On this property which Air has, of being condensed and rarefied by heat, its elasticity or springiness chiefly depends. For if Air were forced by condensation into a less compass than it took up before, and then exposed to a very considerable degree of cold, it would remain quite inactive, without exerting such an effort as it usually makes against the compressing body. On the other hand, the elasticity of heated Air arises only from hence, that being rarefied by the action of Fire, it requires much more space than it occupied before.
Air enters into the composition of many substances, especially vegetable and animal bodies: for by analysing most of them such a considerable quantity thereof is extricated, that some naturalists have suspected it to be altogether destitute of elasticity when thus combined with the other principles in the composition of bodies. According to them, the efficacy of the elastic power of the Air is so prodigious, and its force when compressed so excessive, that it is not possible the other component parts of bodies should be able to confine so much of it, in that state of compression which it must needs undergo, if retaining its elasticity it were pent up among them.
However that be, this elastic property of the Air produces the most singular and important phenomena, observable in the resolution and composition of bodies.
SECTION II.
Of Water.
Water is a thing so well known, that it is almost needless to attempt giving a general idea of it here. Every one knows that it is a transparent, insipid substance, and usually fluid. I say it is usually so; for being exposed to a certain degree of cold it becomes solid: solidity therefore seems to be its most natural state.
Water exposed to the Fire grows hot; but only to a limited degree, beyond which its heat never rises, be the force of Fire applied to it ever so violent: it is known to have acquired this degree of heat by its boiling up with great tumult. Water cannot be made hotter, because it is volatile, and incapable of enduring the heat, without being evaporated and entirely dissipated.
If such a violent and sudden heat be applied to Water, as will not allow it time to exhale gently in vapours, as when, for instance, a small quantity thereof is thrown upon a metal in fusion, it is dissipated at once with vast impetuosity, producing a most terrible and dangerous explosion. This surprising effect may be deduced from the instantaneous dilatation of the parts of the Water itself, or rather of the Air it contains. Moreover, Water enters into the texture of many bodies, both compounds and secondary principles; but, like Air, it seems to be excluded from the composition of all metals and most minerals. For although an immense quantity of Water exists in the bowels of the Earth, moistening all its contents, it cannot be thence inferred, that it is one of the principles of minerals. It is only interposed between their parts; for they may be entirely divested of it, without any sort of decomposition: indeed it is not capable of an intimate connection with them.
SECTION III.
Of Earth.
We observed that the two principles above treated of are volatile; that is, the action of Fire separates them from the bodies they help to compose, carrying them quite off, and dissipating them. That of which we are now to speak, namely Earth, is fixed, and, when it is absolutely pure, resists the utmost force of Fire. So that, whatever remains of a body, after it hath been exposed to the power of the fiercest Fire, must be considered as containing nearly all its earthy principle, and consisting chiefly thereof. I qualify my expression thus for two reasons: the first is, because it often happens, that this remainder does not actually contain all the Earth which existed originally in the mixt body decomposed by Fire; since it will afterwards appear that Earth, though in its own nature fixed, may be rendered volatile by being intimately united with other substances which are so; and that, in fact, it is common enough for part of the Earth of a body to be thus volatilized by its other principles: the second is, that what remains after the calcination of a body is not generally its earth in perfect purity, but combined with some of its other principles, which, though volatile in their own natures, have been fixed by the union contracted between it and them. We shall, in the sequel, produce some examples to illustrate this theory.
Earth, therefore, properly so called, is a fixed principle, which is permanent in the Fire. There is reason to think it very difficult, if not impossible, to obtain the earthy principle entirely free from every other substance: for after our utmost endeavours to purify them, the Earths we obtain from different compounds are found to have different properties, according to the different bodies from which they are procured; or else, if those Earths be pure, we must allow them to be essentially different, seeing they have different properties.
Earth, in general, with regard to its properties, may be distributed into fusible, and unfusible; that is, into Earth that is capable of melting or becoming fluid in the Fire, and Earth that constantly remains in a solid form, never melting in the strongest degree of heat to which we can expose it.
The former is also called vitrifiable, and the second unvitrifiable Earth; because, when Earth is melted by the force of Fire, it becomes what we call Glass, which is nothing but the parts of Earth brought into nearer contact, and more closely united by the means of fusion. Perhaps the Earth, which we look upon as incapable of vitrification, might be fused if we could apply to it a sufficient degree of heat. It is at least certain, that some Earths, or stones, which separately resist the force of Fire, so that they cannot be melted, become fusible when mixed together. Experience convinced Mr. du Hamel that lime-stone and slate are of this kind. It is however undoubtedly true, that one Earth differs from another in its degree of fusibility: and this gives ground to believe, that there may be a species of Earth absolutely unvitrifiable in its nature, which, being mixed in different proportions with fusible Earths, renders them difficult to melt.
Whatever may be in this, as there are Earths which we are absolutely unable to vitrify, that is a sufficient reason for our division of them. Unvitrifiable Earths seem to be porous, for they imbibe Water; whence they have also got the name of Absorbent Earths.
SECTION IV.
Of Fire.
The Matter of the Sun, or of Light, the Phlogiston, Fire, the Sulphureous Principle, the Inflammable Matter, are all of them names by which the Element of Fire is usually denoted. But it should seem, that an accurate distinction hath not yet been made between the different states in which it exists; that is, between the phenomena of Fire actually existing as a principle in the composition of bodies, and those which it exhibits when existing separately and in its natural state: nor have proper distinct appellations been assigned to it in those different circumstances. In the latter state we may properly give it the names of Fire, Matter of the Sun, of Light, and of Heat; and may consider it as a substance composed of infinitely small particles, continually agitated by a most rapid motion, and, of consequence, essentially fluid.
This substance, of which the sun may be called the general reservoir, seems to flow incessantly from that source, diffusing itself over the world, and through all the bodies we know; but not as a principle, or essential part of them, since they may be deprived thereof, at least in a great measure, without suffering any decomposition. The greatest change produced on them, by its presence or its absence, is the rendering them fluid or solid: so that all other bodies may be deemed naturally solid; Fire alone essentially fluid, and the principle of fluidity in others. This being presupposed, Air itself might become solid, if it could be entirely deprived of the Fire it contains; as bodies of most difficult fusion become fluid, when penetrated by a sufficient quantity of the particles of Fire.
One of the chief properties of this pure Fire is to penetrate easily into all bodies, and to diffuse itself among them with a sort of uniformity and equality: for if a heated body be contiguous to a cold one, the former communicates to the latter all its excess of heat, cooling in exact proportion as the other warms, till both come