206 GEOLOGY AND CHEMISTRY OF SOILS. 



capable of sustaining a healthy vegetation, with the exception, 

 perhaps, of limited portions of calcareous and peaty soils, in 

 which the proportion is much less, but generally greater than 

 66 parts in 100. 



The properties of silica render it well fitted to form so large 

 a portion of the soil. It is nearly insoluble in water, and 

 hence is not liable to be washed away by rains. In fact it is 

 not dissolved by any acid found in the soil, unless it be the 

 crenic and hydrofluoric, in which state it may be introduced 

 into the organs of plants. Silica is not an acid, by the chemi- 

 cal tests, because of its insolubility. It however combines 

 with the alkaline bases, with the earths and metallic oxides, 

 and is the most powerful electro-negative element in the com- 

 position of the soil. It acts in the soil as an acid, and bal- 

 ances, by its negative character, almost the entire mass of the 

 electro-positive earths, alkalies and metallic oxides. Its power 

 of absorbing and retaining water, is very slight, and hence 

 when it is the principal ingredient of a soil, it imparts to it a 

 porous, dry and light character. The relations of silica 

 to vegetation are highly interesting. It is almost the only 

 ingredient of soils which gives to them the property of per- 

 mitting the roots of plants to extend themselves in all direc- 

 tions, and forms as we have seen, p. 175, a part of the vege- 

 table structure. Silica thus furnishes the principal support 

 to the cultivated grains and grasses, and defends them from 

 the action of atmospherical and other agents. 



2. Aluminous earths. Alumina, a sesquioxide of alumina, 

 is composed, as we have seen p. 175, of 27.4 parts by weight 

 of the metal aluminium, and 24 parts of oxygen. It is found 

 in every region of the globe, and in the rocks of all ages. It 

 results from the decomposition of the feldspathic minerals or 

 argillaceous rocks. The different kinds of clay of which 

 bricks, pipes and earthen ware are made, consist of hiclratc 

 of alumina, that is, of alumina combined with water, and of 

 a small portion of silica. Aluminous earth is next to silica 



