14 LABORATORY GUIDE IN BACTERIOLOGY 



survive, they will develop in the next 24 hours, and the 

 third exposure to steam will complete sterilization. 



Sterilization is accomplished in a shorter time by 

 the use of steam under pressure. The autoclave is the 

 usual apparatus used for this purpose. Certain bac- 

 teria, some of which are widely distributed in nature, 



FIG. 8 

 Arnold Sterilizer 



have the faculty of forming spores. These spores are 

 highly resistant to heat and do not lose their vitality 

 either by boiling or by application of steam under 

 ordinary atmospheric pressure. By adding the pres- 

 sure of one atmosphere to ordinary pressure, the boiling 

 point is raised to 121 .4 C. 120 C. is sufficient to kill 

 all spores during an exposure of 5 minutes, if the media 

 are in tubes. Larger amounts of media require a 

 proportionately longer exposure. 



The autoclave consists of a strong cylinder made of 



