DISTRIBUTION OF BACTERIA. 13 



tically universal. They are found in the soil, in the Distn- 



<f bution 



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, being 

 more numerous in some' places than others. The soil 

 is the chief home of bacteria on account of the large 

 amount of animal matter in it. They are present 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- AIr 

 portional to the amount of dust. When the wind 

 blows the dust into the air, large numbers of bacteria 

 are carried with it ; but wheni 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. The bacteria 

 in the soil and: air do not exist as a rule in their tru 

 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 water 

 is free from bacteria, but as the rain-drops approach 

 the earth particles of dust adhere to them!. After the 



