From what hath been said concerning the nature of Fire, it is evidently impossible for us to fix and confine it in any body. Yet the phenomena attending the combustion of inflammable bodies shew, that they really contain the matter of Fire as a constituent principle. By what mechanism then is this fluid, which is so subtle, so active, so difficult to confine, so capable of penetrating into every other substance in nature; how comes it, I say, to be so fixed as to make a component part of the most solid bodies? It is no easy matter to give a satisfactory answer to this question. But, without pretending to guess the cause of the phenomenon, let us rest contented with the certainty of the fact, the knowledge of which will undoubtedly procure us considerable advantages. Let us therefore examine the properties of Fire thus fixed, and become a principle of bodies. To this substance, in order to distinguish it from pure and unfixed Fire, the Chymists have assigned the peculiar title of the _Phlogiston_, which indeed is no other than a Greek word for the Inflammable Matter; by which latter name, as well as by that of the Sulphureous Principle, it is also sometimes called. It differs from elementary Fire in the following particulars. 1. When united to a body, it communicates to it neither heat nor light. 2. It produces no change in its state, whether of solidity or fluidity; so that a solid body does not become fluid by the accession of the Phlogiston, and _vice versa_; the solid bodies to which it is joined being only rendered thereby more apt to be fused by the force of the culinary fire. 3. We can convey it from the body with which it is joined into another body, so that it shall enter into the composition thereof, and remain fixed in it. On this occasion both these bodies, that which is deprived of the Phlogiston and that which receives it, undergo very considerable alterations; and it is this last circumstance, in particular, that obliges us to distinguish the Phlogiston from pure Fire, and to consider it as the element of Fire combined with some other substance, which serves it as a basis for constituting a kind of secondary principle. For if there were no difference between them, we should be able to introduce and fix pure Fire itself, wherever we can introduce and fix the Phlogiston: yet this is what we can by no means do, as will appear from experiments to be afterwards produced.