30 On Respiration and Animal Heat. 
but a portion of what was previously in the 
air. . . 
‘We may now deduce one conclusion, which 
indeed Lavoisier was fully aware of, that the 
oxygen which disappears during respiration, 
is not adequate to the formation of the carbo- 
nic acid and the water exhaled. It is only 3 
of the requisite quantity. He conceives only 
a part of the water is formed in the lungs by 
the union of oxygen with hydrogen from the 
blood, while the rest transpires ready formed, 
through the membranes of the blood-vessels, 
and is vapourized by the heat. 
This indeed is the most difficult part of the 
subject. I am inclined to think, that no water 
is formed in the lungs by the union of oxygen 
with hydrogen; but that the whole quantity 
exhaled is an exudation from the blood, 
through the membranes of the lungs, which 
are thereby constantly kept moist.—It is in- 
consistent with the simplicity of the laws of 
nature to employ two causes when one is ade- 
quate to the effect. There is another way 
by which the difficulty may seem to be ob- 
viated ; that is, by supposing that all the water 
exhaled is formed in the lungs by direct com- 
bination of its elements, but that the carbonie 
aeid is formed from carbonic oxide, which has 
previously one half of the oxygen necessary for 
I 
