BACTERIOLOGY OF WATER 243 



non-sporulating bacilli in Cambridge water is usually 

 1 part per 7 ,003,00 J parts of water. Less pure waters 

 may require 0'7 to 2 parts of available chlorine per 

 million. 



The army directs that 2 grammes of bleaching powder 

 shall be added to the contents of water-carts which hold 

 100 to 110 gallons, at the discretion of the medical officer. 



Many water-supplies in America and some in the 

 United Kingdom are now sterilised by bleaching powder. 

 In order to remove any traces of the reagent after treat- 

 ment, the water may be run through iron turnings or 

 charcoal, or an addition of an appropriate quantity of 

 sodium hyposulphite may be made. 



One of us (C. G. Moor) has applied the process of sterili- 

 sation of drinking-water on a large scale for camps and 

 hospitals at a base in France, with satisfactory results. 



At present it appears that sterilisation by minute 

 quantities of bleaching powder, or by chlorine produced 

 electrically, is the coming method, and its cheapness 

 and simplicity make it applicable everywhere. Before 

 it is applied, waters should be strained or filtered to take 

 out all visible particles, otherwise the sterilising solution 

 may fail to kill bacteria enclosed in such particles. For 

 army use a valuable piece of apparatus has been devised 

 by Colonel Horrocks, in which water is clarified and 

 sterilised by bleaching powder at one and the same time. 



Water may be sterilised by allowing it to trickle down 

 a tower through which ozonised air is caused to pass. 

 This is a method which has much to recommend it, inas- 

 much as the water is rendered sterile, but loses none 

 of its salts or gases in the process. The apparatus is, 

 however, expensive, requiring elaborate electrical ap- 

 paratus and skilled attention. It is in use in some towns 

 abroad. 



Sterilisation of water may be effected by the ultra- 

 violet rays created in Cooper-Hewitt lamps made from 

 transparent quartz. A continuous supply of sterile water 

 is available in five minutes. The water is unaffected as 

 far as taste is concerned, as it retains all natural gases 

 and salts in solution. So far the process is not widely 

 adopted. 



In Clark's process for softening hard water, Moor and 

 Hewlett showed that bacteria were reduced, but results 



