46 AMERICAN MANURES. 



when amorphous, it is white, tasteless, and gritty, 

 as in flint, sand, etc. It is insoluble in water, 

 and in acids, hydrofluoric excepted. It enters 

 largely into the composition of glass, porcelain, 

 etc.; and we can safely say that the former 

 contains over half its weight of this acid. 



Silica, when chemically united with water, 

 forms a transparent jelly, known as hydrated or 

 soluble silica. This is soluble in water to a 

 certain extent, and in acids, even in the feeble 

 carbonic. From it, and from alkaline silicates, 

 plants obtain their silica, This silica is prepared 

 for the use of the plant by natural agencies, 

 somewhat in the following manner. Suppose 

 we allow carbonic acid and water to act on a 

 combination of lime, potash, and silica, what 

 takes place? The lime and potash combine 

 with the carbonic acid to form carbonates of 

 these substances, and the silica combining with 

 water becomes plant-food. A long time, how- 

 ever, is required to effect this change, unless the 

 alkaline silicate is in a very minute state of 

 division. 



The part taken by silica in natural operations is 

 chiefly a mechanical one, for which its abundance 

 and stability under ordinary circumstances pe- 

 culiarly fits it ; for it is found to constitute the 

 great bulk of the soil, and serves as a support 

 for the plant, and a reservoir for its food. 



