SECT. XVII. PHYSIOLOGY. 141 
cccxLvill. Having already pointed out the 
real source of animal heat, it is unnecessary to 
resume the subject here; but judging solely from 
facts, I have no hesitation to coincide entirely 
with the opinion of Aristotle: calorem de respi- 
rationegigni, persimile figmento est. 
cccxLix. The weight of the atmosphere so far 
affects the expansibility of the lungs, that the 
freedom of breathing is often sensibly influenced 
by the changes of the weather. 
cect. Travellers who have ascended high 
mountains, have always found their difficulty of 
breathing to increase as the weight of the atmo- 
sphere diminished. Perhaps the difficulty of 
breathing experienced by Moorcroft and Webb, 
on the Hymalaya mountains, as mentioned by 
the Quarterly Review of 1820, may be opposed 
to this position; but, from the animals of these 
elevated regions being affected in the same manner 
as men, there is every reason to suspect, that the 
difficulty experienced by these travellers, was 
chiefly due to the rarity of the atmosphere. 
cccuI. Besides carbon, respiration carries off 
from the lungs a quantity of water in the state of 
vapour, in which some traces of hydrogen have 
been detected. Sanctorius rated pulmonary ex- 
halation at about half a pound in twenty-four 
hours, Hales at twenty ounces, Abernethy at 
nine ounces, and some of the experiments of 
