ne respect they are readily
differentiated, and that is, that practically all of them are capable of
producing their characteristic chemical transformations under
anaesthetic conditions, as in a saturated ether or chloroform
atmosphere.

~Distribution of bacteria.~ As bacteria possess greater powers of
resistance than most other forms of life, they are to be found more
widely distributed than any other type. At the surface of the earth,
where conditions permit of their growth, they are found everywhere,
except in the healthy tissues of animals and plants. In the superficial
soil layers, they exist in myriads, as here they have abundance of
nourishment. At the depth of several feet however, they diminish rapidly
in numbers, and in the deeper soil layers, from six to ten feet or more,
they are not present, because of the unsuitable growth conditions.

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 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 contains generally 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 microbes. 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
infected with soil organisms, as the water issues from the soil. Water
may serve to disseminate certain infectious diseases as typhoid fever
and cholera among human beings, 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 more readily infected. This is
particularly true with reference to the intestinal tract, for in the
undigested residue, bacterial activity is at a maximum. The result is
that fecal matter contains enormous numbers of organisms so that the
possibility of pollution of any food medium s