BACTERIOLOGICAL STUDY OF WATER. 485 



As a result of many observations on both sides of 

 the question, the evidence is greatly in favor of the 

 opinion that polluted water is primarily the underlying 

 cause of these epidemics, and this too, very often, when 

 the state of the soil-water, in the light of the "ground- 

 water" hypothesis, is just the reverse of what it should 

 be in order to render it answerable for them. It is 

 manifest, therefore, that the careful bacteriological study 

 of water intended for domestic use is of the greatest 

 importance, and should be a routine procedure in all 

 communities receiving their water-supply from sources 

 that are liable to pollution. 



The object aimed at in such investigations should be 

 to determine if the water approaches constancy in the 

 number and kind of bacteria contained in it — for all 

 waters, except deep ground- water, contain bacteria; if 

 sudden fluctuations in the number and kind of bacteria 

 occur in these waters, and if so, to what are they due; 

 and finally, and most important, does the water contain 

 constantly, or at irregular periods, bacteria that can be 

 traced to human excrement, not of necessity pathogenic 

 varieties, but bacteria that are known to be present 

 normally in the intestinal canal? For, if conditions 

 are favorable to the presence of these varieties, the same 

 conditions would favor the admission to the water of 

 other forms of bacteria that are concerned in the pro- 

 duction of diseases of the intestines. 



In considering water from a bacteriological stand- 

 point it must always be borne in mind that compari- 

 sons with any general fixed standard are not of much 

 value, for just as normal waters from different sources 

 are seen to present variations in their chemical compo- 

 sition, without being unfit for use, so may the relative 



