50 STERILIZATION AND DISINFECTION. 



under a pressure of two or three atmospheres, which is suffi- 

 cient to destroy the spores. This is done by superheated 

 steam in an autoclave. The medium is placed in the auto- 

 clave, the top is screwed down firmly, and the escape valve 

 left open until the steam has displaced the hot air. The 

 valve is then closed and steam is generated for fifteen minutes 

 or longer if desired (Fig. 15). Cooling must be allowed to 

 take place gradually, as otherwise the cotton plugs will be 



FIG. 15. 



Autoclave for sterilization with live steam under pressure. 



forced into the tubes or flasks by the atmospheric pressure. 

 The principal objection to the autoclave is that the reaction 

 of the medium is altered by the chemical changes brought 

 about by the excessively high temperature. The advantages 

 are rapid and absolute sterilization. 



FILTRATION : Liquids can also be freed from bacteria by fil- 

 tration. They are filtered through unglazed porcelain, gravel, 

 sand, powdered glass, charcoal, or crushed stone. Porcelain is 



