1578 



RURAL HYGIENE 



a fixed method is strictly adhered to. In order to study some of the 

 factors which influence the reaction a series of tests were carried out with 

 river water drawn from the Manilla city reservoir and containing less 

 than 5 parts per million of chlorides. 



The first factor studied was the length of time allowed for the diges- 

 tion. Samples of 200 cc were used and the tests were carried out at loooC. 

 The results are given in Table I. 



Table I. — Reduction of potassium permangate by water ; 

 effect of varying the duration of digestion. 



Duration of digestion 



Permanganate required (i) 



(i) ICC = O.I mgm oxygen. 



The decomposition showed no signs of reaching an end point after 

 several hours digestion. The tests were then repeated at lower tempe- 

 ratures, but even at 10° C. the effect of the time factor was still marked 

 although the difference between the four and eight hour digestion was 

 negUgible for practical purposes. 



The effect of the presence of chlorides was next investigated. Sam- 

 ples of 100 cc were used this time and the digestions were kept going for 

 4 hours at 30"^ C. Some of the results are given in Table II. 



Table II. — Reduction of potassium permanganate by water ; 

 effect of presence of chlorides. 



