THE WATER SUPPLY OF LONDON. 



173 



100 or 1000 in the same volume when, after several hours, 

 it arrives on the consumer's premises. 



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 ? // is 

 simply astounding : water containing thousands of bacteria 

 per cubic centimetre, for a single drop of Thames water 

 sometimes contains nearly 3000 separate living organisms, 

 comes out from the sand filters with fifty, thirty, ten, or even 

 less of these organisms per cubic centimetre, or the number 

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



MICROBES IN RAW AND FILTERED THAMES 

 WATER 1894. 



JANUARY 



FEBRUARY MARCH APRIL 



MAY JUNE 



JULY AUCUST SEPTEMBER 



OCTOBER 



NOVEMBER DECEMBER 



No. 6. 



MEAN 



Rather 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 six per cubic 

 centimetre in 1893, thirteen; in 1894, fifteen; and in 

 1895, eight. 



The diagram No. 6 shows graphically the bacterial 

 improvement of the Thames water by filtration during 



