I/O AGRICULTURAL BACTERIOLOGY. 



about as pure, so far as concerns bacteria, as it was before such 

 contamination. For example : the city of Paris pours its se-w- 

 age into the river Seine in great quantity. But below the city 

 the water gradually becomes purified until finally it is as free from 

 bacteria as it is above the city. Recently the city of Chicago 

 has turned the little Illinois river into an immense drainage 

 canal for the large amount of sewage of that city. This river 

 is a small one and flows very slowly. It finally empties into 

 the Mississippi river after flowing for some 300 miles. It 

 empties a few miles above the point where St. Louis takes its 

 water supply and has naturally excited considerable alarm in 

 the latter city. A careful examination of the bacteria in the 

 river shows that there is a constant decrease in number as the 

 distance from Chicago is increased, and, when it finally empties 

 into the Mississippi, this river has no more bacteria than have 

 the waters of the smaller rivers flowing into it which are not 

 sewage-contaminated. In this flow the river has apparently 

 purified itself of sewage bacteria. 



How this self-purification of streams is brought about is 

 only in part understood. There appear to be several factors 

 concerned in the phenomenon, each of which plays a part. 

 But how great a share each factor has can as yet hardly be 

 stated. Evidently the phenomenon is practically identical with 

 the bacterial purification of sewage already referred to, modi- 

 fied by the different conditions. 



The disappearance of the bacteria has been the subject of 

 investigation and speculation by bacteriologists and the follow- 

 ing factors have been advanced as explaining it : 



1. The agitation of the water and the aeration it receives in 

 its flow. This factor is manifestly confined to swiftly flowing 

 streams and can hardly be of any significance in sluggish 

 rivers, like the Illinois. 



2. The dilution of the water by tributary streams. This 

 doubtless accounts, in part, for the decrease in number of bac- 



