ELEMENTS OF WATER BACTERIOLOGY. 



CHAPTER I. 



THE BACTERIA IN NATURAL WATERS. 



Bacteria are the most numerous and the most widely 

 distributed of living things. They are present not merely 

 at the surface of the earth or in the bodies of water which 

 partially cover it, as is the case with most other living 

 things, but in the soil itself, and in the air above, and in 

 the waters under the earth. 



Probably no organisms are more sensitive to external 

 conditions, and none respond more quickly to slight 

 changes in their environment. Temperature, moisture, 

 and oxygen are of importance in controlling their distri- 

 bution; but the most significant factor is generally the 

 amount of food supply. Bacteria and decomposing or- 

 ganic matter are always associated, and for this reason 

 a brief consideration of the general relation of bacteria 

 to their sources of food supply and to other forms of 

 organic life must precede the study of their distribution 

 in any special medium. 



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