62 BACTERIOLOGY. 



circumstance the temperature to which they are ex- 

 posed becomes more and more elevated as the pressure 

 increases. 



Experience has taught lis that the process of ster- 

 ilization by dry heat is of limited application because 

 of its many disadvantages. For successful sterilization 

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

 high temperature needed, but the substances under 

 treatment must be exposed to this temperature for a 

 comparativel)' long time. The penetration of dry heat 

 into materials which are to be sterilized is, moreover, 

 much less thorough than that of steam. Many sub- 

 stances of vegetable and animal origin are rendered 

 valueless by subjection to the dry method of sterilization. 

 For these reasons comparatively few materials 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 to this degree of heat the objects should be sub- 

 jected 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 wholly unsuitable. In 

 bacteriological work its application is limited to the 

 sterilization 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 poAver of the 



