89 



74. Assuming then, as a facf, that the nitrogen 

 gas of the air neither produces nor suffers change, 

 we have next to inquire, what becomes of the oxy- 

 gen gas, which has been shewn, more or less, in all 

 cases, to disappear. Is it absorbed by any organized 

 structure of the animal adapted to the performance 

 of such an office ? No vessels fitted for such a pur- 

 pose have been yet demonstrated in the animal system. 

 The small size of the stigmata, or breathing pores, 

 in insects, renders them but little suited to be re- 

 ceptacles for containing and decomposing air : and, 

 in many of the venues class, the mucous matter with 

 which their bodies are constantly smeared over, 

 must oppose great difficulties to such an absorption. 

 In the case of aquatic animals, these difficulties are 

 still farther increased ; for the air must be first se- 

 parated from the water before it can be taken up by 

 absorption ; and, after this is effected, it is not easy 

 to conceive how the gills of fishes can be rendered 

 capable of absorbing and emitting air. It is not 

 probable, that this air is taken up in its entire state, 

 for as the nitrogen gas undergoes no change (73.), 

 its absorption can answer no obvious use, but would 

 tend rather to impede the decomposition supposed 

 to go on within the vessels, and the subsequent 

 formation and emission of the carbonic acid. If, on 

 the other hand, the air be considered to be decom- 

 posed previous to its absorption, then a new com- 

 pound must be at once formed ; and, if this be 

 brought about by the union of some substance with 

 the oxygen gas, then that gas, simply as such, can- 

 not be held to enter into the animal system. No 

 one has ever vet detected air in the animal fluids 



