124 Elements of Mineralogy, 



SPECIES XL 



Siliceous Earth intimately mixed and partly 

 united with 0,209 f its weight of Argil t, 

 o, 1 64 of its 'weight of ponderous Earth, and 

 0,12 of its weight of Magnefia. 



Felt-fpar 9 Spathum Pyromachum, Rhombic 

 Quartz, ^uartzum Spathofum^ Spat hum 

 Durum, Petunfe. 



This ftone, which Is generally opake, is 

 found of all colours, white, red, yellow, 

 brown, green, violet, or iridefcent ; fome- 

 tirnes cryftalized in rhombic, cubic, or pa- 

 rallelipedal forms, and often amorphous : its 

 texture clofe but lamellar, and breaks like 

 fpar : its fpecific gravity is from 2,4 to 2,6, 

 and Mr. Gerhard fays he found it even 3,5; 

 but it was then probably loaded with metallic 

 particles : it is harder than fluors, but not fa 

 as quartz. 



It melts per fe more quickly and perfectly 

 than fluors, into a whitifh glafs, and doe& 

 .not, like them, attack the crucibles. Borax 

 and microcofmic fait entirely diflblve it with- 

 out effervefcence, but it does not eafily enter 

 into fixed alkalis. When cryftalized, it 

 decrepitates in a moderate heat a otherwife 

 not* 



it 



