422 BACTERIOLOGT. 



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 analy- 

 ses 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 intes- 

 tines 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 diseased intestinal contents ; and 

 though efforts may result in failure to detect the specific 

 bacillus of typhoid fever, the finding of the other 

 organism, the baderium coli commune, justifies one 

 in expressing the opinion that the water under con- 

 sideration has been polluted by intestinal 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 \n connection with 

 the value of chlorine as indicative of contamination by 

 human excrement. It is commonly taught that an 

 excessive amount of chlorine in water points to con- 

 tamination by human excreta. This may or may not 

 be true according to circumstances. A high propor- 

 tion of this substance in a sample of water from a 

 locality the neighboring waters of which are poor in 

 chlorine, is unquestionably a suspicious indication, but 



