METHODS OF CULTURE. 



41 



Second. To maintain this degree of disinfection throughout 

 the whole course of the growth, and prevent, by proper con- 

 tainers, the entrance of foreign germs. 



Disinfectants, Corrosive sublimate (bichloride of mercury), 

 which is the most effective agent we possess, cannot be gene- 

 rally used because it renders the soil unproductive and therefore 

 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 

 not too moist with the sublimate. 



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

 Drv heat and moist heat are the two forms employed. 



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, 



