I might apply this theory successively to all combustions, but as I shall have frequent occasion to return to this subject, I will content myself at this time with these general illustrations. So, to resume, the air is composed, according to my idea, of fire matter as a dissolvent combined with a substance which serves it as a base, and which in some way neutralizes it; whenever a substance for which it has a greater affinity is brought into contact with this base, it leaves its solvent; then the fire-substance regains its rights, its properties, and appears to our eyes with heat, flame and light. Pure air, the dephlogisticated air of Mr. Priestley, is then, according to this opinion, the real combustible body, and perhaps the only one of that nature, and it is seen that it is no longer necessary, in order to explain the phenomena of combustion, to suppose that there exists a large quantity of fire fixed in all the substances which we call combustible, but that it is very probable, on the contrary, that very little of it exists in metals, in sulphur, phosphorus, and in most of the very solid, heavy and compact bodies, and, perhaps even that there exists in these substances only free fire matter, in virtue of the property which this matter has of putting itself in equilibrium with all surrounding bodies. Another striking reflection which comes to the support of the preceding ones, is that almost all substances may exist in three different states: under a solid form, under a liquid form, that is to say melted, or in the state of air or vapor. These three states depend solely on the quantity, more or less, of fire matter with which these substances are interpenetrated and with which they are combined. Fluidity, vaporization, elasticity, are then properties characteristic of the presence of fire and of a great abundance of fire; solidity, compactness, on the contrary, are evidences of its absence. By so much then as it is demonstrated that aeriform substances and air itself contain a large quantity of fire in combination, by so much it is probable that solid bodies contain little of it. For the rest, I repeat, in attacking here the doctrine of Stahl, it was not my purpose to substitute for it a rigorously demonstrated theory, but only an hypothesis which seemed to me more probable, more in conformity with the laws of nature, and one which appeared to involve less forced explanations and fewer contradictions. FOOTNOTES: [Footnote 19: _On Combustion_, Vol. II, p. 225.] XVIII ALESSANDRO VOLTA 1745-1827