THE DESTRUCTION OF BACTERIA 71 



In noting the time of an exposure in an Arnold sterilizer, it is 

 important to time the process from the time when the temperature 

 has reached 100 C. and not from the time of lighting the flame. 



The principle of fractional sterilization at low temperatures is ap- 

 plied also to the sterilization of substances which can not be sub- 

 jected to temperatures as high as 100 C. This is especially the case 

 in the sterilization of media containing albuminous materials, when 

 coagulation is to be avoided, or when both coagulation of the medium 

 and sterilization are desired. 



In such cases fractional sterilization may be practiced in simply con- 

 structed sterilizers, such as a Koch inspissator or, in the case of fluids, 

 such as blood serum, by immersion in a water-bath at a temperature 



FIG. 10. Low TEMPERATURE STERILIZER (!NSPISSATOR). 



varying above 55 C., according to circumstances. Exposures at such 

 low temperatures may be repeated on five or six consecutive days, usu- 

 ally for an hour each day. 



The use of steam under pressure is the most powerful method of heat- 

 disinfection which we possess. It is applicable to the sterilization of 

 fomites, clothing, or any objects of a size suitable to be contained in the 

 apparatus at hand, and which are not injured by moisture. In labora- 

 tories this method is employed for the sterilization of infected appa- 

 ratus, such as flasks, test tubes, Petri plates, etc., containing cultures. 

 The device most commonly used in laboratories is the so-called auto- 

 clave, of which a variety of models may be obtained, both stationary 

 and portable. The principle governing the construction of all of these 



