128 SALTS OF ALUMINA, 



24 parts of water to 1 part of sulphate of alumina, and 1 part 

 of some other sulphate. In potash-alum, this sulphate is a 

 sulphate of potash. This is the common alum of the shops. 



The corresponding sulphate in the other alums is as fol- 

 lows : — 



Soda-alum, sulphate of soda ; 



Magnesia-alum, sulphate of magnesia ; 



Ammonia-alum, sulphate of ammonia p\cxxX*x*^ 



Iron-alum, sulphate of iron ; 



Manganese-alum, sulphate of manganese. 



Besides these there is also a hydrous sulphate of alumina 

 without any other sulphate ; it is called feather-alum, and ia 

 even of more common occurrence than any of the true 

 alums. 



These alums are formed from the decomposition of pyrites, 

 in contact with clay. Iron pyrites is a compound of sulphur 

 and iron ; in decomposition, its sulphur and iron unite with 

 oxygen derived from the moisture present, and it then be- 

 comes sulphate of iron, or a compound of sulphuric acid and 

 oxyd of iron. This sulphuric acid, or part of it, by uniting 

 with the alumina of the clay rock, produces a sulphate of 

 alumina. To form a true alum, a little potash, or soda, &c. 

 must be present, in the clay. The iron of the iron alum pro- 

 ceeds from the pyrites which undergoes the decomposition 



These compounds differ but little in taste and appear- 

 ance. 



Obs. Potash alum and more abundantly the sulphate of 

 alumina (or feather alum), and sulphate of alumina and iron, 

 impregnate frequently clay-slates, which are then called 

 aluminous slates or shales. These alum rocks are often 

 quarried and lixiviated for the alum they contain. The rock 

 is first slowly heated after piling it in heaps, in order to de- 

 compose the remaining pyrites and transfer the sulphuric 

 acid of any sulphate of iron to the alumina and thus produce 

 the largest amount possible of sulphate of alumina. It is 

 next lixiviated in stone cisterns. The lye containing this sul- 

 phate is afterwards concentrated by evaporation, and then 

 the requisite proportion of potash (sulphate or muriate, alum 

 containing potash as well as alumina) is added to the lix- 



What is the composition of common potash alum? What of a soda 

 alum ? What are alum shales ? Whence the alum or sulphate of alum- 

 ina they contain 1 How is alum obtain from alum shale 1 



