CHEMISTRY OF SOIL. 43 



48. Silex or silica, or the earth of flints, as it has been 

 called, is in its pure state a perfectly white, insipid, tasteless 

 powder. In various combinations of minerals', it unites with 

 the bases (41), forming neutral salts, termed silicates, from 

 the silicic acid. Thus is formed, as in the case of vinegar, or 

 acetic acid (47), a large class in which are found silicates of 

 soda, of potash, of lime, of magnesia, of alumina, of iron, 

 and of manganese. This class forms the great bulk of all 

 rocks and soil. 



49. The seven substances last mentioned (42) are all 

 metals united to oxygen. They are metallic oxides. If the 

 oxygen is removed, and replaced by carbon, sulphur, phos- 

 phorus or silicon, combinations are formed, called sulphur- 

 ets, carburets, phosphurets, siliciurets. 



50. Metalloid compounds are combinations of metalloids 

 with metals, in their pure, or unoxidated state. 



51. Salts are combinations of metalloids with oxygen, 

 and the metals in their rusted or oxidated state. 



52. The formation of carbonic, sulphuric, phosphoric acids, 

 has been explained (41). When these acids unite to the 

 bases, salts are formed, called carbonates, sulphates, phos- 

 phates. 



53. Hence, when a substance is named, for example, sul- 

 phate of lime, a definite idea of the nature of this is con- 

 veyed. It is, on the principles stated, at once known to be 

 a salt, that is a sulphate, that is, sulphur and oxygen united 

 to lime. So, too, phosphate of lime is seen to be a salt of 

 lime. 



54. If the thirteen elements which enter into the compo- 

 sition of rocks, had each an equal tendency to unite with the 

 other ; or, in other words, if their affinities were mutual, 

 then there would be as many different combinations as it 

 would be possible to form with thirteen different substances. 



