226 ELEMENTS OF WATER BACTERIOLOGY 



importance. In other words, when pollution is to be 

 avoided, because the decomposition of chemical sub- 

 stances causes a nuisance, it must be studied by chem- 

 ical methods. When the danger is sanitary and comes 

 only from the presence of bacteria, bacteriological 

 methods furnish the best index of pollution. 



In the study of certain special problems the para- 

 mount importance of bacteriology is generally recognized. 

 The distribution of sewage in large bodies of water 

 into which it has been discharged may thus best be 

 traced on account 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 Dis- 

 trict of Boston was studied in this way by E. P. Osgood 

 in 1897, and mapped 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, preserving 

 its own bacterial characteristics and not mixing per- 

 fectly with the water of the river for a distance of 

 more than 24 miles (Heider, 1893). Jordan (Jordan, 

 1900), in studying the self -purification of the sewage 

 discharged from the great Chicago drainage canal, 



