1 86 Elements of Mineralogy. 



reckoned them among the argillaceous con- 

 tents, which muft alfo have contained a 

 quantity of vitriolic acid- To make this 

 ore produce alum, it is neceflary that it ihould 

 firft be torrefied to decompofe the fulphur, 

 whofe acid then re-acts on the argill, and 

 being moiftened, or expofed to the air, 

 it foon fwells, efflorefces, and forms alum. 

 This ore was probably at firft a clay, mixed 

 with fulphur, and hardened by volcanic fire, 

 and derives its alkali from vegetables incine- 

 rated by the volcano. Mr. Monnet found 

 alfo a little magnefia in this ore. Miner alog. 

 p. 1 60. The red colour of the alum pro- 

 ceeds from iron in a particular flate. 3 Bergm. 

 250. 



2 d - Pyritaceons Clay. This is found at 

 Sch'wemfal in Saxony at the depth of 10 or 

 12 feet ; it is a black, hard, yet brittle fub- 

 ftance, confifting of clay, pyrites and bitu- 

 men ; after it is dug it is left expofed to the 

 air for two years, by which means the pyrites 

 are decompofed, and alum formed. 3 Jars 

 Voy. Metallurg. p. 293. The alum ores of 

 Hejfe and Liege are alfo of this fpecies, yet 

 they are torrefied; a practice which Jars 

 condemns. According to Monnet this ore 

 .contains alfo magnefia. Mineralog. p. 164. 

 64 pounds of this ore yield from 5 to 

 7 of alum. An earth of this fpecies, of a 



foliated 



