amount of the change induced in the relative quantities of attractive and repulsive matter around it, is inconsiderable. If, again, a particle have such range of action that it may be made to alternate between a very hot state and a very cold state, a proportionally great change will be induced upon the relative quantities of its attractions and repulsions, and more easily may it be made to change from the aëriform to the liquid or solid state. This change, however, may ob riously be effected otherwise than by a change of temperature. Thus, if by an electric discharge, the intensity or quantity of the electric or repulsive matter, investing each one in a volume of particles in the aëriform state, be lessened, a vapour may condense in consequence of its attractive influences being laid naked by the loss of a certain depth in its investing repulsive sphere. Compression may obviously effect the same; and it may be anticipated that this is an agent of great force. It is, in fact, equivalent to inducing a difference in electric state among the particles; the atomic parts are brought sufficiently contiguous to clasp each other by their attractions, which being consecutive, stretch out towards each other the more the particles are made contiguous; and do not suffer compression towards the solid matter which they invest, as is the case with the nonconsecutive repulsive medium which insulates them. Compression, then, effected without violence, so as to excite heat, which is calculated to sustain the aëriform state, must be a powerful agent in effecting the condensation of aëriform bodies. Some aëriform bodies, in which there is a very critical balance between the attractive and repulsive energy, may yield to a small pressure; and yet, while uncompressed, be most eminently elastic. But such is the harmony of things, that media of this structure must also have a great capacity for heat, and, consequently, a proportional tendency to sustain the aëriform state by the exhilarated state of their electricity, and the depressed state of their attractions and the pressure which forces the particles towards each other may heat them, and consequently increase their elastic force.

F

Considered in a state of repose, and apart from bodies to compress them, or towards which they may fall, there is no generic distinction between matter in the solid, liquid, gaseous, or vapoury state. All are elastic not spontaneously expansible tissues of atomic particles, occupying a definite volume, and more or less completely insulated, and retained in position by the subtile matter which invests them. Solids and liquids cannot be compressed into smaller volumes to any great extent. At that degree of depth by which their atomic parts are separated from each other, their subtile matter is very dense, and requires an immense force to make it yield. Aëriform bodies, again, may be compressed into very small volumes, because the medium which insulates them is at so great a height above the atomic surfaces of the particles, rare and easily resisted.

At the surface of the earth, aëriform bodies are always in a state of compression, and hence, their true characters cannot be discovered by experiment. Immediately on removing the force which oppresses them, they expand, as nearly as they are permitted, to a state of natural tenuity; and, in this state, they can only be obtained at the surface of the earth, in a perfect Boylean vacuum. This circumstance, that aëriform media are always presented to us in a state of compression, naturally leads us to regard them as more peculiarly elastic and expansible than other bodies, and that they possess these properties different in kind, as well as in degree, from more dense bodies, that, whereas the particles of other bodies must first be made to approach each other before their elasticity is exhibited, aëriform bodies expand first. We are apt to forget that they are already compressed to our hand, and that their expansion is merely the recoil from a compression previously inflicted on them. An uncompressed aëriform medium, then, is a symmetrical tissue of atomic particles, which are more free to move, independently of each other, than particles in any other state. If several be removed, the volume is proportionally diminished, and the others come in, and, occupying their positions, restore the symmetry

of the whole. A volume of an uncompressed gas is like a vessel full of small ivory balls, each similarly to another traversed by magnetic needles, suspended in a liquid medium, whose specific gravity is less than their own, by the least quantity. If one of them be thrown into a state of elastic vibration, which is propagated equally around from the centre, radii of elastic tremor will be propagated in the direction of all those which touch it around. Each of these receives the impulse which excites it on a point; its elastic movement is, therefore, determined between that point and one diametrically opposite. The next ball, or that second from the central source, is agitated in a like manner. Thus, in an uncompressed gas, an elastic impulse, propagated from a centre, is soon divided into radii, while the intervals remain more or less quiescent. In a compressed gas, on the other hand, the whole tissue resembles one ivory ball. The whole is in a state of mutual dependence and connexion. When one part is moved, there is a re-action through the whole. There is undulation, while, in the other case, there is radiation.

BOOK II.

OF

RADIANT MATTER.

THAT the intervals between the dense bodies of the universe are filled with particles of some sort, no one will dispute. This celestial medium philosophers have sometimes called Light, sometimes Lumeniferous Ether; and while all agree that it consists of particles possessing mechanical properties, some think that these particles move through two hundred thousand miles in a second of time; others, that their excursions of utmost amplitude are never more than twenty-five millionth parts of an inch. Moreover, these particles or molecules are admitted to have perfect uniformity of action in the same circumstances, to submit to the crdinary laws of the impact of elastic bodies,-to have sides and properties peculiar to certain regions, and still they are treated of with some reservation as if they were different from matter and every thing else. This has arisen, in no small measure, from the reluctance of philosophers to admit any thing possessing inertia into the planetary spaces, lest such a medium might occasion a retardation in the heavenly bodies, which are commonly believed to move without any mechanism to sustain their motions.

But without a knowledge of the mechanism of such a

medium, we cannot infer a priori, whether its inertia shall be of avail to bring a body in contact with it to rest, or to cause it move through it. Let us see what we find around us, and in the sky between the stars, and, in the mean time, leave planets, moons, and double stars to themselves, assured, that, though our philosophical notions respecting their movements may be a retardation to our progress in knowledge, they can be no retardation to them in their orbits.

According to the general principles which have been already advanced, it follows, that the lumeniferous ether, medum or substance of fire and light, radiant medium, or universal ether, to be any thing possessing physical properties at all, must only be regarded as atomic invested with its proper quantity of subtile matter; and, consequently, it must be a ponderable body, unless its specific heat be so great that its attractive energy is reduced to zero; for it is never to be forgotten, that the weight of a body is merely the effect of its attractive energy, causing it to tend towards the earth. Farther, it must obviously be either a gas or a vapour, and it must conduct itself like other gases and vapours, in as far as its atomic form is analogous..

We are, therefore, at once excluded from the consideration of all the doctrines of the system of emanation. All excessive penetrating power is likewise denied to the particles. Light must find its way through solid yards of quartz, otherwise than by the transmission of hard particles. There can be no doubt, however, that the particles of the radiant medium are more minute than those of all other bodies; but still it is incredible that quartz, glass, and such like bodies, can be so porous as to permit any particles to pass through them in right lines.

From an investigation of the phenomena of physical optics, afterwards to be illustrated, it is inferred in this work, that the radiant medium consists of particles symmetrically reated and fixed in their positions by their mutual attractions; and that they are uniform in size, and that their shape is the regular tetrædron (Fig. 2.). These atoms are, like the