Having thus ascertained the absorption of nitrous oxide in respiration, and the evolution of nitrogene and carbonic acid from the lungs during its absorption: considering atmospheric air as a compound in which principles identical with those in nitrous oxide existed, though in different quantities and looser combination, I was anxious to compare the changes effected in this gas by respiration, with those produced in nitrous oxide and oxygene; particularly as they are connected with the health and life of animals. The ingenious experiments of Lavoisier and Goodwyn, prove the consumption of oxygene in respiration, and the production of carbonic acid. From many experiments on the respiration of common air, Dr. Priestly suspected that a certain portion of nitrogene, as well as oxygene, was absorbed by the venous blood. _b._ In the following experiments on the respiration of atmospheric air in the mercurial airholder; the composition of the gas before inspiration and after expiration, was ascertained in the following manner. Forty measures of it were agitated over mercury in solution of caustic potash, and suffered to remain in contact with it for two or three hours. The diminution was noted, and the gas absorbed judged to be carbonic acid. Twenty measures of the gas, freed from carbonic acid, were mingled with thirty of nitrous gas, in a tube of,5 inches diameter; they were not agitated,[200] but suffered to rest for an hour or an hour and half, when the volume occupied by them was noticed: and 50-_m_ the volume occupied, divided by 3 considered as the oxygene _x_, and 20-_x_ considered as the nitrogene. [200] When they are agitated, a greater proportion of nitrous gas is absorbed, condensed in the nitric acid by the water; and to find the oxygene, (50 - _m_) (50 - _m_) _x_ = —————————— or ————————— . (3,4) (3,5) _c._ To ascertain the changes effected in atmospheric air by single inspirations, I made, after a compleat voluntary exhaustion of my lungs, at temperature 61°, one inspiration and expiration of 141 cubic inches of atmospheric air. After expiration, they filled a space equal to 139 cubic inches nearly. These 139 cubic inches analised were found to consist of Nitrogene 101 Oxygene 32 Carbonic acid 6 The 141 cubic inches before inspiration, were composed of 103 nitrogene, 1 carbonic acid and 37 oxygene. The time taken to perform the inspiration and full expiration, was nearly a quarter of a minute. I repeated this experiment seven or eight times, and the quantity of oxygene absorbed was generally from 5 to 6 cubic inches, the carbonic acid formed from 5 to 5,5, and the quantity of nitrogene apparently diminished by from 1 to 3 cubic inches. E. 2. I made, after a voluntary expiration of common air, one inspiration and full expiration of 100 cubic inches of atmospheric air. It was diminished nearly to 98¾ or 99 cubic inches, and analised, was found to consist of