Mayow made an important addition to Boyle's hypo thesis, by supposing that the air, besides carrying off these vapours, imparts something to the blood; and he even attempted to explain the nature of this substance which is abstracted from the atmosphere. By performing compa rative experiments on portions of air before and after it had been respired, he concluded that, during its passage through the lungs, it imparts to the blood a certain vola tile matter, which he calls nit 0-aerial spirit, and which was also concerned in combustion, as well as in many other chemical operations. Most of the contemporaries of Nlayow, as Boredi, Lower. and Willis, adopted opinions that were not very dissimilar with respect to the part which the air acts in the process of respiration, some of them imagining that a portion of the whole mass of air, and others that some pattleular part of it. entered the blood. This hypothesis was, however, after some time, gradually abandoned, and what may appear more re markable, the discoveries and experiments of Nl•yo•, which were extremely curial'', and calculated to throw great light upon natural philosophy, were entirely for gotten.
Our knowledge respecting the change which air un dergoes by respiration remained without any addition, or rather may ne considered as retrograde for some years, when the inquiry was again resumed by Hales. He in stituted a number of experiments upon this point, and was led to form a conclusion nearly similar to that of Boyle, that the change consists in the air acquiring a nox ious vapour, and losing part of its elasticity ; the same doctrine was also maintained by Haller. It was shortly after this period that Black commenced his investigations on the chemical nature of air ; and after discovering the existence of carbonic acid, as a gaseous substance, pos sessed of distinct and specific propertie . he found that a quantity of it was generated in the lungs by respiration. The labours of Black in this department of science were zealously seconded b) Cavendish. Scheele, and Priestley, whose successive discoveries led to the knowledge of the chemical composition of the atmosphere, as consisting principally of two aeriform substances to which the names of oxygen and azote have been since applied. Among the great variety of subjects to which the last of these philosophers directed his attention, he carefully examined the nature of the effect produced upon the air by respira tion, when he found that it had lost a part of its oxygen, or had experienced nearly the same kind of change as combustion, fermentation, and other analogous operations, which, in conformity to the hypothesis then prevalent, he styled phlogistic processes ; he therefore concluded that the air, in passing through the lungs, loses oxygen and acquires phlogiston and aqueous vapour.
• Not long after the publication of Priestley's experi ments, the subject of respiration was taken up by Lavoi sier. He examined the opinions of his predecessors with his accustomed address; he agrees with Priestley in the essential circumstance of the diminution of the oxygen is air that has been respired ; but he points out an impor tant distinction between the different phlogistic processes that had been hitherto confounded together; and with respect to the effect of respiration, he concludes that the air loses pat t of its oxygen and receives an addition of carbonic acid. The opinion of Lavoisier has been gene
rally assented' to by succeeding physiologists, and the ob ject of the experiments which have been lately made has been, eithe to ascertain the amount of these several changes, or to account for the operation of the lungs in effecting them.
With respect to the first of these changes, the con sumption of oxygen, there are two points to be ascertained. 1st, What proportion of it in air that is respired is ne cessary for the support ol life ? and, in the 2c1 place, what is the actual quantity that is consumed under ordinary circumstances ? Although our experiments have enabled us to acquire much information on these subjects, yet considerable difficulty attends all our investigations, and we are obliged, in most cases, to make allowance for va rious intervening circumstances, before we can apply our results to the solution of the question under discussion. We find that different classes of animals affect the air in very different degrees, and that this is likewise the case with different individuals of the same class, or even with the same individual under different circumstances. We find that the degree of purity in the atmosphere necessary for the continuance of life, depends very much upon thy constitution of the animal as to its ordinary temperature, and the structure of its organs of respiration. Birds would appear to require the greatest proportion of oxygen, and to be incapable of subsisting when more than two thirds of the usual proportion is removed. A mouse can live until nearly three-fourths is consumed; frogs, and the cold-blooded quadrupeds, will exist in an atmosphere that contains considerably less oxygen, while worms and snails are not destroyed until very nearly the whole is ab stracted. It is, however, necessary to bear in mind, that the death of the warm-blooded animals, in these cases, de pends not so much upon the deficiency of oxygen, as upon the presence of carbonic acid ; and, accordingly, it is found, that when the experiments are so conducted, that the acid is absorbed as fast as it is produced, a mouse can live, without apparent uneasiness, when no more than one-fifteenth of the original proportion of oxygen is left in the air. The temperature of the human subject, and the general structure of his organs of respiration and circulation, are similar to those of the mouse, so that it is probable a man might exist in air of this standard ; but it would only serve for his support while in a state of per fect rest, and without the unnecessary action of any of the functions, for, as we shall afterwards find, the consump tion of oxygen is materially augmented by these circum stances.