140 Chemistry. [Lecture 29. 



performing them: as in the separation of com- 

 mon salt from saltpetre, the saltpetre sometimes 

 still contains a portion of common salt, which 

 makes it necessary to repeat the operation. From 

 common salt we may generally obtain a quan- 

 tity of saltpetre ; and sometimes we must crystal- 

 lize it three or four times before we have it 

 pure. 



Notwithstanding all salts are, as we have seen, 

 compound bodies, yet a distinction was establish- 

 ed by chemists ; and some, such as the alkalies 

 and acids, were called simple salts, because as 

 salts they were really so, though they might be 

 compounded of other matters : the others, which 

 are a combination of two salts with each other, 

 or of some saline substance with another matter, 



combinations of the acids with alkalies, earths* 

 and the oxides of metals. Later chemists have 

 wished to confine the term salts to these last 

 only. It will, however, facilitate the matter to 

 the student to treat them in some measure ac- 

 cording to the old system, which will at least be 

 always necessary foi the methodical arrangement 

 or classification of salts, even if we exclude from 

 that denomination the pure acids and alkalies. 

 I shall therefore first take a short view of what 

 are called the simple salts, which are divided 

 into the two orders of alkalies and acids. 



Of alkalies there are only three soda, potass, 

 and ammonia. The characteristic common to 



