or ATH, a strong little town in the Austrian Netherlands, and province of Hainault, situated on the river Dender, about 20 miles south-west of Brussels.

ÆTHALIA, or ILUA, in Ancient Geography, now Elba, an island on the coast of Etruria, in compass a hundred miles, abounding in iron. It was so called from aëthi smoke, which issued from the shops of Vulcan.

ÆTHELSTAN, see ATHELSTAN.

ÆTHER is usually understood of a thin, subtle matter or medium, much finer and rarer than air, which, commencing from the limits of our atmosphere, possesses the whole heavenly space. The word is Greek, aëthēr, supposed to be formed from the verb aëthō, to burn, to flame; some of the ancients, particularly Anaxagoras, supposing it to be of the nature of fire.

The philosophers cannot conceive that the largest part of the creation should be perfectly void; and therefore they fill it with a species of matter under the denomination of æther. But they vary extremely as to the nature and character of this æther. Some conceive it as a body sui generis, appointed only to fill up the vacancies between the heavenly bodies, and therefore confined to the regions above our atmosphere. Others suppose it of so subtle and penetrating a nature as to pervade the air and other bodies, and to possess the pores and intervals thereof. Others deny the existence of any such specific matter, and think the air itself, by that immense tenacity and expansion it is found capable of, may diffuse itself through the interstellar spaces, and be the only matter found therein.

In effect, æther being no object of our sense, but the mere work of imagination, brought only upon the stage for the sake of hypothesis, or to solve some phenomenon real or imaginary, authors take the liberty of modifying it as they please. Some suppose it of an elementary nature, like other bodies, and only distinguished by its tenacity, and the other affections consequent thereon; which is the philosophical æther. Others will have it of another species, and not elementary, but rather a sort of fifth element, of a purer, more refined, and spiritious nature, than the substances about our earth, and void of the common affections thereof, as gravity, &c. The heavenly spaces being the supposed region or residence of a more exalted class of beings, the medium must be more exalted in proportion. Such is the ancient and popular idea of æther, or ethereal matter.

The term æther being thus embarrassed with a variety of ideas, and arbitrarily applied to so many different things, the later and severer philosophers choose to set it aside, and in lieu thereof substitute other more determinate ones. Thus, the Cartesians use the term materia subtilis, which is their æther: and Sir Isaac Newton, sometimes a subtle spirit, as in the close of his Principia; and sometimes a subtile or æthereal medium, as in his Optics.

Heat, Sir Isaac Newton observes, is communicated through a vacuum almost as readily as through air; but such communication cannot be without some interjacent body, to act as a medium. And such body may be subtle enough to penetrate the pores of glass, and may permeate those of all other bodies, and consequently be diffused through all the parts of space.

The existence of such an æthereal medium being settled, that author proceeds to its properties; inferring it to be not only rarer and more fluid than air, but exceedingly more elastic and active; in virtue of which properties he shows, that a great part of the phenomena of nature may be produced by it. To the weight, e.g. of this medium, he attributes gravitation, or the weight of all other bodies; and to its elasticity, the elastic force of the air and of nervous fibres, and the emission, refraction, reflection, and other phenomena of light; as also sensation, muscular motion, &c. In fine, this same matter seems the primum mobile, the first source or spring of physical action in the modern system.

The Cartesian aether is supposed not only to pervade, but adequately to fill, all the vacuities of bodies; and thus to make an absolute plenum in the universe.

But Sir Isaac Newton shows that the celestial spaces are void of all sensible resistance; and hence it follows, that the matter contained therein must be immensely rare, in regard the resistance of bodies is chiefly as their density: so that if the heavens were thus adequately filled with a medium or matter, how subtle soever, they would resist the motion of the planets and comets much more than quicksilver or gold. But it has been supposed that what Newton has said of aether is to be considered only as a conjecture, and especially as no new proofs of its existence have been adduced since his time.

The late discoveries in electricity have thrown great light upon this subject, and rendered it extremely probable that the aether so often talked of is no other than the electric fluid, or solar light, which diffuses itself throughout the whole system of nature.

ÆTHEREAL, ÆTHEREUS, something that belongs to, or partakes of, the nature of ÆTHER. Thus we say, the aethereal space, aethereal regions, &c. Some of the ancients divided the universe, with respect to the matter contained therein, into elementary and aethereal. Under the aethereal world was included all that space above the uppermost element, viz. fire. This they supposed to be perfectly homogeneous, incorruptible, unchangeable, &c. The Chaldees placed an aethereal world between the empyreum and the region of the fixed stars. Besides which, they sometimes also speak of a second aethereal world, meaning by it the starry orb; and a third aethereal world, by which is meant the planetary region.

ÆTHIOPIA. See ABYSSINIA, and ETHIOPIA.

ÆTIANS, in Church History, a branch of Arians, who maintained that the Son and Holy Ghost are in all things dissimilar to the Father. See AETIUS.

ÆTOLOGY is that part of pathology which is employed in exploring the causes of diseases.

ÆTION, a celebrated painter, who left an excellent picture of Roxana and Alexander, which he exhibited at the Olympic games. It represents a magnificent chamber, where Roxana is sitting on a bed of a most splendid appearance, which is rendered still more brilliant by her beauty. She looks downwards in a kind of confusion, being struck with the presence of Alexander standing before her. A number of little Cupids flutter about, some holding up the curtain, as if to show Roxana to the prince, whilst others are busied in undressing the lady; some pull Alexander by the cloak, who appears like a young bashful bridegroom, and present him to his mistress. He lays his crown at her feet, being accompanied by Hephaestion, who holds a torch in his hand, and leans upon a youth, who represents Hymen. Several other little Cupids are represented playing with his arms: some carry his lance, stooping under so heavy a weight; others bear along his buckler, upon which one of them is seated, whom the rest carry in triumph; another lies in ambush in his armour, waiting to frighten the rest as they pass by. This picture gained Action so much reputation, that the president of the games gave him his daughter in marriage.

ÆTITES, or EAGLE-STONE, in Natural History, a flinty or crusted stone, hollow within, and containing a nucleus, which, on shaking, rattles within. It was formerly in repute for several extraordinary magical as well as medical powers; such as preventing abortion, discovering thieves, and other ridiculous properties. The word is formed from æros, eagle, the popular tradition being, that it is found in the eagle's nest, whither it is supposed to be carried while the female sits, to prevent her eggs from being rotten. It is found in several parts. Near Trevoux, in France, one can scarcely dig a few feet without finding considerable strata or beds of the coarser or ferruginous kind. They are originally soft, and of the colour of yellow ochre. But the finest and most valued of all the eagle-stones are accidental states of one or other of our common pebbles.