_Gravity must be caused by an agent acting constantly according to certain laws._"[274] 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?