The Nature of Combustion

Antoine Laurent Lavoisier

Emboldened by these reflections, I venture to submit to the Academy today a new theory of combustion, or rather, to speak with that reserve to whose law I submit myself, an hypothesis, by the aid of which all the phenomena of combustion, calcination, and even to some extent those accompanying the respiration of animals are explained in a very satisfactory manner. I had already laid the foundations of this hypothesis p. 279–280 of vol. I. of my Opuscules physiques et chimiques; but I admit that trusting little to my own knowledge, I did not then dare to put forward an opinion which might seem singular, and which was directly opposed to the theory of Stahl and of many celebrated men who have followed him.

Though perhaps some of the reasons which then checked me still remain today, nevertheless, the facts which have multiplied since that time, and which seem to me favorable to my views, have confirmed me in my opinion: though hot, perhaps, any stronger, I have become more confident, and I think I have sufficient proofs, or at least probabilities, so that even those who may not be of my opinion cannot blame me for having written.

In general in the combustion of bodies four constant phenomena are observable, which seem to be laws from which nature never departs. Though these phenomena may be found implicitly stated in other memoirs, yet I cannot dispense with recalling them here in a few words.

First Phenomenon. All combustion sets free matter either of fire or light.

Second Phenomenon. Bodies can burn only in a very small number of kinds of gases (airs), or rather there can be combustion only in one kind of air, that which Mr. Priestley has named dephlogisticated air, and which I should call pure air. Not only will the bodies which we call combustibles not burn in a vacuum or in any other kind of air, they are, on the contrary, extinguished there as promptly as if they had been plunged into water or any other liquid.

Third Phenomenon. In all combustion there is destruction or decomposition of the pure [p.301] 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 (matiere 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 (matiere 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. [p.302]

No doubt you will not fail to ask me first what I understand by fire matter. I reply with Franklin, Boerhaave and some of the philosophers of old, that the matter of fire or of light is a very subtle, very elastic fluid, which surrounds every part of the planet we live on, which penetrates with more or less ease the substances which compose that, and which tends, when it is free, to come to a state of equilibrium in all.

I will add, borrowing the chemical phraseology, that this fluid is the solvent of a large number of substances; that it combines with them in the same way that water does with salt, and the acids with metals, and that the bodies thus combined and dissolved by the igneous fluid lose in part the properties which they had before the combination and acquire new ones which bring them nearer (make them more like) the fire matter.

It is thus, as I have shown in a memoir deposited with the secretary of this Academy, that every aeriform fluid, every kind of air, is a resultant of the combination of some substance, solid or fluid, with the matter of fire or of light; and it is to this combination that aeriform fluids owe their elasticity, their specific volatility, their rarity, and all the other properties which ally (rapprochent) them to the igneous fluid.

Pure air, according to this, what Mr. Priestley calls dephlogisticated air, is an igneous compound into which the matter of fire Or of light enters as solvent, arid into which some other substance enters as a base; but if, in any solution whatever, a substance is presented to the base with which that has greater affinity, it unites with this instantly and the solvent which it leaves is set free.

The same thing happens with the air in combustion; the substance which burns steals away the base; then the fire matter which served as its solvent becomes free, regains its rights and escapes with the characteristics by which we know it; that is to say, with flame, heat and light.

To elucidate whatever may seem obscure in this theory let us apply it to some examples: when a metal is calcined in pure air, the base of the air, which has less affinity for its own solvent than for the metal, unites with the latter as it melts and converts it into metallic calx. This combination of the base of the air with the metal is proved 1st, by the increase in weight which the latter undergoes in calcination; 2nd, by the almost total using up of the air under the receiving bell. But, if the base of the air was held in solution by the fire-matter, in proportion as this base combined with the metal, the fire-matter should become free and produce, in freeing itself, flame and light. You understand that the more speedy the calcination of the metal, that is to say, the more fixation of the air takes place in a give time, the more fire-matter will be liberated, and, consequently, the more marked and obvious the combustion will be.

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 it 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 fore, 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.—On Combustion, II. 225.