STERILIZATION. 63 



coli communis, may grow through the pores in the walls of both 

 the Berkefeld and Pasteur filters if sufficient nutrient material 

 is present to permit of their multiplication.* 



Filters of these kinds are widely used for water, and will be 

 spoken of in connection with the chapter on water. Similar 

 tubes are employed for the filtration of certain organic nutrient 

 media whose ingredients would be damaged by sterilization 

 with heat, chiefly extracts of organs. The soluble "toxins" 

 of bacteria may be obtained by filtration of fluid-cultures 

 through such tubes, which remove the bacteria (Fig. i6) 

 These fluids usually filter very slowly, and filtration has to be 

 assisted by some form of vacuum-pump; usually the filter- 

 pump, which is used in connection with a stream of running 

 water, is employed. Compressed air or carbonic acid may be 

 used to assist in forcing fluids through the filter. The filter 

 bougies, the flasks and all parts of the apparatus must, of 

 course, be sterilized by heat before and after using. 



♦Wherry. Journal oj Medical Research. Vol. VIII., 1902. 



