BACTERIOLOGICAL STUDY OF WATER 443 



called " water bacteria," and of more highly organized 

 water plants ; the effect of mechanical precipitation ; and 

 of great importance, the disinfecting action of direct 

 sunlight. 



Though it is so rare as to be almost never, that 

 typhoid bacilli are found in drinking-water, it must, 

 nevertheless, not be supposed that bacteriological anal- 

 yses 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 all human beings, 

 and many other mammals, as well as associated with the 

 specific disease-producing bacterium in the intestines of 

 typhoid-fever patients, is an organism that is frequently 

 found in polluted drinking-waters, and whose presence 

 is proof positive of pollution by either normal or dis- 

 eased intestinal contents ; and though efforts may result 

 in failure to detect the specific bacillus of typhoid fever, 

 the finding of the other organism, the bacterium coli com- 

 mune, justifies one in expressing the opinion that the 

 water under consideration has been polluted by intes- 

 tinal evacuations from either human beings or animals. 

 Waters so located as to be liable to such pollution can 

 never be considered as other than a continuous source of 

 danger to those using them. 



Another point to be remembered is in connection with 

 the value of chlorine as indicative of contamination by 

 human excrement. It is commonly taught that an ex- 

 cessive amount of chlorine in water points to contamina- 

 tion by human excreta. This may or may not be true 

 according to circumstances. A high proportion of this 

 substance in a sample of water from a locality, the neigh- 

 boring waters of which are poor in chlorine, is unques- 



