Argillaceous Genus. 85 



have a fplendid metallic appearance) fmooth, 

 but not greafy to the touch, which diftin- 

 guifhes it from talc. Its texture is always 

 lamellar or fcaly, and the lamellae, or fcales, 

 are flightly flexible and often elaftic; thefe 

 fcales are fometimes parallel to each other, 

 fometimes interwoven, fometimes wavy, or 

 undulated, and fometimes they reprefent fila- 

 ments. Its fpecific gravity is from 2,535, to 

 3,000 when loaded with iron. 



It does not effervefce with acids, and is in- 

 foluble in them without particular manage- 

 ment, but after it has been calcined with 4 

 times its weight of fixed alkali, it effervefces 

 ftrongly, and is in great meafure foluble. 



The pure colourlefs mica is infufibie ferfe^ 

 and fcarcely melts even with mineral alkali, 

 but yields more readily to borax or microcof- 

 mic fait, with fcarce any effervefcence ; but 

 the coloured forts were found by Mr. Saujfure 

 to be fufible per fe, though with difficulty, 

 for they require a ftronger heat than ihoerl 

 does. 



100 parts of the colourlefs kind contain 

 38 of filex, 28 of argill, 20 of magnefia, 

 and 14 of the moft dephlogifticated calx 

 of iron. Martial mica contains befides, 

 1Q or 12 per cenj. of a more phlogifticated 



@ 3 calx 



