80 BACTERIOLOGY 



injury whatever when intelligently 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 mate- 

 rials to be sterilized, their action toward the organisms to 

 be destroyed is quite different. The penetrating power of 

 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 purposes 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 glassware such 

 as flasks, test-tubes, culture-dishes, pipettes, plates, etc. 



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. 



Discontinued Sterilization. The manner in which heat is 

 employed in processes of sterilization varies with circum- 

 stances. When used in the dry form its application is always 

 continuous i. e., the objects to be sterilized are simply 

 exposed to the proper temperature for the length of time 

 necessary to destroy all living organisms which may be upon 

 them. With the use of steam, on the other hand, the articles 

 to be sterilized are frequently of such a nature that a pro- 

 longed application of heat might materially injure them. 

 For this and other reasons steam is usually applied inter- 



