Methods of Sterilization 



205 



turning up the edge or putting in a pin. The paper is placed 

 over the stopper before the sterilization, after which no 

 contamination of the cotton can occur. 



II. Sterilization and Protection of Culture=media. 

 As almost all of the culture-media contain about 80 per 

 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 



Fig- 39- Arnold's steam sterilizer (Boston Board of Health form). 



placed in previously sterilized flasks or tubes, carefully 

 plugged with cotton- wool, and then sterilized in an Arnold's 

 steam sterilizer. 



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

 the spores, so that one exposure of the culture-media to 

 streaming steam is of little use. The sterilization must be 

 applied in a systematic manner intermittent sterilization 

 based upon a knowledge of sporulation. 



In carrying out intermittent sterilization the culture- 

 medium is exposed for fifteen minutes to the passage of 



