. 78 Elements of Mineralogy. 



Blue Clays. 



Thefc fometi mes lofe their colour and become 

 white when heated, and confequently contain 

 but little of iron, but owe their colour to an 

 oily matter, which is diflipated by heat ; others 

 owe their colour, according to Mr. Mantlet, 

 to a mixture of pyrites. Mineralogie, p. 339. 

 Mr. Beaume fays, that blue clays contain the 

 vitriolic acid, but he probably found it only 

 in the pyritical clays, for in other forts Mr. 

 Woulfe could difcover none. Phil. Tranf. 

 I 779 P- 20 ' Mr. Bergman found fome which 

 contained copper and a little of cobolt. Rin- 

 man mentions a blue lithomarga, which con- 

 tained 1 3 per cent, of iron, 4 of lead, and fome 

 veftiges of zinc. Hifl. Ferri, 201, p. 712. 



Umber. 



A brown orblackifh fubftance, which ftains 

 the fingers, and is very light; it has long been 

 taken for a clay, but Mr. Hupfch, Mem. Ber- 

 lin, 1777, has difcovered it to confiil of par- 

 ticles of decayed wood, mixed with bitumen ; 

 yet it is certain that this name hath alfo been 

 given to a fort of brown ochre, of the fame 

 colour, which becomes red when flightly heat- 

 ed, but in a ftronger heat is again brown and 

 magnetic, and in a ftill ftronger melts into a 

 black glafs. It does not effervefce with acids 



before 



