[Sidenote: General remarks on subsoil and its properties.]
Before dealing more in detail with the processes taking place in the
pores of the subsoil of sewage farms, it may not be out of place to
make here a few general observations on the mechanical structure of
soil, its permeability, water capacity, retentive power, the capillary
movements in the same, its temperature, the subsoil air, the movement
of water in and through the same, the micro-organic life in soil, and
its absorbing powers.