Mr. Kirwan, who had acquired a high reputation, partly by his _mineralogy_, and partly by his experiments on the composition of the salts, undertook the task of refuting the antiphlogistic theory, and with that view published a work to which he gave the name of "An Essay on Phlogiston and the Composition of Acids." In that book he maintained an opinion which seems to have been pretty generally adopted by the most eminent chemists of the time; namely, that phlogiston is the same thing with what is at present called _hydrogen_, and which, when Kirwan wrote, was called light _inflammable air_. Of course Mr. Kirwan undertook to prove that every combustible substance and every metal contains hydrogen as a constituent, and that hydrogen escapes in every case of combustion and calcination. On the other hand, when calces are reduced to the metallic state hydrogen is absorbed. The book was divided into thirteen sections. In the first the specific gravity of the gases was stated according to the best data then existing. The second section treats of the composition of acids, and the composition and decomposition of water. The third section treats of sulphuric acid; the fourth, of nitric acid; the fifth, of muriatic acid; the sixth, of aqua regia; the seventh, of phosphoric acid; the eighth, of oxalic acid; the ninth, of the calcination and reduction of metals and the formation of fixed air; the tenth, of the dissolution of metals; the eleventh, of the precipitation of metals by each other; the twelfth, of the properties of iron and steel; while the thirteenth sums up the whole argument by way of conclusion. In this work Mr. Kirwan admitted the truth of M. Lavoisier's theory, that during combustion and calcination, oxygen united with the burning and calcining body. He admitted also that water is a compound of oxygen and hydrogen. Now these admissions, which, however, it was scarcely possible for a man of candour to refuse, rendered the whole of his arguments in favour of the identity of hydrogen and phlogiston, and of the existence of hydrogen in all combustible bodies, exceedingly inconclusive. Kirwan's book was laid hold of by the French chemists, as affording them an excellent opportunity of showing the superiority of the new opinions over the old. Kirwan's view of the subject was that which had been taken by Bergman and Scheele, and indeed by every chemist of eminence who still adhered to the phlogistic system. A satisfactory refutation of it, therefore, would be a death-blow to phlogiston and would place the antiphlogistic theory upon a basis so secure that it would be henceforth impossible to shake it.