1 82 Elements of Water Bacteriology. 



of the ready response of the bacterial counts to slight 

 proportions of sewage, particularly since the ease and 

 rapidity with which the technique of plating can be carried 

 out make it possible to examine a large series of samples 

 with a minimum of time and trouble. The course of the 

 sewage carried out by the tide from the outlet of the 

 South Metropolitan District of Boston was studied in 

 this way by E. P. Osgood in 1897, and map^ped out by 

 its high bacterial content with greater accuracy than 

 could be attained by any other method. Some very 

 remarkable facts have been developed by similar studies 

 as to the persistence of separate streams of water in 

 immediate contact with each other. Heider showed that 

 the sewage of Vienna, after its discharge into the Danube 

 River, flowed along the right bank of the stream, pre- 

 serving its own bacterial characteristics and not mixing 

 perfectly with the water of the river for a dista;nce of 

 more than twenty-four miles (Heider, 1893). Jordan 

 (Jordan, 1900), in studying the self -purification of the 

 sewage discharged from the great Chicago drainage 

 canal, found by bacteriological analyses that the Des 

 Plaines and the Kankakee Rivers could both be distin- 

 guished flowing along in the bed of the IlUnois, the two 

 streams being in contact, yet each maintaining its own 

 individuality. Finally, the quickness with which slight 

 changes in the character of a water are marked by fluc- 

 tuations in bacterial numbers renders the bacteriological 

 methods invaluable for the daily supervision of surface 



