ecause of the
lack of proper food supply and oxygen. The fertility of the soil is
closely associated with their presence.

The bacteria are found in the air because of their development in
the soil below. They are unable to grow even in a moist atmosphere,
but are so readily dislodged by wind currents from the soil that
over land areas the lower strata of the air always contain them.
They are more numerous in summer than in winter; city air contains
larger numbers than country air. Wherever dried fecal matter is
present, as in barns, the air contains many forms.

Water generally contains enough organic matter in solution, so that
certain types of bacterial life find favorable growth conditions.
Water in contact with the soil surface takes up many impurities, and
is of necessity rich in bacteria. As the rain water percolates into
the soil, it loses its germ content, so that the normal ground
water, like the deeper soil layers, contains practically no
bacterial life. Springs, therefore, are relatively deficient in germ
life, except as they become contaminated with soil organisms, as the
water issues from the ground. Wells vary in their germ content,
depending upon manner of construction, ease of contamination at
surface, etc. Wells are too frequently insufficiently protected from
surface leachings, and consequently may contain all kinds of
organisms found in the surface soil. Typhoid fever is very
frequently disseminated in this way, as is cholera and a number of
animal maladies.

While the inner tissues of healthy animals are free from bacteria,
the natural passages, as the respiratory and digestive tracts, being
in more direct contact with the exterior, become readily infected.
This is particularly true with reference to the intestinal tract,
and in the undigested residue of the food, bacterial activity is at
a maximum. The result is that fecal matter of all kinds contains
enormous numbers of organisms so that the pollution of any food
medium, such as milk, with such material is sure to introduce
elements that seriously affect its quality.

CHAPTER II.

METHODS OF STUDYING BACTERIA.

=Necessity of artificial cultivation.= The bacteria are so extremely
small, that it is impossible to study individual germs separately
without the aid of powerful microscopes. Little advance was made in
the knowledge of these lower forms of plant life until the
introduction of culture methods, whereby a single organism coul