240 THE VITAL FUNCTIONS. 



heart, therefore, it is loaded with carbon, a principle, which, 

 when in excess, becomes noxious, and requires to be re- 

 moved from the blood, by combining it with a fresh quan- 

 tity of oxygen obtained from the atmosphere. It is not yet 

 satisfactorily determined whether the whole of the oxygen, 

 which disappears during respiration, is employed in the for- 

 mation of carbonic acid gas: it appears, probable, however, 

 from the concurring testimony of many experimentalists, 

 that a small quantity is permanently absorbed by the blood, 

 and enters into it as one of its constituents. 



A similar question arises with respect to nitrogen, of 

 which, as I have already mentioned, it is probable that a 

 small quantity disappears from the air when it is respired; 

 although the accounts of experimentalists are not uniform on 

 this point. The absorption of nitrogen during respiration 

 was one of the results which Dr. Priestley had deduced from 

 his experiments: and this fact, though often doubted, ap- 

 pears, on the whole, to be tolerably well ascertained by the 

 inquiries of Davy, Pfaff, and Henderson. With regard to 

 the respiration of cold-blooded animals, it has been satisfac- 

 torily established by the researches of Spallanzani, and more 

 especially by those of Humboldt and Provengal, on fishes, 

 that nitrogen is actually absorbed. A confirmation of this 

 result has recently been obtained by Messrs. Macaire and 

 Marcet, who have found that the blood contains a larger 

 proportion of nitrogen than the chyle, from which it is 

 formed. We can discover no other source from which chyle 

 could acquire this additional quantity of nitrogen, during its 

 conversion into blood, than the air of the atmosphere, to 

 which it is exposed i'n its passage through the pulmonary 

 vessels.* 



According to these views of the chemical objects of res- 

 piration, the process itself is analogous to those artificial 

 operations which effect the combustion of charcoal. The 

 food supplies the fuel, which is prepared for use by the di- 



* See the note at page 238. 



