SECT. II, PHYSIOLOGY. 7 
This fact successively occupied the attention of 
Hales and Cigna. Priestley repeated their experi- 
ments, and examined the air, after having agitated 
it with blood, and found it to be the same as 
respired air; or, in other words, the oxygen of the 
atmospheric air had been changed into carbonic 
acid gas. It was previously well known, that the 
blood undergoes a remarkable change, from a 
purple to a scarlet colour, in its passage from the 
pulmonary arteries to the veins; and the changes 
which take place in the properties of the blood 
and air being apparently the same in respiration, 
as when the blood and air are agitated together in 
a bottle, the inference, that respiration is the 
source of animal heat, was almost unavoidable. 
One formidable objection was still to be overcome, 
the temperature of the lungs was not found to 
differ from that of the other cavities. 
tv. The important chemical fact observed and 
illustrated by Black, that bodies, at the same 
temperature, contain unequal quantities of caloric, 
was ingeniously applied by Dr. Crawford, to 
explain away the objection in question. He un- 
dertook a series of experiments for the pur- 
pose of showing, that the difference of capacity 
for caloric, in venal and arterial blood, and in 
atmospheric air and carbonic acid gas, might 
account for the absence of augmentation of free 
calorie in the lungs, and explain, at the same 
