METHODS OF CULTURE 



45 



moist heat at 100 C. killed the spores of the anthrax bacillus 

 in one hour, it required three hours of dry heat at 140 C. to 

 produce death. 



For obtaining dry heat that is, a temperature of 150 C. 

 (about 300 F.) a sheet-iron oven (Fig. 14) is used, which can 

 be heated by a gas-burner. If it have double walls (air circulat- 

 ing between), the desired temperature is much more quickly 

 obtained. A small opening in the top to admit a thermometer 

 is necessary. These chests are usually about i foot high, 

 i \ feet wide, and f foot deep. In them glassware, cotton, and 



Fig. 14. Hot air oven. 



paper can be sterilized. When the cotton is turned slightly 

 brown, it usually denotes sufficient sterilization. All instru- 

 ments, where practicable, should be drawn through the flame 

 of an alcohol lamp or Bunsen burner. One hour in the oven 

 at 170 C. usually sterilizes glassware, while the ordinary 

 germs in liquids may be killed by boiling for five minutes if 

 no spores are present. The boiling of any fluid at 100 C. 

 for one and one-half hours nearly always insures sterilization. 



Moist Heat. Steam at 100 C. in circulation has been 

 shown to be a very effective application of heat. 



