72 BACTERIA IN WATER 



pass on the second or third day, B. coli and the typhoid bacillus 

 do not appear with them in the filtrate until a subsequent date. 

 Probably, if reliance is to be placed upon such a filter from a 

 bacteriological point of view, daily sterilisation is advisable. 



Experiments of a similar nature have been done by Horrocks,* 

 who arrives at the following conclusions. First, the B. typhosus is not 

 able to grow through the walls of a Pasteur-Chamberland candle, and 

 if proper care be taken to prevent the direct passage of organisms 

 through flaws in the material and imperfections in the fittings, the 

 Pasteur-Chamberland filter ought to give complete protection from 

 water-borne disease. Secondly, typhoid bacilli can grow through the 

 walls of Berkefeld candles, the time required for the passage being 

 largely dependent on the nutriment supplied to the organisms by the 

 filtering fluid. Possibly the weakness of the candle from a bacteri- 

 ological point of view is due to the large size of the lacunar spaces, 

 which cannot be avoided if a fair delivery is to be obtained, but 

 which " appears to militate against the immobilising and devitalising 

 influences which operate so strongly in filters made with very narrow 

 lacunar spaces." Thirdly, Horrocks concluded that when a highly 

 polluted liquid containing typhoid bacilli is filtered through a 

 Berkefeld candle the bacilli may appear in the filtrate in four days. 

 Consequently, it is necessary to sterilise these candles every third day. 

 The method of sterilisation of filters is not washing or brushing or 

 any other kind of cleansing or soaking in water, but by exposing 

 them to steam or boiling them. 



* Bacteriological Examination of Water, 1901, pp. 273-280. 



