PURIFICATION OF WATER FOR DRINKING PURPOSES 119 



indicating that sand-filtration had comparatively little 

 effect on the dissolved matters in water, led to a priori 

 speculation that it would be unlikely to remove bacteria 

 and other minute forms of life. During the early 

 growth of the belief in the communication of zymotic 

 diseases by micro-organisms the process of filtration in 

 general was viewed with great distrust, for it was not 

 unnaturally assumed that the extremely small dimensions 

 of these living creatures would admit of their passing 

 through the comparatively large interstices of ordinary 

 filtering media, with the same sort of facility that 

 vehicles can thread their way along a crowded thorough- 

 fare. 



It is less than ten years ago since this latter point 

 the bacteriological side of the question was for the first 

 time put to the test of accurate experimental investiga- 

 tion, and in the interval a large amount of work has 

 been done both in this country and on the Continent, 

 which has placed the subject of water-purification on 

 an entirely new basis. 



London Water Supply: Rivers Thames and Lea. 

 The process of sand-filtration was first practised in 

 connection with the London water supply in the year 

 1839, and although it was admitted that by its means 

 the water was rendered clear and bright to the eye, 

 yet, as already pointed out, beyond that there was no 

 public confidence in its efficiency as a purifying agent. 

 In the year 1885 Koch's gelatine-process of water- ex- 

 amination was first introduced into this country by one 

 of us x and applied to the London water supply, and 

 the following table 2 gives the results of these first 

 investigations : 



1 ' The Removal of Micro-organisms from Water,' Percy Frankland. 

 Proc. Boy. Soc. p. 379. 1885. 



2 ' Water-purification, its Biological and Chemical Basis,' Percy 

 Frankland. Transactions of Institution of Civil Engineers, 1886. 



