BACTERIAL CONTENT OF WATERS 573 



illustrates the number of organisms that may be met with 

 in water from different sources : 



Source Number of organisms 



per cubic centimetre. 



Freshly fallen snow . . . 34-38 



Ice (very variable) 30-1700 



Rain water (Paris) . . . 4-5 

 Rhone, above Lyons . . 75 

 Rhone, below Lyons . . 800 

 Rhine, at Miihlheim . . . average about 20,000 

 Thames, at Hampton (Frank- 

 land) (variable) 2000-90,000 



Deep well in the chalk (Kent 



Company) .... 3-19 

 Surface well .... 1200 

 Spring water, Reigate (Frank- 

 land) 8 



Lake of Lucerne . . . 8-50 



Loch Katrine (Frankland) . 74 

 Filtered water supplied to London 



(Houston) .... average rarely exceeds 100 



Sewage (Frankland) . . 26,000,000 



The number of bacteria in a natural water varies con- 

 siderably with its source, at different seasons, and under 

 different climatic conditions. The Table x on p. 575 

 illustrates the seasonal variation in certain raw London 

 waters. 



The following factors modify the number of organisms 

 present in the water : 



(1) Storage of unfiltered water. A large storage capacity 

 permits of the water being admitted when the source 

 (river, etc.) is in its best condition, so that foul water, in 

 flood time or drought, may be avoided. Moreover, storage 

 alone usually markedly diminishes the number of organisms, 

 partly by subsidence, partly by lack of aeration, and partly 

 probably owing to the struggle for existence going on 

 among them (see also p. 361). 



1 Houston, Seventh Ann. Rep. Hetropol. Water Board, 1913. 



