NATURE OF RESPIRATION. 259 
Plants than to Animals; for they also die when excluded 
from it: and thus its presence may be stated to be a general 
condition, necessary for the continuance of the life of all 
organised beings.—There is, however, a marked difference in 
the rapidity with which the deprivation of air occasions 
death in different animals (§ 310). 
298. At first sight it might be thought that Animals which 
always live beneath the surface of the water, such as Fishes, 
Zoophytes, and many Mollusca, are removed from the influence 
of the air ; and that they hence constitute an exception to this 
general law. But such is not the case; for the liquid which 
they inhabit has the power of absorbing, and of holding dis- 
solved in it, a certain quantity of air, which they can easily 
separate from it, and which is sufficient for the maintenance 
_ of their life. They cannot exist in water which has been 
_ deprived of air (as by boiling, or by being placed under the 
receiver of an air-pump); for they then become 
Imsensible and die, just as do Mammalia and Birds when 
prevented from inhaling air in the ordinary manner. 
299. The changes which result from the exposure of the 
blood or nutritious fluid of Animals to the air, either in the 
atmosphere, or through the medium of water, form a very 
important part of their vital actions ; and the changes them- 
selves, together with the various mechanical operations by 
which they are effected, constitute the function of Respiration. 
The nature of these changes will be first explained ; and the 
structure and operations of the organs by which they are 
performed will be afterwards described. 
Nature of the Changes essentially constituting Respiration. 
300. Atmospheric air, it has been stated, is necessary to 
the continued life of all animals; but this fluid is not com- 
" of one element alone. By the science of Chemistry, 
it is shown to be a mixture of three gases possessing very 
differen properties. Besides the watery vapour with which 
P ‘the atmosphere is always more or less charged, the air con- 
BS sels about 42 of oxygen, and 79 parts of nitrogen or 
; with about 1-5000th part of carbonic gcid gas. The 
ion immediately presents itself, therefore, whether these 
have the same action on animals; or, if their actions 
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