gravitation is transmitted through the same medium as light, or indeed through any medium, is encumbered with serious if not insuperable difficulties. All transmission of whatever kind—of a letter by the post, a gunshot, a sound, a wave of light, an electro-magnetic disturbance—occupies time. It has a velocity—sometimes a very great one, as in the case of light; still it is a measurable velocity. But, according to Herschel, the pull which the sun exerts on the earth is delivered instantaneously. Were it not so there would be "a continually progressive increase of the major axis of the earth's orbit, and therefore of the length of the year."[280] Surely it must be obvious to every one that the instantaneous transmission of the sun's attractive force to the planet Neptune, three thousand millions of miles distant, through a physical medium like the ether, would be as great a miracle as action at a distance through a perfect void. But the advocates of this hypothesis have not thereby escaped the difficulties of action at a distance. The majority of physicists regard the luminiferous ether as consisting of "discrete particles"—"elementary molecules of inconceivable minuteness and tenuity." These ultimate particles or atoms of highly attenuated matter must have some magnitude, some extension, however inconceivably minute. If extended, they must have some form, and must occupy separate positions in space. If they are capable of motions—undulatory, rotatory, or spiral motions—they can not be in mutual contact. Conceive, then, two such atoms, and draw around each an imaginary circle. Let these circles touch at the middle point between the two, and ask yourself the question, What exists there? On the hypothesis under consideration you are bound to answer pure, empty space—that is, pure nothing. "But if there is no matter between the atoms, then all their actions, one upon the other, must be exerted across a void—that is, through a medium of nothingness;" in other words, through no medium at all. Now the size of the interval makes no difference in the argument. "Whether that interval be the 92 billionth of an inch, or the 92 millions of miles or thereabouts between the earth and the sun, it is still action at a distance, and no escape."[281]
The physicist who regards the ether as consisting of discrete particles not in bodily or actual contact, and at the same time finds himself logically compelled to reject this "mystical action at a distance," has no alternative but to accept the doctrine of Newton that the action of one particle of matter upon another is mediated by an agent which is not material. "If it be true that the conception of force as the originator of motion in matter without bodily contact ... is essential to the right interpretation of phenomena; and if it be equally true, on the other hand, that its exertion makes itself manifest to our personal consciousness by that peculiar sensation of effort which is not without its analogue in purely intellectual acts of the mind, it [i. e., force] comes not unnaturally to be regarded as affording a point of contact, a connecting link between these two great departments of being—between mind and matter—the one as the originator, the other as the recipient of force."[282]
There are distinguished physicists—as Helmholtz, Thomson, Challis, and Maxwell—who seek to escape the difficulties of action at a distance by the assumption that the ether is absolutely continuous (and therefore does not consist of atoms)—a perfectly homogeneous, incompressible, frictionless fluid which fills the universe. This fundamental presupposition as the basis of a physical theory of the universe necessitates the further assumption that "motion is the very essence of what has been hitherto called matter."[283] All quantitative and qualitative phenomena, all statical and dynamical phenomena, are due solely to varied modes of motion in the primordial fluid. "By various motions of the nature of eddies [ring-vortices], the qualities of matter—cohesion, elasticity, hardness, weight, mass, or other universal properties of matter—are given to small portions of the fluid which constitute the chemical atom, and these, by modifications in their combinations, form, and motion, produce the accidental phenomena of gross matter.... On this view, gross matter would be merely an assemblage of parts of the medium moving in a peculiar way, groups of ring-vortices having inertia.... The primary fluid by other motions transmits light, radiant heat, magnetism, and gravitation."[284]
It may be regarded as an act of presumption in an obscure critic to offer an opinion of the theories of these great masters in science. We venture, however, to suggest that most men will find a difficulty in conceiving how space absolutely full of matter can be made to contain more, or how a truly continuous substance can be capable of condensation. The most tenuous ether, if it be absolutely continuous, occupies the whole of the space in which it lies—that is, there is no point of the space which is not occupied by a point of matter.[285] But the hardest iron can do no more than this, and, therefore, on this hypothesis it seems impossible to account for its greater density. It is suggested that if molecules are mere assemblages of parts of the ether moving in a peculiar way, then greater density may be due to a modification in the motion of molecules, and not merely to the greater frequency of the eddying molecules in a given space. But how can a truly continuous substance have parts, and how can relative motion occur in an absolute plenum? The very notion of particles is quite inconsistent with the continuity of matter; and in a universe absolutely full no motion whatever would be possible. We are told that Sir William Thomson and Professor Tait find no difficulty in all these, to our minds, contradictory conceptions, and therefore we must conclude that our intellect is not properly "focussed so as to give definition without prenumbral haze."
Granting, then, the absolute continuity of all matter, and the possibility of motion in an absolute plenum, the question which concerns us most in this essay is, How is motion generated and sustained? One of the greatest lights of this new school tells us that "all we can affirm of matter is that it is the recipient of impulse and of energy."[286] They no longer regard the atom "as a mystic point endowed with inertia and the attribute of attracting and repelling other such centres with forces depending on the intervening distances."[287] They have "no faith whatever in attractions and repulsions acting at a distance between centres of force."[288] Force, then, is not regarded by these leading physicists as an inherent attribute of matter. The primary fluid, originally inert and motionless, must have been set in motion by some force, by some agency external to and distinct from itself. An "original impetus" from without, according to Maxwell,[289] or a "pressure" of the universal ether "from somewhere outside the world of stars," according to Challis,[290] must be the source of all motion and all forms of energy in the universe.
It is a fundamental principle of dynamics that "force is wholly expended in the action it produces,"[291] therefore, if all the forms of energy in the universe are the result of pressure, that pressure must be continuous; if they are the result of impulses, these impulses must be incessantly renewed, and must recur with immeasurable rapidity. On either supposition, "the universe is not even temporarily automatic, but must be fed from moment to moment by an agency external to itself," and "the preservation of the universe is effected only by the unceasing expenditure of enormous quantities of work;"[292] that is, it is ceaselessly sustained by Divine Omnipotence—"He upholdeth all things by the word of his power."