PURIFICATION OF WATER FOR DRINKING PURPOSES 175 



recorded in the above table tlie filtrate was quite sterile, 

 an examination of another cylinder (of the same age, 

 and working under similar circumstances) yielded a 

 non-sterile liquid. Again, that when the filter is kept 

 at a low temperature, the filtrate remains sterile over a 

 much longer period of time, the conditions not being so 

 favourable for the development and multiplication of 

 the micro-organisms arrested in the pores of the filter. 

 A coating of slime forms on the porcelain, often reach- 

 ing a thickness of 1 mm. and more ; this tends to re- 

 tard the rate of filtration which we have seen takes 

 place after the filter has been in use for a short 

 time, and necessitates the cylinders being frequently 

 thoroughly washed and sterilised ; in fact, Preuden- 

 reich, in recommending these filters, states that they 

 should not be kept in use for longer than eight days 

 without being sterilised, care being at the same time 

 taken that the temperature of the place where they are 

 kept is as low as possible. 



These results raise, of course, the important ques- 

 tion as to whether pathogenic organisms, such as 

 cholera and typhoid bacilli, are able to grow through 

 the pores of such a filter, and Freudenreich has also 

 conducted some experiments with a view to deter- 

 mining this point. For this purpose a Chamberland 

 porcelain cylinder was immersed in sterile broth, which 

 latter of course penetrated also into its interior ; typhoid 

 bacilli were then introduced into the outer broth, in 

 which they abundantly multiplied, but the internal 

 broth remained sterile even after twenty-two days, 

 although the apparatus was maintained throughout at 

 the temperature of 35° C. It would thus appear that 

 the typhoid bacilli were not able to grow through the 

 pores of the porcelain ; but the result of the experi- 

 ment bears, as Freudenreich points out, another in- 



