In 1778, Lavoisier having been incessantly engaged on the subject of gases and combustion, announced another great discovery, “that the respirable portion of the atmosphere is the constituent principle of acids,” which he therefore denominated _oxygen_. The question as to “the acidifying principle” had long formed the subject of discussion. The prevalent theory was that of Beccher with various modifications, which made the acid principle a compound of earth and water regarded as elements. Lavoisier found in the instance of a great number of the acids, that they consisted of a combustible principle united with oxygen. He showed this both analytically and synthetically, and hence proceeded to the conclusion that oxygen is the acidifying principle in all acids. Berthollet opposed this doctrine, and contended that, in general, acidity depended on the manner and proportion in which the constituents are combined. The fact is, that, in this instance, Lavoisier had advanced a little too rapidly to his conclusion. Had he contented himself with stating it as applying to a _great number_ of acids, it would have been strictly true; but he had certainly no proof of its being _universally_ the case. When Sir H. Davy, some years after, showed that one of the most powerful acids (the muriatic) does not contain a single particle of oxygen, and when the researches of Guy Lussac and others had exhibited other proofs of the same thing, it became evident that Lavoisier’s assertion required considerable modification. And though _nearly_ all acids have been since included under the general law of containing _some supporter of combustion_, yet there appear to be exceptions even to this; the cautious language of Berthollet has been completely justified; and a perfect theory of acidity is perhaps yet wanting. Nevertheless, Lavoisier’s discovery is one of first-rate magnitude and importance, and with this qualification, certainly forms the basis of all our present knowledge of the subject. Another important research in which Lavoisier engaged, in conjunction with Laplace, was the determination of the specific heats of bodies, by means of an ingenious apparatus, which they denominated the calorimeter: these were by far the most precise experiments on the subject which had as yet been made, though some inaccuracies in the method have since been pointed out.