BACTERIOLOGICAL EXAMINATION. 117 



not mixing perfectly with the water of the river for a dis- 

 tance 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 Illinois, 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 

 fluctuations in bacterial numbers renders the bacterio- 

 logical methods invaluable for the daily supervision of 

 surface supplies or of the effluents from municipal filtra- 

 tion plants. 



In the commoner case, when normal values obtained 

 by such routine analyses are not at hand, the problem of 

 the interpretation of any sanitary analysis is a more diffi- 

 cult one. The conditions which surround a source of 

 water-supply may be constantly changing. No engineer 

 can measure the flow of a stream in July and deduce the 

 amount of water which will pass in February; yet the 

 July gauging has its own value and significance. So a 

 single analysis of any sort is not sufficient for all past and 

 future time. If it gives a correct picture of the hygienic 

 condition of the water at the moment of examination it 

 has fulfilled its task, and this the bacteriological analysis 

 can do. The evidence furnished by inspection and by 

 chemical analysis should be sought for and welcomed 

 whenever it can be obtained, yet we are of the opinion 



