WATER. 399 



Sources and Mode of Water-supply. In the practical 

 application of bacteriologic examination of water for the 

 construction of conduits, wells, etc., it is, in the first place, 

 important to determine whether the water used contains 

 pathogenic bacteria or not. If a given water contains 

 many putrefactive bacteria (proteus) or the bacterium coli 

 commune, it should be entirely excluded from use ; for 

 such conditions indicate with certainty that the source of 

 supply is polluted. Formerly, great weight was attached 

 to the number of germs present. Some difficulty was en- 

 countered in this connection in the establishment of a limit, 

 beyond which water should no longer be considered pure. 

 Some observers made this limit one hundred, others fifty, 

 and still others five hundred. The absolute number of 

 germs contained in a given water depends, however, upon 

 such varied factors that an opinion based solely upon the 

 number of colonies formed is not to be depended upon. It 

 was then proposed to attach the greatest importance to the 

 number of different varieties present. A large number of 

 different species, it was thought, would, to a certain degree, 

 be an indication of the greater probability of the occur- 

 rence of contamination of the water. This belief is not 

 without justification, but too much significance must not be 

 attached to the number of different varieties of bacteria in 

 water. The fact can not be evaded that bacteriology has 

 thus far not attained the importance in the hygienic deter- 

 mination of the usefulness of water that was originally 

 attributed to it. 



As a central source of water-supply for large communi- 

 ties bacteria-free spring-water or subsoil-water is most to be 

 recommended, when this is obtainable in at all sufficient quan- 

 tities. In the selection of a source of supply care should 

 be taken to avoid undue proximity to communities or 

 manured land. If spring-water or subsoil-water is not 

 available, there is no alternative but to employ river-water 

 or lake-water, which naturally must be obtained from a 

 point above the city to be supplied. Such water is, as has 

 been pointed out, exposed to numerous sources of contami- 

 nation, and must, therefore, unconditionally be subjected to 

 some process of filtration before being used. This maybe 

 effected by means of sand-filtration, which is carried out in 

 large, cemented, covered reservoirs. The utility of sand-fil- 

 ters has been studied in recent years with especial care by 



