PURIFICATION OF WATER FOR DRINKING PURPOSES 141 



tainly have contained much more than 676 micro- 

 organisms per cubic centimetre. 



Summary. The results arrived at in this further 

 investigation may be summarised as follows : 



(1) That there is a most striking reduction in the 

 number of suspended micro-organisms during storage 

 in large reservoirs. The strongest evidence of this was 

 afforded in the case of the large reservoir of the Grand 

 Junction Company at Hampton, the storage reservoirs 

 of the West Middlesex Company at Barnes, and those 

 of the New Eiver Company at Stoke Newington. 



(2) The most searching examination which has yet 

 been instituted into the efficiency of the filtration carried 

 out by the London water companies shows that out of 

 61 samples of filtered water collected at the several 

 works only one sample yielded more than 100 colonies 

 per cubic centimetre, whilst the average number was 

 only 29. 



(3) The average numbers in the filtered waters of 

 the several companies were as follows : 



New River (excluding one exceptional sample) . 13 



Chelsea 17 



East London ....... IB 



West Middlesex 25 



Lambeth ........ 27 



Southwark 35 



Grand Junction (excluding the samples from the 



general filter weU) 21 



These numbers show how very uniform, from a 

 bacteriological point of view, was the filtered water 

 produced by the several companies. 



(4) In almost every case the number of micro- 

 organisms in the filtered water appeared to be inde- 

 pendent of the age of the filter bed, but only on two 

 occasions (one at the New Eiver, and one at the South- 



