the end of this time he found them not altered.. They have since, however, been brought to emit very sensible vapours from the more intense heat of a burning glass. Hence we have some reason to conclude, that with sufficient heat the earthy substances also might be fused and volatilized. V. The last of the more general effects of caloric is COMBUSTION; but this effect is not so general as the former, since there is only one class of bodies susceptible of it, hence called combustible bodies. The distinction between them and others is, that the bodies which are not combustible are not altered by heat in a permanent manner, nor is the caloric which they receive at all increased, but is readily transmitted. Combustible bodies, on the contrary, are, when inflamed, sources of light and heat. Their capacity of producing light and heat is, however, in time exhausted, and when we examine what remains we find them greatly changed; they appear to be different substances, and are no longer combustible. All bodies that are not combustible are ready to receive caloric, and part with it again, giving out the same quantity, neither more nor less. If a red-hot stone is thrown into a quantity of water, the heat seems to be extinguished or annihilated, but this heat is not lost. I do not. positively assert that caloric is never destroyed, or, more properly, changed in its nature; I only say it is a fallacious way of judging of the loss of heat by our senses. It is very clear, that when bodies receive caloric in the usual manner, they retain or part with it in a sensible manner, except in the case of combination, or latent caloric. We may cause a body not inflammable to retain heat longer by surrounding it with bodies of a looser texture; but whatever pains we take the body will cool in time, and the caloric will communicate itself to the surrounding matter: but such is the nature of combustible bodies, that, when heated to a certain degree in the air, they not only become hot, but by proper management they may be heated to any degree, and the heat which is thus generated may be communicated to other bodies without any loss of heat to the combustible bodies: they are in general also luminous; hence their uses in chemistry and the arts. While the stream of heat and light flows from them, they are consumed or changed into a different matter, which cools or heats in the usual manner, and is no longer combustible. Some combustible substances have been thought exceptions to this, as spirit of wine highly rectified. To a rude observer it seems to be totally consumed during its inflammation, and he is apt to conclude that the spirit of wine is not converted into a matter no longer com⚫ bustible *. The reason of the phænomenon is, * This was even Boerhaave's opinion. that when we set the spirit of wine on fire, the flame continues clear without smoke till the last drop of the fluid is consumed, and nothing remains in the vessel. But here is a deception, for a great quantity of matter escapes from the spirit that evades the cognizance of our senses. This may be proved by placing a hollow vessel over the flame, which confining and cooling it, the internal surface of the vessel will be bedewed with a watery moisture. Some have imagined it to be one-third of the spirit, others one-half, and others three-fourths: the fact is, however, that highly rectified spirit affords rather more than its own weight of water by combustion. The reason we do not see the watery part evaporate is, that the fluid is converted into vapour in so gradual a manner, that it is impossible to discover it by our senses, unless we use those means of collecting and condensing it. Sulphur is another substance which former chemists imagined was consumed or destroyed wholly by fire; but by an easy experiment we find that a quantity of acid is thrown off during the combustion, which is equal in weight to the substance which produced it. The theory of combustion is now indeed as well understood as most facts in chemistry. Combustible bodies are such as will readily combine with oxygen gas; consequently, when any such body is raised to a certain temperature, it begins to be decomposed, and to combine with the oxygen of the atmosphere, and this oxygen during its combination lets go the caloric with which in the state of air or gas it was combined. Hence combustion consists of two things, a decomposition, and a combination; and the substance which has undergone combustion is essentially altered; it is, in fact, a compound of the body which has been subjected to combustion and oxygen. It is of course incombustible, because its base being already saturated with oxygen, cannot combine with any more. Upon the same principles, if by any process the oxygen is taken away, the substance will again be rendered combustible. To illustrate this whole theory, take a familiar instance. Sulphur, we have just seen, is reduced to a corrosive acid by burning in the open air; but by charcoal applied in a particular manner with the assistance of fire, this acid may be again converted into inflammable sulphur. It is the same thing whether we use charcoal made of blood, flesh, or bone, or whether the charcoal from any vegetable matter. Nor is it necessary to have recourse to charcoal, as pit-coal and other combustible bodies have the same effect. The fact is easily explained upon the principles just laid down. The sulphur, when burned, extracts the oxygen, or acidifying principle, from the air, |