[Sidenote: Permeability depends first on the size of the pores, and
secondly on the pore-volume.]
The permeability of a soil for the passage of air and water depends,
in the first instance, on the size of the pores, and is further to
some extent influenced by the pore-volume.
[Sidenote: Effect of large and small pores.]
Soil with large pores will offer but little resistance to the passage
of air and water, but when the pores are small these movements will be
greatly impeded.
[Sidenote: Permeability is proportional to the fourth power of the
pore-diameter.]
It has been ascertained that the permeability of soils is proportional
to the fourth power of the diameter of the pores, so that it decreases
very rapidly with the diminishing size of the pores.
[Sidenote: In frozen soil permeability decreases rapidly.]
In subsoil with small pores all movements of air practically cease
when it is half full of water, and in frozen soil the decrease of the
permeability is still more marked.