It must, I apprehend, be sufficiently obvious, that upon this plan the respiration was not carried on in the natural manner, and the observations of the experimentalists, although intended to convey the contrary impression, prove that this was the case. They give us the result of ten experiments, the longest of which lasted eleven minutes, and in each of which between 3 and 4000 cubic inches of atmospheric air were respired. They remark that "the operator was scarcely fatigued, and his pulse not raised more than about one beat in a minute; the respirations, however, were deeper and fewer than natural, amounting only to about 58 in 11 minutes, whereas, from repeated observations at different and distant times, he makes 19 in a minute." I conceive it will be thought that a mode of respiring, which could produce such considerable effects in the short period of 11 minutes, must have been very far from exhibiting the state of the lungs in their natural action. So far as respects the present question, the absolute amount of oxygen consumed in a given time, it was found that air, after having been once respired, contained only 12.5 per cent. of oxygen, 8.5 per cent. having been consumed and its place supplied by an equal bulk of carbonic acid; hence

8 Phil. Trans. for 1808, p. 253.

9 As a proof that the mechanical action of the lungs was not performed in a perfectly natural state, I may remark that the quantity of air taken in at a single respiration was no more than 16 cubic inches, p. 256.

■ P. 255, 279. This remark only applies to the respiration

we deduce that the average quantity in 24 hours is, under ordinary circumstances, 39534 cubic feet," equal to 13343 grs. It may appear somewhat remarkable, that although according to the method in which these experiments were performed, the lungs were more completely emptied than in natural respiration, and therefore that the change produced in the air should have been proportionally greater, yet the quantity indicated is less than in the experiments either of Menzies, Lavoisier, or Davy.3

Since the experiments of Messrs. Allen and Pepys,

of atmospheric air under ordinary circumstances; when the same air was respired as frequently as possible, until symptoms of suffocation were produced, the quantity of oxygen absorbed is 10 per cent. p. 262.

But this apparent discrepancy will probably be removed by the consideration, that when these philosophers performed their experiments, it was generally supposed that the atmosphere consisted of 27 oxygen and 73 nitrogen, and as they would no doubt employ the same method of analysis in all cases, we must diminish the proportion of oxygen in all the steps of the calculation. The 27 per cent. of oxygen before respiration, which was supposed to be diminished to 22.5, leaving a deficiency of 4-5, will therefore become 21 before, and 174 after respiration, leaving a deficiency of 3.6 only; if we apply this scale of proportion to Sir H. Davy's estimate, it will reduce it from 15337 grs. tó 12272 grs., leaving, as might be expected, a superiority in quantity to Messrs. Allen and Pepys's experi ments; and the same remark of course applies to the others.

the chemical effects of respiration have been very minutely examined by Dr. Edwards, and he has given us the result of his investigations in a treatise of uncommon merit, whether we regard the decisive nature of the experiments, the clearness and simplicity with which they are narrated, or the extensive information which is displayed on all points connected with the animal economy. Among the most valuable facts of Dr. Edwards's essay is the conclusion which we are led to form respecting the difference in the effect of the respiration of the different animals, and of the same animal under different circumstances. This, while it encreases the difficulty of ascertaining the absolute amount of the changes that are produced in the air, shows us that many of the discrepancies which had been noticed between the results of former experiments, depend not so much upon any inaccuracy in the process, as upon an actual difference in the effect produced, of which the experimentalists were not aware. Although we shall have occasion, in numerous instances, to profit by Dr. Edwards's labours, he does not give us any information respecting the immediate subject of our present inquiry, the absolute quantity of oxygen consumed by a man in a given time."

4 De l'Influence des Agens physiques sur la Vie.

s Thenard informs us, in a general way, and without specifying any particular authority, that air, after being once respired, contains from 18 to 19 per cent. of oxygen, a quantity which is greater than what has been assigned by any of the experimentalists who have minutely attended to the subject; Chimie, t. iii. p. 666.

With respect to the question which was alluded to above, what proportion of oxygen must there be in air, in order to render it fit for the support of life, we have no facts that can lead us to any very decisive conclusion. As a general principle we find that animals enclosed in a given portion of air die long before the consumption of the whole of the oxygen," and that the fatal effect, in this case, immediately depends not upon the absence of oxygen, but upon the presence of the carbonic acid which is substituted in its place. The only experiment that I have met with, on which we can depend, as giving us any accurate information on this point, is one of Lavoisier's, in which he found, that when the carbonic acid was carefully removed by caustic potash, as fast as it was formed, a guinea pig could live, without any apparent inconvenience, in air that contained only 6.66 per cent. of oxygen, and even when the proportion was still farther diminished, the only obvious effect produced upon the animal was a degree of drowsiness. As

6 From this remark we must except many of the lower tribes of animals; Vauquelin found that certain species of limax and helix have the power of completely deoxidizing the air in which they are confined; Ann. Chim. t. xii. p. 278. et seq. Spallanzani also obtained the same results with various kinds of worms; Mem. sur la Respiration, p. 62.

7 Mem. Acad. Scien. pour 1789, p. 573. Messrs. Allen and Pepys also witnessed the same effect in a similar kind of experiment, in which a guinea pig was inclosed in a portion of air consisting of a mixture of oxygen and hydrogen; Phil. Trans. for 1809, p. 424, 428.

the temperature of the guinea pig, and structure of its lungs, is the same with that of man, it may be presumed that, under the same circumstances, human life might be supported by air of similar composition; but this, we must bear in mind, could probably only be the case under the most favourable circumstances, where every extraordinary source of expenditure was guarded against, or did not exist.s

The next point which we proposed to examine ist the quantity of carbonic acid produced by respiration. The fact of its presence in air that has been respired, I have already mentioned, as one of the most interesting of the discoveries of Black, but it does not appear that he made any attempt to ascertain its quantity. Lavoisier, in his first memoir on respira

Halley, in his proposal for improving the construction of the diving bell, Phil. Trans. v. xxix. p. 492. et seq. observes that a gallon of air will become unfit for respiration in a little more than a minute. Lavoisier, Ann. Chim. t. v. p. 261, informs us that a man cannot live more than an hour in 5 cubic feet of air. Sir G. Blane gives us an important practical obser vation on this subject, which is deduced from very extensive observation; that in calculating for the arrangements of a hospital, each individual should be allowed a space of 600 cubic feet, below which it will be found impossible to maintain the requisite purity of the air; Med. Chir. Trans. v. iv. p. 115. Dr. Edwards has given us the result of his experiments on the extreme limits of the rarefaction of the air, when it appears incapable of supporting life for any perceptible length of time; this for birds he found to be a pressure of a little more than 5 inches, and for guinea pigs of a little more than 34; De l'Influence &c. p. 495.