Hydrogen is likewise capable of combining with many combustible substances. In the state of gas, it dissolves charcoal, sulphur, phosphorus, and several metals; we distinguish these combinations by the terms, _carbonated hydrogen gas_, _sulphurated hydrogen gas_, and _phosphorated hydrogen gas_. The sulphurated hydrogen gas was called _hepatic air_ by former chemists, or _foetid air from sulphur_, by Mr Scheele. The virtues of several mineral waters, and the foetid smell of animal excrements, chiefly arise from the presence of this gas. The phosphorated hydrogen gas is remarkable for the property, discovered by Mr Gengembre, of taking fire spontaneously upon getting into contact with atmospheric air, or, what is better, with oxygen gas. This gas has a strong flavour, resembling that of putrid fish; and it is very probable that the phosphorescent quality of fish, in the state of putrefaction, arises from the escape of this species of gas. When hydrogen and charcoal are combined together, without the intervention of caloric, to bring the hydrogen into the state of gas, they form oil, which is either fixed or volatile, according to the proportions of hydrogen and charcoal in its composition. The chief difference between fixed or fat oils drawn from vegetables by expression, and volatile or essential oils, is, that the former contains an excess of charcoal, which is separated when the oils are heated above the degree of boiling water; whereas the volatile oils, containing a just proportion of these two constituent ingredients, are not liable to be decomposed by that heat, but, uniting with caloric into the gasseous state, pass over in distillation unchanged. In the Memoirs of the Academy for 1784, p. 593. I gave an account of my experiments upon the composition of oil and alkohol, by the union of hydrogen with charcoal, and of their combination with oxygen. By these experiments, it appears that fixed oils combine with oxygen during combustion, and are thereby converted into water and carbonic acid. By means of calculation applied to the products of these experiments, we find that fixed oil is composed of 21 parts, by weight, of hydrogen combined with 79 parts of charcoal. Perhaps the solid substances of an oily nature, such as wax, contain a proportion of oxygen, to which they owe their state of solidity. I am at present engaged in a series of experiments, which I hope will throw great light upon this subject.