1896.] on the Past, Present and Future Water Supply of London. 61 



be taken immediately after it has passed through the filters ; for 

 if it be obtained from the delivery mains in town, that is to say, 

 after the water has passed through many miles of pipes, the rapid 

 multiplication of these organisms, excejjt in very cold weather, is 

 such that a water which contains only a single living organism 

 per cc, as it issues from the filter, may contain 100 or 1000 in the 

 same volume when, after several hours, it arrives on the consumers* 

 premises. Fig. 11 shows isolated bacteria, enormously magnified, 

 taken from one of the towns or colonies. The scale at the foot of 

 this figure represents thousandths of an inch. 



Now what is the effect of sand filtration as carried out by the 

 various water companies supplying London, upon the living matter 

 contained in the raw river water ? It is simply astounding — water 

 which, when poured upon sand filters, contains thousands of bacteria 

 per c.c. — for a single droj) of Thames water sometimes contains 

 nearly 3000 sej^arate organisms — comes out from those filters with 

 50, 30, 10, or even less of these organisms per c.c. ; or the number 

 of microbes in a single drop is reduced to two or even to zero. 



Eather less than one-tenth of the total volume of water supplied 

 to London is derived by the Kent Water Company from deep wells 

 in the chalk. As it issues from the porous rock into the fissures and 

 headings of these wells, this water is, in all probability, absolutely 

 sterile ; but, by the time it has been pumped up to the surface, it 

 usually contains a certain number, though small, of microbes. 

 Thus, during the year 1892 it contained on the average 6 per c.c. ; 

 in 1893, 13 ; in 1894, 15 ; and in 1895, 8. 



The diagram (Fig. 12) shows, graphically, the bacterial improve- 

 ment of the river water by filtration during the j^ear 1894. In this 

 diagram, the black squares and white centres represent the relative 

 numbers of microbes in the unfiltered and filtered waters respec- 

 tively. 



Thus, although the deep-well water has, from a bacterial point of 

 view, a decided advantage, the filtered river waters are not very far 

 behind ; and there is every reason to believe that with the improve- 

 ments which are now being carried out by the various river-water 

 companies, the Kent company's water will before long be run very 

 hard by the other supplies. 



By the examination of the water as it issues from the filters, the 

 utmost freedom from microbes, or maximum degree of sterility, of 

 each sample is recorded. This utmost freedom from bacterial life, 

 after all sources of contamination have been passed, is obviously the 

 most important moment in the history of the water ; for the smaller 

 the number of microbes found in a given volume at that moment, the 

 less is the probability of pathogenic or harmful organisms being 

 present ; and although the non-pathogenic may afterwards multiply 

 indefinitely, this is of no consequence in the primary absence of the 

 pathogenic ; but it is only fair, in describing the character of the 

 present water supply of London, to say that not a single pathogenio 



