Ethelwulf, second king of the Anglo-Saxons, son and successor of Egbert, ascended the throne about 836, and died in 857 or 858. He began his reign by transferring the provinces of Kent, Essex, and Sussex to the government of his eldest son Athelstan. For many years he waged incessant contests with the Danes, who annually made inroads into England, and though repulsed and defeated always carried off booty. In 850-'51 a part of them dared for the first time to pass the winter in England. Strongly reenforced in the spring, they sailed up the Thames, sacked Canterbury and London, and met Ethelwulf at the head of the West Saxons at Okely. After an obstinate battle the Danes were defeated with a loss greater, it is said, than they had ever before suffered, and other divisions of their forces were defeated by Ceorle in Devonshire, and by Athelstan at sea. Yet they maintained their settlement on the ! isle of Thanet, but were cautious during the remaindcr of Ethelwulf's reign. In 853 the king marched against the Welsh, and reduced them to subjection. Shortly afterward he made a visit to Koine, accompanied by his son Alfred, who there received the regal unction and the sacrament of confirmation. He returned through France, where he tarried to marry Judith, daughter of the French king.

His son Athel-stan meantime had died, and Ethelbald was usurping the kingdom, when he returned and yielded to the latter the government of Wessex. He survived this partition only two years, which he passed in charity and devotion. ETHER, or AEther (Gr.Ethelwulf 0600578 the upper air, fromEthelwulf 0600579 to burn), in physics, a hypothetical fluid of great tenuity, which by ancient as well as by modern philosophers has been supposed to occupy all space; hence called the cosmic or luminiferous ether. Among the ancient Greeks it was the personified idea of cosmic material as well as force. According to Hesiod, AEther was the son of Erebus and Night and brother of Day. The children of AEther and Day were the heavens, the land, and the sea, from which we are naturally led to conclude that it was considered one of the elementary substances out of which the universe was formed. In the Orphic hymns AEther is represented as the soul of the world, from which all life emanates. Anaxagoras called ether the principle of fire, and Plato described it as a matter purer and lighter than air, which being diffused throughout space, it would be impossible to ascertain if it had weight. The modern ideas of the interstellar ether differ in many respects from those of the ancients, and have assumed a more precise and scientific form ; but the speculations of the old philosophers seem to be a foreshadowing of modern theories.

Descartes, in his treatise on dioptrics, entertains the idea of an ether, or a "subtile medium" for the transmission of light, which he believed to be "a certain motion or most rapid and lively action coming toward the eye." Huygens was the first to give system and mathematical order to the theory. He conceived that from every point of a luminous body undulations are propagated through an ethereal fluid diffused throughout space, which possesses the property of extreme tenuity and infinite elasticity, and by the most refined mathematical demonstrations accounted for so many of the phenomena of light as to lay the broadest foundations for the subsequent labors of his followers. Newton did not accept the undulatory theory of light, although in the 22d query of his third book of optics he does propound the question as to whether vision may not be caused by a propagation through the fibres of the optic nerve of vibrations communicated by an ethereal medium, thereby stating his belief in the existence of such a medium, as he also does when treating of the subject of heat.

His theory that light is caused by the emission of luminous corpuscles from all incandescent bodies was so generally accepted for more than 100 years, that the doctrine of a luminiferous ether, which seems to have been generally considered as peculiarly a fundamental part of the theory of Huygens, did not occupy much attention until the investigations of Young, Fresnel, Euler, and Cauchy placed the undulatory theory of light in the position of a demonstrated fact. The luminiferous ether is now supposed not only to occupy all space between the heavenly bodies, but to permeate all ponderable matter, and to move in undulations between its particles, which undulations may be modified by the nature of the matter, and transformed into heat or other force. It is supposed to be uninfluenced by gravitation, to preserve a uniform density when not contained within the interstices of ponderable bodies, and has also been supposed to offer no resistance to their passage; but the diminution in the periodical revolution of some of the comets, as Encke's, has been ascribed to a retardation possibly due to the resistance of interstellar ether.

From being at first considered as a medium for the transmission of light, it has within the last half century, upon the undulatory theory, been adopted as that of heat also, since it has been demonstrated that both heat and light are the effects of undulations of different amplitudes; and at the present time the question is raised as to whether it may not also be the medium, or at least an important agent, in the manifestation of electricity, since this force is now regarded as a correlation of light and heat. It must be observed that Euler maintained only the doctrine of undulations, and rejected that of an ethereal medium, as do several physicists of to-day, among whom is Prof. Grove, contending that the undulations are produced in the ponderable matter, whatever it may be, which constitutes the medium of transmission. The arguments of these philosophers are stated by Prof. Grove in his work on the "Correlation of Physical Forces." Among them may be briefly noted the following: It has been impossible hitherto to produce a perfect vacuum; in other words, to create a space which shall be void of ponderable matter.

The tendency of the particles of bodies to fly from their surfaces, even under ordinary circumstances, is so great that the odor of many metals, as iron, copper, tin, and zinc, is always plainly perceptible. Water in a vessel within the exhausted receiver of an air pump will acquire a taste of the tallow used between the edge of the receiver and the pump plate. The tendency of gaseous matter to fly off into space is so great that it is contended that no interstellar vacuum could continue for an unlimited time. Another argument is that light is lost in the interstellar spaces; for if not, all the stars being suns, there ought to be no night. To account for this loss, the light must be supposed to be transmuted into another force, and this requires the intervention of matter, which is believed to be furnished by the mingling of portions of the atmospheres of worlds in the interstellar spaces; and the retardation of comets has been ascribed to this matter. This argument is not affected by the law that light decreases in proportion to the square of the distance, because, although the rays diverge in all directions from every source, the field of the heavens, as Prof. Grove remarks, is everywhere studded with suns, so that the sum of the rays which would reach the earth, if they retained their luminous property, would furnish more light than is actually received.

Again, when as near an approach to a vacuum as is possible has been formed, it has been found to offer an effectual barrier to the passage of the electric spark; and regarding electricity as a correlation of light and heat, it is asked why it will not employ the ethereal medium which upon that hypothesis must pervade the vacuum. This objection loses weight from the fact that the space which contains no medium for the transmission of electricity, nevertheless offers one for the transmission of light and heat. The difficulty of accounting for the polarization of light by assuming that the undulations take place in the molecules of the polarizing body, is greatly diminished by adopting the doctrine of a special medium, as the luminif-erous ether. A further consideration of the subject will be found in the articles Light and Heat. (See also Elasticity.)