BACTERIOLOGY OF WATER 239 



washed through, the bacterial content of the water in- 

 creasing until sometimes it is greater than that of the 

 unfiltered water. Sponge filters form an excellent nidus 

 for bacterial growth, and are particularly reprehensible. 

 Bacterial filters of the earthenware type are by far the 

 most satisfactory. These are made in tubes or ' candles,' 

 to offer as large a surface as possible to the water. 



The Pasteur-Chamberland candle is a composition of 

 unglazed porcelain, the Berkefeld of a diatomaceous earth 

 (kieselguhr); the Slack and Brownlow, and the Doulton 

 filter, are also composed of porcelain. None continues to 

 give sterile water indefinitely, and should be submitted 

 to a weekly scrubbing with a nailbrush, and subsequent 

 boiling in water containing sodium carbonate. Kieselguhr 

 filters vary considerably in the length of time for which 

 they will give sterile water, some allowing direct con- 

 tamination, while others are satisfactory. The Berkefeld 

 filter has a small portion of its surface removed every 

 time it is scrubbed, and thus may in time become faulty. 



The inevitable appearance of organisms in the filtrate, 

 in the case of all filters after shorter or longer periods of 

 effectiveness, is, according to Craw, partly dependent on a 

 mechanical acceleration, caused by the water current 

 sweeping the micro-organisms through the filter mass. 

 This indirect contamination appears to depend partly on 

 the grain of the filter. The most efficient makes of filters 

 examined by Craw the Pasteur - Chamberland and 

 Doulton contain very small-sized pores; the Berkefeld 

 pores are larger, and the Slack and Brownlow larger still. 

 The characters of the organisms (motility, size, etc.) and 

 the chemical composition of the water may also be factors 

 in determining the period of sterility. When water is 

 filtered under pressure an extra strain is put on the filter. 

 The soundness of the Pasteur-Chamberland filter can be 

 ascertained by compressing air in the candle at a pressure 

 of one-half to one atmosphere, when, if held' beneath 

 water, no air will escape from a sound tube. A stream of 

 bubbles will issue from any spot capable of passing 

 bacteria. 



Recognising their limitations, bacterial filters, especially 

 the Pasteur - Chamberland, offer a perfect protection 

 against infection from drinking-water. The slow rate of 

 filtration is not surprising considering the size of bacteria, 



