with regard to the mortar of the house. That degree of humidity of the soil is called ground-water; it begins at the lowest limit of the air in the soil.
It is well known that water becomes solid at a temperature below freezing-point. In becoming ice it changes its consistency totally, but its volume not very much, increasing it by about six per cent., one hundred volumes of water becoming one hundred and six of ice. In a frozen soil there must have been a certain quantity of water. This water in freezing has become a kind of cement for the particles of the soil, and gives it a solidity which the liquid water could not
Fig. 1.
impart. Although such frozen soil is as hard to work as stone, we have no right to assume that it is impermeable to air or gases of any kind.
Those pores of the soil which were free from water cannot be narrowed much by the expansion of the neighboring pores through the
