285 
quences, to which such a conclusion would conduct us. 
If the mere disappearance of any gas, received into 
the lungs, be sufficient evidence of its absorption, then 
every gas, which is not returned, must be held to be 
absorbed. Are we then prepared to admit that hy- 
drogen and nitrogen gases are absorbed by the blood ? 
for, when their respiration is carried to its full extent, 
they too equally disappear. This supposed absorp- 
tion, however, cannot proceed from the operation of 
chemical attraction, for little or no affinity (98. 1O4.) 
subsists between these gases and the blood. Neither 
can it arise from the operation of the living system ; 
for it occurs only when the living powers are about 
to cease. To us there appears but one way ofescap* 
ing from these manifold difficulties, which is simply 
to conclude, that the inspired air, which is not return- 
ed, is retained in the cells of the lungs. Such a sup- 
position dissipates at once all anatomical and chemi- 
cal difficulties, and explains why no air disappears 
in natural respiration, when the expiratory powers 
are in full vigour and able to expel it, and why its 
disappearance increases in proportion as the actions 
of these powers decline and cease. 
577. It is, however, worthy of remark, that, in 
these last experiments, not only was there a diminution 
in the whole bulk of air, but its relative proportions 
likewise varied ; for, in 10O parts, the oxygen and 
carbonic acid amounted together only to about two- 
thirds of the usual quantity of oxygen, and the de- 
ficiency was supplied by a superabundance of nitro- 
gen gas. We are not prepared to say why, in this 
very embarrassed state of the respiratory function, 
