32 NOURISHMENT OF- PLANTS. 



plies, that they consist mainly of potash^ soda, mag'' 

 nesia, oxide of iron, silica, phosphoric acid, sulphuric 

 acid, muriatic acid [chlorine), and carbonic acid. Of 

 these substances the chemist places the first five 

 amongst the bases or oxides, the last five amongst 

 the acids, and understands by the former those 

 bodies which, if soluble, have an alkaline taste, as, 

 for example, wood-ashes or burnt lime, and by the 

 latter, those which in a state of solution have a sour 

 or acid taste. Potash and soda are also called 

 alkalies ; lime and magnesia, alkaline earths. If a 

 base or alkali unites chemically with an aoid, the 

 peculiar properties of both disappear, and the com- 

 bination, which presents itself as a wholly different 

 body with entirely new properties, is no longer alka- 

 line or acid to the taste, but saline. In this condi- 

 tion it is called a salt. Hence we obtain from the 

 caustic potash and the corrosive aquafortis (nitric 

 acid) a mild salt, the well-known saltpetre ; and 

 from the caustic soda and the corrosive oil of vitriol 

 (sulphuric acid), an innocuous salt, the well-known 

 Glauber salts. Wherever bases and acids come into 

 connection with each other, they combine to form salts. 

 This fact is exemplified in the mineral constituents 

 of plants, and hence we meet with them in vegetable 

 ashes, not in a free and uncombined state, but in a 

 state of combination ; therefore, as salts. 



Of these combinations there are principally, — 

 1. Soluble in water : the alkaline salts (salts of 

 potash and soda). 



