321 
b 
gen consumed in the same time, will be equal in Vo* 
lume to the carbonic acid gas ; but it is evident that 
the quantity of acid expelled will depend much upon 
the circumstances under which respiration is perform- 
ed *. This estimate falls considerably below fliat 
given by Lavoisier, Menzies, and Davy (126.) ; and 
we must be allowed to observe, that, although we 
give entire credit to the accuracy and correctness of 
these experiments, in determining the actual chemi- 
cal changes induced by respiration on the air, yet, 
from many circumstances which have been already 
stated, we do not think them equally calculated to 
ascertain the quantities of air consumed in the natu- 
ral exercise of this function. 
621, From the foregoing series of facts, concern- 
ing the respiration of the higher classes- of animals, we 
feel ourselves entitled to repeat with increased confi- 
dence, that " the whole of the oxygen gas which 
disappears in respiration is employed (123.) to form 
the carbonic acid produced in that process :" And 
that " the nitrogen gas of the air neither suffers any 
change itself (129.), nor produces any direct opera- 
tion on the animal system.' 7 Or, in the words of 
Messrs Allen and Pepys, " When atmospheric air 
alone is respired, no other change takes place in it, 
than the substitution of a certain portion of carbonic 
acid gas for an equal volume of oxygen f." Con- 
sequently in man, as well as in the lower animals, 
* Phil. Trans/1807, p. 27& 280. 
f Phil. Trans. 1809, p. 427. 
X 
