XI, A, 3 Heise and Aguilar: Purification of Swimming Pools 115 



se, but in order to prevent the destruction of the hypochlorites 

 to which the purifying action is due. 



There are, then, two distinct actions or effects: the first, the 

 germicidal action of chlorine or hypochlorites; the second, the 

 specific interaction between the chlorine and the substances in 

 water. There is evidently a minimum concentration below which 

 effective purification does not occur ; if this is reached in a short 

 time, purification will not be adequate or lasting in its effect. 

 It thus becomes necessary to maintain at all times in the water 

 of a swimming pool an excess of "free chlorine" sufficient to keep 

 up effective purifying action. 



With these conditions in mind, the purification of a swimming 

 pool becomes a comparatively simple matter. A relatively small 

 amount of hypochlorite will effectively purify the water, after 

 which it becomes necessary to keep the bacterial content within 

 safe limits by means of repeated additions of disinfectant. That 

 this is true is evinced by the results obtained during a series 

 of tests in which chloride of lime was used in quantities repre- 

 senting a daily addition of 0.5 part of "available chlorine" per 

 million parts of water. Throughout this series the bacterial 

 content was kept below 200 and no B. coli was found. In all 

 cases the water remained in the pools for two weeks and was 

 safe during the entire period. That there was no cumulative 

 effect and that there was no large excess of chlorine at any 

 time were shown by omitting chlorination for a single day, when 

 the bacterial content immediately increased to dangerous propor- 

 tions. Throughout the two weeks qualitative tests for the 

 presence of free chlorine and quantitative determinations of the 

 chlorine-consuming power were made, with the results listed in 

 Table X. 



At first the chlorine consumption remained almost constant, 

 next it passed through a minimum, and then, as people used the 

 pool, it rose rapidly. Evidently it takes more and more 

 chlorine to produce a given effect as time goes on; and there- 

 fore it becomes advisable to change the water after the chlorine 

 consumption reaches a certain maximum, although just where 

 that maximum should be placed is a matter of conjecture. The 

 efficiency of the purifying action was such that it is clear that 

 the maximum was not exceeded during the first week, nor in the 

 case of tank I even during the second week. Chloride of lime, 

 therefore, will keep a tank clean and reasonably safe for ten days 

 or two weeks, even under the unfavorable circumstances existing 



