267 
the foregoing experiment make a near approxima- 
tion. 
556. ' In like manner, from the same table, we 
learn, that, in some experiments, the loss of nitrogen 
amounted to about A ; in other instances, to one- 
sixth ; and in one case, it extended to about T X T of 
that gas. This disappearance of nitrogen is, like- 
wise, in every instance, ascribed to absorption ; and 
the circumstance is pointed out as constituting an 
important difference between the respiration of fishes, 
and of the mammaKa, in whom no absorption of nitro- 
gen is conceived to take place. No other evidence, 
however, of this absorption of nitrogen is afforded, 
than the mere disappearance of a portion of that gas ; 
and all the differences in the results are at once as- 
cribed to variation in the exercise of this supposed 
absorbing power. In any investigation purely chemi- 
cal, we are persuaded that similar discrepancies would 
have led to some attempts to discover their cause ; 
but, in the application of chemistry to physiology, the 
most contradictory results obtain equal credit, and all 
the anomalies which may arise are at once charged, 
not to errors in experiment, but to some unknown 
operation of the animal system, which is called into 
existence for the mere purpose of carrying on some 
other operation, that is equally improbable and un- 
known. 
557. As the air dissolved in water*loes not exceed 
T V of the volume of that liquid, and only ^Vo of this 
air are pure oxygen, the relation of fishes to the 
oxygen contained in water corresponds with that of 
an animal breathing in a gaseous mixture, which 
