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 1-100th 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 
