BACTERIAL CONTENT OF WATER 699 



is remarkably free from organisms. The following table 

 illustrates the number of organisms that may be met with 

 in water from different sources : 



Number of organisms 

 Source. per cubic centimetre. 



Freshly fallen snow . . . 34-38 



Ice ...... (very variable) 30-1,700 



Rain water (Paris) . . . 4-5 

 Rhone, above Lyons ... 75 

 Rhone, below Lyons . . . 800 

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

 Thames, at Hampton (Frankland) . (variable) 2,000-90,000 

 Deep well in the chalk (Kent Com- 

 pany) ..... 3-19 



Surface well .... 1,200 



Spring water, Reigate (Frankland) . 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, particularly the rainfall. 



As regards seasonal variation the figures for raw Thames 

 water at Hampton may be quoted (the year being April- 

 March inclusive). The average gelatin count for 1915-16 

 was 10,315 per c.c. ; for 1918-19, 11,449 ; and for 1919-20, 

 3,506. For 1919-20, the minimum count was 1,005 in 

 October, 1919, and the maximum 7,900 in December, 1919. 

 In June, 1915, when there was an excessive rainfall, 

 the number was 17,724. By various methods of treat- 

 ment of a raw water, the number of organisms may be 

 reduced : 



(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, 



