ALUMINUM. 



quenee of which octohedral crystals of this double salt precipi- 

 tate, whenever any salt of potash is added to a strong solution 

 of sulphate of alumina. Alum is a salt of which large quan- 

 tities are consumed in dyeing. It is prepared by.several pro- 

 cesses, or derived from different sources. It may be prepared 

 by decomposing clay with sulphuric acid ; the decomposition is 

 effected in the most complete manner by igniting pure clay, 

 grinding it afterwards to powder and mixing it with 0.45 of 

 sulphuric acid,, of 1.45 density. This mixture is heated in a 

 reverberatory furnace till the mass becomes very thick ; after- 

 wards left to itself for at least a month, and then treated with 

 water to wash out the sulphate of alumina formed. The addi- 

 tion of sulphate of potash converts the last salt into alum. 

 But the mode of manufacture just described has not been found 

 so advantageous as the following, which alone is practised in 

 this country. A series of beds occur low in many of the coal 

 measures, which contain much bisulphuret of iron. One of 

 these known as alum slate is a siliceous clay, containing a con- 

 siderable portion of coaly matter, and of the metallic sulphuret 

 in a state of minute division. When this mineral is exposed to 

 air and moisture, it soon exfoliates, from the formation of sul- 

 phate of iron, the bisulphuret of iron absorbing oxygen like a 

 pyrophorus. The excess of sulphuric ac;d formed attacks the 

 other bases present, of which the most considerable is alumina. 

 Aluminous schists often require to be moderately calcined or 

 roasted, before they undergo this change in the atmosphere. 

 The mineral being lixiviated, after a sufficient exposure, affords 

 a solution of sulphate of alumina and protosulphate of iron, 

 from which the latter salt is first separated by crystallization. 

 The subsequent addition of sulphate of potash to the liquor, 

 causes the formation of alum ; the chloride of potassium answers 

 the same purpose, and has the advantage over the sulphate that 

 it converts the remaining sulphates of iron into chlorides, which 

 are very soluble, and from which the alum is most easily sepa- 

 rated by crystallization. A very pure alum is obtained in the 

 Roman states from alum-stone., which is simply heated till sul- 

 phurous acid begins to escape from it, and the residue of this cal- 

 cination treated with water. This mineral contains an insoluble 

 subsulphate of alumina with sulphate of potash. The heating has 

 the effect of separating the excess of alumina, so that a neutral 



