440 AGRICULTURAL AND INDUSTRIAL BACTERIOLOGY 



polluted at some portion of their course, will eventually 

 apparently regain their purity at some distance down 

 stream. The factors which have to do with such purification 

 have been carefully studied in connection with several of 

 the rivers and streams in this country. The most important 

 of the factors which have to do with self-purification are 

 as follows: 



First, the oxidation and removal of organic material 

 through the activity of the decay and putrefactive micro- 

 organisms. This occurs relatively rapidly, particularly if 

 the water in the streams is sufficiently agitated to receive an 

 abundant supply of oxygen. 



Second, antibiosis. Microorganisms which develop most 

 abundantly in the water of streams and sewage are more or 

 less unfavorable for the life, .certainly for the development, 

 of the pathogenic bacteria such as the Bacterium typhi. As 

 a result the latter organisms die off more or less rapidly. 

 This is an example of what has been termed antibiosis, the 

 antagonistic influence of one form of life upon another. 



Third, sedimentation. Microorganisms are constantly fall- 

 ing to the bottom of the stream and remaining there, 

 usually dying off rather rapidly. This sedimentation occurs 

 particularly rapidly when there is more or less sediment in 

 the stream which settles out. Bacteria show a decided ten- 

 dency to cling to the surface of solid particles in suspension. 

 As these settle out, the bacteria would of .course pass down 

 with them. 



Fourth, the antagonistic action of protozoa. It is an 

 easily demonstrated fact that many of the protozoa live 

 upon bacteria. A sample of sewage, for example, allowed 

 to stand for a few hours usually will be found to swarm 

 with protozoa feeding upon the bacteria. These protozoa 

 are quite abundant in the water of polluted streams and 

 assist materially in its purification. 



Fifth, filtration. The water in any stream constantly 



