ELEMENTS OF SOILS. 227 



be disengaged from the base with which it is combined ; thus wo have only to heat lime- 

 stone, to obtain quicklime. It is a solvent'of rocks and soils. 



Silkx. Ii is a solid, the purest form of which is known as rock crystal. White sand is 

 often nearly as pure. It is hard, and, in these natural states, resists the atmospheric in- 

 fluences, and is insoluble in water. Its specific gravity is -2.66. 



Silex or silica is a compound of oxygen and silicon : it is the only compound known of 

 these bodies in a state of purity. Silica, in consequence of its peculiar composition, and 

 the compounds it forms with other bodies, is regarded as an acid; and hence its combi- 

 nations ,ur termed silicates, after the manner of carbonates and sulphates. 



Silex is the largest constituent of the earth. It not only forms large masses, or thick 

 strata in the earth's crust, but it is very frequently combined with the other elements, 

 forming with them the extensive class of bodies called silicates, as silicate of lime, of 

 magnesia, of potash, of soda, etc. 



The silicates are important bodies, notwithstanding they are apparently so insoluble. 

 Their feeble insolubility serves an important end. Were the case reversed, and were the 

 elements so necessary to vegetables quite soluble, they would he speedily removed from 

 the soil ; but with their present constitution, they remain and are dissolved slowly, and 

 no faster than the necessities of plants demand. 



Soils are principally silicates. They are probably more so in this country than in some 

 parts of Europe, where chalk or some other calcareous rocks enter largely into the com- 

 position of the soil. In New-York, calcareous soils are unknown, notwithstanding large 

 areas of limestone exist. 



Silex is known by its harsh gritty feel ; and where it predominates, it imparts the same 

 grittiness to the soil. It differs in feel from chalk ; the sensation in the latter case being 

 described as meagre, while that from silex is sharp and gritty. It has no adhesiveness, 

 and hence never coheres ; and when its particles are fine and smooth, the mass flows like 

 a liquid. This character in soils requires to be understood. 



Alumina. Clay and alumina, although often used as synonimes, ought not to be used 

 in the same sense. Alumina is the pure earth, the oxide of aluminum. Clay is a silicate 

 in part of alumina, mixed probably with both alumina and silex. Alumina is white, like 

 pure silica, but, unlike that, it is soluble in acids. 



Adhesiveness is a striking property of alumina, and also of clay ; hence the latter holds 

 together the substances in mixture with it. Soils are close and compact in proportion to 

 the quantity of clay present. In the arts, this property, or one allied to it, is highly im- 

 portant ; for instance, a fibre of cotton, immersed in a solution of acetate of alumina, 

 attracts the clay and detaches it from its acid : it is thus covered with a coating of alumina. 

 Clay or alumina, when contained in bodies or in soils, gives to them a smooth feel, 

 which covers the trritty feel of silex. Such soils exhale the peculiar odor called argilla- 

 ceous, when they are breathed upon. The excess or deficiency of alumina is indicated 

 where a soil is wet, and it is capable of being rolled or kneaded ; when there is a de- 

 ficiency of alumina, the soil falls to pieces by its own weight. 



29* 



