METHODS OF STUDY 



21 



oo o o o o 



plished by means of an autoclave, which is a strong 



iron cylinder standing upright, closed at the 



bottom and furnished with a tight- 



fitting lid, and provided with a ther- 



mometer, a steam valve, a safety 



valve, and a gauge whereby the tem- 



perature and pressure may be regu- 



lated, heat being furnished by Bunsen 



gas burners or by indirect steam from 



a central boiler. For culture media 



in tubes an exposure of five minutes 



at 120 C. (248 F.) is sufficient to 



destroy all bacteria, but for media in 



bulk an exposure of fifteen minutes 



is necessary. 



Certain culture media are injured 



by exposure to great heat 

 under pressure, and this fact, 

 together with the extraordi- 

 nary resistance of the spores 

 of certain species of bacteria, 

 led to the employment of frac- 

 tional or discontinuous sterili- 

 zation. The ordinary Arnold 

 steam sterilizer used in the 

 FIG. 7. -Bunsen burner, kitchen for the sterilization of 



. 6. -Autocave. 



a. Safety-valve. 



b. Blow-off pipe. 

 o. Gauge. 



