106 



origin do not, however, deposit masses of native 

 alum ; to yield which the rocks have need of 

 torrefaction. I know not of any deposits ana- 

 logous to those I brought from Cumana* for 

 the capillary and fibrous masses found in veins 

 traversing the beds of lignites (banks of the 

 Egra, between Saatz and Commothau in Bohe- 

 mia # ), or efflorescing in cavities (Freienwalde,in 

 Brandenburg; Segario in Sardinia), are impure 

 salts, often destitute of potash, mixed with sul- 

 phats of ammonia and magnesia. A slow de- 

 composition of the pyrites, that act perhaps as 

 so many little galvanic piles, renders the waters 

 alumiferous, that circulate across the bitumin- 

 ous lignites and carbu retted clays These 

 waters, in contact with carbon at of lime, even 

 give rise to the deposits of subsulphat of alumin 

 (destitute of potash) which is formed near Halle, 

 and was formerly believed erroneously to be 

 pure alumin, belonging, like the porcelain earth 



* Feder-alaun, haarsalz, mehliger and stangliger alaun of 

 Freienwalde, Tcherning, &c. (Klaproth, Beitrage, Tom. i, 

 p. 311 j Tom. iii, p. 102, Ficinus, in the Schriften der Dres- 

 dener Gesellschaftfuer Mineralogie, Tom. i, p. 266; Tom.ii, 

 p. 232). From what formation is the native alum drawn, 

 which the Goubanians carry to Syena from the interior of 

 Africa? {Decade Egypt. Tom. iii, p. 85). I regret, that I am 

 not able, at a distance from my own collections, to determine 

 the quantity of potash, which the native alum of Robalo 

 contains. 



t Braunkohle and Alaunerde. 



