OF AGRICULTURE. 193 



BASES. 



The oxydcs of metals are termed bases* Potash, 

 soda, lime and magnesia are termed alkiline bases ; 

 the others, metalic bases. Acids and bases unite and 

 form salts in the soil. Thus, these unmetallic sub- 

 stances unite with metals, and form a class of 

 compounds of the highest importance in agriculture. 

 It is seen, therefore, that the elements composing 

 rocks, are reduced to salts and metalloid compounds. 

 The peculiar character of the salts formed by silicic 

 acid, will be easier understood by separating these 

 from the others under the name of silicates. These 

 form the great bulk of the earths crust The com- 

 pounds of silicic acid would hardly be recognized as 

 salts, in the common and popular sense of the term; 

 with which is associated the idea of softness and 

 solubility. Carbonic, sulphuric and phosphoric acids, 

 form with bases, salts, in the usual sense of the term. 



The elements which compose the silicates may be 

 named in pairs, to aid the memory. First, the alkalies, 

 potash and soda. Second, alkaline earths lime and 

 magnesia. Third, earths silica or sand, and alu- 

 mina. Thus, we have two alkalies, two alkaline 

 earths, and two earths. 



EXPERIMENTS SILICA. 



The term salt, silicate and metalloid compound, 

 may need further explanation. Pearl ash and vinegar, 

 are well known substances. One is an alkali, the 

 othjer an acid. Pearl ash has the alkaline properties 

 of a bitter, burning taste, and the power of changing 

 vegetable blues to green, and pink to blue. Vinegar 

 has the acid properly of a sour taste, and of causing 

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