I have already shown, that phosphorus is changed by combustion into an extremely light, white, flakey matter; and its properties are entirely altered by this transformation: From being insoluble in water, it becomes not only soluble, but so greedy of moisture, as to attract the humidity of the air with astonishing rapidity; by this means it is converted into a liquid, considerably more dense, and of more specific gravity than water. In the state of phosphorus before combustion, it had scarcely any sensible taste, by its union with oxygen it acquires an extremely sharp and sour taste: in a word, from one of the class of combustible bodies, it is changed into an incombustible substance, and becomes one of those bodies called acids.
This property of a combustible substance to be converted into an acid, by the addition of oxygen, we shall presently find belongs to a great number of bodies: Wherefore, strict logic requires that we should adopt a common term for indicating all these operations which produce analogous results; this is the true way to simplify the study of science, as it would be quite impossible to bear all its specifical details in the memory, if they were not classically arranged. For this reason, we shall distinguish this conversion of phosphorus into an acid, by its union with oxygen, and in general every combination of oxygen with a combustible substance, by the term of oxygenation: from which I shall adopt the verb to oxygenate, and of consequence shall say, that in oxygenating phosphorus we convert it into an acid.
Sulphur is likewise a combustible body, or, in other words, it is a body which possesses the power of decomposing oxygen gas, by attracting the oxygen from the caloric with which it was combined. This can very easily be proved, by means of experiments quite similar to those we have given with phosphorus; but it is necessary to premise, that in these operations with sulphur, the same accuracy of result is not to be expected as with phosphorus; because the acid which is formed by the combustion of sulphur is difficultly condensible, and because sulphur burns with more difficulty, and is soluble in the different gasses. But I can safely assert, from my own experiments, that sulphur in burning absorbs oxygen gas; that the resulting acid is considerably heavier than the sulphur burnt; that its weight is equal to the sum of the weights of the sulphur which has been burnt, and of the oxygen absorbed; and, lastly that this acid is weighty, incombustible, and miscible with water in all proportions: The only uncertainty remaining upon this head, is with regard to the proportions of sulphur and of oxygen which enter into the composition of the acid.
Charcoal, which, from all our present knowledge regarding it, must be considered as a simple combustible body, has likewise the property of decomposing oxygen gas, by absorbing its base from the caloric: But the acid resulting from this combustion does not condense in the common temperature; under the pressure of our atmosphere, it remains in the state of gas, and requires a large proportion of water to combine with or be dissolved in. This acid has, however, all the known properties of other acids, though in a weaker degree, and combines, like them, with all the bases which are susceptible of forming neutral salts.
The combustion of charcoal in oxygen gas, may be effected like that of phosphorus in the bell-glass, (A. Pl. IV. fig. 3.) placed over mercury: but, as the heat of red hot iron is not sufficient to set fire to the charcoal, we must add a small morsel of tinder, with a minute particle of phosphorus, in the same manner as directed in the experiment for the combustion of iron. A detailed account of this experiment will be found in the memoirs of the academy for 1781, p. 448. By that experiment it appears, that 28 parts by weight of charcoal require 72 parts of oxygen for saturation, and that the aëriform acid produced is precisely equal in weight to the sum of the weights of the charcoal and oxygen gas employed. This aëriform acid was called fixed or fixable air by the chemists who first discovered it; they did not then know whether it was air resembling that of the atmosphere, or some other elastic fluid, vitiated and corrupted by combustion; but since it is now ascertained to be an acid, formed like all others by the oxygenation of its peculiar base, it is obvious that the name of fixed air is quite ineligible[11].
By burning charcoal in the apparatus mentioned p. 60, Mr de la Place and I found that one lib. of charcoal melted 96 libs. 6 oz. of ice; that, during the combustion, 2 libs. 9 oz. 1 gros. 10 grs. of oxygen were absorbed, and that 3 libs. 9 oz. 1 gros. 10 grs. of acid gas were formed. This gas weighs 0.695 parts of a grain for each cubical inch, in the common standard temperature and pressure mentioned above, so that 34,242 cubical inches of acid gas are produced by the combustion of one pound of charcoal.
I might multiply these experiments, and show by a numerous succession of facts, that all acids are formed by the combustion of certain substances; but I am prevented from doing so in place, by the plan which I have laid down, of proceeding only from facts already ascertained, to such as are unknown, and of drawing my examples only from circumstances already explained. In the mean time, however, the three examples above cited may suffice for giving a clear and accurate conception of the manner in which acids are formed. By these it may be clearly seen, that oxygen is an element common to them all, which constitutes their acidity; and that they differ from each other, according to the nature of the oxygenated or acidified substance. We must therefore, in every acid, carefully distinguish between the acidifiable, base, which Mr de Morveau calls the radical, and the acidifiing principle or oxygen.
FOOTNOTES:
[11] It may be proper to remark, though here omitted by the author, that, in conformity with the general principles of the new nomenclature, this acid is by Mr Lavoisier and his coleagues called the carbonic acid, and when in the aëriform state carbonic acid gas. E.