EXAMINATION OF WATER. 425 



given volume, the more likely is it .to be free from in- 

 jurious forms. On the other hand, if the number of bac- 

 teria is subject to considerable fluctuation and is usually^ 

 high, it indicates conditions favorable to decomposition. 

 The bacteria in a given specimen of water may be ex- 

 tremely numerous, and yet they may be mere harmless 

 saprophytes. The fact that such water is a good nutrient 

 medium should make it suspicious inasmuch as intestinal 

 bacteria, if once introduced, will be favored in like manner. 

 They may indeed be present but, masked by the large 

 number of common water bacteria, they may escape detec- 

 tion. 



The chief interest is attached to the kind of bacteria 

 present. The recognition of certain well known intestinal 

 bacteria in a given water may be taken as a good indica- 

 tion of the existence of pollution by animal excreta. Thus, 

 the Bacillus coli communis is invariably present in the in- 

 testinal contents of man and animals, and hence its detec- 

 tion in water, especially in appreciable numbers, points to 

 the source of the contamination. The fact, however, 

 should not be overlooked that this or related species are 

 widely distributed in nature. It makes its appearance in 

 the intestines of the new-born within a few hours after 

 birth and this fact in itself indicates the wide prevalence 

 of the colon bacillus. Consequently, the detection of the 

 colon bacillus in a water should arouse suspicion and cause 

 a thorough examination of the premises with reference to 

 the close presence of polluting material. The finding of 

 the colon bacillus, therefore, does not indicate more than 

 the possibility of the existence of pollution, which, as a rule, 

 will also be indicated by a chemical examination. 3D 



The recognition of a specific pathogenic germ in a 

 water is of prime importance inasmuch as it demonstrates 

 actual pollution as well as the possibility of infection. The 

 typhoid bacillus is the one most frequently looked for. 

 Under exceptional conditions the cholera, anthrax and 



