C. 84 3 



formed by Mr. Hatchett, by the action of heated dilu- 

 ted nitric acid on charcoal, and evaporation of the 

 mixture to dryness. From 100 grains of charcoal 

 Mr. Hatchett obtained 120 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 render oak bark 

 more efficient as a tanning material by infusion in lime 

 water, are consequently founded on erroneons princi- 

 ples. Lime forms with tannin, a compound not so* 



Juble 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 (Isatis tinc- 

 torla^) by digesting alcohol on it, and evaporating the 

 solution. White crystalline grains are obtained, 

 which gradually become blue by the action of the at- 

 mosphere : these grains are the substance in question. 



The indigo of commerce is principally brought 

 from America. Jt is procured from the Indigofera 

 argentea, or wild indigo, the Indigofera disperma, or 

 Gautimala indigo, and the Indigofera tinctoria, or 

 French indigo. It is prepared by fermenting the 

 leaves of those trees in water. Indigo in its common 



