CHEMICAL ACTION IN THE SOIL. 331 
place, a mechanical analysis, as described on page 147, and 
subjecting the fine portion, which consists entirely or in 
great part of clay, to the action of these acids, the quan- 
tity of clay may be approximately estimated. Or, by 
melting the portion insoluble in acids with carbonate of 
soda, or acting upon it with hydrofluoric acid, the whole 
may be decomposed, and its elementary composition be 
ascertained by further analysis. 
Notwithstanding an immense amount of labor has been 
expended in studying the composition of soils, and chiefly 
in ascertaining what and how much, acids dissolve from 
them, we have, unfortunately, very few results in the way 
of general principles that are of application, either to a 
scientific or a practical purpose. In a number of special 
cases, however, these investigations have proved exceed- 
ingly instructive and useful. 
§ 5. 
REACTIONS BY WHICH THE SOLUBILITY OF THE ELEMENTS 
OF THE SOIL IS ALTERED. SOLVENT EFFECT OF 
VARIOUS SUBSTANCES THAT ARE COMMONLY 
BROUGHT TO ACT UPON SOILS. THE AB- 
SORPTIVE AND FIXING POWER OF SOILS. 
Chemical Action in the Soil.—Chemistry has proved 
that the soil is by no means the inert thing it appears to 
be. It is not a passive jumble of rock-dust, out of which 
air and water extract the food of vegetation. It is not 
simply a stage on which the plant performs the drama of 
growth. It is, on the contrary, in itself, the theater of 
ceaseless activities; the seat of perpetual and complicated 
changes. 
A large share of the rocks now accessible to our study 
at the earth’s surface have once been soil, or in the condi- 
tion of soil. Not only the immense masses of stratified 
limestones, sandstones, slates, and shales, that cover so 
