In all combustion there is destruction or decomposition of the pure air in which the combustion takes place, and the body burned increases in weight exactly in proportion to the quantity of air destroyed or decomposed. FOURTH PHENOMENON In all combustion the body burned changes to an acid by the addition of the substance which has increased its weight: thus, for example, if sulphur is burned under a receiver the product of the combustion is vitriolic acid; if phosphorus be burned the product is phosphoric acid; if a carboniferous substance, the product is fixed air, otherwise known as acid of lime (carbonic acid, etc.). (Note: I would remark in passing that the number of acids is infinitely greater than has been supposed.) The calcination of metals is subject to exactly the same laws, and it is with very great reason that Mr. Macquer has treated it as a slow combustion; thus, 1°, in all metallic combustion there is a liberating of fire matter (_matière du feu_); 2°, veritable calcination can take place only in pure air; 3°, there is a combination of the air with the substance calcined, but with this difference, that in place of forming an acid with it there results from it a particular combination known as metallic calx. This is not the place to point out the analogy which exists between the respiration of animals, combustion and calcination; I shall return to that in the sequel to this memoir. These different phenomena of the calcination of metals and of combustion are explained in a very happy manner by Stahl’s hypothesis; but it is necessary with him to suppose the existence of fire matter (_matière du feu_) or of fixed phlogiston in the metals, in sulphur and in all bodies which he regards as combustibles; yet if the partisans of Stahl’s doctrine are asked to prove the existence of fire matter in combustible bodies, they fall necessarily into a vicious circle and are obliged to reply that combustible bodies contain fire matter because they burn, and that they burn because they contain fire matter. It is easy to see that in the last analysis this is explaining combustion by combustion. The existence of fire matter, or phlogiston, in metals, in sulphur, etc., is then really only an hypothesis, a supposition which, once admitted, explains, it is true, some of the phenomena of calcination and combustion; but if I show that these very phenomena may be explained in quite as natural a way by the opposite hypothesis, that is to say, without supposing the existence of either fire matter or phlogiston in the substances called combustible, Stahl’s system will be shaken to its foundations.