Salts. 187 



foliated texture, is alfo found in Burgundy, 

 and is by fome called Ampelytes. i Chym 

 de Dijon, p. 107. 



3 d - Shiftus Alumlnaris. This differs from 

 roof fhiftus in this, that it contains a variable 

 proportion of pyrites intimately mixed with 

 it, and alfo Petrol. Bergm. Sciagr. and I 

 Bergm. 292. Its colour is blue or black, 

 but when the proportion of petrol is very fmall, 

 grey. When the proportion of pyrites is fo 

 large as to form a vifible mafs, the ore is 

 commonly reje&ed as containing too much 

 iron, though it might be worked to ad- 

 vantage, by adding a proportion of clay; a 

 valuable improvement fuggefted by Mr. Berg- 

 man. When the proportion of petrol is con- 

 fiderable, it does not efflorefce by expofure to 

 the air, and therefore muft be torrefied to 

 burn off the petrol, and extricate the acid 

 from the fulphur of the pyrites ; but when 

 the proportion of petrol is fmall, the pyri- 

 tical part is decompofed by long expofure to 

 the air and moifture; and thus alum is 

 formed. In Sweden, if 100 pounds of the 

 ore yield 4 of alum, it is worth working ; 

 but it generally yields more. The mine of 

 Be eke t in Normandy, and thofe of Whit by in 

 Torkfhire, are of this fpecies. 



4 th * Volcanic Aluminous Ore. This is 

 found at Solfatcra near Naples, and elfe- 



where, 



