. Earths'. 185 



appear like indurated clays, of a dark black 

 colour. They have a sulphureous smell : when 

 exposed to the air for some time they grow hot, 

 swell, and crumble down into a powder, emit 

 turned like brimstone, and sometimes take fire. 

 Some do not undergo this change merely from 

 exposure to the air, but must be first burnt or 

 roasted, and then exposed to the air a good 

 while before any alum can be obtained. After 

 they have been crumbled down in this manner, 

 they are steeped in water, an alkali is then added, 

 and the alum crystallizes and subsides. The 

 fixed and volatile alkalies are both employed ; 

 but the volatile alkali or fetid, urine is most used, 

 which attracts the superfluous acid. 



At Hurlet, near Glasgow, there is a remark- 

 able bed of alum ore, which, having been an- 

 tiently laid open by a coal-mine carried through, 

 is now, by long contact with the air, prepared 

 for yielding alum, and is wrought for that pur- 

 pose. 



These ores, therefore, are natural mixtures of 

 clay and sulphur, and when exposed to the air, 

 the oxygen in the atmosphere is attracted, and 

 converts the sulphur into sulphuric acid. After 

 a proper quantity of alkali has been added to the 

 solution, it is crystallized in the common man- 

 ner. The crystals are at first tolerably distinct, 

 but of no considerable size. A very strong so- 

 lution being prepared from them a second time, 

 it is put, after clarification, into large tubs, 



