AMOUNT OF CARBONIC ACID EXHALED. 295 united with its proportional of carbon, forms one volume of carbonic acid, it was conceived that the greater part of the oxygen had united with the carbon of the blood, so as to form carbonic acid, which was expired in the gaseous form; while the rest of the oxygen, combining with hydrogen contained in the blood, had given rise to the watery vapour which is exhaled from the lungs in considerable quantity. Taking the mean result of the experiments of Lavoisier, Menzies, Abernethy, Thomson, and Hales, it would appear that the quantity of watery vapour exhaled by an adult in twenty-four hours amounts to 7963 grains. This water contains some animal matter. (Gmelin’s Chemie, iv. p. 1524.) Quantity of carbonic acid generated.—Sir Humphry Davy having ascertained that a portion of air, which measured 161 cubic inches, was composed of 117 cubic inches nitrogen, 42.4 cubic inches oxygen, and 1.6 cubic inch carbonic acid, respired it for the space of a minute, during which time he performed nineteen respirations. At the termination of the experiment he found the air to consist of 111.6 cubic inches nitrogen, 23.0 cubic inches oxygen, and 17.4 cubic inches carbonic acid. Consequently 15.8 cubic inches of carbonic acid had been generated in his lungs in one minute. (Sir H. Davy, Researches on Nitrous Oxide, p. 435.) Messrs. Allen and Pepys* afterwards investigated the subject in an admirable manner. The air was inspired from one gasometer and expired into another. As the mean result of their numerous observations they adopt their eleventh experiment, in which 302 cubic inches of carbonic acid were exhaled in eleven minutes, or about 27.2 cubic inches in one minute. After frequent repetition of their experiments, they found that air, after being once respired, con¬ tains 8 or Si per cent, of carbonic acid; and that however often the same air is respired, even if until it will no longer sustain life, it does not become charged with more than 10 per cent, of this gas. Allen and Pepys ascertained, moreover, that a man respiring oxygen generates more carbonic acid than when breathing common air. Thus, for 100 parts of oxygen inhaled, 12 parts of carbonic acid were expired, and a considerable quantity of nitrogen was de¬ veloped at the same time. Gmelin (Chemie, iv. 1525,) has collected the results of the different experiments of Davy, Berthollet, Allen and Pepys, Menzies, and Prout; and, taking the mean of all these, it would appear that 100 parts of air, once respired, contain 5.82 parts of carbonic acid. Dr. Prout’s (Ann. Phil. vol. ii. p. 330; and vol. iv. p. 331—334,) experiments seem to show that the quantity of carbonic acid gene¬ rated in a given time is greatest between the hours of eleven, a. m., and one, p. m.; smallest between the hours of half-past eight, p. m., and half-past three, a. m. If the quantity of carbonic acid generated in respiration is from any cause increased for a period, it afterwards sinks in the same proportion below the usual quantity. The amount * Philos. Transact. 1808, 1809. Schweigger’s Journal, t. 1; and Meckel’s Archiv, iii. 233.