CHEMISTRY OF SOIL. 39 



gist bestows on the assemblage of external characters, which 

 determines the species. 



37. The mineralogy of agriculture is no more than this, 

 that the farmer be able ever to connect with a certain name 

 a certain chemical composition. Hearing mica (which is 

 isinglass) named, he immediately connects with that, the 

 chemical properties which belong to the species, as he would 

 connect with the term isinglass, the physical properties of 

 that substance ; such as transparency, divisibility into thin 

 plates, which are flexible and elastic. 



38. The amount of this mineralogical knowledge is very 

 limited. Seven simple minerals compose all rocks, viz. : 

 quartz, mica, felspar, hornblende, talc, serpentine, carbonate 

 of lime. Other minerals are found in rocks, but these seven 

 compose all those termed geological formations, and which 

 form the crust of the globe. 



39. The chemical constitution of rocks, the nature, prop- 

 erties and relations of their elements, prove to be of the 

 highest value, when it is known that the elements of these 

 seven minerals are also the earthy part or ashes of all 

 plants. The farmer should therefore be so far a chemist as 

 to understand the results to which the analysis of minerals 

 conducts. 



40. The number of elements which chemistry has detected 

 is sixty-two ; probably sixty-three. All are either metallic 

 or unmetallic. Of these some are gaseous, others earthy, 

 others combustible. The last are also called metalloids, by 

 which term they are designated in these pages. 



Of the simple or elementary bodies, thirteen chiefly 

 compose all rocks, and the mineral portion of soil. Six of 

 this number are unmetallic and seven are metallic sub- 

 stances. 



