LABORATORY STERILIZATION 243 



in test-tubes is more quickly sterilized than media in flasks. At the 

 end of the allotted time, the pressure is gradually reduced until 

 equilibrium is reached with the atmospheric pressure; a sudden release 

 of pressure would cause violent ebullition of fluid, and a wetting or 

 even expulsion of cotton plugs from test-tubes or flasks. 



TABLE OF PRESSURE AND TEMPERATURE. 



Pressure, Temperature, 



pounds. Centigrade. 



100.0 



5 107.7 



10 115.5 



15 121.5 



20 126.5 



2. Live Steam. Many solutions are injured by temperatures above 

 100 C. Media containing sugars (particularly bioses) milk and gela- 

 tin are partly decomposed by prolonged sterilization in the autoclave. 

 An exposure to live steam at lOQ^.C. for thirty minutes on each of 

 three successive days usually suffices to effect sterilization of these 

 media without injury to the constituents of the medium. This method 

 of fractional sterilization depends upon the destruction of all vegetative 

 cells during the heating process, and the germination of spores into 

 vegetative organisms between heatings. It is assumed that all viable 

 spores will have germinated before the third exposure to heat, but 

 Theobald Smith 1 has shown that spores of anaerobic bacteria may not 

 vegetate within the specified time. A fourth exposure to heat after 

 two or three days may be required to insure sterilization. The Arnold 

 sterilizer is widely used for fractional sterilization with live steam. 

 It consists essentially of a double-walled copper chamber surmounting 

 a double-bottomed water reservoir, the lower compartment of which 

 is shallow and contains but little water. A flame applied to this 

 shallow reservoir soon generates steam, which rises through a central 

 passage to the chamber in which the material to be sterilized is placed. 

 Condensed steam flows by gravity to the upper water compartment, 

 and from thence to the lower heated reservoir to replace the evapora- 

 tion. It takes but a few minutes to generate sufficient steam to fill 

 the sterilizing chamber. The sterilizing process begins when the 

 contents of the sterilizing chamber have reached 100 C. 



3. Fractional Sterilization at temperatures from 60 to 80 C. is fre- 

 quently made use of for materials such as blood serum, which would 

 be injured by exposure to 100 C. The sterilizing process is repeated 



1 Jour. Exp. Med., 1898, iii, 647. 



