ABSORPTIVE POWER OF THE SOIL. 349 
ide of iron which the soils yielded to hydrochloric acid. 
Heiden traced a similar relation between the silica set free 
by the action of acids on eleven soils and their absorptive 
power. Rautenberg and Heiden further confirmed what 
Way and Peters had previously shown, viz., that treat- 
ment of soil with acids diminished their absorbent power. 
These facts admit of interpretation as follows: Since 
neither silica, hydrated alumina, nor hydrated oxide of 
iron, as such, have any absorptive or decomposing power 
on sulphates, nitrates, etc., and since these bodies do not 
ordinarily exist as such to much extent in soils, therefore 
the connection found in twenty cases to subsist between 
their amount (soluble in acids) in the soil, and the ab- 
sorptive power of the latter points to a compound of 
these (and other) substances (silicate of alumina, iron, 
lime, etc.), as the absorptive agent. 
That the absorbing compound is not necessarily hydra- 
ted, is indicated by the fact that calcination, which must 
remove water, though it diminishes, does not always alto- 
gether destroy the absorptive quality of a soil. (See p. 
343.) Eichhorn, as already stated, found that the anhy- 
drous silicates, chlorite and labradorite, were acted upon 
by saline solutions, though but slowly. 
Do Zeolitic Silicates, hydrated or otherwise, exist 
in the Soil 2—When a soil which is free from carbonates 
and salts readily soluble in water, is treated with 
acetic, hydrochloric, or nitric acid, there is taken up a 
quantity (several per cent.) of matter which, while con- 
taining all the elements of the soil, consists chiefly of 
alumina and oxide of iron. Silica is not dissolved to much 
extent in the acid, but the soil which before treatment 
with acid contains but a minute amount of uncombined 
silica, afterwards yields to the proper solvent (hot solution 
of carbonate of soda) a considerable quantity. This is our 
best evidence of the presence in the soil of easily decom- 
