n6 ELEMENTS OP WATER BACTERIOLOGY. 



In studying the heavy pollution of small streams, the 

 treatment of trades wastes, and the purification of sewage, 

 the relations of nitrogeneous compounds and of oxygen 

 compounds are of prime importance. In other words, 

 when pollution is to be avoided, because the decompo- 

 sition of chemical substances causes a nuisance, it must 

 be studied by chemical methods. When the danger is 

 sanitary and comes only from the presence of bacteria, 

 bacteriological methods furnish the true index of pollution. 

 In the study of certain special problems the paramount 

 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 District 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 



