271 
to live in common air, and convert its oxygenous 
portion into carbonic acid. They became lethargic 
from cold, and the heart then beat very slowly or 
not at all ; the respiration was then also suspended, 
and little or no effect was produced in the air. The 
skin of these animals acted upon the air as well as 
the lungs * ; and when their blood was reddened by 
exposure to the air, its oxygen also disappeared, and 
carbonic acid was produced ! Similar results were 
obtained in experiments on the respiration of vipers, 
tortoises, lizards, and salamanders J. 
561. The preceding facts sufficiently shew, that 
various animals, in all the foregoing classes, and in 
every stage and form of their existence, require the 
presence of oxygen gas to maintain the functions of 
life ; that this gas, by the exercise of these functions, 
is converted into carbonic acid ; and that the degree 
in which this conversion proceeds, depends much on 
the healthy condition of the animal, and the vigour 
of its circulating system. Since, also, in every in- 
stance where the experiments have been made with 
the requisite accuracy, the bulk of carbonic acid pro- 
duced, nearly or exactly equalled that of the oxygen 
which disappeared, we may conclude, from analogy, 
that such is universally the extent to which this change 
in the air takes place in animal respiration; and 
since, farther, the nitrogen gas of the air appears to- 
suffer no necessary change (73.) in the exercise of 
this function, we may also conclude, that, as far as 
* Rapports, &c. torn. i. p. 249. t Ibid. p. 239. 2(>3< 
J Ibid. p. 275. 287. 295. 353. 
