1 74 On ah artificial fanning Sut/stc/ncej 



greatest extension and some of the most valuable addiiionJ 

 From the ingenious labours of JNlr. Davy, particularly th« 

 discovery of the important fact, that catechu or terra japo-* 

 nica consists principally of tannin*. 



The united results of the experiments made bv these and 

 other eminent chemists, appear tlierefore to have fully esta- 

 blisiied that tannin is a peculiar subs'lance or principle 

 which is naturally formed, and exists in a great number oi' 

 vegetable bodies, such as oak-bark, galls, sumach, catechu, 

 &;c. &c. commonly accompanied by extract, gallic acid, 

 and mucilage. 



But no one has hitherto supposed that it could be pro- 

 duced by ait, unless a fact noticed l)y M. Chenevix may 

 he considered as some indication of it. 



M. Chevenix observed that a decoction of coffee-berries 

 did not precipitate gelatine unless thcv had been previously 

 roasted ; so that tannin had in this case either been formed 

 or had been developed from the other vegetable principles 

 by the effects of heat f. 



Some recent experiments have however convinced me 

 that a substance possessing the chief characteristic proper- 

 ties of tannin may be formed by very siujple means, not 

 only from vegetable, but even from mineral and animal 

 substances. 



§ II. 



In the course of my experiments on lac, and on some of 

 the resins, I had occasion to notice the powerful effects 

 produced on them by nitric acid; and I have since .observed, 

 that by long digestion, almost every species of resin is dis- 

 solved, and is so completely changed, that water does not 

 cause any precipitation, and that bv evaporation a deep 

 yellow viscid substance is obtained, v^hich is equally soluble 

 in w ater and in alcohol, so that the resinous characters are 

 obliterated. 



When I afterwards had discovered a natural substance, 

 which was composed partly of a resin similar to that of re- 

 cent vegetables, and partlv of asphaltumj, I was induced 

 to extend the experiments already mentioned to the bitu- 

 mens, in the hope of obtaining some characteristic proper- 

 ties by which the probable original identity of these bodies 

 with vegetable substances might be furtlier corroborated. 

 In this resjiect I siicceedcd, in some measure, better than I 

 expected ; but I observed a very material difference between 



• Phi!. Trans. 1803, p 233. 

 f Nicholson's Journal for 1802, Vol. ii. p. 174, 

 Phil.Tians. 180J. p. 385. 



3 thft 



