ABSORPTIVE POWER OF THE SOIL. 353 
moved the acid,) the 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 ordinary 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- 
water, 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 Dict. of 
Solubilities, p. 551.) 
The fact that free bases, as ammonia, potash and lime, 
are absorbed by and fixed in soils or clays that contain no 
organic acids, and to adegree different, usually greater than, 
when presented in combination, would indicate that they 
directly unite 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 carbonate of lime, or of 
other lime-salts with the soil, usually enhances its absorbent 
power, is not improbably due, as Rautenberg 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 from 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, an 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 
