184 



146. To the life of water-snails, muscles, fishes, 

 and the amphibia, a supply of fresh air has been 

 shewn to be equally necessary, its oxygenous portion 

 being, in every instance, more or less completely 

 consumed and changed into carbonic acid ; the car- 

 bon for which purpose could be derived only from 

 the animal system. We have not, in this instance, 

 the evidence of experiment to prove that the carbon 

 was emitted by the living power of the animal, that 

 is, only while its circulation continued : nor, although 

 some of these animals are known to suffer torpidity, 

 can we experimentally shew that they then produce 

 no change on the air contained in the water in which 

 they are placed. In every view of the subject, how- 

 ever, analogy is so strongly in our favour, that we 

 cannot hesitate to admit both suppositions to be true : 

 and the termination of the branchial artery in exha- 

 lent orifices (65.) on the gills of fishes, would seem 

 to point it out as the structure by which, in that 

 class of animals, the carbon, in combination with the 

 exhaled fluid, escapes. At all events, the blood of 

 a very large number of these animals being devoid of 

 colour, is at least a proof that carbon does not cause 

 the blackness of that fluid ; or, if it still be contend- 

 ed that it does, then blood, which is white, and may 

 be presumed therefore to contain no carbon, can ef- 

 fect the same changes on the air, as that which owes 

 its blackness to this substance. 



147. Analogous to the emission of carbonic mat- 

 ter from the surface of the bodies of snails (145.), 

 seems to be that which is carried on by the external 

 and internal surfaces of the human body. The 

 Count de Milly first observed small bubbles of air 



