STERILIZATION BY HEAT. 51 



ture at which sterilization is ordinarily accomplished is, 

 as a rale, not destructive to the objects under treat- 

 ment. This is conspicuously seen in the work of the 

 laboratory; the culture media, composed in the main 

 of decomposable organic materials that would be ren- 

 dered entirely worthless if exposed to the dry method 

 of sterilization, sustain no injury whatever when intel- 

 ligently subjected to an equally effective sterilization 

 with steam. The same may be said of cotton and 

 woollen fabrics, bedding, clothing, etc. 



Aside from the relations of the two methods to the 

 materials to be sterilized, their action toward the organ- 

 isms to be destroyed is quite different. The penetrating 

 power of the steam renders it by far the more efficient 

 agent of the two. The spores of several organisms 

 which are killed by an exposure of but a few moments 

 to the action of steam, resist the destructive action of 

 dry heat at a higher temperature for a much greater 

 length of time. 



These differences will be strikingly brought out in 

 the experimental work on this subject. For our pur- 

 poses it is necessary to remember that the two methods 

 have the following applications : 



The dry method, at a temperature of 150-180 C. 

 for one hour, is employed for the sterilization of glass- 

 ware : flasks, test-tubes, culture-dishes, pipettes, plates, 

 etc. 



The sterilization by steam is practised with all culture 

 media, whether fluid or solid. Bouillon, milk, gelatin, 

 agar-agar, potato, etc., are under no circumstances to 

 be subjected to dry heat. 



The manner in which heat is employed in processes 

 of sterilization varies with circumstances. When used 



