BACTERIOLOGICAL STUDY OF WATER 609 



have been detected, still waters in which such pollutions 

 are possible are a constant menace to the health of those 

 who use them for domestic purposes. 



A sudden variation from the normal, mean number of 

 bacteria, or from the normal chemical composition of a 

 water, calls at once for a thorough inspection of the supply, 

 while at the same time the organisms present are to be sub- 

 jected to the most careful study. In many instances, even 

 after the most thorough bacteriological and chemical study 

 of a suspicious water, one is forced to admit that informa- 

 tion of but limited usefulness has been obtained through 

 the employment of such analytical methods. In these 

 cases too much stress cannot be laid upon the importance 

 of a systematic inspection of the supply, and its relation 

 to soiu'ces of pollution. Optical evidence of more or less 

 dangerous contamination may often be obtained when 

 laboratory methods fail to detect them. The reasons for 

 such failure, in addition to those already given, are obvious — 

 the polluting matters are often so diluted by the large 

 mass of water into which they find their way as to be beyond 

 recognition by the tests usually employed in such work, 

 and still be present in amounts sufficient to originate 

 disease. 



The Qualitative Bacteriological Analysis of Water. — The 

 qualitative bacteriological analysis of water entails much 

 labor, as it requires not only that all the different species 

 of organisms found in the water should be isolated, but 

 that each representative should be subjected to systematic 

 study, and its pathogenic or non-pathogenic properties 

 determined . 



For this purpose a knowledge of the methods for the 

 isolation of indi^-idual species which have already been 

 39 



