' STERILIZATION 85 



be destroyed by this second heating. A third heating, twenty-four 

 hours later, will quite certainly destroy all the bacteria which may 

 have been present. This process is called intermittent steriliza- 

 tion. It finds its principal application in the sterilization of 

 materials which would be changed or broken down by heating 

 at a higher temperature. Among such materials are media con- 

 taining sugars which undergo incipient caramelization when 

 heated too hot. 



Sterilization by Steam under Pressure. This is generally 

 accomplished in the autoclave or digester, which consists essentially 



Fig. 49. Arnold steam sterilizer (Fowler). 



of a chamber into which steam under pressure can be introduced 

 (Fig. 50). Many different types of these autoclaves have been 

 put upon the market. Live steam under a given pressure unmixed 

 with air has a constant temperature; therefore, if the pressure 

 of the steam is known, one can determine easily the temperature 

 as well. It is necessary, however, that all air be first eliminated. 

 This is accomplished by allowing the stop-cock, which is always 

 present upon the steam chamber in the autoclave, to remain open 

 until all the air has escaped and the steam issues in a constant 

 stream. This cock is then closed and the pressure caused to 



