BACTERIOLOGICAL EXAMINATION 227 



found by bacteriological analyses that the Des Plaines 

 and the Kankakee Rivers could both be distinguished 

 flowing along in the bed of the Illinois, the two streams 

 being in contact, yet each maintaining its own indi- 

 viduality. Finally, the quickness with which slight 

 changes in the character of a water are marked by 

 fluctuations in bacterial numbers renders the bacteri- 

 ological methods invaluable for the daily supervision 

 of surface supplies or of the effluents from municipal 

 filtration 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 

 difficult one. The conditions which surround a source 

 of water supply may be constantly changing. No en- 

 gineer 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 fur- 

 nished by inspection and by chemical analysis should 

 be sought for and welcomed whenever it can be obtained, 

 yet we are of the opinion that, on account of their 

 directness, their delicacy, and their certainty, the 

 bacteriological methods should least of all be omitted. 



