PURIFICATION OF WATER FOR DRINKING PURPOSES 125 



and the vague terms " turbid," " slightly turbid," 

 " clear," and the like, which have hitherto been em- 

 ployed to describe whether the filtration of water has 

 been satisfactory or not, must now be replaced by this 

 scientific and important standard which I have de- 

 scribed.' 



These investigations further brought to light some 

 very interesting points in connection with sand-filtra- 

 tion, and have indeed placed that process on a sound 

 basis by exhibiting what are the principal factors in 

 determining its efficiency. Thus, amongst the London 

 water companies there are seven employing sand- 

 filtration, and in the works of each the process has 

 undergone to a great extent an independent evolution, 

 so that in no two of them is the process at the present 

 time carried on under precisely similar conditions. 

 This, for experimental purposes, peculiarly fortunate 

 circumstance enabled one of us to institute a comparison 

 between the results achieved in the several modifications 

 of the general process. In this connection we may 

 quote from a paper read by one of us before the Insti- 

 tution of Civil Engineers in the year 1886, the conclu- 

 sions which we then arrived at having been fully verified 

 by further observations made both in this country and 

 abroad since that date. 



An examination of the tables of bacteriological re- 

 sults shows ' that there is a certain uniformity in the 

 position which the various companies occupy as regards 

 freedom from micro-organisms, and on referring to the 

 statistics of the various companies published in Sir 

 Francis Bolton's "Manual of the London Water Supply,'' 

 it is found that there is an unmistakable relationship 

 between this position of each company and certain 

 factors in the mode of working, which might be anti- 

 cipated from theoretical considerations. 



