1 68 SCIENCE PROGRESS. 



These observations graphically represented in the dia- 

 gram show that, at the time they were commenced, the 

 filtering operations were carried on with considerable care- 

 lessness, and that this continued, though to a less extent, 

 down to the year 1883, since which time, and especially 

 since 1884, the efficiency of filtration of all the river waters 

 supplied to the Metropolis has left little to be desired. 



What is it then that separates the past from the present 

 water supply of London ? In the first place there is the 

 change of source — I mean the change in position of the 

 intakes of the several Companies drawing from the Thames 

 and Lea — and the total abandonment of the much-polluted 

 river Ravensbourne by the Kent Water Company. So 

 long as the water supply was derived from the tidal reaches 

 of the Thames and Lea, receiving as these reaches did the 

 drainage of immense populations, the risk of infection from 

 water-borne pathogenic organisms could scarcely be other- 

 wise than imminent ; for, although we now know efficient 

 filtration to be a perfect safeguard, anything short of effi- 

 ciency must be attended with risk in the presence of such 

 extreme pollution. 



Nevertheless, the line of demarcation between the past 

 and the present water supply of the Metropolis is, in my 

 opinion, to be drawn, not when the intakes of the river 

 companies were removed to positions beyond the possibility 

 of pollution by the drainage of London, but it must be drawn 

 at the time when efficient filtration was finally secured and 

 ever since maintained, that is to say, in the year 1884. 



The removal of turbidity by sand filtration, however, 

 refers only to suspended matters ; but there are sometimes 

 objectionable substances in solution of which organic matter 

 is the most important. River water and mountain water, 

 even when efficiently filtrated, contains more organic matter 

 than spring or deep well water; but this is reduced in quantity 

 by storage and especially by filtration, although these waters 

 can perhaps never be brought up to the standard of organic 

 purity of spring and deep well water. 



