ABSORPTIVE POWER OF THE SOIL. 359 
form. Dung liquor, rich in carbonate of potash, on the 
other hand, would dissolve silica from the soil, 
Sulphuric acid, existing in considerable quantities in 
dung liquor as a readily soluble salt of ammonia or potash, 
would be partially retained by a soil rich in carbonate of 
lime by conversion into sulphate of lime, which is com- 
paratively insoluble. 
Absorption of Bases, from their Hydrates, Carbonates 
and Silicates.—1. Incidentally it has been remarked that 
free bases, among which ammonia, potash, soda and lime 
are specially implied, may be retained by combining with 
undissolved silica. Potash, soda (and ammonia?) may 
at once form insoluble compounds if the silica be in large 
proportion ; otherwise they may produce soluble silicates, 
which, however, in contact with lime, magnesia, alumina 
or iron salts, will yield insoluble combinations. As is 
well proved, gelatinous silica and lime at once form a . 
nearly insoluble compound. It is probable that gelatinous 
silica may remove magnesia from solution of its bicarbon- 
ate, forming a nearly insoluble silicate of magnesia. 
2. It has long been known that hydrated owide of iron 
and hydrated alumina may unite with and retain free 
ammonia, potash, etc. Rautenberg experimented with 
both these substances as freshly prepared by artificial 
means, and found that, under similar conditions, 
10 grms. of hydrated 10 grms. of hydrated 
~ oxide of i iron, alumina. 
Avarbed of free ammonia 0.046 grm. 0.066 grm. 
& “ free potash - 0.147 “ not det. 
Long continued washing with water removes the alkali 
from these combinations. That oxide of iron and alumina 
commonly occur in the soil in quantity sufficient to have 
appreciable effect in absorbing free alkalies is extremely 
improbable. 
Liebig has shown (Ann. Ch. u. Ph. 105, p. 122,) ‘that 
hydrated alumina unites with silicate of potash with great 
