Salts. 169 



potass and soda. Where there is an excess of 

 acid in the salt, it is expressed by prefixing the 

 preposition super. Thus sulphat of potass ex- 

 presses the combination of the sulphuric acid 

 with potass, and super-sulphat of potass denotes 

 that there is a redundancy of acid. On the con- 

 trary, the preposition sub implies that there is an 

 excess of base; thus sub-sulpkat of potass is 

 the salt before described, but with a deficiency 

 of the acid, and a redundancy of the potass or 

 base. 



Salts have also been divided into the alkaline, 

 the earthy, and the metallic, according to the 

 nature of their respective bases. But it is un- 

 necessary to enter into any particular description 

 of them here, since the nomenclature itself de- 

 fines the nature of the salts. Dr. Thomson 

 calculates that there may be about 2000 different 

 species of compound or neutral salts; and he 

 remarks that " some idea may be formed of the 

 progress which this branch of chemistry has 

 made, by recollecting that forty years ago not 

 more than thirty salts in all were known." 



TOL. II. 



