Chap. 13,] Lapis Lazuli ', Rotten Stone, 6?r. 83 



V ZEOLITE is in general of a cryftnlline form, com- 

 pofed of imperfed pyramids turned towards a com- 

 mon' center. Their form is fometimes globular,- but 

 feldom prifmatic. The lapis lazuli belongs to the 

 zeolites. Thefe foflils are harder than fluors, but may 

 be fcratched by fteel. The filiceous earth predomi- 

 nates much in them. The lapis lazuli contains filver 

 and iron, which gives the blue colour. Zeolite melts 

 ferfe with ebullition into a white frothy flag, which in 

 the moment of fufiori affords a phofphoric light. The 

 lapis lazuli firft becomes brown when melted, and theri 

 produces a clear glafs with blue clouds. By rectifi- 

 cation with lead, one hundred weight of it has afforded 

 two ounces of filver. 



VI. TRIPOLI feems to be of a volcanic origin. It 

 is an earth confifting of very fine particles, and is 

 known by its quality of rubbing or wearing hard bo- 

 idies, which gives them a polifh. Other fine clays> 

 however, have the fame property when a little burnt. 

 The tripoli grows forriewhat harder in the fire, and is 

 fufed with great difficulty. When crude it imbibes 

 water, but is not foluble in it. It tafles like chalk, 

 and is ro^gh or fandy between the teeth, though no 

 fand can be feparated from it. That which has been 

 juft defcribed is of a yellow colour, and is fold by 

 druggifts. This kind of tripoli has been lately dif- 

 covered in Scotland. Another variety, called the 

 totten-ftone, is found in Derbyfhire. It is in common 

 ufe in England for all forts of finer grinding and po- 

 lifh'mg, and is alfo fometimes ufed by lapidaries for cut- 

 ting of ftones, 



VII. Common or BRICK CLAY is found of various 

 colours, as red, pale red, grey, and blue. It acquires 



G 2 a red 



