JO MANUAL OF BACTERIOLOGY. 



munis, may grow through the pores in the walls of both the 

 Berkenfeld and Pasteur filters if sufficient nutrient material 

 is present to permit of their multiplication. 1 



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

 be spoken of in connection with the chapter on water. Simi- 

 lar tubes are employed for the filtration of certain organic 

 nutrient media whose ingredients would be damaged by 

 sterilization with heat, chiefly extracts of organs, such as 

 the thymus gland. The soluble " toxins " of bacteria may 

 be obtained by filtration of fluid-cultures through such tubes, 

 which remove the bacteria (Fig. 16). These fluids usually 

 filter very slowly, and filtration will have 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. 



1 Wherry, Journal of Medical Research. Vol. VIII., 1902. 



