42 CHEMISTRY OF SOIL. 



45. The elements w hich compose silicates may be enumer- 

 ated in pairs, as a help to the memory — 



The alkalies — potash and soda. 



" alkaline earths — lime and magnesia. 



" metallic acids or earths — silex and alumina. 

 2 alkalies, 2 alkaline earths, 2 metallic acids. The last have 

 all the characters of earths, and silica, whose acid proper- 

 ties were first noticed by Smithson, was and is still called 

 the earth of flints, while alumina is known as the earth of 

 clay. 



46. The terms, salt, silicate, and metalloid compound may 

 need a further explanation. Pearlashes and vinegar are 

 well-known substances. One is an alkali, the other an acid. 

 Pearlash has the alkaline properties of a bitter, burning 

 taste, the power of changing vegetable blues to green, and 

 pinks to blues. Vinegar has the acid property of sour 

 taste, of causing a hissing or effervescence, when poured on 

 pearlash. This action ceasing, there are neither acid taste 

 nor alkaline properties. The characters of the vinegar and 

 pearlash have disappeared. These substances have united ; 

 they have formed a new substance called a salt. Their 

 properties are neutralized, and lost in the salt. This is no 

 longer either pearlash or vinegar. 



47. The fact to be observed in the action (46) is, that an 

 acid and alkali mutually neutralize each other. The vinegar 

 is said, in this case, in common language, to "kill" the 

 pearlash. So soda, potash, lime, magnesia, iron, and man- 

 ganese would all be killed or neutralized by vinegar ; they 

 would all be dissolved by it, and lose their distinguishing 

 characters. In either case, a neutral salt would be formed. 

 Such a class of salts is termed acetates, being formed of 

 alkalies, alkaline earths, or metallic oxides united with acetic 

 acid. • 



