i6o 



Sterilization and Disinfection 



cent, of water, which would evaporate in the hot-air closet, 

 and so destroy the material, hot-air sterilization is inap- 

 propriate for them, sterilization by streaming steam being 

 the only satisfactory method. The prepared media are 

 placed in previously sterilized flasks or tubes, carefully 

 plugged with cotton- wool, and then sterilized in what is 

 known as Koch's steam apparatus (Fig. 25) or in Arnold's 

 steam sterilizer (Fig. 23), which is more convenient and 

 more generally useful. 



The temperature of boiling water, 100 C., does not kill 

 the spores, so that one ex- 

 posure of the culture media 

 to streaming steam is of little 

 use. The sterilization must 

 be applied in a systematic 

 manner intermittent sterili- 

 zation based upon a knowl- 

 edge of sporulation. 



Fig. 24. Flask with the cot- 

 ton stopper protected with a 

 paper cap. 



Fig. 25. Koch's steam sterilizer. 



In carrying out intermittent sterilization the culture 

 medium is exposed for fifteen minutes to the passage of 

 streaming steam or to some temperature judged to be 

 sufficiently high, so that the adult micro-organisms con- 

 tained in it are killed. As the spores remain uninjured, 

 the medium is stood aside in a cool place for twenty-four 

 hours, and the spores allowed slowly to develop into adult 

 organisms. 



When the twenty-four hours have passed, the medium is 

 again exposed to the same temperature, until these newly 



