136 The Principles of Part III. 



heap. This fact fhows, that this fait is 

 not the natural fait of the earth, but formed 

 by its expofition to the air. After thefe 

 earths have been expofed a fufficient time 

 to the air, they are put into different cafks, 

 and water poured over them, to lixiviate or 

 diffolve the falts. The materials muft be 

 frequently ftirred about in the water, that 

 it may diffolve the falts. After the water is 

 fufficiently impregnated with the falts, they 

 draw it off, and add to it fome quick-lime, 

 and the aihes of vegetables or alkaline falts, 

 if it has not already got enough of them ; 

 the former to feparate the oil from the fait, 

 the latter to give it a fixed alkaline bafe in- 

 flead of an earthy one. They add alkaline 

 falts, as long as their addition makes the 

 liquor take a milky appearance, and yield a 

 precipitation. When no more precipitation 

 happens, then they decant the liquor, and 

 boil it up. 



LET us now inquire into the origin of 

 nitre. There are various opinions about 



this 



