The Bacteria in Natural Waters. 



9 



exerts an opposite effect, and after the main impurities 

 which can be washed away have been removed, may 

 dilute the stream with water purer than itself. In the 

 Kennebec River at WaterviUe, for example, A^Tiipple 

 (1907), foimd that bacterial counts were highest at the 

 times of largest stream flow. What the net effect of rain 

 may be depends on the character of the stream. A river 

 of fairly good quality shows its highest numbers in rainy 

 periods. With a highly polluted stream, on the other 

 hand, the constant influx of sewage overbalances occa- 

 sional contributions of surface contamination. Thus Gage 

 (1906) shows in the following table that the bacterial con- 

 tent of the Merrimac is highest when the stream is lowest, 

 that is when its sewage content is least subject to dilution. 



RELATION BETWEEN VOLUME OF FLOW AND BAC- 

 TERIAL CONTENT IN THE MERRIMAC RIVER. 

 (GAGE, 1906.) 



The contrast between the two classes of rivers is well 

 brought out in a study of the Lahn and the Wieseck, by 

 Kisskalt (1906); and the table below, compiled from his 

 data, gives an excellent idea of the total numbers of bac- 



