76 Elements of Mineralogy. 



power arifes from the property of clay to ab- 

 forb oils, the finenefs of its particles, which 

 do not injure the cloth, and its eafy diffufi- 

 bility in water, which renders it capable of 

 being wafhed off. 



Terra lemma is ayellowifh or flefh-colour- 

 ed clay of this fort. 



Pipe Clay. 



Any moderately pure and fine white clay* 

 which retains its colour in fire, is diftinguifh- 

 ed by this appellation. That of Cologne is 

 the pureft, and is infufible in the ftrongeft 

 heat. Many of thefe clays become grey in a 

 weak degree of heat, becaufe the mineral oil 

 with which they are mixed burns to a coal* 

 but in a ftronger heat they again become 

 white, this coal being confumed. 



Porcelain Earthy or Kaolin. 



This differs from the former chiefly in the 

 finenefs or fubtility of the filiceous ingredient* 

 It fometimes effervefces with acids, either be- 

 caufe it contains cryftalized argill, or mag- 

 nefia, or calcareous Earth : it alfo abounds in 

 talcofe particles. It never contains any oily 

 matter, and therefore retains its colour in 

 every degree of heat. 



Brick 



