86 
of the most generally diffused of the vegetable 
principles. 
A substance very similar to tannin has been 
formed by Mr. Hatchett, by the action of heated 
diluted nitric acid on charcoal, and evaporation of 
the mixture to dryness. From 100 grains of char- 
coal Mr. Hatchett obtained 1^0 grains of artificial 
tannin, which, like natural tannin, possessed the 
property of rendering skin insoluble in water. 
Both natural and artificial tannin form com- 
pounds with the alkalies and the alkaline earths ; 
and these compounds are not decomposable by 
skin. The attempts that have been made to ren- 
der oak bark more efficient as a tanning material by 
infusion in lime-water, are consequently founded 
on erroneous principles. Lime forms with tannin 
a compound not soluble in water. 
The acids unite to tannin, and produce com- 
pounds that are more or less soluble in water. It 
is probable, that in some vegetable substances 
tannin exists, combined with alkaline or earthy 
matter ; and such substances will be rendered more 
efficacious for the use of the tanner, by the action 
of diluted acids. 
9. Indigo may be procured from woad {I sat is 
tinctoria ), by digesting alcohol on it, and eva- 
porating the solution. White crystalline grains 
are obtained, which gradually become blue by the 
action of the atmosphere : these grains are the 
substance in question. 
The indigo of commerce is principally brought 
from America. It is procured from the Indigo- 
fer a argentea , or wild indigo, the Indigofer a dis - 
