ORIGIN AND FORMATION OF SOILS. 107 
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THE CHEMICAL ELEMENTS OF ROCKS. 
The chemical elements of rocks, i. e., the constituents 
of the minerals which go to form rocks, include all the 
simple bodies known to science. Those, which, from their 
universal distribution and uses in agriculture, concern us 
immediately, are with one exception the same that have 
been noticed in a former volume as composing the ash of 
agricultural plants, viz., Chlorine, Sulphur, Carbon, Silicon, 
Potassium, Sodium, Calcium, Magnesium, Iron, and Man- 
ganese. The description given of these elements and 
of their most important compounds in “ How Crops Grow ” 
will suffice. It is only needful to notice further a single 
element. 
Aluminum, Symbol Al, Aé. wt. 27.4, is a bluish silver- 
white metal, characterized 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 chloridz 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 NaCl + 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—When alum (double sul- 
phate of alumina and potash) is dissolved in water and 
ammonia added to the solution, a white gelatinous body 
separates, which is alumina combined with water, Al, O,, 
3H,O. By drying and strongly heating this hydrated 
‘alumina, a white powder remains, which is pure alumina, 
