Royal Society of London. ^71 



vitrification ; and in regard to the size of the masses, it 

 may be said that it has no bounds, sit;ice ^tna, a volcano 

 much less considerable than those of Peru, throws up lava 

 several leagues ui extent, and of a very great thickness. 



Obsidian, therefore, or the black glass of the volcanoes 

 of Peru, is as certainly a production of their fires as the lava 

 which is seen to issue from the bottom of every crater. 



[To be continued.] 



XLT. Proceedings of Learned Societies. 



ROYAL SOCIETY OF LONDON. 



An the sitting of 25th April last there was read an inte- 

 resting paper on an artificial substance possessing the prin- 

 cipal characteristic properties of tannin, by Charles Hat- 

 chett, Esq. a member of the society. 



The author, after mentioning the labours of Mr. Deyeux 

 and Mr. Seguin, the former of whom first separated this 

 substance from galls, the characteristic property of which, 

 to precipitate gelatin from water, was ascertained by the 

 latter ; and after mentioning the experiments of Mr. Big- 

 gin, Mr. Proust, and Mr. Davy, remarks that no one had 

 hitherto supposed that it could be produced by art, unless 

 the fact mentioned by Mr. Chenevix, that " a decoction of 

 coffee berries did not precipitate gelatine, unless they had 

 been previously roasted," might be considered as an indi- 

 cation of it. Recent experiments have, however, shown 

 him that tannin may be formed by very simple means, not 

 only from vegetable, but from mineral and animal sub- 

 stances. 



In the course of his experiments on lac and resins, he 

 observed the powerful effects of nitric acid on these sub- 

 stances, and has since observed that by long digestion 

 almost every species of resin is dissolved, and so completely 

 altered that water does not cause any precipitation, and that 

 by evaporation a deep yellow viscid substance is obtained, 

 equally soluble in water and in alcohol. In his experi- 

 ments afterwards, on the bitumens, he observed a material 

 difference between their solutions and those of resiijs. 

 With bitumens, nitric acid, by long digestion, formed a 

 dark brown solution ; a deep yellow coloured mass was se- 

 parated, which, by subsequent digestion in another portion 

 of nitric acid, was completely dissolved, and, by evapora- 

 tion. 



