STERILIZATION 45 



by means of heat,, either dry or moist. For the killing 

 of bacteria by other means see Chapter V. 



The most widely applicable and efficient physical 

 agent for sterilization is heat. A certain amount of 

 heat is necessary for the life of bacteria, but there are 

 certain temperatures beyond which they cease to live. 

 While 38 C. or 98.5 F. is their optimum, they find it 



FIG. 19 



Laboratory hot-air sterilizer. 



increasingly difficult to live as the temperature rises 

 to 50 C. or 122 F. Beginning there and extending to 

 62 C. or 144 F. the commoner pathogenic organisms 

 are killed by ten minutes' exposure. For example, the 

 typhoid bacillus dies when heated to 56 C. or 133 F. 

 for ten minutes, the pneumonia coccus at 52 C. or 

 126 F. for ten minutes. The tubercle bacillus is much 

 more resistant, and requires from ten to twenty minutes' 



