PURIFICATION OF WATER FOR DRINKING PURPOSES 155 



effect upon the two filters : that, whereas (a), after thirty 

 •days' work, was practically useless, owing to its having 

 become clogged up ; (b) was hardly affected at all, and 

 could have been kept in operation for weeks longer. 

 Further, that the number of micro-organisms in the 

 filtrate bore a distinct relationship to the rate at which 

 the filtration took place, for the number of B. violaceus 

 ■colonies was in every case larger in the filtered water 

 from (a) than in that from (b). Taking the results of 

 ■observations extending over a whole month, the number 

 of B. violaceus colonies was found for (a) to be 894, for 

 (b) 361 in the c.c. Again, in another series of experi- 

 ments carried on simultaneously the number for (a) 

 was 1,578, for (b) 607. Another series of experiments 

 was carried on with filters in which the sand was not 

 fresh, but taken from a real filter bed which had been 

 some time in use, and the water employed was the raw 

 river-water. These materials were selected in order to 

 procure the slimy deposit which forms on the surface 

 of sand filters, and the raw river-water was substituted 

 for the filtered in order,' by providing more organic 

 material, to stimulate the formation of this slime. The 

 rate of filtration for the two filters (a and b) was kept 

 respectively at 300 mm. and 50 mm. per hour. 

 Bacillus violaceus was added to the unfiltered water as 

 in the previous experiment. 



Here again it was found that in the filtrate from (a) 

 nearly six times as many violaceus colonies were obtained 

 as in that from (b), and the more rapid rate of filtration 

 so completely clogged (a) that after eight days it had to 

 be stopped and cleaned, whilst (b) went on working for 

 thirty days. It is, moreover, particularly noteworthy 

 that the numbers of bacteria found in the filtered water 

 were markedly less in this second set of filters (a and b) 

 than in the first, although the number of bacteria in the 



