clay soil, it will take decades for it to
disappear. The vegetable matter in soil is usually produced by the decay
of plants which have either grown on the soil or have been washed down
into its voids. A great deal was formerly written on the relation
between this organic matter and the prevalence of malaria, and some
earlier writers believed that the amount of malaria in a district was
dependent upon the amount of vegetable debris in the soil. Since we have
learned that malaria is carried by mosquitoes, we are less interested in
the amount of organic matter in the soil. Its mere presence is not
likely to be injurious.

_Water in the soil._

Only the hardest rocks are entirely solid, the others containing a
certain percentage of voids or interstices. These voids are filled with
air or water, as the case may be, and we may stop for a moment to
inquire the effect of the presence of this air and water. In loose sands
the amount of voids is 40 to 50 per cent of the total volume, in
sandstone about 20 per cent, and in other rock reduced amounts. The
volume of air, therefore, in the soil under a cellar to a depth of four
or five feet, amounts to a good many cubic feet and would not be worth
inquiring into except for the fact that it is continually in a state of
motion. When the ground water, perhaps normally five feet below the
cellar bottom, rises in the spring, this ground air is forced out, and
in a cellar without a concrete foundation it rises into the cellar and
penetrates into the house.

A house artificially warmed by stoves is continually discharging heated
air from the tops of the rooms and colder air is being brought in from
below to take its place. This air comes from the ground below, and in
open soil may come from a great depth. A case has been noted where gas
escaping from a main in a city street twenty feet from a cellar wall
was, by the suction due to heat, drawn into the cellar and thence into
the rooms of the house. It is possible that air from cesspools and
broken drains in the vicinity of a house may, in this same way,
contribute to the atmosphere breathed within the walls of the house.
Gravelly and sandy soils, therefore, in order to maintain the
superiority which they furnish for building construction, should not be
polluted, since any pollution in the vicinity influences the quality of
air which may get into the house. The method of preventing such ingress
is plainly to water-proof the outside walls