324 



PHILOSOPHY OF ZOOLOGY. 



generated in the blood, secreted into this organ, to be af- 

 terwards thrown into the stomach or intestines, to promote 

 the digestion of the food. 



. The nature of the air contained in the air-bag, was never 

 investigated until pneumatic chemistry had opened up new 

 fields of discovery. In 17T4, Dr Pkiestley turned his at- 

 tention for a short time to the subject ; and in the air-bag 

 of the roach he found azote in one instance unmixed, and 

 in another, in union with oxygen. FouiiciiOY afterwards 

 examined the gaseous contents of the air-bag of the carp, 

 and found them to consist, in a great measure, of pure azote. 

 The most accurate and extended experiments on this 

 subject, are those of M. Biot, published in the Mem. 

 (TArciieil, i. 252. and ii. 8. He found the proportion be- 

 tween the oxygen and the azote (for he was unable to de- 

 tect tlie presence of hydrogen, or any sensible quantity of 

 carbonic acid), to vary according to the species *. 



