50 



CHEMISTRY OF SOIL. 



division of fractional equivalent numbers. They are easily 

 apprehended, and, for all practical agricultural purposes, 

 correct. 



59. Viewed by the light of chemistry, rocks are masses 

 of silicates. The simple minerals composing rocks are truly 

 only silicates in fixed proportions. The simple minerals are 

 quartz, felspar, mica, hornblende, talc, serpentine. Their 

 comnosition is presented in the following 



In each the silex acts as an acid. This is not only the 

 most constant, but the most abundant ingredient of rocks. 

 Next is alumina. The average quantity of these elements 

 in the most important rocks, is silica 62.79, alumina 25.15 

 per cent. 



60. In each simple mineral, the bases (41 ) being combined 

 with silica, a compound, or silicate is formed. Jn this case, 

 the few simple minerals forming rocks, may be arranged in 

 three classes, and it will be perceived that, notwithstanding 

 their great variety of external appearance, their ultimate 

 chemical composition resolves itself into classes of double, 

 or simple silicates, in which silicate of alumina 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. 



