Sterilization by Filtration 



163 



The media to be sterilized should be placed in the auto- 

 clave, the top firmly screwed down, but the escape-valve 

 allowed to remain open until steam is freely generated within 

 and replaces the hot air. The valve is then closed, and 

 the temperature maintained for fifteen minutes or longer 

 if the media be in bulk in flasks. The cooling must take 

 place gradually, for if the pressure be suddenly relieved 

 the fluids boil rapidly and the cotton plugs may be forced 

 into the tubes or flasks by the air pressure. The chief ob- 

 jection to the use of the autoclave is that the high tern- 



Fig. 28. Kitasato's filter: a, 

 Porcelain bougie; b, attachment 

 for suction pump; c, reservoir; d, 

 sterile receiver. 



Fig. 29. Reichel's bacterio- 

 logic filter of unglazed porcelain : 

 A, Sterile receiver; B, porce- 

 lain filter; c, d, attachments for 

 vacuum pump. 



perature brings about certain chemic changes in the media 

 by which the reaction is altered. 



Sterilization by Filtration. Liquids that cannot be 

 subjected to heat without the loss of their most important 

 qualities may be sterilized by filtration i. e., by passing 

 them through unglazed porcelain or some other material 

 whose interstices are sufficiently fine to resist the passage 

 of bacteria. This method is largely employed for the 

 sterilization of the unstable bacterial toxins that are de- 

 stroyed by heat. Various substances have been used for 

 filtration, as diatomaceous earth (Berkefeld filters), stone, 



