DISTRIBUTION OF BACTERIA. 
13 
The distribution of bacteria in nature is prac¬ 
tically universal. They are found in the soil, in the 
air, in the food we eat, and in the water we drink. In 
fact, wherever plants and animals live, bacteria are 
found. Their distribution, however, is not equal. The 
soil is the chief home of bacteria on account of the 
large amount of animal matter in it. They are pres¬ 
ent in greatest number at the surface and diminish in 
the deeper layers. The reason for this is that the 
closely packed particles of the soil will not permit the 
bacteria to penetrate beyond the superficial layers. 
Surface water which contains bacteria in great number 
is rendered practically free from them by this filtering 
action of the soil. 
In the air the number of bacteria is directly pro¬ 
portional to the amount of dust. When the wind 
blows the dust into the air, large numbers of bacteria 
are carried with it; but when the air is quiet, the bac¬ 
teria by force of gravity settle to the ground. It is a 
well-known fact that bacteria will not leave a moist 
surface; so in wet weather the number of bacteria in 
the air is considerably less than at other times. At 
high altitudes and far out at sea there are practically 
no bacteria in the air, as there is no dust. Many bac¬ 
teria in the soil and air do not exist as a rule in their 
true form, but as spores which develop into bacteria 
when the conditions for growth become favorable. 
Water as it leaves the clouds in the form of rain 
is free from bacteria, but as the rain-drops approach 
the earth particles of dust adhere to them. After the 
