ORIGIN AND FORMATION' OF SOILS. 107 



THE CHEMICAL ELEMENTS OF ROCKS. 



The chemical elements of rocks, i. e., ths constituents 

 of the minerals which go to form rocks, include all the 

 simple bodies known to science. Those, which, fiom their 

 universal distribution and uses in agriculture, concern us 

 immediately, are with one exception the same th:it have 

 been noticed in a former volume as composing the ash of 

 agricultural i)lants, viz., Chlorine, Sulphur, Carbon, Silicon, 

 Potassium, Sodium, Calcium, Magnesium, lion, and Man- 

 ganese. The description given of these elements and 

 of their most important compounds in " IIow Crops Grow " 

 Avill suffice. It is only needful to notice further a single 

 element. 



Allliuinuin, Symbol Al., At. wt. 27.4, is a bluish silver- 

 white metal, ciiaracterized by its remarkable lightness, 

 having about the specific gravity of glass. It is now 

 manufactured on a somewhat large scale in Paris and New- 

 castle, and is employed in jewelry and ornamental work. 

 It is prepared by a costly and complex process invented 

 by Prof. Deville, of Paris, in 1854, which consists essen- 

 tially in decomposing chlorido of aluminum by metallic 

 sodium, at a high heat, chloride of sodium (common 

 salt) and metallic aluminum being produced, as shown by 

 the equation, Al„ Cl^ + 6 Na = 6 Na CI + 2 Al. 



By combining with oxygen, this metal yields but one 

 oxide, which, like the highest oxide of iron, is a sesqui- 

 oxide, viz.: 



Alumina, Al^ O^, Eq. 102.8. — ^Vhen alum (double sul- 

 phate of alumina and i)otash) is dissolved in water and 

 ammonia added to the solution, a white gelatinous body 

 separates, which is alumina combined with water, Al^ O3, 

 3 H^O. By drying and strongly heating this hydrated 

 alumina, a white powder remains, which is pure alumina. 



