488 BACTERIOLOGY. 



larly the so-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 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 of 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 bacil- 

 lus of typhoid fever, the finding of the other organism, 

 the bacterium coli commune, justifies one in expressing 

 the opinion that the water under consideration 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 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 contam- 

 ination 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 

 neighboring waters of which are poor in chlorine, is 



