CHEMISTRY OF SOIL. 35 



great variety of external appearance, their ultimate 

 chemical composition resolves itself into classes of 

 double, or simple silicates, in which, silicate of alum- 

 ina is united with potash, or lime, or with magnesia, 

 forming thus, three classes only of simple minerals 

 which compose rocks and soil. 



1st. Silicate of Alumina and Potash forms Felspar and Mica. 

 2d. " " and Lime with Magnesia form Hornblende. 



3d. " Magnesia forms Serpentine and Talc. 

 And Silica almost pure is Quartz. 



61. The iron and manganese in the table, (59) 

 are regarded as accidental mixtures of silicates of 

 these metals. Silicate of soda is often present in 

 place of potash, and this constitutes an extensive 

 variety of the felspar family. From the analysis of 

 ashes of plants, it is probable that the soda felspar 

 is widely diffused in rocks, though in too small a 

 quantity to be distinguished from the potash variety ; 

 it is probably chemically combined with that. 



It will be observed, by the chemical reader, that 

 truly eleven elements, excluding those of water, are 

 found in soil. The division into twelve substances, 

 including oxygen, is more consonant with popular 

 ideas, and is adopted ; though by this mode, silicon 

 occupies a double position. 



