Black then gives a clear account of the experiments of Priestley and Lavoisier (see pp. 58, 59, and 87-89), which established the presence, in common air, of a peculiar kind of gas which is especially concerned in the processes of combustion; he emphasizes the fact that a substance increases in weight when it is burned; and he gives a simple and clear statement of that explanation of combustion which is now accepted by all, and which does not require that the existence of any principle of fire should be assumed.
It is important to note that Black clearly connects the _physical_ fact that heat is absorbed, or evolved, by a substance during combustion, with the _chemical_ changes which are brought about in the properties of the substance burned. He concludes with an admirable contrast between the phlogistic theory and the theory of Lavoisier, which shows how wide, and at the same time how definite, his conceptions were. Black never speaks contemptuously of a theory which he opposes.
"According to this theory" (_i.e._ the theory of Lavoisier), "the inflammable bodies, sulphur for example, or phosphorus, are simple substances. The acid into which they are changed by inflammation is a compound. The chemists, on the contrary" (_i.e._ the followers of Stahl), "consider the inflammable bodies as compounds, and the uninflammable matter as more simple. In the common theory the heat and light are supposed to emanate from, or to be furnished by, the burning body. But, in Mr. Lavoisier's theory, both are held to be furnished by the air, of which they are held to be constituent parts, or ingredients, while in its state of fire-supporting air."
Black was not a brilliant discoverer, but an eminently sound and at the same time imaginative worker; whatever he did he did well, but he did not exhaust any field of inquiry. Many of the facts established by him have served as the basis of important work done by those who came after him. The number of new facts added by Black to the data of chemistry was not large; but by his lectures--which are original dissertations of the highest value--he did splendid service in advancing the science of chemistry. Black possessed that which has generally distinguished great men of science, a marked honesty of character; and to this he added comprehensiveness of mental vision: he saw beyond the limits of the facts which formed the foundations of chemical science in his day. He was not a fact-collector, but a philosopher.