444 NATURAL HISTORY. 



chemical purposes and preparations ; ground up it is em- 

 ployed as white paint ; white lead and arsenic are fre- 

 quently adulterated with it, an imposition not easily 

 detected on account of its high specific gravity. 



SECOND CLASS. 

 SALTS. 



Mineral salts are distinguished by their solubility in 

 water, and peculiar taste. 



FIRST ORDER. 

 SALTS OF A L U M I X A . 



Native Alum occurs in regularly formed crystals. 

 Colorless very seldom colored ; luster vitreous ; fracture 

 conchoidal ; taste sweetish astringent ; soluble in water. 

 Sometimes it is found in compact masses, but of silky, 

 fibrous structure ; at others in efflorescent crusts, as if 

 composed of flour ; but mostly on the surfaces of rocks 

 containing sulphur ; in clay-slate, also called alum-slate, 

 gneiss, and in most of the other formations. Met with in 

 all volcanic regions Silesia and Norway; also at the 

 Solfataras, in South America. Alum is an important 

 material in the arts, on account of its caustic nature ; 

 used in dyeing, paper-making, tanning, and also in the 

 preparation of medicine. H. = 2.0 to 2.5; G. =1.7 

 to 1.8. 



