PURIFICATION OF WATER FOR DRINKING PURPOSES 195 



minutes, and then allowed to subside for nearly five 

 hours : — 



Untreated water contained 8,000 micro-organisms in 1 c.c. 

 After 15 minutes' agitation 

 with animal charcoal 

 and nearly 5 hours' 



subsequent subsidence 60 „ „ 



Reduction in the number of micro-organisms = 99 per cent. 



Agitation with vegetable charcoal. — Water containing 

 soil-extract was shaken with one-fiftieth of its weight 

 of ordinary wood-charcoal for fifteen minutes, and was 

 then allowed to subside for twenty-seven hours : — 



Untreated water . . 3,000 micro-organisms in 1 c.c. 

 After 15 minutes' agitation 



with wood-charcoal and 



27 hours' subsequent 



subsidence . . 120 „ „ 



Reduction in the number of micro-organisms = 9tj per cent. 



Agitation with coke. — Water to w^hich a little stale 

 urine had been added was shaken with one-fiftieth of 

 its weight of fine coke for fifteen minutes, and then 

 allowed to subside for forty-eight hours : — 



Untreated water . . Micro-organisms in 1 c.c. too 



numerous to count. 

 After 15 minutes' agitation 



with coke and 48 hours' 



subsequent subsidence . None. 

 Reduction in the number of micro-organisms = 100 per cent. 



Further experiments made with coke and water 

 containing soil-extract illustrate what we have said as 

 to the reascension and multiplication of micro-organisms 

 in some cases. Thus : — 



Untreated water (containing soil extract), 3,000 micro-organisms in 1 c.c. 

 After agitation with coke and 26 hours' 



subsidence .... 20,000 „ „ 



When less time was allowed for subsidence, the 

 results were more satisfactory. Thus : — 



Untreated water (containing soil extract) 655 micro-organisms in 1 c.c. 

 After agitation with coke and 5 hours' 



subsidence 28 „ „ 



[Reduction in the number of micro-organisms = 96 per cent. 



2 



