244 BIOLOGY. [Book ii. 



liquid. Thus, the deficit in the quantity of oxygen given back; 

 to the air by the exhalation of carbonic acid, is greater in the 

 carnivora than in the herbivora. If animals are fed on grain,, 

 the proportion is sometimes inverse; there is then an excess. 

 in the exhaled oxygen. 



The exchange of gases through the pulmonary membrane 

 is much more a physical phesaomenon than a physiological 

 one ; the azote must, then, also penetrate the blood through 

 the wall of the capillaries ; this is, in effect, what happens. But 

 here, reversing what, takes place in the, case of oxygen, the 

 pulmonary exhalation restores to the air more azote than it 

 has borrowed from it, at least in most cases. 



Eegnault and Reiset found that all animals subjected to then- 

 habitual regimen always generated an, excess of azote. Despretz 

 demonstx'ated the same thing by more than two hundred experi- 

 ments.^ Boussingault, proceeding by the indirect method, that 

 is to say, by comparing the quantity of azote introduced by 

 the aliments with the quantity expelled in the solid and liquid 

 matters, observed that the first was always greater than the 

 second.^ We can thence infer that a certain portion of the azote 

 of the aliments is exhaled by the lungs. In effect, if an animal 

 is made. to respire in an atmosphere composed of seventy-nine 

 parts of hydrogen, and twenty-one parts of oxygen, there are 

 seen to be a simultaneous absorption of hydrogen, and an exhala- 

 tion of azote. In this case the production of azote must 

 evidently be ascribed to the organism. Besides, the absorption 

 of hydrogen changes into certainty the great probability of the 

 absorption of azote in the normal air. "We must, then, admit, with 

 Edwards, that in the ordinaiy' atmosphere the disengagement and 

 absorption of azote by the. pulmonary surface always take place 

 simultaneously, and that the only variation is caused by the 

 augmentation or diminution iji the quantity of azote exhaled. 



In effect, it is the quantity of azote exhaled which varies ; 



' Annales de Ghimie et de Physique, 2" serie, XXVI. 

 ^ Economie Eurale, t. II. 



