Though some of the phenomena of combustion were known from the earliest ages, yet no rational attempt to explain them was made till about the middle of the 17th century.

Before that period, it was supposed by alchymists and physicians, that sulphur was the inflammable principle, on which all the phenomena of combustion depended.

But Beecher perceiving, that sulphur did not exist in several combustible bodies, asserted it was not the principle of inflammability; which, he maintained, was a different substance, common to sulphur and other combustible bodies: this principle he supposed to be of a dry nature, and called it inflammable Earth.

Stahl refined this hypothesis by supposing the inflammable earth of Beccher to be pure fire, fixed in combustible bodies, and constituting an essential part of them.

According to Stahl, a combustible body is a compound, containing fixed fire, or phlogiston; and combustion is the disengagement of this phlogiston from a fixed to a free state, attended sometimes with heat and light: when these phenomena cease, the body becomes incombustible; if this calcined body be now heated with charcoal, or any other inflammable substance, capable of giving it phlogiston, or fixed fire, it returns again to the class of inflammable bodies.

This hypothesis is so simple and plausible, that, since the year 1736, it was received, though differently modified, all over Europe.

The principal objections to it are, that the existence of phlogiston cannot be demonstrated; and that it does not explain, why bodies become heavier after oxygenation, and lighter after reduction.

M. Lavoisier discovering, that the increase of weight, which bodies acquire during combustion, is equal to the weight of the combustible substance, and that of the vital air employed, denied the existence of phlogiston, or fixed fire, in combustible bodies; though he grants the existence of combined caloric in them. He seems then to allow the existence of phlogiston, but to have changed its name to that of caloric, and denied it the power of producing combustion.

However, after discovering that the increase of weight, which bodies acquire during combustion, depended on the quantity of the air absorbed, he was led to the following beautiful hypothesis, equally simple as that of Stahl; and if ever any hypothesis deserved the name of theory, surely it is that of M. Lavoisier.

According to M. Lavoisier, “an inflammable body is nothing else but a body which has the property of decomposing vital air, and taking the base from caloric and light; that is to say, the oxygene, which was united to them; and that a body ceases to be combustible when its affinity for the oxygene is satisfied, or when it is saturated with that principle; but that it becomes again combustible, when the oxygene has been taken from it by another body, which has a stronger affinity with that principle.

“When this decomposition of the air is rapid, and, as it were, instantaneous, there is an appearance of flame, heat, and light; when, on the contrary, the decomposition is very slow, and quietly made, the heat and light are scarcely perceptible⁠[1].”

Thus all the phenomena of oxygenation, which Stahl ascribed to the disengagement of phlogiston, M. Lavoisier ascribes to the union of oxygen with combustible bodies.

And, on the other hand, all the phenomena of reduction, which Stahl attributed to the union of phlogiston to calcined bodies, M. Lavoisier attributes to the separation of oxygen from the same. Combustion, then, according to Stahl, is the transition of phlogiston from one body to another; and, according to M. Lavoisier, ’tis nothing but the transition of oxygen from one substance to another.

Stahl supposed, that the heat, and light, attending combustion proceed from the burning body; but M. Lavoisier supposes, that the heat, and light, proceed principally, and almost entirely, from the vital air.

However, this opinion of M. Lavoisier is far from being established; for Doctor Crawford has shown, that inflammable air contains a much greater quantity of heat, than vital air does.

It is evident, then, that the great advantage, which M. Lavoisier’s hypothesis has over that of Stahl is, that the former seems to account for the increase and decrease of weight, which bodies suffer during calcination, and reduction.

Notwithstanding this brilliant discovery, which seems to account, with such ease, for the increase of weight, that bodies acquire during combustion, M. Macquer was of opinion, that M. Lavoisier’s hypothesis was insufficient to explode that of Stahl, and endeavoured to remove the objection urged against the nonexistence of phlogiston, by supposing it to be light; and that in every case of combustion, light, and vital air, mutually precipitate each other: so that, according to M. Macquer, when a body is oxygenated, vital air unites to it, and disengages light; and, on the other hand, when an oxygenated substance is reduced to its inflammable state, light unites to it, and disengages vital air.

M. Scheele also modified the hypothesis of Stahl, by supposing that heat, light, and inflammable air, were compounds of empyreal, or vital air, and phlogiston. Light, he thought, contained more phlogiston than heat did; and inflammable air more than either: he farther supposed, that these compounds of vital air and phlogiston were convertible into one another, by the addition, or subtraction, of phlogiston; and that, when a metal was reduced by them, they were decomposed; their phlogiston uniting to the substance reduced.

His idea of calcination is, that metals either attract empyreal air by means of their phlogiston, and thus form heat, or that they communicate their phlogiston to the air, and attract heat from the fire; and that, either way, there is empyreal air in the calces, which makes the overplus of their weight⁠[2].

The last modification of Stahl’s hypothesis is that of M. Kirwan, who supposes that inflammable air exists in a fixed, or concrete, state in all combustible bodies, and is the true phlogiston of the antient chymists; and, as it can be exhibited in the form of air, its existence is no longer doubtful.

M. Kirwan supposes, that, when combustible bodies are calcined, or changed into acids, they combine most commonly with fixed air, formed during the operation, by the union of their phlogiston to vital air; and sometimes to other substances, and water, which is either absorbed in specie, or formed by the union of their phlogiston to vital air, during calcination.

He supposes, that some of these are restored to their combustible state by the decomposition of their fixed air; some partly by the decomposition of their fixed air, and partly by its expulsion, and that of the other foreign bodies, they had absorbed; and lastly, that some recover their combustibility by the expulsion of water, and others possibly by its decomposition in high degrees of heat. In all these reductions he supposes a simultaneous reunion of these bodies to phlogiston, or the inflammable principle⁠[3].

Though M. Kirwan’s hypothesis seems to account for the increase of weight, which bodies acquire during combustion, yet he has not proved the existence of inflammable air, in a concrete state, in all combustible bodies; nor has he shown, that fixed air is a compound of inflammable, and vital air; neither has he proved, that fixed air is the acidifying principle.

All these objections have been made, with great justice, to his hypothesis, by the antiphlogistians; who also object, that M. Kirwan’s account of calcination, and reduction, is complex, and repugnant to the simplicity of nature; “for,” says M. De Fourcroy, “as the author had admitted three general species of calcination, it was necessary, that he should, likewise, admit three kinds of reduction⁠[4].”

This criticism is very just; but it will appear, that the antiphlogistic account of calcination, and reduction, is no less complex, erroneous, and repugnant to the simplicity of nature: for when we consider the various sources, whence they derive the oxygen, which oxygenates bodies; and the long list of metallic reducers, which they suppose; it must be allowed, that if simplicity be a recommendation, their hypothesis is destitute of that advantage.

Thus, according to M. Lavoisier, the oxygen, which oxygenates combustible bodies, is sometimes derived from vital air; sometimes from atmospheric air; sometimes from acids; sometimes from water; and sometimes from metallic oxids, &c.

On the other hand, the catalogue of substances, which reduce bodies to their combustible state, is no less numerous.

“Heat,” says M. De Fourcroy, “separates oxygene from some; one metal takes it from another; hydrogene, or inflammable gas, takes it from most metals; and carbone perhaps from all⁠[5].”

To this list, may be added phosphorus, sulphur, and compounds of these with hydrogen; also light, and the electric fluid, &c., as will appear in the sequel.

But, I shall endeavour to show, that the hydrogen of water is the only substance, that restores oxygenated bodies to their combustible state; and that water is the only source of the oxygen, which oxygenates combustible bodies.

Want of simplicity is not the only defect in Mr. Lavoisier’s hypothesis: for he supposes, that the increase of weight, which bodies acquire during combustion, depends on the absorption of the oxygenous principle alone.

Thus, when a bit of phosphorus, sulphur, or charcoal, is burnt in vital air; the increase of weight, which these bodies acquire, he attributes entirely to oxygen, or the base of vital air.

These combustions he explains, by supposing, that the phosphorus, sulphur, and charcoal, decompose oxygen gas, by absorbing its base from caloric, and light, which are set free.

“There is,” says M. Lavoisier, “a total absorption of vital air, or rather of the oxygene, which forms its base in the combustion of phosphorus, and the weight of the phosphoric acid obtained, is found to be rigorously equal to the weight of the phosphorus, added to that of the vital air employed in its combustion. The same agreement of weights is observed in the combustion of inflammable gas and vital air, in the combustion of charcoal⁠[6]”, &c.

His idea of the calcination of metals is the same.

“We do not,” says M. Lavoisier, “affirm that vital air combines with metals to form metallic calces, because this manner of enunciating would not be sufficiently accurate: but we say, when a metal is heated to a certain temperature, and when its particles are separated from each other to a certain distance by heat, and their attraction to each other is sufficiently diminished, it becomes capable of decomposing vital air, from which it seizes the base, namely oxygene, and sets the other principle, namely caloric, at liberty.

“This explanation of what passes during the calcination is not an hypothesis, but the result of facts. It is upwards of twelve years, since the proofs have been laid by one of us, before the eyes of the academy, and have been verified by a numerous commission. It was then established, that when the calcination of metals is effected beneath an inverted glass vessel, or in closed vessels containing known quantities of air, the air itself is decomposed, and the weight of the metal becomes augmented by a quantity accurately equal to that of the air absorbed. It has since been found, that when the operation was performed in very pure vital air, the whole might be absorbed.”

“No supposition enters into these explanations; the whole is proved by weight and measure⁠[7].”

Hence it is evident, that M. Lavoisier confounds oxygen with oxygen gas; and considers the latter as a compound of oxygen, light, and caloric.

But since the dryest oxygen gas contains a large proportion of water, as Dr. Priestley and M. Kirwan have shown⁠[8]; and since the whole of the gas, except the caloric, and light, is absorbed, it necessarily follows, that the increase of weight, which bodies acquire during combustion, depends not only on the oxygen, but also on the water, contained in vital air.

Therefore oxids are compounds of combustible bodies united to oxygen, and water.

Another great objection to M. Lavoisier’s hypothesis, is, that he supposes both oxygenation, and reduction, effected by a single affinity.

Thus, according to him, when iron reduces a sulphate of copper, the iron does nothing more, than separate the oxygen from the copper, by its superior attraction for that principle.

From this view of combustion, grounded on the most accurate experiments in chymistry, it is manifest, that the antiphlogistic hypothesis does not account fully for the increase of weight, which bodies acquire during combustion; and consequently, that it cannot be admitted as a just theory.