ABSORPTIVE POWER OF THE SOIL. 335 
the soil just as a mass of clean wool separates indigo from 
the liquor of a dye-vat, or as bone-charcoal removes the 
brown color from syrup. 
Chemical absorptions depend upon the formation of 
new compounds, and in many cases occasion chemical 
decompositions and displacements in such a manner that 
while one ingredient is absorbed, and becomes in a sense 
fixed, another is released from combination and becomes 
soluble. Brief notice has already been made of the 
chemical absorption of ammonia by the soil (p. 243). 
We shall now enter upon a fuller discussion of this and 
allied phenomena. 
When solutions of the various soluble acids and bases 
existing in the soil, or of their salts, are put in contact 
with any ordinary earth for a short time, suitable exami- 
nation proves that in most cases a chemical change takes 
place,—a reaction occurs between the soil and the 
substance. 
If we provide a number of tall, narrow lamp-chimneys 
or similar tubes of glass, place on the flanged end of each 
a disk of cotton-batting, tying over it a piece of muslin, 
then support them vertically in clamps or by strings, and 
fill each of them compactly, two-thirds full of ordinary 
loamy soil, which should be free from lumps, we have an 
arrangement suitable for the study of the absorptive 
power in question. 
Let now solutions, containing various soluble salts 
of the acids and bases existing in the soil, be pre- 
pared. These solutions should be quite dilute, but 
still admit of ready identification by their taste or by 
simple tests. We may employ, for example, any or all of 
the following compounds, viz., saltpeter, common salt, sul- 
phate of magnesia, phosphate of soda, and silicate of soda. 
If we pour solution of saltpeter on the soil, which 
should admit of its ready but not too rapid percolation, 
we shall find that the first portions of liquid which pass 
