SECRETIONS OF THE SKIN. 105 
4. Carbon.—W hen 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 SpaLLANzANI, Provencat and Humpoupr. 
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 Daniex Exxis, 8vo, Edin. part i. 1807, and part ii. 
1811 ;—a work which exhibits accurate information and great caution. 
