BACTERIOLOGICAL STUDY OF WATER. 585 



presence of an excess of chlorine in water, while often 

 indicating pollution from human evacuations, may 

 nevertheless, sometimes arise from other sources; but 

 the presence in water of bacteria normally found in the 

 intestinal canal can manifestly admit of but one inter- 

 pretation, viz., that fsecal matters from either man or 

 animals have at some time been deposited in this water, 

 and that while no specific disease-producing organisms 

 may have been detected, still M-aters in wliich such pol- 

 lutions 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 cora2>osi- 

 tion of a water, calls at once for a thorough in- 

 spection of the sujiply, while at the same time the 

 organisms present are to be subjected to the most care- 

 ful study. In many instances, even after the most 

 thorough bacteriological and chemical study of a sus- 

 picious water, one is forced to admit that information 

 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 im- 

 portance of a systematic inspection of the supply, and 

 its relation to sources 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 w^ork, and still be present in 

 amounts sufficient to originate disease. 



The Qualitative Bacteriological Analysis 



