260 CHANGES IN AIR BY RESPIRATION. 



be different, to which, of them specially belongs the property 

 of thus contributing to the maintenance of life. This question 

 may be decided by a few simple experiments. If we place 

 a Bird or small Mammal in a jar filled with air, and cut off 

 all communication with the atmosphere, it perishes by suffo- 

 cation in a longer or shorter time ; and the air in the vessel, 

 which has thus lost the power of maintaining life, is found 

 by chemical analysis to have lost the greater part of its 

 oxygen. If we then place another animal in a jar filled with 

 nitrogen gas, it perishes almost immediately; whilst if we 

 place a third in pure oxygen, it breathes with greater activity 

 than in air, and shows no sign of suffocation. It is then 

 evident, that it is to the presence of oxygen that atmospheric 

 air owes its vivifying properties. 



301. But the change produced in the atmosphere by animal 

 respiration is not limited to this. The oxygen which disap- 

 pears is replaced by carbonic acid ; which, instead of being 

 favourable to the maintenance of life, causes the death of 

 animals which inhale it, even in small quantities. The 

 exhalation of this substance is an action not less general in 

 the Animal kingdom than the absorption of oxygen ; and it 

 is in these two changes that the act of respiration essentially 

 consists. 



302. The quantity of nitrogen or azote in the air that has 

 been respired, varies but very little. There appears, however, 

 to be a continual absorption of nitrogen by the blood, and 

 as continual an exhalation of it. When the quantity exhaled 

 and the amount absorbed are equal, or nearly so, no change 

 manifests itself in the air which has been breathed ; when the 

 quantity absorbed is the greater, there is a diminution in that 

 which the respired air contains; and when the quantity 

 exhaled is the greater, there is a corresponding increase. An 

 exhalation of nitrogen seems to be ordinarily taking place in 

 warm-blooded animals, to an extent varying between l-50th 

 and 1-1 00th of the oxygen consumed; but when the same 

 animals are partially or wholly deprived of food, an absorption 

 of nitrogen usually occurs. 



303. The differences in the character of the blood which 

 are produced by its exposure to the air, have already been 

 noticed ( 227); and we now see that they are essentially due 

 to the absorption of oxygen, and the setting free of carbonic 



