AETHER is usually understood of a thin, subtile 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, αι6ηξ, supposed to be formed from the verb αι6ειv, 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 aether. But they vary extremely as to the nature and character of this aether. Some conceive it as a body sui generis, appointed only to fill up the vacuities between the heavenly bodies, and therefore confined to the regions above our atmosphere. Others suppose it of so subtile 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 tenuity and expansion it is found capable of, may diffuse itself through the interstellar spaces, and be the only matter found therein.

In effect, aether 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 tenuity, and the other affections consequent thereon; which is the philosophical aether. Others will have it of another species, and not elementary, but rather a sort of fifth element, of a purer, more refined, and spiritous 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 aether, or aethereal· matter.

The term aether 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 aether: and Sir Isaac Newton, sometimes a subtile spirit, as in the close of his Principia; and sometimes a subtile or aethereal 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 maybe subtile 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 aethereal 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 [7:2:199]motion, &c. In fine, this same matter seems the primum mobile, the first source or spring of physical action in the modem 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 subtile 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.