BACTERIOLOGICAL STUDY OF WATER. 589 



weeks after the pollution probably occurred. These 

 intervals of time are ordinarily sufficient for the deli- 

 cate, non-resistant bacillus of typhoid fever to succumb 

 to the unfavorable conditions under which it finds itself 

 in water. By unfavorable conditions are meant the 

 absence of suitable nutrition ; unfavorable temperature ; 

 probably the antagonistic influence of more hardy 

 saprophytic bacteria, particularly the so-called " water- 

 bacteria," and of more highly organized water-plants ; 

 the effect of precipitation and of sedimentation ; and, 

 of great importance, the disinfecting action of direct 

 sunlight. 



Though the positive demonstration of typhoid bacilli 

 in drinking-water by bacteriological methods is of ex- 

 treme rarity, it must not be concluded that bacteriological 

 analyses of suspicious waters shed no light upon the exist- 

 ence of pollution and the suitability or non-suitability 

 of the water for drinking-purposes. 



In the normal intestinal tract of human beings and 

 domestic animals, as well as associated with the specific 

 disease-producing bacillus in the intestines of typhoid- 

 fever patients, is an organism that is frequently found 

 in polluted drinking-waters, and whose presence is in- 

 dicative of pollution by either normal or diseased intesti- 

 nal contents ; and though efforts may result in failure 

 to detect the specific bacillus of typhoid-fever, the find- 

 ing of the other organism, bacillus coli, justifies one in 

 concluding that the water under consideration has been 

 polluted by intestinal evacuations from either human 

 beings or animals. Waters so exposed as to be liable to 

 such pollution should never be considered as other than 

 a continuous source of danger to those using them, 



