50 BACTERIOLOGY. 



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 

 materials 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 materials that 

 can be sterilized in this way without seriously impair- 

 ing their further usefulness. 



Successful sterilization by dry heat cannot usually be 

 accomplished at a temperature lower than 150 C., and 

 to this degree of heat the objects should 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, and leather 

 articles, this method is entirely unsuitable. In bac- 

 teriological work its application is limited to the ster- 

 ilization of glassware principally such, for example, 

 as flasks, plates, small dishes, test-tubes, pipettes and 

 such metal instruments as are not seriously injured by 

 the high temperature. 



Sterilization by moist heat steam offers conditions 

 much more favorable. The penetrating action of the 

 steam is not only more energetic, but the temperature 

 at which sterilization is ordinarily accomplished is, as a 

 rule, not destructive to the objects under treatment. This 

 is conspicuously seen in the work of the laboratory ; the 



