352 HOW CROPS FEED. 



The water of the soil holds silica, lime, magnesia, alka» 

 lies, and oxide of iron, often alumina, in solution. In- 

 stances are numerous in Avhich the evaporation of water 

 containing dissolved salts lias left a solid residue of sili- 

 cates. Thus, Kersten has described {Jour, fur praki. 

 Chem., 22, 1) a hydrous silvate of iron and manganese 

 that occurred as a hard incrustation upon the rock, in one 

 of the Freiberg mines, and was deposited where the water 

 leaked from the pum2)s. Kersten and Berzelius have lio- 

 ticed in the evaporation of mineral waters which contain 

 carbonates of lime and magnesia, together with silica, that 

 carbonates of these bases are first deposited, and finally 

 silicates separate. {B's:hof^s Chem. Geolo^/y^ Car. Ed.^ 

 Vol. 1, p. 5). Bischof (loc. cit., p, 6) has found that silica, 

 even in its most inactive form of quartz, slowly decom- 

 poses carbonate of soda and potash, forming silicate when 

 boiled with their aqueous solutions. Undoubtedly, simple 

 contact at ordinary temperature has the same effect, 

 though more slowly and to a slight extent. 



Such facts make evident that silica, lime, the alkalies, 

 oxide of iron and alumina, when dissolved in water, if they 

 do not already exist in combination in the water, easily 

 combhie when adverse affinities do not jirevent, and may 

 react upon the ingredients of the soil, or upon rock dust, 

 with the formation of zeolites. 



The " pan," which often forms an impervious stratum 

 under peat bogs, though consisting largely of oxide of iron 

 combined with organic acids, likewise contains consider- 

 able quantities of hydrated silicates, as shown by the 

 analyses of Warnas and Michielsen {Mulder'' s Chem. d. 

 Ackerkrume, Bd. 1, p. 566.) 



Mulder found that when Portland cement (silicate of 

 lime, alumina, iron, cte.) was treated with strong hydro- 

 chloric, acid, whereby it was decomposed and in part dis- 

 solved, and then with ammonia, (which neutralized and re* 



