466 IOWA ACADEMY OF SCIENCE Vol. XXV, 1918 



seven hundred million gallons) ehlorination is the only means 

 of purification applied. 



Such then, is the history of ehlorination as applied to munic- 

 ipal supplies. Cantonment supplies, (as has been mentioned be- 

 fore), take on many of the characteristics of city supplies. At 

 least sixteen of our cantonments and large camps in the country 

 are using liquid chlorine and are applying it in the manner used 

 by municipal plants. For the smaller quantities of water spe- 

 cial procedures are employed — ^though the principles of the ac- 

 tion are the same. 



The great difficulty in the ehlorination by hypochlorites is the 

 loss of strength due to the escape of the free chloride or its com- 

 bination with other substances. This difficulty is increase:! in 

 warm climates — a, fact which has suggested to certain Indian 

 army officers some methods of using the chlorine as a gas. Cal- 

 cium hypochlorite is not completely soluble. The sludge left 

 after making up the solution is bothersome and retains some of 

 the active chlorine. To avoid as much as possible of the loss of 

 chlorine from decomposition the chemical is usually packed in 

 air-tight cans. 



When an excess of chlorine is used, an objectionable taste and 

 odor will result. Different waters contain various sorts of or 

 ganic matter which albsorb varying quantities of the reagent. 

 Sometimes the chlorine may combine with these substances giv- 

 ing rise to compounds which themselves have odors and tastes 

 similar to those developed in the presence of an excess of the 

 chemical. For this reason, the mere taste of the chemical in the 

 water is not always an indication that a sufficient quantity has 

 been used. The employment of coagulants — or their use in larger 

 amounts — by removing much of this organic matter (including 

 the bacteria) will in many cases tend to reduce this difficulty 

 and at the same time give better bacterial removal. 



Inasmuch as it is essential for good results that there be a res- 

 idue of free chlorine in the water after the absorption, the dif- 

 ferent waters will require different amounts of the hypochlorite. 

 The usual quantity applied in ordinary water works practice is 

 from one to two parts per million of the calcium hypochlorite 

 Iby weight. This is equivalent to .3 to .6 parts per million of free 

 chlorine. (Weight U. S. gallon 8.3 pounds, British Imperial gal- 

 lon 10 pounds.) It is customary in field work to add a sufficient 

 amount of the chemical to give the potassium iodide-starch test 



