STERILIZATION — ANTISEPSIS — POOD PRESERVATION 65 



for one hour, 170° C. for 30 minutes, or 200° C. for one minute will 

 kill all bacteria. Such exposure browns cotton of good grade only 

 slightly. One fallacy in hot-air sterilization needs to be guarded 

 against. Glassware and other apparatus must be dry before it 

 is put into the oven to sterilize. A tube containing water may be 

 left m the oven until the thermometer records a temperature of 

 200° C. in the uppet corner of the sterilizer, and subsequently the 



Pig. 28. — Hot-air sterilizer. 



tube may be removed from the oven with the most of the water still 

 in it. Hot-air sterilization is employed for glassware, tubes with 

 cotton plugs, granite-ware, stone-ware, and for metals not injured 

 by heat. 



Moist heat or heat in the presence of liquid water must be used 

 whenever drying is to be avoided, especially in the sterilization of 

 culture media and various solutions. It is employed as continu- 

 ous sterilization at a single exposure and as discontinuous ster- 

 ilization, heating for a short time on several consecutive days. 

 5 



