74 Natural Hi/lory of Alum. (Book VI. 



the quantity of earthy bafis is eafily determined by pre- 

 cipitation with fixed or volatile alkali. 



Mr. Pott firft obferved that when clay is mixed with 

 vitriolic acid, and boiled a confiderable time, the fo- 

 lution affords alum, though he obtained but little in 

 this way. MargrarF found that the vitriolic acid dif- 

 folves one half or more of the pure ft clay ; from the 

 folution he obtained much alum, but always found it 

 neceflary to add a certain portion of alkaline fait before 

 the alum would appear j and he could never obtain 

 it without adding a greater quantity of vitriolic acid 

 than was neceflary to compofe the alum. The *>ther 

 half, which did not difTolve, he concludes to be a 

 very fine fand, or an earth of the flinty kind, The 

 fhorteft way is to add a fmall quantity of fixed or 

 volatile alkali to the folution, which immediately be- 

 comes muddy, the alkali- attracting fome of the fuper- 

 fluous acid, a powder precipitates, confiding of the cryf- 

 tals of alum. 



The alum, however, which is employed in the arts> 

 is not prepared in this way. Nature produces but a 

 very fmall quantity of actual alum, and. this is mixed 

 with, heterogeneous matters, or efflorescences, in va- 

 rious forms, upon ores during calcination, but rarely 

 occurs cryftallized. In this latter ftate, it is reported 

 to be found in Egyyt, Sardinia, Spain, Bohemia, and 

 other places > it is fometimes generated in the alumi- 

 nous fchift of Lapland and Weft Gothland, by a fpon- 

 taneous decompoution of the ore; bcfides it is found 

 (but rarely) in mineral fprings. Alum is moftly pre- 

 pared from certain foITils, or ores of alum, as they are 

 called. Thefe ores are generally found in ftrata, which 

 appear like indurated clays, of a dark black colour* 

 and have a fulphureous fmell. When expofed to the 

 air for fome time, they grow hot, fwell, and crumble 



dowa 



