Earths. 195 



a heat very intense into an imperfect kind of 

 glass, and is very apt to break on pouring hot 

 water into it, especially if it stands upon a cold 

 substance. This has for its basis the clayey earth 

 mixed with a proportion of glass. The Saxon 

 manufacture has subsisted longest in Europe, 

 and is most famous, its productions equalling the 

 best foreign china. 



In England many persons used the soap rock 

 (whose principal ingredient is magnesia) ; but 

 Mr. Cookworthy, of Plymouth, found out that 

 the true materials of china are in great abund- 

 ance in Cornwall. The kaolin, being a clay of 

 uncommon whiteness and lustre, contains a very 

 great quantity of a very fine talc. The petuntze 

 is a stone of the granite kind *, which contains a 

 great quantity of the fusible stone ; this must be 

 powdered for use. The kaolin, when melted 

 alone produces the stone china, or Japan porce- 

 lain, which is less transparent, and harder than 

 the ordinary kind. 



V. BARYTES or BARYTA is the heaviest of all 

 the earths, yttria, and perhaps zirconia, excepted. 

 It is obtained from a mineral not uncommon in 

 England, called ponderous spar, which is a sul' 

 phate of baryta. By reducing the mineral to 

 powder, and keeping it for some hours red-hot 

 in a crucible, with an eighth part of its weight 



* The common granite contains iron, which would 

 discolour the china. 



