CIRCULATING SYSTEM. 351 
be necessary to attend, in the first place, to the changes 
produced in the air itself. 
In man, regarding whose respiration the greatest num- 
ber of experiments have been performed, it has been ob- 
served, that the air which is alternately inspired and ejected, 
becomes unfit for future use ; and is likewise rendered im- 
capable of supporting combustion. The analysis of this 
altered air indicates the change to have taken place in its 
oxygenous portion *. A part thereof has disappeared, and 
an equal bulk of carbonic acid is found occupying its place. 
The quantity of oxygen in this carbonic acid is equal to 
that which has been abstracted from the air. In this case, 
either carbonic acid escapes from the blood, and an equiva- 
lent bulk of oxygen is absorbed; or, the blood furnishes 
the carbon only, with which the oxygen of the air unites. 
The former supposition was long countenanced by chemists; 
the latter is at present the prevailmg opinion. In the 
adoption of the former, many difficulties present them- 
selves. There is no apparent cause to produce the expul- 
sion of the carbonic acid, as there is no substance in the 
blood with which it could be combined, and from which it 
could so easily escape; neither is there the slightest reason 
to suppose, that it could be displaced by the same bulk of 
oxygen, since these two gases have very different combin- 
ing values. But when it is considered, that, in all cases 
where carbon unites with oxygen, the carbonic acid pro- 
duced is equal in bulk to the oxygen consumed, we are 

* There is no reason to believe that any change is produced on the azote, 
In some experiments on the respiration of fishes, by Humsorpt and Pro- 
VENGAL, a loss of azote was indicated. But the sources of error in perform- 
ing such experiments are so many, and the results which were obtained 
differ so much from one another, that no confidence can be placed in the 
conclusions which have been drawn from them. 
