CHEMICAL EFFECTS OF RESPIRATION. 30 



o 



gas, regains its natural vernriillion colour, and is 

 now qualified to be again transmitted to the dif- 

 ferent parts of the body for their nourishment and 

 growth. As the blood contains a greater propor- 

 tion of carbon than the animal solids and fluids 

 which are formed from it, this superabundant 

 carbon gradually accumulates in proportion as its 

 other principles, (namely, oxygen, hydrogen, and 

 nitrogen) are abstracted from it by the processes of 

 secretion and nutrition. By the time it has re- 

 turned to the heart, therefore, it is loaded with 

 carbon, a principle, which, when in excess, becomes 

 noxious, and requires to be removed from the blood, 

 by combining it with a fresh quantity of oxygen 

 obtained from the atmosphere. It is not yet satis- 

 factorily determined whether the whole of the 

 oxygen, which disappears during respiration, is 

 employed in the formation 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 pro- 

 bable 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, appears, 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 satisfactorily 



