It was known, that if an animal was confined beyond a certain limited time in a given volume of atmospherical air, it died of suffocation, in consequence of the air becoming unfit for breathing; and that if another animal was put into this air, thus rendered noxious by breathing, its life was destroyed almost in an instant. Dr. Priestley had thrown some light upon this subject by showing that air, in which an animal had breathed for some time, possessed the property of rendering lime-water turbid, and therefore contained carbonic acid gas. He considered the process of breathing as exactly analogous to the calcination of metals, or the combustion of burning bodies. Both, in his opinion acted by giving out phlogiston; which, uniting with the air of the atmosphere, converted it into phlogisticated air. Priestley found, that if plants were made to vegetate for some time in air that had been rendered unfit for supporting animal life by respiration, it lost the property of extinguishing a candle, and animals could breathe it again without injury. He concluded from this that animals, by breathing, phlogisticated air, but that plants, by vegetating, dephlogisticated air: the former communicated phlogiston to it, the latter took phlogiston from it. After Lavoisier had satisfied himself that air is a mixture of oxygen and azote, and that oxygen alone is concerned in the processes of calcination and combustion, being absorbed and combined with the substances undergoing calcination and combustion, it was impossible for him to avoid drawing similar conclusions with respect to the breathing of animals. Accordingly, he made experiments on the subject, and the result was published in the Memoirs of the Academy, for 1777. From these experiments he drew the following conclusions: 1. The only portion of atmospherical air which is useful in breathing is the oxygen. The azote is drawn into the lungs along with the oxygen, but it is thrown out again unaltered. 2. The oxygen gas, on the contrary, is gradually, by breathing, converted into carbonic acid; and air becomes unfit for respiration when a certain portion of its oxygen is converted into carbonic acid gas. 3. Respiration is therefore exactly analogous to calcination. When air is rendered unfit for supporting life by respiration, if the carbonic acid gas formed be withdrawn by means of lime-water, or caustic alkali, the azote remaining is precisely the same, in its nature, as what remains after air is exhausted of its oxygen by being employed for calcining metals. In this first paper Lavoisier went no further than establishing these general principles; but he afterwards made experiments to determine the exact amount of the changes which were produced in air by breathing, and endeavoured to establish an accurate theory of respiration. To this subject we shall have occasion to revert again, when we give an account of the attempts made to determine the phenomena of respiration by more modern experimenters.