Chap. 13.] Procefs m making Roch Alum. 75 



down into a powder, emit fulphureous fumes, and 

 fometimes take fire. Some do not undergo this change 

 merely from cxpofure to the air, but mud be firft 

 burnt and waihed, and then expofed to the air a good 

 while before alum can be obtained. After they have 

 been crumbled down in this manner, they are fleeped 

 in water ; an alkali is then added, and the alum cryf. 

 tallizes and fubfides. 



Thefe ores are evidently natural mixtures of clay 

 and fulphur. By expofure to air, according to the 

 old fyftem of chcmiftry, it was fuppofed that the clay 

 acted on the fulphur in fuch a manner as to make it 

 part with its phlogiilon, whence they explained the 

 production of heat. This fact, however, is now much 

 better explained, according to the new fyftem, by fup- 

 pofing that the oxygenous gas of the atmofphcre is 

 decompofed j and while the oxygen is abforbcd by the 

 fulphur (with which it forms vitriolic acid) the heat is 

 fet at liberty, and becomes fenfible. 



After a proper quantity of alkali has been added 

 to the folution, it is cryftallized in the common man- 

 ner, that is by evaporation. The cryfrals are at firft 

 tolerably diftinct, but of no confiderable fize. A 

 great quantity of them being heaped together until 

 they undergo a watery fufion, they unite into a mafo, 

 which is called roch alum, and in this form it is ex- 

 pofed to fale. 



That accurate and attentive obferver, Bifliop Wat- 

 fon, in his Chemical F.ffays, relates, that being one day 

 engaged, in evaporating fpmething or other from a 

 faucer made of Stafford fhire yellow ware, he was 

 lurprifed to fee a white fubftance bubbling through a 

 crack in the faucer ; upon tailing it, he found it to be 

 a fait, and, upon further examination, difcovered that 

 it was a perfect alum. The n:e a he obferves, was a 



very 



