SOILS. 251 



ure and carbonic acid in the air, these compounds 

 are disintegrated, carbonate of potash or soda is 

 formed, and in part washed away bj the rain, whilst 

 the insoluble earthy bases are left, in the form of a 

 very line powder. 



652. The most abundant constituent of soil is com- 

 monly silica, which frequently forms nearly nine- 

 tenths of the whole of its weight ; but this is by no 

 means always the case, for in calcareous or limestone 

 countries, we frequently find soils containing a very 

 large quantity of lime ; whilst others, again, contain 

 a large proportion of alumina. These differences in 

 the proportion of the earthy components of the soil, 

 give rise to the varieties of light or free, and stiff or 

 clayey soils, which are also modified by the presence 

 of a greater or less quantity of organic substances. 



653. Silica and alumina, which are generally the 

 two principal constituents of soils, differ very greatly 

 in their respective uses. The former is of import- 

 ance, both chemically and mechanically. Chemically, 

 as forming a soluble compound with alkali, and thus 

 being absorbed by the roots of plants, and confer- 

 ring strength and stability to their structures; and 

 mechanically, by diminishing the extreme closeness 

 and tenacity of alumina, and thus, by making the 

 soil more porous and open, allowing the passage of 

 air and water through it, and enabling the roots of 

 plants more easily to penetrate it than they could 

 were it wholly composed of alumina. 



654. On the other hand, the use of alumina is 



