CHAPTEE III. 



Principles of sterilization by heat — Different methods employed — 

 Principles of discontinued sterilization — Sterilization under pressure 

 — Apparatus employed. 



By the term sterilization, as employed here, is under- 

 stood the destruction of bacteria by heat. It is accom- 

 plished in two ways : either by dry heat, or by moist 

 heat in the form of steam. 



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 ster- 

 ilization by the method of dry heat, not only is a rela- 

 tively high temperature essential, but the substances 

 under treatment must be exposed to this temperature for 

 a comparatively long time. Its penetrating action into 

 the substances which are to be sterilized is, moreover, 

 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 substances which 

 may be sterilized in this way without serious damage to 

 their further usefulness. 



Successfiil sterilization by dry iieat cannot usually be 

 accomplished at a temperature lower than 150° C, 

 and to this degree of heat must the objects be subjected 

 for not less than one hour. For the sterilization, 

 therefore, of the organic materials of which the media 

 employed in bacteriological work are composed, and 

 of domestic articles, such as cotton, woollen, wooden, 



