Lavoisier, in the course of his investigations, had proved that carbonic acid is a compound of carbon and oxygen; sulphuric acid, of sulphur and oxygen; phosphoric acid, of phosphorus and oxygen; and nitric acid, of nitrous gas and oxygen. Neither the carbon, the sulphur, the phosphorus, nor the nitrous gas, possessed any acid properties when uncombined; but they acquired these properties when they were united to oxygen. He observed further, that all the acids known in his time which had been decomposed were found to contain oxygen, and when they were deprived of oxygen, they lost their acid properties. These facts led him to conclude, that oxygen is an essential constituent in all acids, and that it is the principle which bestows acidity or the true acidifying principle. This was the reason why he distinguished it by the name of oxygen.[5] These views were fully developed by Lavoisier, in a paper inserted in the Memoirs of the Academy, for 1778, entitled, "General Considerations on the Nature of Acids, and on the Principles of which they are composed." When this paper was published, Lavoisier's views were exceedingly plausible. They were gradually adopted by chemists in general, and for a number of years may be considered to have constituted a part of the generally-received doctrines. But the discovery of the nature of chlorine, and the subsequent facts brought to light respecting iodine, bromine, and cyanogen, have demonstrated that it is inaccurate; that many powerful acids exist which contain no oxygen, and that there is no one substance to which the name of acidifying principle can with justice be given. To this subject we shall again revert, when we come to treat of the more modern discoveries. [5] From ὀξυς, sour, and γινομαι, which he defined the _producer of acids_, the _acidifying principle_. Long as the account is which we have given of the labours of Lavoisier, the subject is not yet exhausted. Two other papers of his remain to be noticed, which throw considerable light on some important functions of the living body: we allude to his experiments on _respiration_ and _perspiration_.