ETHER (Lat. (ether, Gk. a1B4p, either, upper air, from atOctr, niacin, to glow. Skt. idh, to kindle). It may he regarded as proved that the sensation light is due to wave-motion, and that all the thermal effects attributed to 'radiation' arc due to the absorption of waves. A train of waves is the advance into a medium of a periodic disturbance: and therefore a medium is required for the• waves which produce luminous and thermal effects. This medium is called the `Iuminiferous, ether,' or. more simply. 'the ether.' The medium which was imagined by Faraday as a necessary part of his theory of electric and magnetic actions has also been identified by Maxwell with the ether. The fact that the ether is distinet from ordinary matter as known to us is shown by the transmission of radiation through interstellar space and through vacua, as well as by the magnitude of the velocity of sueh waves-3 X l0'° cent imetera, or about 106. 000 miles, per seeond—whieh is greater than would be possible with any matter of properties with ordinary matter. The ether has inertia, because time is required for the propagation of waves; but there is no evidence that it has weight. In fact, the passage of radiation through all bodies, to a greater or less degree, proves that the ether is a per meating all space, and that portions of ordinary matter, i.e. molecules and atoms, are immersed in it., as particles of dust float in the air, or small solid particles exist in water. The lengths of ether-waves may be measured by suitable means (see Limn') ; and those that appeal to physical instruments are found to vary from o any miles to less than two-millionths of a milli meter. If waves as short as these last are prop agated in a medium, it shows that the structure of that medium must be extremely minute, its portions—if there are any—being much smaller than the smallest wave-length known; otherwise waves as short. as these could not be produced. Nothing of the actual structure of the ether is known; but from analogy with matter its `elas ticity' and 'density' are spoken of, simply mean ing those properties of restitution and inertia by virtue of which waves may be transmitted. These waves are transverse, meaning that, what ever the disturbance is, it is perpendicular to the direction of propagation. This shows that the ether must have properties analogous to the material properties of an elastic jelly, be cause the only form of matter which can carry transverse waves is one with rigidity. It should be observed that some forms of matter behave like solids for sudden forces, but like fluids for slow ones; thus shoemaker's wax is brittle for quick blows, but a piece of lead put on top of the wax will in the course of time pass through, the wax flowing around it.
Waves in the ether are produced by electric os cillations, and they are emitted also by all forms and conditions of matter (see RADIATION) and the statement that these waves have identical properties--except as to wave-number—is the so-called `electro-magnetic theory' of light. The phenomena of radiation and absorption prove that if a minute portion of matter—an `atom'— is vibrating extremely rapidly, it produces waves in the ether. This establishes the fact that there
is sonic connection between ether and ordinary matter in this case. Whether a large piece of matter moving with motion of translation drags the ether with it, or simply allows the ether to pass through it—like wind through a tree—is still to a certain extent an open question. There is, however, no decisive experimental fact in favor of the idea that the ether is dragged alone, (wept in the experiments of Michelson Morley to be referred to later.
It has been shown by Fizeau and by Michelson and Morley that a beam of light is accelerated by its passage through a current of water moving in the direction of the beam, and retarded by an opposing current. This can he explained on the hypothesis of Fresnel that, in addition to the free ether which exists equally everywhere, there is in any transparent body an amount of ether ff'-1 times that of the free ether occupying the same volume, 21 being the index of refraction. and that. this extra amount of ether is attached to the body and moves with it. On this hypothesis the density of the ether in the body is therefore a' times that of the ether in free space, meaning by density that property of the medium which measures its inertia.
The phenomena of stellar aberration (q.v.) seem to prove that the ether near the earth roust he independent of the' earth's motion, or it least that there should not be produced in it what is called in hydrodynamics •rotationar motion. On the other hand, Michelson and Morley have shown that the ether near the surface of the earth moves with at least very nearly the velocity of the earth, assuming that their ap paratus is not affected by the motion. Lodge, 'however, has performed most careful experiments from which he concludes that the veltwity of light between two steel plates moving together in their own planes an inch apart is not altered by an appreciable quantity. It is extremely difficult to reconcile these exia•rimental results; and there are many others, equally eonfifsing. It may be that, in order to explain them, it will be sary to assume that the dimensions of solids change as they mote through the ether.
BIBLIOGRAPHY. The suhjeet of the ether is one Bibliography. The suhjeet of the ether is one that presents many difficulties; and, while much has been written in this connection, it is not al ways in a shape to be of assistance to the aver age reader. The following references, however, may be consulted by the physicist: Larmor, .Ether and Alatter (London, 1901) ; Lodge, "Aber ration Problems," in Philosophical Transactions (London, 1S92-93) ; id., in Philosophical Trans actions (London, 1897) : ',armor, "Dynamical Theory of the Electric and Luminiferous Ether," in Philosophical Transactions (London, 1S94, 1S9S) ; Michelson and Morley, in Philosophical Magazine, vol. xxiv. (London, 1SS7 I ; Lorenz, Vcrsuch ciner Throne der clektri.s.chen und opti schen Erscheinunyc• in ben-eaten Kiirpern (Ley den, 1873) : Wien. Refcrat, 70. Fcrsammlung deutsche,- Naturforscher surd Acrztc in. Diissel dorf (Dusseldorf, 1895).