ETHER, The, or COSMICAL ETHER, in physics and astronomy, postulated material substance, which is assumed to fill all space, and to penetrate freely among the ultimate particles of which all matter is composed. It is not in any way related to the substance known as nether" to the chemist, and the identity in name is unfortunate. The physicist has the ad vantage of priority, however, and cannot be ex pected to change the name because the chemist subsequently appropriated it for something else. Although it has not been possible to determine the properties of the ether of physics, the ad mission of its existence seemed a necessity of scientific reasoning. For we know that light is some kind of a periodic disturbance, and we know that it travels through interstellar space with a definite; finite speed. It appears absurd to suppose that a motion of any kind could take place in a void, in which there was nothing to be moved; and hence, as has been said, it appears to be a logical necessity to assume the existence of some kind of a luminiferous (light-bearing) ether throughout space. As soon as we begin to inquire closely into itS nature, however, we encounter difficulties that have proved insuper able. Obviously our conclusions in this respect depended to a large extent upon a study of the phenomena of light and, later, of electricity, and of the kind of motion that would be com petent to produce those phenomena. Naturally the assumption was first made that the ether, when submitted to stress, conforms to the same laws of elasticity that hold true in ordinary mat ter. (See ELASTICITY). In that case the full mathematical theory of the motion of the ether would involve no less than 21 numerical coeffi cients, if the ether were anisotropic. And it is as reasonable to believe that, whatever its nature may be, it is the same in all its parts, and that its properties, whatever they may be, are the same in all directions. If these two facts are admitted—that is, if the ether be ad mitted to be isotropic—then the number of con stants involved in the theory reduces to two. These, as is explained in the article EtAsTicrrt, are (1) the modulus of compressibility, and (2) the modulus of rigidity. If the ether were analogous to a liquid or a gas, its modulus of rigidity would be zero. It is found, however, that the equations of motion that are obtained by making the modulus of rigidity zero are not at all competent to explain the acual phenom ena of light; for in this case the ether-waves would be merely waves of alternate compres sion and rarefaction, like those of sound in the air, and there could be no such phenomenon as polarization. It must therefore be admitted that
the modulus of rigidity of the ether has a defi nite, finite value, if the ether itself is to be re garded as analogous to other kinds of matter, so far as its general mechanical deportment is concerned. If it be also admitted that the modulus of compressibility of the ether has a definite, finite value, the conclusion is reached that the ether can transmit two essentially dif ferent kinds of waves, one of which involves distortions of its parts, while the other involves changes in its density. Of these the first would admit of polarization, while the second would not. Moreover, the two kinds of waves would have, in general, different velocities of propa gation; and the fact that all ether-disturbances appear to be propagated at the same speed indi cates that only one kind exists, and that we must therefore make one of the three following assumptions with •regard to the compressibility of the ether: (I) The modulus of compressibil ity of the ether is infinite; or (2) it is zero; or (3) the circumstances under which the atoms (or their component electrons) impress their motions upon the ether are such that the modu lus of compressibility is not involved in any way. The first of these alternatives implies that the ether is absolutely incompressible, and this is the one that has been most favorably re garded by physicists in general. The second implies that the ether yields indefinitely, even to the smallest compressive forces, so that it is essentially unstable. This view has been de veloped in recent years by Lord Kelvin, but it is hard to regard it as more than a mathematical possibility. The mind cannot be brought to ad mit that it corresponds to the actual state of affairs in space. The third of the suggested alternatives must also be regarded as improb able, although, for lack of exact knowledge, we can hardly pronounce it impossible. On the whole, therefore, it is plain that if the elastic behavior of the ether is analogous to that of ordinary bodies, we have to admit (tenta tively, at least) that so far as elastic properties are concerned, the ether resembles an absolutely incompressible solid.