STERILIZATION BY HEAT. ^\ 



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 cul- 

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

 gelatin, agar-agar, potato, etc., are under no circum- 

 stances to be subjected to dry heat. 



The way in which heat is employed in processes 

 of sterilization varies with circumstances. In its em- 

 ployment as dry heat its application is always contin- 

 uous — i. e., the objects to be sterilized are simply exposed 

 to the proper temperature for the length of time neces- 

 sary to destroy all living organisms which may be upon 

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

 objects to be sterilized are frequently of such a nature that 

 a prolonged application of the heat would materially 

 injure them. For this and other reasons steam is usually 

 applied intermittently and for short periods of time. 

 The principles involved in this method of sterilization 

 depend upon differences of resistance toward heat which 

 the organisms to be destroyed are seen to possess at 

 different stages of their development. During the life 

 histol^ of many of the bacilli there is a time in which 

 the resistance of the organism toward the action of both 

 chemical and thermal agents is much higher than at 

 other stages of its development. This increased power 

 of resistance is seen to exist when these organisms are 

 in the spore or reding stage, to which reference has 



