ABSOEPTIVE POWER OF THE SOIL. 353 



moved the acid,) tlie gelatinous precipitate, consisting 

 chiefly of free silica, free oxide of iron, free alumina, with 

 smaller quantities of lime and magnesia, contained never- 

 theless a portion of silica, and of these bases in combina- 

 tion, because it exhibited absorbent power for bases, like 

 Way's artificial silicates and like orilhiary soil. Mere 

 contact of soluble silica or silicates, with finely divided 

 bases, for a short time, is thus proved to be sufficient for 

 chemical union to take place between them. 



Recently precipitated silicic acid being added to lime- 

 Avater, unites with and almost completely removes the lime 

 from solution. The small portion of lime that remains 

 in the liquid is combined with silica, the silicate not being 

 entirely insoluble. (Gadolin, cited in Storer's Diet, of 

 Solubilities, p. 5.51.) 



The fict that free bases, as ammonia, potash and lime, 

 are absorbed by and fixed in soils or clays that contain no 

 organic adds, and to a degree different, usually greater th.in, 

 when presented in combination, would indicate that they 

 directly nnite either with free silica or with simple sili- 

 cates. The hydrated oxide of iron and alumina are in- 

 deed, under certain conditions, capable of retaining free 

 alkalies, but only in minute quantities. (See p. 359.) 



The fact that an admixture of cai'b onate of lime, or of 

 other lime-salts with the soil, usually enhances its absorbent 

 ])Ower, is not improbably due, as liautenberg first suggest- 

 ed, to the formation of silicates. 



A multitude of additional considerations from the his- 

 tory of silicates, especially from the chemistry of hydraulic 

 cements and fiom geological metamorphism, might be 

 adduced, were it needful to fortify our position. 



Enough has been written, however, to make evident 

 that silica, which is, so to speak, ;m accident in the plant, 

 being unessential (we will not affirm useless) as one of its 

 ingredients, is on account of its extraordinary capacity for 

 chemical union with other bodies in a great variety of 



