STUDY OF WATER. 419 



question, the evidence is greatly in favor of the opinion 

 that polluted drinking-water is primarily the under- 

 lying 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 of bacteria occur in 

 these waters, and if so, to what they are 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 

 thfe 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 production of diseases 

 in the intestines. 



In considering water from a bacteriological stand- 

 point, it must always be borne in mind that comparisons 

 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 composition, 

 without being unfit for use, so may the number of 

 bacteria per volume in water from one source be always 



