Eth´elwulf, King of England, succeeded his father, Egbert, about A.D. 837, died 857. His reign was in great measure occupied in repelling Danish incursions; but he is best remembered for his donation to the clergy, which is often quoted as the origin of the system of tithes. Alfred the Great was the youngest of his five children.
Eth´endun, Battle of, the victory which Alfred the Great gained over the Danes (878), and which led to the treaty with Guthrum, the Danish king of East England. The locality is doubtful.
Ether, or Æther, sometimes called luminiferous ether to prevent confusion with the well-known volatile liquid of the same name, a hypothetical medium filling the whole of what seems to be empty space, and even the interstices between the atoms of material bodies. Most thinkers believe that such a medium must be postulated if we are to explain the transmission of physical actions between bodies at a distance from one another. With the exception of ordinary mechanical pressures and tensions, the simplest examples of influences that can pass across space are sound and light. Sound, we know, is carried by the air, a medium more subtle than solid or liquid bodies, but still easily recognizable by its effects on our senses, and by its mechanical, physical, and chemical properties. We know a good deal about air, and about the process that goes on when sound is passing through it. But the ether is incomparably more elusive than air. It affects the sense of sight, indeed, as the air affects the sense of hearing; but, so far as we know, it has no weight, no specific heat, no chemical affinity. Except that it is the medium which conveys light, electric and magnetic actions, and possibly gravitation, we know extremely little about it. An extreme school of modern physicists is even inclined to deny, or at least to ignore, its existence altogether.
Early speculators regarded the ether as a species of fluid, which could be displaced by ordinary matter, so that upholders of the wave theory of light necessarily thought of waves like those of sound, in which the direction of vibration is in the line of transmission, for no other kind of wave can occur in a fluid. Young and Fresnel, however, insisted on the view that the movements of the medium are at right angles to the direction of propagation, and pointed out that this might be explained by supposing the medium to possess elasticity of shape. The obvious objection to the conception of a solid which permits the planets to move through it with apparently perfect freedom was met long afterwards by Stokes and Kelvin, who instanced such substances as shoemaker's wax and jelly, which are rigid enough to be capable of elastic vibration, and yet permit bodies to pass through them with more or less ease. Fresnel's work called attention to the subject of the elasticity of bodies, and led to the discovery of the general equations of vibration of an elastic solid by Navier in 1821. Navier's equations, slightly generalized, were used by Cauchy with a certain amount of success to explain reflection, refraction, and the phenomena of crystal-optics. In 1837 George Green published a variety of elastic solid theory which was a decided improvement on Cauchy's, but many difficulties remained, and it is now almost universally agreed that the vibrations of an ordinary elastic solid do not furnish an exact parallel to the vibrations which constitute light. One of the chief difficulties is that in an ordinary elastic solid two types of waves can occur, one distortional, with the displacement of a particle perpendicular to the direction of transmission, and the other dilatational, with the displacement along the line of transmission, as in sound. Waves of light must be of the distortional kind, and the velocity of the other kind of wave may be quite different from the velocity of light. A kind of ether in which this difficulty of the longitudinal wave does not occur was imagined by Cauchy and afterwards discussed by Lord Kelvin, who called it the contractile, or labile, ether. This is an elastic body with negative compressibility, like homogeneous foam which is prevented from collapsing by attachment to the sides of a containing vessel. Another type of quasi-elastic solid was brought forward by James MacCullagh in 1839. MacCullagh's solid possesses what may be called elasticity of rotation, but offers no resistance to deformations in which elementary parts of the solid preserve their orientation. The equations of motion of this ether devised by MacCullagh are very similar to those obtained much later from a very different physical point of view by Clerk Maxwell. Elastic solid theories, however, have fallen into the background before the advancing popularity of the electromagnetic theory of James Clerk Maxwell. Maxwell's equations of the electromagnetic field are deduced from easily demonstrable experimental facts, supplemented by the characteristic hypothesis that the electric current always travels in a closed circuit, even in cases where, as in the discharge of a condenser, the material circuit is open, so that the path of the current has to be completed through the ether. Other essential features of Maxwell's view are that electric, magnetic, and electromagnetic action is transmitted by means of stresses in a medium which possesses some sort of elasticity and inertia not exactly of an ordinary mechanical kind, and that the energy of all such action resides in the medium. 'Maxwell's equations', especially as modified by H. A. Lorentz so as to take account of the atomic structure of electricity, are fundamental in modern electrodynamics and the electron theory of matter. The form of Maxwell's equations shows that electromagnetic action can be propagated in waves with a definite velocity, which depends on the specific inductive capacity and the magnetic permeability of the medium. Maxwell had no difficulty in showing from experimental data that the velocity given by his theory, which turns out to depend on the ratio of the electrostatic and electromagnetic units of charge, is identical with the known velocity of light. He concludes that waves of light are electric waves. The actual production of waves by electrical means was experimentally demonstrated by Sir Oliver Lodge, and more completely by Heinrich Hertz, and is now a commonplace of wireless telegraphy and telephony. The question of the nature of the mechanical process by which physical actions are carried on in the ether weighed heavily on Maxwell, as on other nineteenth-century physicists. Mechanical models of many kinds have been devised to represent ethereal action. Were it sufficient for the purpose, certainly nothing could be simpler than the elastic solid model. Other models of much interest are the gyrostatic ether and the vortex sponge ether of Lord Kelvin, and the molecular vortex ether of Maxwell. It is recorded that the celebrated mathematician Gauss had made out a theory of electrodynamics, but always declined to publish it because he was unable to devise a mental picture of the physical action represented by his mathematics; and it was probably a similar reason that led Lord Kelvin to declare, so late as 1904, that "the electromagnetic theory has not helped us hitherto". Sir J. J. Thomson has developed a theory of moving tubes of electric force, which produce magnetic fields by their motion. Possibly light may consist of tremors in these tubes, and if the tubes are
discrete, it may become practicable to reconcile the modern quantum theory (q.v.) with the phenomena of interference of light, with which at present it seems to be utterly inconsistent.
The extraordinary developments in both theoretical and experimental physics during recent years have diverted attention to some extent from the question of the constitution of the ether, and the problem of its mode of working is more frequently considered from a mathematical and pseudo-metaphysical point of view than from the old standpoint of Newtonian dynamics. It was from a question about the ether, however, that the theory of relativity, the most important of recent speculations, took its origin. Is the ether fixed, or does it move? Is it carried along with the earth in its motion round the sun, or does the ether pass through the atoms of material bodies as the sea passes through the meshes of a net? The elastic solid analogy, and the simplicity of the classical explanation of the aberration of light, are evidence in favour of a fixed ether. But the celebrated interference experiment of Michelson and Morley, which was capable of detecting a comparatively small relative velocity of earth and ether, gave a null result. Various electrical experiments also point to the conclusion that the medium in which optical and electrical effects take place is carried along with the earth in its motion. We are thus placed in a dilemma. We must either reconcile the idea of a fixed ether with the Michelson-Morley and kindred experiments, or we must explain aberration on the supposition that earth and ether move together. Both alternatives have had their supporters. Those who, like Sir Joseph Larmor and Sir Oliver Lodge, believe in a fixed ether rely on the hypothesis of the 'Fitzgerald contraction', according to which bodies moving through the ether with velocity v are contracted in the direction of their motion by the fraction √(1 - v2/c2) of their length, c being the velocity of light. This contraction is in ordinary cases very small, amounting only to a few inches for the diameter of the earth when moving round the sun. The hypothesis follows naturally enough from the accepted theory of the motion of electrons, and leads to a perfectly simple explanation of the Michelson-Morley result. The most prominent champion of a moving ether was Sir George Stokes. He assumed that, so far as the earth's motion through it is concerned, the ether behaves as a perfect liquid, so that it moves along with the earth, and he proved that aberration would be unaffected by this motion, provided it is everywhere irrotational, or free from spin. Stokes's theory has been extended by Larmor so as to cover a very important set of phenomena found by Arago and Airy, and explained in a general way by Fresnel. These phenomena relate to the velocity of light in material media which are in motion relative to the earth, running water for example. Fresnel proved that all the experimental results are explained if the velocity of light in the water, with respect to the earth, is given by the formula c´ + v(1 - 1/m2), where c´ is the velocity of light in still water, v is the velocity of the water relative to the earth, and m is the index of refraction of water. At present the fashionable view of all the phenomena is that taken in Einstein's theory of relativity (q.v.), which makes revolutionary suppositions with respect to the measurement of space and time, and assumes that the velocity of light is a universal constant, independent of the motion either of the source of light or of the observer. Once its initial assumptions are granted, the theory undoubtedly gives simple and natural explanations of the chief optical and electrical phenomena, and in particular leads at once to Fresnel's formula given above. Most English writers on the subject, among whom A. S. Eddington, E. Cunningham, and A. N. Whitehead are prominent, continue to believe that an ether exists, in spite of the fact that as relativists they hold that no experiment can ever enable us to determine our motion through it.—Bibliography: E. T. Whittaker, History of the Theories of Æther and Electricity; Sir Joseph Larmor, Æther and Matter; A. S. Eddington, Space, Time, and Gravitation; O. W. Richardson, Electron Theory of Matter; R. W. Wood, Physical Optics.
Ether, or Ethyl Ether, (C2H5)2O, a colourless, inflammable liquid produced by distillation of alcohol with concentrated sulphuric acid. It is almost immiscible with water, lighter than alcohol, has a sweet taste, and evaporates rapidly in air, producing extreme cold. The vapour of ether mixed with air forms an explosive mixture. Ether is a valuable solvent for many organic substances, fats, oils, &c., and is also used in surgery as an anæsthetic.
Etherege (eth´ė-rej), Sir George, English writer of comedy, born about 1635, died about 1691. He studied at Cambridge, travelled afterwards on the Continent, and then returned to enter himself at one of the Inns of Court. Devoting himself less to legal studies than to literature and society, he wrote several plays. In 1664 he had his first comedy represented, The Comical Revenge, or, Love in a Tub, which was well received. Four years later his She Would if She Could appeared, a brilliant play, though frivolous and immoral. Eight years afterwards (1676) he produced his best comedy, The Man of Mode, or Sir Fopling Flutter. Etherege's plays are witty and sparkling, and the characters, genuine portraits of the men and women he saw, are vividly if lightly drawn.