5 generally attributed to some accidental occurrence. Priestley's conclusion has, however, been powerfully confirmed by subsequent experiments; Sir H. Davy conceived it to be the case in his experiments, and he estimated that 5.2 cubic inches of nitrogen were absorbed per minute, or about 7488 inches in the 24 hours, a quantity equivalent to 2240 grains. The absorption of a portion of nitrogen is maintained by Cuvier, and has been proved by the researches of Dr. Henderson, and Prof. Pfaff, each of whom instituted a series of well conducted experiments, which nearly coincided in their results. They both of them indicated a deficiency of nitrogen in the air of expiration, although they differed somewhat in the amount; Dr. Henderson supposing it to be less, and Prof. Pfaff more, than the estimate of Sir H. Davy. There are indeed certain points in which these experiments would appear not to be altogether unexceptionable, but they fully warrant the conclusion which the authors deduce from them with respect to the question now under consideration. It may be observed also that they both of them agree in supposing that the total bulk of the air is diminished by respiration. To add to the apparent confusion of opinion on this subject, Jurine was induced to conclude, from the result of his experiments, that nitrogen is gene Researches, p. 434.. 3 Leçons d'Anat. Comp. t. iv. p. 303. 6 Ibid. v. vii. p. 43, 4; and v. xii, p. 251, 2. 8 rated by respiration; and the same result was obtained by Berthollet, and Nysten. The experiments of Berthollet and Nysten seem to warrant the conclusion that is drawn from them; but with respect to those of Jurine, it may be doubted whether they are equally conclusive, as the nitrogen which he supposed to be generated may, with more probability, be referred to a portion of the residual air of the lungs mixed with the air of expiration. The experiments which have been referred to above were performed either on man or on some of the warm-blooded vertebrated animals, whose respiration may be conceived to produce similar changes on the air. But we are in possession of many very curious facts respecting the respiration of the coldblooded animals, which are not to be disregarded in forming our judgment upon this subject. Spallanzani's researches on the respiration of the coldblooded quadrupeds appear to show very clearly that they absorb nitrogen in respiration; and the experiments of Humboldt and Provençal, on fishes, place the fact beyond all doubt, so far as these animals are concerned. The quantity of nitrogen varied very considerably in the different experiments, from 20 to as much as 89 per cent., while the relation which it bore to the carbonic acid produced was also variable, 7 Encyc. Meth. "Medecine," t. i. p. 493..7. 8 Mem. d'Arcueil, t. ii. p. 454.. 463. 9 Recherches, p. 186, 215; from p. 187 to p. 200 is an account of his experiments. Mem. sur la Respir. p. 184, 258. although, for the most part, an increase in the one was attended with an increase in the other.2 The researches of Dr. Edwards on this point have been no less successful than on the other objects to which he directed his attention. By immersing small animals in a large quantity of air, for a limited period, and calculating what effect the air contained in their lungs before and after the experiment would have upon the whole mass on which he operated; he found, that in many instances, there was an evident increase in the quantity of nitrogen, while in others there was a deficiency of it. He observed that the former change took place when the experiments were performed in spring or summer, or when young animals were employed, while the latter occurred during the winter. Hence we have the important fact established, that nitrogen is, according to circumstances, either exhaled or absorbed in respiration; the probability is, that in all cases, both these operations are going forwards, that they are often exactly balanced, so as to show neither excess nor deficiency of nitrogen in the expired air, while in other cases, depending, as it would appear, principally upon temperature, or upon the age of the 2 Mem. d'Arcueil, t. ii. p. 359. et seq.; p. 378 consists of a tabular view of the results of the experiments. I shall refer my reader to Mr. Ellis's judicious observations on these experiments, which may lead us to doubt whether we can implicitly rely upon the exact quantity of effect produced; they do not, however, appear to me, in any degree, to invalidate the general conclusion; Farther Inquiries, p. 264. et seq. animal, either the absorption or the exhalation is in excess, producing a corresponding effect upon the. composition of the expired air. It now remains for me to offer some remarks upon the aqueous vapour which is contained in the air of expiration. The discharge of water from the lungs was a circumstance which must have been noticed by the most cursory observers, and we shall accordingly find that 'it was much insisted upon by the earlier physiologists, who indeed regarded it as one of the principal uses of the function of respiration. Sanctorius appears to have been among the first who attempted to estimate the amount of the pulmonary exhalation with any degree of accuracy; he supposed it to be half a pound in the 24 hours; but neither the mode in which he conducted his experiments, nor the reasoning which he employed respecting them, were calculated to produce any correct conclusion. Hales adopted the method of passing the air that was emitted from the lungs through a flask filled with wood-ashes, and by observing what addition of weight it had acquired, he ascribed this to the moisture which the potash contained in the ashes had imbibed; this he estimated at 9792 grs., or about 20 oz. in the 24 hours. Menzies received 3 De l'Influence &c. p. 420. et seq.; Tab. 62. .66. 4 Medicina Statica, by Quincy, Aphor. v. p. 45. s Statical Essays, v. ii. p. 322. . 4. I may observe that Haller has deviated from his usual accuracy in speaking of the estimates that have been formed on this subject. Home states, not quite correctly, that Hales obtained 23 oz. of water in 24 hours; Water supposed to be generated in the Lungs. 107 the air of expiration in an allantoid, and by weighing it before and after the experiment, ascertained what additional weight it had acquired; in this way he calculated, that the quantity of water discharged in 24 hours is equal to 2880 grs. or about 6 oz. Mr. Abernethy breathed into a glass vessel, adapted for the purpose, and collected 180 grs. in an hour, which will give us 4320 grs. or 9 oz. in 24 hours; but he supposes that the fluid contains a quantity of mucus dissolved in it, the proportion of which he did not ascertain, but which must be deducted from the total amount.7 The difficulty which there is in actually collecting the water exhaled from the lungs may probably have induced Lavoisier, in his later and more elaborate experiments, to endeavour to ascertain the quantity by an indirect method. He first determined the quantity of oxygen consumed, and of carbonic acid produced; and as he always supposed that the oxygen which had disappeared was more than sufficient to form the carbonic acid which he obtained, he conceived that the excess of oxygen was employed in uniting with hydrogen that was given off by the lungs, and thus generating water." The Med. Facts, p. 238; and Haller, in relation to Home's estimate, says, "ad uncias 23 æstimat Cl. Home," and refers to the above passage in Home's work; El. Phys. viii. 5. 40. 7 Essays, p. 141. 6 Dissertation, p. 54. 8 I have already stated that Lavoisier, in his first memoir, does not advert to the aqueous vapour which is exhaled from the lungs; it is in the memoir on the respiration of the guinea |