CULTIVATION OF BACTERIA 63 



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 glassware that comes in contact with the nutrient 

 media free from the sublimate. 



Fig. 15. Hot-air sterilizer. The gas-jets are inclosed within the 

 space between the outer and middle walls, C, and can be seen at F. The 

 heat ascends, warming the air between the two inner walls, which ascends 

 between the walls, K, K, then descends over the contents, /, and escapes 

 through perforations in the bottom, B, to supply the draft at F, and 

 eventually escapes again at S; R, gas regulator; T t thermometer. 



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 



