METHODS OF CULTURE. 



43 



must only be employed in washing dishes, to destroy the old 

 cultures. Even after washing, a few drops of the solution may 

 remain and prevent growth, so that one must be careful to have 

 the glass-ware that comes in contact with the nutrient media 

 free from the sublimate. 



Heat. Heat is the best agent we possess for general use. 

 Dry heat and moist heat are the two forms employed, but these 

 differ greatly in effectiveness. Thus Koch found that while 

 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. 



FIG. 14. 



Hot Air Oven. 



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

 (about 300 F.) a sheet-iron oven is used which can be heated 

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

 tween), the desired temperature is much more quickly obtained. 

 A small opening in the top to admit a thermometer is neces- 

 sary. These chests are usually about 1 foot high, 1| foot wide, 

 and f foot deep. In them, glassware, cotton, and paper can be 

 sterilized. When the cotton is turned slightly brown, it usually 



