28 WATER 



is constantly fed by small streams containing soil washings, 

 drainage of cultivated and uncultivated land and often drainage 

 from human habitations. In this way vast numbers of bacteria 

 are being added to the water and if samples are collected at 

 favourable situations they show a great increase in the bacterial 

 content. Acting all the while, however, are the agencies men- 

 tioned above and making for the diminution of the bacteria 

 present, so that samples taken lower down the same stream 

 may show a much smaller number of bacteria with an absolute, 

 or at least a relative, diminution of the unsatisfactory organisms 

 possibly very prevalent higher up. 



Water, unless highly charged with organic matter or unless 

 other conditions are favourable, is not a medium which readily 

 allows bacterial multiplication. 



It will be shown later on that the numerical estimation of 

 certain special bacteria is of far greater importance than general 

 bacterial enumerations. The presence of these special bacteria 

 in water supplies is subject to the same influences as affect the 

 total number of bacteria. 



In view of the varying influences which affect the bacterial 

 content of water and which are at work unequally for different 

 classes of water it is essential in all cases to carefully consider 

 the nature of the water supply under investigation. 



Analysis of water supplies is undertaken for two purposes: 



(a) To obtain reliable information in regard to the fitness 

 of a given water supply for drinking purposes. 



(b) To judge the suitability of a supply for domestic or 

 medicinal purposes. 



Bacteriology plays no part in the second object, which has 

 to be judged on chemical grounds, but is of very great value 

 in ascertaining suitability for drinking purposes. 



From the bacteriological examination of water data can be 

 obtained which not only give information as to existing con- 

 ditions, but from which deductions can be drawn as to recent 

 harmful pollution. 



Bacteriological examinations are also very useful in connec- 

 tion with the testing . of the efficient working of sand and 

 other filters. 



