on a Substance possessing the Properties of Tannin. 511 
evident, that this tanning substance is formed in consequence of 
part of the oxygen of the nitric acid becoming combined with 
the hydrogen of the original body, so as to form water ; and 
the carbon being thus in some measure denuded, is rendered 
capable of being gradually acted upon by the nitric acid in a 
manner nearly similar to that, which takes place when it has 
been previously converted into coal. 
The colour of the precipitates which this tanning substance 
yields with gelatine is constantly pale or deep yellow, whilst 
that of the precipitates formed by the first variety is always 
brown ; I am therefore induced to believe, that the different 
colours of the precipitates produced by the varieties of tannin 
depend on the state of their carbon. 
When resin and the other bodies were treated as above 
described with nitric acid, the quantity obtained of the tanning 
substance was much less than when an equal quantity of coal 
was employed, or even when these bodies had been previously 
converted into coal in the humid way by sulphuric acid. 
The cause of this seems to be, that a number of other pro- 
ducts are simultaneously formed, all of which require more or 
less of carbon as a constituent ingredient, so that, in consequence 
of the affinities which prevail under the existing circumstances, 
formation of the new wood : that there is not a continual accumulation of tannin in 
the vegetables which afford it, as it is successively formed in and with the white cor- 
tical layers, and is successively decomposed by concurring to form new wood : and, 
lastly, that as the vegetable approaches more nearly to the full maturity of its growth, 
when wood is less rapidly and less plentifully formed, so in like manner less tannin is 
secreted, for the fabric being nearly completed, fewer materials are required. 
Such I am inclined to suspect, from the facts which have been adduced, to be the 
cause and mode by which tannin is formed in oaks and other vegetables, but I make 
this statement only as a probable conjecture, which may be refuted or confirmed by 
future observations. 
