being complicated by the gas originally contained in the lungs.3

3 Mr. Ellis, in his "Farther Inquiries," has offered many valuable observations on the mechanical part of Messrs. Allen and Pepys's experiments, p. 280. et seq.

Nysten and Spallanzani, from their experiments, the former on the human subject, Recherches, p. 214, the latter on various species of the mollusca, Memoires, p. 66. et alibi, come to the same conclusion, respecting the want of correspondence between the oxygen and carbonic acid. Legallois also found this to be the case in his experiments, and although they were performed under particular circumstances, and for a specific object, yet they are generally applicable so far as this question is concerned; Ann. de Chim. et Physique, t. iv. p. 115. Dr. Thomson, Chem. vol. iv. p. 619, and Mr. Dalton, Manch. Mem. vol. ii. 2d ser. p. 25, likewise obtained a surplus quantity of oxygen, although, from certain considerations, they were induced to ascribe this difference to incidental circumstances, not essentially connected with respiration; see Dalton, p. 36.

The specific gravity of the serum of arterial blood has been generally found to be less than that of venous; in the experiments of Dr. Davy, the proportion was as 1047 to 1050; Phil. Trans. for 1814, p. 591. It has been supposed that, as during the change from the venous to the arterial state, there is an absorption of oxygen and a discharge of carbonic acid, a point which will be more fully investigated hereafter; we might conclude from this change of the specific gravity, that the quantity of gaseous matter absorbed is greater than that discharged. This observation proceeds upon the idea, which is probably correct, that the water which is discharged from the lungs does not immediately proceed from the blood in the pulmonary vessels, but that it is the result of secretion. But it also takes for granted, that the blood loses nothing by serous transudation; for if we conceive that this process takes place as the blood passes through the lungs, it accounts for the difference in the specific gravity of the two kinds of serum. It may be farther

The third point which I proposed to examine, whether the bulk of the air be diminished by respiration, is essentially connected with the question which has been discussed above, and indeed almost resolves itself into the same inquiry. All the earlier physiologists supposed the diminution to take place, and they accounted for it upon the idea, that the air had lost part of its elasticity, or, as they termed it, its spring. Mayow appears to have been the first who attempted to ascertain the exact amount of the diminution; he estimated it at while Hales, in different experiments, found it to vary from

30

4

5

ta

6 But all these statements are much over-rated; a circumstance that depends, in part at least, upon the air of expiration being passed through, and confined over water, which would necessarily absorb a part of the carbonic acid, Lavoisier, Goodwyn, and Sir H. Davy, however, in their more correct experiments, although they found the diminution to be much less, did not fail to recognize it. Lavoisier, in

remarked upon this subject, that the specific gravity of the serum of different individuals differs at least as much as the difference indicated by Dr. Davy, between the arterial and venous blood; See Med. Chir. Tr. v. ii. 170, 363.

Boyle informs us, that in one experiment, in which a mouse was confined in a portion of air over mercury, the volume of the air was not diminished; Works, v. iii. p. 380.

s Tract. p. 105.

6 Stat. Essays, v. ii. p. 238, 320.

7 Crawford found that when the air was confined over water, 4th of the whole was absorbed; on Animal Heat, p. 146.

his Memoir of 1777, fixes the amount at

9

wyn obtained the same result, Sir H. Davy found that air, which had once passed through the lungs, as is the case in ordinary respiration, suffered a diminution which varied, in his different experiments, from

1 •

2

1

to Berthollet always found a diminution, although somewhat less in quantity, and the same result was obtained by Jurine and by Spallanzani. It might be supposed that these results afforded sufficient proof of the general fact of the diminution of the air by respiration, yet we have here, as on the former occasion, a great diversity of opinion. Crawford

In the experiments where the guinea-pig was confined in oxygen, the diminution was, in one case,, and in the other of the volume of the air, the greater diminution in these experiments probably depending upon the increased consumption of oxygen, in consequence of the greater purity of the air employed; Mem. Acad. Scien, pour 1780. p. 401; Ann. Chim. t. v. p. 261; Mem. Soc. Roy. Měd. pour 1782, 3. p. 572. › Connexion of Life &c. p. 51.

Researches, p. 431..3.

• Mem. Soc. d'Accueil, t. ii. p. 454. .463.

3 Mem. Soc. Roy. Med. t. x. p. 25.

• Mem. sur la Respir. p. 102. Cuvier also states the fact of the diminution of the volume of air, and fixes it at; it does not, however, appear, that he himself performed any experi ments on this subject; Leçons d'Anatomie Comp. t. iv. p. 303. Dr. Thomson also found a diminution in the volume of the air, but it varied so much in his different experiments, that he was disposed to ascribe it to some accidental cause; Chem. v. iv. p. 617. Dr. Henry also agrees in the general fact; Elements, vol. i. p. 293.

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expressly states that he could not observe any diminution of the volume of the air, when the process was conducted over mercury; it is not adverted to by Lavoisier and Seguin in their conjoined experiments, while Messrs. Allen and Pepys, in their elaborate researches, although they generally found a slight diminution, attributed it to some accidental eircumstance connected with the management of the apparatus, or the nature of the process, and concluded that the bulk of the air is not essentially affected by respiration. But the experiments of Dr. Edwards again most fortunately relieve our embarrassment, by showing us that the diminution really takes place in a great majority of cases, although, in such various degrees, that we are not able to reduce it to any fixed amount."

s On Animal Heat, p. 147.

The average diminution of the ten first experiments, p. 253, is not; in the eleventh experiment, which they appear to regard as the most correct, it is about , p. 254; but they state that the general average of all their experiments is about 6 parts in 1000, p. 281, or Tr. It may be presumed, from various expressions in Dr. Prout's papers, that he did not suppose there was any diminution of the volume of the air; Ann. Phil. vol. ii. p. 330. et alibi. Mr. Ellis likewise concludes that the volume of the air is not diminished, Inquiry, p. 99,0; and M. Magendie's experiments led him to the same opinion; Mem. sur la Transp. Pulm. p. 7..9. Mr. Abernethy, on the contrary, supposes that the volume of the air is increased; Essays, p. 147.

7 De l'Influence &c. p. 411. et alibi; it follows from the view which Dr. Edwards takes of the action of the lungs, that occasionally the bulk of the air may be increased by respiration, that at other times the bulk may be unaffected, but that in a majority of cases it will be diminished.

8

The fourth point that we proposed to examine respects the absorption of nitrogen, and on this we shall find as much diversity of opinion as on those that have already passed under our review. Lavoisier's experiments led him to conclude that the nitrogen is entirely passive in respiration, or that it serves no other purpose than to dilute the oxygen; and Messrs. Allen and Pepys deduced the same conclusion from their experiments. Priestley, on the contrary, supposed that there was an absorption of nitrogen; but his experiments being performed in an early stage of the pneumatic chemistry, and with a less perfect apparatus, notwithstanding the confidence with which he maintained his opinion, the result was

8 Mem. Acad. Scien. pour 1777. p. 193; and he still continued to support this opinion in his later essays; see Mem. pour 1789. p. 574. where he says that he has proved this by very decisive experiments.

9 Phil. Trans. for 1808. p. 264. et alibi; and Phil. Trans. for 1809. p. 412..5. Messrs. Allen and Pepys conceive that in natural respiration the nitrogen is not affected, but that when the same portion of air is frequently respired, a quantity of nitrogen is discharged; Phil. Trans. for 1808. p. 263. The same effect was also produced by the respiration of pure oxygen, Phil. Trans. for 1809. p. 404, 415..421, 427. They remark, with justice, that an apparent increase in the portion of nitrogen may depend upon the quantity of it which exists in the lungs before the experiment; they proved, however, by causing an animal to respire a mixture of oxygen and hydrogen, that, in certain cases at least, nitrogen is actually evolved, p. 420.. 427. Cuvier, Tabl. Elem. p. 46, supposes that the nitrogen is not affected by respiration.

On Air, v. iii. p. 380.