CHAPTER XII. 



IMPURE WATER AND ICE AS A SOURCE OF 

 BACTERIAL DISEASE. 



Impure Water. 



WE have seen in another part of this 

 book that natural surface waters always 

 contain considerable numbers of living bacteria 

 of various kinds, which are growing and pro- 

 liferating there, and no doubt actually purify- 

 ing the water in a certain way by feeding upon 

 and removing from it organic material which 

 has collected or been dissolved. Now these 

 bacteria in moderate numbers, under ordinary 

 conditions, are not at all harmful to the con- 

 sumer of the water for drinking or culinary 

 purposes. 



But, on the other hand, if the water 

 becomes stagnant and the ordinarily harm- 

 less bacteria collect in very large numbers, it 

 has been abundantly shown by most bitter 



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