351 
pletely proving the fact, that no chemical change is 
effected in the air by the respiratory organs, during 
the continuance of complete torpor ; and Spallan* 
zani found, also, that neither bats, dormice, nor 
hedgehogs, produced any change in atmospheric air 
during the suspension of the circulating r.nd respira- 
tory functions. The results thus obtained, by ex- 
periments on the lower animals, may be extended, in 
all their circumstances, to man ; for although we 
possess no direct proof of the air remaining un- 
changed during the suspension of respiration, yet the 
actual cessation of that function may be regarded as 
evidence of it ; and we have also seen that man, like 
other animals, consumes more oxygen while he is 
vigorous, and during a state of exertion, than under 
the opposite circumstances of debility and inaction. 
660. But if it thus appear, that, so long as the 
blood, in all animals, continues to move, the oxy- 
gen gas of the air continues to be changed ; and con- 
versely, if no change be effected in the air when this 
motion in the blood has ceased ; we must suppose 
some necessary connection to subsist between these 
events : and farther, as the change in the air follows 
always the motion of the blood, and gradually ceases 
to be produced as that motion declines and ceases, 
we must also suppose the effect in the air to depend 
on this antecedent motion of the blood, which, there- 
fore, in common philosophical language, we are en- 
titled to consider as its cause. In the order of events, 
however, this motion of the blood is not the imme- 
diate cause of the change in the air, but only enables 
