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pose, the establishment of the ether with any demonstrated properties might aid our conceptions of matter, and be concatenated with it as one of its higher forms. Maxwell has pointed out ("Encyclopædia Britannica," ninth edition, article "Ether") that, though many ethers have been proposed for various purposes, none have survived except that which was invented by Huygens to explain the propagation of light. Evidence accumulates for this hypothesis, in some form, for we have no other way of accounting for the facts, but the mechanism is still a mystery.

The very property of the supposititious ethers which is so fatal to all explanation of a static stress, like gravity, namely, the requirement of time for their functions, qualifies them so far as a vehicle of radiant manifestations. Were it not for the transmission of radiant energy in specific time, doubtless it would be far simpler and more satisfactory to explain the whole effect as actio in distans, under the necessary law of conservation, or on the Cartesian principle of contact. The phenomenon of electro-magnetic induction—which is believed to occur between the earth and sun, as a real material effect manifest in converted energy, and yet acting in lines transverse to the lines of transmission, and apparently simultaneous—even now outstands as unexplainable in any other way, for its mechanism certainly can not at present be comprehended. Nor is the mechanism for the transmission of the radiant forms of energy yet clearly made out, though some postulates about it having consistency and probability have been laid down. The fact that something supra-material is necessary and probable on other grounds gives encouragement to the idea that a basis for the atom can eventually be found.

The ether has been conceived under four principal modes of structure, all fashioned out of our concepts of matter. Two of these are static, and two kinetic. The first is the pseudo-concept of a continuous, colloidal plenum. This is a metaphysical, not a physical, concept; derived from an idealization of a false observation of matter which can not be realized consistently in thought with what is postulated of it afterward. As Maxwell happily remarks about the notion of homogeneous and continuous matter ("Encyclopædia Britannica," ninth edition, article "Atom"), "it is in its extreme form a theory incapable of development."

The second concept is that of a solid. This has been assumed as a conceivable way of accounting for the very high co-efficient of elasticity required by the undulatory theory, and also for the transverse mode of transmitting vibrations exhibited. The word "solid," however, can not have any meaning such as we ordinarily attach to it; and under any signification it is admitted that the theory is encumbered with several difficulties, some of which have been set forth by Professor G. G. Stokes, in his "Report on Double Refraction" ("British Association Report," 1862, p. 253).