PURIFICATION OF WATER FOR DRINKING PURPOSES 187 



sedimentation, which the following experiments, 1 quite 

 recently carried out by one of us, demonstrate as taking 

 place under such circumstances. Samples of water 

 were taken from the Eiver Thames at Hampton, where 

 the intake of the Grand Junction Company is situated ; 

 secondly, from the first small storage reservoir nearest 

 the river ; thirdly, from the second small storage reser- 

 voir further from the river ; and fourthly, from the 

 large storage reservoir, the greater part of the water in 

 which had been stored for six months, and none for less 

 than one month. 



The following results were obtained : 



Number of Colonies 

 obtained from 

 1 c.c. of water 



1. Intake from Thames, June 25, 1892 . . . 1,991 



2. First small storage reservoir .... 1,703 



3. Second small storage reservoir .... 1,156 

 4 ( Large storage reservoir, (a) leeward side . 464 



( (6) windward side . 368 



These figures bring out most strikingly the diminu- 

 tion in the number of suspended bacteria which is 

 effected by storage. In the small storage reservoirs 

 referred to above, the capacity is so limited that the 

 water has but little chance of depositing anything, and 

 we find accordingly the number of micro-organisms to 

 be practically identical with that in the river-water 

 feeding them, although there is a slight reduction per- 

 ceptible. On the other hand, in the large storage reser- 

 voir the prolonged subsidence has brought about a most 

 remarkable diminution in the number. There was a 

 strong wind on the day the experiments were made, 

 the surface of the water being considerably disturbed. 

 It was anticipated, therefore, that the surface water 

 on the lee side would probably contain more micro- 



1 ' Reinigung des Wassers durch Sedimentirung,' Percy Frankland. 

 Centralbl f. Bakter. xiii. p. 122, 1893. 



