STERILIZATION BY HEAT. 49 



tissues, etc., are a matter of no consequence. On the 

 other hand, for the sterilization of all materials to be 

 used as culture media heat only is employed. 



The two processes will be explained in this chapter, 

 beginning with 



STERILIZATION BY HEAT. 



Sterilization by means of high temperature is accom- 

 plished in several ways, viz., by subjecting the sub- 

 stances to be treated to a high temperature in a properly 

 constructed oven, this is known as dry sterilization ; by 

 subjecting them to the action of streaming or live steam 

 at the temperature of 100° C ; and by subjecting them 

 to the action of steam under pressure, under which 

 circumstances the temperature to which they are ex- 

 posed becomes more and more elevated as the pressure 

 increases. 



Experiments have taught us that the process of steril- 

 ization by dry heat has a relatively limited application 

 because of its many disadvantages. For successful 

 sterilization by the method of dry heat, not only is a 

 relatively high temperature essential, but the substances 

 under treatment must be exposed to this temperature 

 for a comparatively long time. Its penetration into 

 the substances which are to be sterilized is, more- 

 over, much less energetic than that of steam. Many 

 substances of vegetable and animal origin are rendered 

 useless by subjection to the dry method of sterilization. 

 For these reasons there are comparatively few materials 

 that can be sterilized in this way without seriously 

 impairing their further usefulness. 



Successful sterilization by dry heat cannot usually be 

 accomplished at a temperature lower than 150° C, and 



