Nitre comes under the first of these classes: in detonation it emits great quantities of air, not afterwards condensible to the like space. This may be seen by firing a few grains of gunpowder in an unblown bladder, or in a vessel nearly full of water with its mouth inverted. The same is true of all the solid parts of animals and vegetables, when subjected to fire; as appears from the experiments of that learned philanthropist, Dr. Hales. But of water the contrary is evident. In the steam-engine, a jet of cold water, we find, instantly condenses that immense rarefaction; which I apprehend could not be, if it was constituted of escaped elastic air. And altho’ this steam must be acknowledged to put on some properties of air; such as ventilating a fire; or that a taper blown out by it, is capable of being again lighted immediately, and that without a crackling noise, which occurs when touched with water; this does not in the least invalidate our opinion, tho’ it has certainly conduced very much to propagate the former one: since from this way of reasoning, the whole must be air, and we should have no water at all in vapour. From considering this power of expansion, which the constituent parts of some bodies acquire by heat; and withal, that some bodies have a greater affinity to heat, that is, acquire it sooner and retain it longer than others; which affinity appears from experiments, and which, I apprehend, is in some ratio of their specific gravities and their powers of refraction, reflexion, or absorption of light; or at least in some ratio much greater than that of their specific gravities alone. From considering these, I say, many things, before utterly inexplicable, became easily understood by me. Such as, Why when bismuth and zinc are fused together, and set to cool, the zinc, which is specifically heavier, is found above the bismuth? Why the buff covering of inflammatory blood, the skum of heated milk, the sedative salt of borax, which are all specifically heavier than the liquids in which they are formed, are still formed at the surface of them? How benzoin, sulphur, and even the ponderous body mercury, may be raised into vapour, again to be condensed unaltered? And, lastly, how water, whose parts appear from the æolipile to be capable of immeasurable expansion, should by heat alone become specifically lighter than the common atmosphere, without having recourse to a shell inclosing air, or other assistant machinery? and when raised, I am persuaded we shall find, that to support them floating, perhaps many days, in the atmosphere, is not a knot so intricate, as to oblige us to conjure up a new divinity to unravel it.