76 BACTERIOLOGY 



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 comparatively long time. The penetra- 

 tion of dry heat 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 

 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 subjected for not 

 less than one hour. For the sterilization, therefore, of the 

 organic materials of which the media employed in bacterio- 

 logical 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. 



Methods Employed. — Sterilization by moist heat — steam — 

 offers conditions much more favorable. The penetrating 

 power of the steam is not only more energetic, but the tem- 

 perature at which sterilization is ordinarily accomplished is, 

 as a rule, not destructive to the objects under treatment. 

 This is conspicuously seen in the work pi the laboratory; 

 the culture-media, composed in the main of decomposable 

 organic materials that would be rendered entirely worthless 

 if exposed to the dry method of sterilization, sustain no 



