“40 On Respiration and Animal Heat. 
“ methods have been discovered of rendering 
“it fit for breathing again. It is evident, - 
“ however, that there must be some provision 
“in nature for this purpose, as well as for 
“that of rendering the air fit for sustaining 
“ flame ; for without it the whole mass of the 
“atmosphere would, in time, become unfit 
“ for the purpose of animal life; and yet there 
“is no reason to think that it is at present at all 
“ less fit forrespiration than it has ever been.” 
In the sequel, he concludes, from certain ex- 
periments on vegetation, that it is one of the 
processes employed by nature for the great 
purpose of restoring the atmosphere to a fit 
state for the support of respiration and com- 
bustion. How far this conclusion is correct, 
namely, that the growing of vegetables ab- 
stracts the carbonic acid from air, I have had 
no opportunity to observe. But the necessity 
of this, or some other process, for the pur- 
pose, has, I believe, been generally adopted 
by the later writers on this subject. No one, 
that I know of, has undertaken to calculate 
the quantity of carbonic acid, which is pro- 
bably thrown into the atmosphere in any 
given time, in order to compare it with the 
whole quantity of the atmosphere. Now, if 
we state the diameter of the earth to be 8000 
miles, and the circumference 25000, in round 
