its own emptiness, is a symptom of health; but when it seems full, though it has nothing in it but wind, the habit is vitiated by some distemper.

Of Liquefaction.

Liquefaction is a loosening of the parts of bodies with a certain degree of heat, which are fixt with an inferior degree. This is effected by the matter of fire introduced between the parts of bodies, and driving them farther asunder by its expansive force, till they acquire a free motion amongst themselves. Experiments have now taught us, that fire is both resident and active in all bodies at all times; but, in order to liquefy them, it must act with a force which is more or less according to the subject it has to work upon. Some vegetable oils expressed from the seeds of plants continue fluid with a tem: perature of the air far below the point of freezing; which is wisely so ordained, that the seeds containing this oil may endure the severest frosts of the winter without being hurt in their vital principle, and so continue fit to be opened by the powers of vegetation at the proper season. Water requires a far

ther

gree

ther degree of heat to keep it fluid: animal fat, bees-wax, and resinous matters, a farther still. The metals, according to their several degrees of hardness, require a different deof fire to loosen their parts; but none of them can resist the force of it: we are accustomed to say of bodies naturally hard, that when they flow they are melted; and of bodies naturally fluid, that when they grow hard they are congealed or frozen: but the effects are similar in thèm all; and whatever custom may require, philosophy will justify us if we consider all water as melted ice, and a pig of lead as a mass of congealed metal. All that fluidity which is ordinary, is a lower kind of liquefaction; the difference being only this, that some substances require more, some less heat, to keep them in this state whence it follows, that if the motion of fire were to cease, universal rigidity and stagnation would ensue: all the qualities of the ancient schools, and all attractions and repulsions of later philosophy, would be buried together in one grave, and sealed up under a monument of impenetrable brass.

As nature is now constituted, we may consider natural bodies, with the minutest of their parts, as floating in an ocean of what the

poet properly terms. axaμalov avg, indefatigable fire, which is moved with the same vigour now as five thousand years ago; and that the bodies sustained in it are hard or soft, fluid or solid, lax or firm, rare or dense, expanded or contracted, according to those changes in the temperature of this element, which keep up a constant systole and diastole through the whole frame of nature.

Of Evaporation.

When fire transpires from fluids which it has heated, its course is upwards, and it carries continually with it into the air that exceeding thin plate or stratum of the liquor which lies at the surface, where the fire, as it escapes, comes into contact with the air. Fluids which are light and thin yield a vapour which is spread into the air and dissipated; but others which are ponderous, as 'melted metals of some kinds, are separated at the surface; but instead of being carried aloft, their particles fall back upon the mass, and rest there in form of a dry dust or powder. This is the case with lead: and the powder so detached is not to be looked upon as dross or fæces, but the metal itself pulve

rized,

rized, and which will return to its fluidity with the application of any flaming matter of oil or sulphur.

The raising of vapour has always been a subject much inquired into by philosophers. The usual way of understanding it in the last age, was to suppose that the matter of light or fire, insinuating itself into the particles of water, turns them into vesicles, or inflated spherules, which, being specifically lighter than air, are rendered buoyant. Others have asserted that fire is no element, and that the particles of water are expanded by a repulsive power, but yet allowing that the rarefaction of the vapour is always in proportion to the heat. Of late it has been supposed, that air acts as a menstruum on water by the power of attraction; but if this is adopted, then air must be supposed to act downwards, and draw water upwards; which is not agreeable to the laws of motion, all motion being in the direction of the moving cause. If the rays of the sun are supposed to draw up the particles of water, this again is liable to the same objection, unless we mean it of his reflected rays, which conspire with the effect of evaporation; but the effect is of such a kind that it must be owing to a

cause

cause which diffuses itself every way, and acts in all directions; and such is the nature both of heat and the electric medium.

When a vapour rises from the surface of an heated fluid, and goes up into the air, all is consistent and rational; the fire goes off the same way, and so the cause and effect are in one direction; and therefore if we would account for evaporation consistently, we must reduce other cases to this, and argue that all slower and gentler evaporations are produced on the same principle with this, in which the operation is more quick as the cause is more violent. It is allowed by all, that heat is the general cause of vapour; and as the vapour raised by the sun's heat is in proportion to his heat, which diffuses itself in all directions near the surface of the earth and ocean, we have a cause adequate to the effect, and need not fly to any supplemental repulsions in water, or attractions in air. That vapour will not rise but in air, is very certain, because if it is ever so much rarefied, it must have air to sustain it; but the air no more raises it, than it raises the smoke which is carried upwards from a fire. The air is the vehicle, but is itself driven upwards by the fire, in common with the smoke. Without