66 r Mica andMufcovy Glafs. [Book VL 



earthy or ftony bodies, the texture and compofition of 

 which confifts of thin flexible particles, divifible into 

 plates or leaves, having a fhining furface. Thefe 

 plates, by being expofed to heat, feparate into fmaller 

 ones, but their flexibility is much diminifhed. By a 

 ftrong heat they curl or crumple, but it is very diffi- 

 cult to reduce them to perfect fufion without addition. 

 The plates of mica, when of the pureft kind, are tranf- 

 parent, and there is one variety, the plates of which, 

 from their near refernblance to glafs, are called Muf- 

 covy glafs. Micas, however, are often tinged with a 

 variety of colours, or are more or lefs opake. Some- 

 times they have a . luftre refembling that of metallic 

 fubftances. Sometimes they are in the form of min- 

 ing powder, like that which is ufed in bronzing figures. 

 This appearance has often impofed upon miners, who 

 have thought they met with gold and filver, whereas 

 there is never any metal in thefe fubftances but iron. 

 They are found alfo mixed with other ftones, as the 

 granite, which frequently contains a great quantity of 

 talk. Freeftone alfb contains more or lefs; its hori- 

 2ontal layers have between them a thin ftratum of talk 

 where the ftone more eafily leparates. It is alfo found 

 in lome kinds of flate, which, when expofed to the air, 

 moulder into talky po-.vder. The tranfparent Muf- 

 covy glafs is ufed for windows, and for thofe Ian- 

 thorns which are employed in powder magazines, as 

 this fubftance is not fo liable to break as glafs, and is 

 unaffailable by fire. The twifted, or crumpled mica, 

 which is found at Harcuil in Jempland, is there manu- 

 factured into kettles and other veflels, as alfo for 

 hearths of chimnies ; and the powder which falls in the 

 working may be mixed witli common fait for the 

 diftillation of the muriatic acli. The fhining appear- 

 ance of the micaceous earths has obtained them the 

 9 names 



