SECRETIONS OF THE SKIN. 105 



4. Carbon. When the human hand is confined for some 

 hours in a vessel filled with atmospheric air, a portion of 

 carbonic acid makes it appearance. When the same hand 

 is confined in a jar with mercury, lime-water, or hydrogen 

 gas, no carbonic acid makes its appearance. The skin does 

 not, therefore, give out the carbonic acid. But in the first 

 experiment, a portion of the oxygen disappears, equal to the 

 quantity contained in the carbonic acid that is produced. 

 We here see the origin of the oxygen of the acid ; and as 

 no other body is present, capable of furnishing the other in- 

 gredient, the carbon, the conclusion that it is given out by 

 the skin is irresistible. In the lower animals, the same 

 emission of carbon from the skin has been ascertained by 

 the labours of SPALLANZA:NI, PIIOVE^AL and HUMBOLDT. 

 They found, that the skins of fishes produced carbonic 

 acid, as well as the gills ; and that when frogs were de- 

 prived of their lungs, atmospheric air was still decompos- 

 ed*. 



It is, perhaps, difficult to point out the purposes which 

 this emission of carbon from the skin serves in the animal 

 economy. It appears to be a secondary kind of respiration, 

 which, while it removes from the system the superfluous 

 carbon, probably regulates the distribution of animal heat. 

 But of this we shall speak more at large, when we come to 

 treat of the organs of respiration. 



There are several circumstances which prove, that the 

 skin of the human body, in particular states, is capable of 

 exerting an absorbing power. Whether the absorption takes 

 place by peculiar vessels, or by the exhaling vessels having 

 their motions reversed, or whether absorption ever takes 



* We refer the reader for a minute and candid account of this cutaneous 

 emission of carbon, to " An Inquiry into the changes induced in Atmosphe- 

 ric Air, by the germination of Seeds, the Vegetation of Plants, and the Res- 

 piration of Animals ;" by DANIEL ELLIS, 8vo, Ediri, part i. 1807, and part M 

 1811 ; a work which exhibits accurate information and great caution. 



