200 



tion are in kind precisely similar, we are justified in 

 ascribing their formation to similar laws : and, since 

 it seems to have been demonstrated by direct expe- 

 riment (L5O.), that no transpiration of aeriform fluids 

 takes place through the skin, we may presume that 

 none is able to be carried on through the cells and 

 blood-vessels of the lungs. Not only is the cellular 

 surface of rhe lungs furnished with absorbent and 

 exhalent vessels like that of the skin, but it is sup- 

 plied from within by the same blood, and exposed 

 from without to the same atmospheric air. It has 

 been shewn also, that the colourless fluids of various 

 animals, are able to effect the same change on the 

 air, as that which is produced by the blood : and that 

 the serum of the blood itself, (which is especially 

 destined to supply- the exhalent function), produces 

 on the air the same identical change as it experien- 

 ces in the lungs : all which circumstances strongly 

 incline us to suppose, that the function of the lungs 

 resembles in kind that of the skin. The proofs 

 likewise already adduced, that the carbon furnished 

 by vegetables (14O.), and by the inferior animals, as 

 well those which respire by the skin (142.), as those 

 which breathe by lungs (J43.), depends wholly on 

 the due circulation of their fluids, and is, consequent- 

 ly, the result of a living action, are strong presump- 

 tive evidence, that the same law obtains in the supe- 

 rior animals, and in man : and seem to authorise the 

 conclusion, that the carbon supplied in human respir 

 ration, is truly an animal excretion, carried on by 

 the exhalent vessels of the lungs ; and therefore, 

 that it primarily depends, like other excretions, on 



