The first attempts to estimate the absolute quantity of oxygen consumed were those of Menzies. He found that one•twentieth part of air that has once passed through the lungs is removed, and this he supposed to be oxygen; hence he concludes that 52,000 cubic inches, or between 17,000 and 18,000 grains, arc consumed in 24 hours. This calculation, however, being derived from insufficient data, can only be regarded as an approximation to the truth ; and the results of some experiments which were performed by Lavoisier, in conjunction with Seguin, seem to be much more entitled to our confidence. They were conducted upon a large scale, with an apparatus more complete than had hitherto been employed in physiologi cal researches, and every circumstance seems to have been attended to which could ensure their accuracy. We learn from them, that a man, under ordinary circum stances, consumes in 24 hours about 46,000 cubic inches, or 15,500 grains. Since the death of this philosopher, who, as is well known, was sacrificed to the barbarous fury of Robespierre, some experiments on the same subject have been performed by Sir H. Davy ; and although he made use of a very different kind of process, his results nearly coincide with those of Lavoisier. We may, there fore, conclude, with considerable confidence, the quantity to be between 45,000 anti 46,000 cubic inches, or about 2lbs. 8 oz. Troy.
The next point to be ascertained with respect to the chemical change produced in the air by respiration, is the quantity of carbonic acid that is produced. We have al ready stated, that the general fact of its existence in air that is emitted from the lungs is due to Black ; but he does not appear to have made any attempt to estimate its quantity. Menzies calculated that nearly 4lbs. of carbo nic acid are produced by a man under ordinary circum stances in twenty-four hours ; whereas Lavoisier and Se guin, in the experiments referred to above, reduce the quantity to rather more than 3lbs.; and in their later ex periments, which, it may be presumed, were performed with greater accuracy, the carbonic acid was still farther diminished to only half that quantity. The estimate of Sir H. Davy, on the contrary, nearly agrees with the first of Latoisier's.
In these experiments, as well as in all the others that were performed until lately, the oxygen consumed always appeared to be greater than the carbonic acid that was generated ; and although the difference between these amounts varied considerably in different experiments, yet the difference was found to exist in all those upon which the greatest confidence was to he placed. We have, how ever, a series performed by Messrs. Allen and Pepys, which were conducted, as it would appear, with the most scrupulous attention to accuracy, where the amount of oxygen consumed exactly coincided with the carbonic acid produced ; and, in consequence of these experiments, the opinion has been pretty generally adopted, that these quantities always coincide. It is, indeed, difficult to con ceive how experimentalists of acknowledged address and dexterity, such as Lavoisier and Davy, could have fallen into so considerable a mistake, yet it is, perhaps, still more so, to detect any probable source of error in the opera tions of Allen and Pepys.
Besides the general causes of uncertainty that must at tach to all experiments performed upon the living body, there is a specific difficulty in ascertaining the amount of the chemical changes produced upon the air by respira tion depending upon the circumstance, that different states of the functions cause the changes to be produced in different degrees. This fact was first noticed by
Priestley and Crawford, with respect to the operation of external temperature ; they observed that an animal con sumed less oxygen, and produced less carbonic acid/ when exposed to a warm medium, than the same animal when placed in a lower temperature. Subsequent experiments have confirmed this discovery, and Jurine of Geneva ex tended the observations to other circumstances besides temperature. He found that the production of carbonic acid was increased by the process of digestion, by violent exercise, and by the state of fever ; and Lavoisier met with the same results in his elaborate researches. The experiments in this case were performed upon the per son of Seguin ; and, so far as the proportionate quantities are concerned, they appear to deserve great confidence. When the stomach was empty, the body at rest, and the temperature considerably elevated, it was found that 1210 cubic inches of oxygen gas were consumed in an hour ; whereas, when the temperature was reduced to 25° of Fahr., the other circumstances remaining the same, the amount of oxygen was increased to 1344 cubic inches. During digestion the oxygen was found to be between 1800 and 1900 inches ; and, if violent exercise had'been used when digestion was going forwards, the quantity tvas increased to 4600 cubic inches in an hour. It is to be observed, that, in all these different cases, the tempera ture is not affected, but that the circulation and respira tion are much accelerated.
We meet with the same kind of discordance of opinion respecting another change which the air has been sup posed to experience by its passage through the lungs, viz. the diminution of bulk. The earlier physiologists very much over-rated this diminution in bulk, in conse quence of obvious sources of inaccuracy in the mode of conducting their experiments ; but the same change, al though in a less degree, has been admitted to take place by Goodwyn, Lavoisier, and Davy. Lavoisier, in the first of his memoirs, stated it at 1-60th, and Davy found it to vary from 1-70th to 1-100th. But, powerful and direct as this evidence must appear in favour of the fact, a contrary result was obtained by Messrs. Allen and Pepys ; who, in all cases, found the bulk of the air to be the same before and after the experiment. The difference of opinion that exists respecting this matter of fact is the more important, as it very materially affects our hypo thesis of the nature of the action of the lungs. When oxygen is converted into carbonic acid by the addition of carbon, it experiences no change of volume ; therefore we are led to conclude, that, if the bulk of the air remain unaltered after passing through the lungs, the only effect of respiration will be, to remove a portion of carbon from the blood; whereas, if the quantity of oxygen which dis appears be greater than what is necessary for the forma tion of the carbonic acid, a portion of it must be employed for some other purpose in the animal economy.