nges of the external temperature; also according to the character
of the soil, its color, composition, depth, degree of organic
oxidation, ground-water level, and degree of dampness. In hot weather
the surface soil is cooler, and the subsurface soil still more so,
than the surrounding air; in cold weather the opposite is the case.
The contact of the cool soil with the warm surface air on summer
evenings is what produces the condensation of air moisture which we
call dew.

=Bacteria.=--Quite a large number of bacteria are found in the soil,
especially near the surface, where chemical and organic changes are
most active. From 200,000 to 1,000,000 bacteria have been found in 1
c.c. of earth. The ground bacteria are divided into two
groups--saprophytic and pathogenic. The saprophytic bacteria are the
bacteria of decay, putrefaction, and fermentation. It is to their
benevolent action that vegetable and animal _debris_ is decomposed,
oxidized, and reduced to its elements. To these bacteria the soil owes
its self-purifying capacity and the faculty of disintegrating animal
and vegetable _debris_.

The pathogenic bacteria are either those formed during the process of
organic decay, and which, introduced into the human system, are
capable of producing various diseases, or those which become lodged in
the soil through the contamination of the latter by ground water and
air, and which find in the soil a favorable lodging ground, until
forced out of the soil by the movements of the ground water and air.

=Contamination of the Soil.=--The natural capacity of the soil to
decompose and reduce organic matter is sometimes taxed to its utmost
by the introduction into the soil of extraneous matters in quantities
which the soil is unable to oxidize in a given period. This is called
contamination or pollution of soil, and is due: (1) to surface
pollution by refuse, garbage, animal and human excreta; (2) to
interment of dead bodies of beasts and men; (3) to the introduction of
foreign deleterious gases, etc.[11]

_Pollution by Surface Refuse and Sewage._--This occurs where a large
number of people congregate, as in cities, towns, etc., and very
seriously contaminates the ground by the surcharge of the surface soil
with sewage matter, saturating the ground with it, polluting the
ground water from which the drinking water is derived, and increasing
the putrefactive changes taking place in the soil. Here the pathogenic
bacteria abound, and,