CHAP. XIV. 



SILICEOUS EARTHS. 



General Arrangement of Flinty Subftances.~Ge?ns.Dianiond.-~R.ul>j, 

 Sapphire, Topaz Emerald. -r- Hyacinth.- Amethyjl. Garnet. 

 Tourmalin. ~Qpal. L-ompoJition and Properties of precious Stones* 

 -Quartz. Rock Cry ft ah, Pe bbles. Flints .Lapis Nephriticus. 

 Cat's -Eye. Hydroplanes. Explanation of the Phenomena of the 

 Hydrophanous Stone. Moon-Stone. Chalcedony. Onyx.Carne- 

 iion. Sardonyx.' Agate.* Common Flint. Chert. Sand and 

 Gra-veL Jafper. Feldt Spar.-> Labrador Stone. Scbirl and 

 Whetjlone. Art of making Glafs. Prince Rupert's Drop and 

 PhihifophicqJ Phial. Curious Phenomenon, 



THE filiceous earths are chiefly diftinguifhed by 

 their hardnefs, by which they ftrike fire with 

 fteel, and their property of forming good glafs with 

 alkali. They may be divided into, i, Gems, or 

 precious ftones 3 2, Quartz j 3, Flints ; 4, Jafpers ; 

 5, Feldt fpars. 



1. GEMS. The diamond is the hardeft of all bo- 

 dies, and the moft valuable of all the gems j it is 

 however not an earthy but an inflammable fubftance, 

 and therefore will be fully treated of under that head*. 

 It is mentioned on the prefent occaiion only, that the 

 catalogue of gems may not appear incomplete. 



2. The ruby has been confounded with the dia- 

 mond, on account of its hardnefs and luftre, but fome 

 late experiments mew that it is effentially different. 

 According to Cronftadt, there are four principal kinds 

 of rubies : i. The ruby of a deep colour inclining tq 



* See Chap. 44. 

 G 3 



