WATER SUPPLY FOR TROOPS 467 



after half an hour. Sometimes this result is secured by makino: 

 the test on the treated water and sometimes the use of an over- 

 whelming quantity of chemicals is depended upon. The excess 

 is then removed by sodium thiosulphate. 



,. A point of practical importance that is seldom mentioned in 

 this connection, is the influence of low temperature on the treat- 

 ment of water. It is well known, of course, that chemical reac- 

 tions usually proceed more slowly or with decreased intensity at 

 a low teniiperature. The influence of this retarded action in the 

 coagulation of water is very easily noticed. The flakes of coagu- 

 lant can no longer be seen floating in the water, or as the water 

 works operator says, you have "pin-point coagulation.'' In the 

 chlorination of water the retarding of the reaction is not so read- 

 ily seen. The odor and taste are the cause of more complaints in 

 the winter time and although the water will give the test for free 

 chlorine after half an hour, the bacterial removal is not always 

 satisfactory. The difficulties in the winter appear, however, to 

 be more troublesome when calcium hypochlorite is the germicide 

 than when the liquid chlorine is used. 



In applying the hypochlorite, it is very convenient to make up 

 a small amount of a stock solution iLsing a known weight of the 

 chemical and a definite volume of water. The desired amount of 

 this solution may then be measured into known volumes of the 

 water to be treated and carefully mixed. The Thresh method 

 (130) provides sealed quarter pound tins of calcium hypochlorite 

 and half pound packages of sodium thiosulphate. One tin of the 

 hypochlorite is mixed into a gallon of water and the package of 

 sodium thiosulphate is dissolved in another gallon of water to 

 furm the antichlor. The hypochlorite is sufficient for 8,000 gal- 

 lons of water, making one part of the chemical in 320,000 parts 

 of water (since the British Imperial gallon weighs ten pounds) . 

 The free chlorine employed is therefore a little more than one 

 part per million. After fifteen minutes a volume of the thio- 

 sulphate solution equal to that of the bleaching powder solution 

 is added to destroy the excess chlorine. It might be of interest 

 t(. note that while ordinary bleaching powder contains 25 to 37 

 per cent of its active ingredient — available chlorine, the Bayer 

 Company, a German concern, at the opening of the war pro- 

 duced a similar compound containing 75 per cent available chlo- 

 rine. 



