STERILIZATIOJSt— ANTISEPSIS — FOOD PRESERVATION 



69 



consists of a metal cylinder with a movable top, which is fastened 

 down tightly during sterilization. It is furnished with a pressure 

 gauge, a stop-cock, and a safety-valve which is set to allow the 

 steam to escape when the desired pressure is attained and thus 

 prevents it from running too high. Heat is furnished by a gas- 

 burner underneath. The lower part of the cylinder contains water. 

 The objects to be sterilized are supported 

 above this water on a perforated bottom 

 or shelf. 



It is necessary to observe certain pre- 

 cautions in the use of the autoclave. 

 Allusion has already been made to the 

 necessity for having the steam saturated 

 with moisture. This is effected by allow- 

 ing the air to escape after the heat is 

 applied, and in order to be sure that all 

 the air has really been expelled, the stop- 

 cock, with which all autoclaves are pro- 

 vided, is left open until the steam escapes 

 freely. The stop-cock is then closed, and 

 the pressure begins to rise. After leaving 

 the articles to be sterilized in the auto- 

 clave for the length of time desired, the 

 apparatus must not be opened while the 

 steam contained within it is still under 

 pressure, as there may be a sudden evolution of steam upon 

 the removal of the pressure which may blow the media out of 

 ■ their tubes and flasks. After the pressure has fallen to zero it is 

 well to open the stop-cock only a little way so that air may not 

 be drawn in too rapidly to replace the condensing steam. The 

 autoclave may be opened as soon as the internal and external 

 pressure become equal. 



The length of exposure necessary to accomplish sterilization 

 in the autoclave depends upon the protection which the article 

 to be sterilized affords the bacteria. In sterilizing agar, a con- 



Snicir:: 



Pig. 32. — Autoclave. 



