50 BACTERIOLOGY. 



circumstance the temperature to which they are ex- 

 posed becomes more aud more elevated as the pressure 

 increases. 



Experiments have taught us that the process of ster- 

 ilization by dry heat is of limited application because 

 of its many disadvantages. For successful steriliz- 

 ation 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 penetration into 

 materials which are to be sterilized is, moreover, much 

 less thorough 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 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 exam- 

 ple, 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 power of the 

 steam is not only more complete, but the tempera- 



