PRINCIPLES OF STERILIZATION BY HEAT. 39 



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

 etc. 



The sterilization by steam is practised with all media, 

 whether fluid or solid. Bouillon, milk, gelatin, agar- 

 agar, potato, etc., are under no conditions to be subjected 

 to the dry heat. 



The methods by which heat is employed in processes 

 of sterilization vary with circumstances. In its em- 

 ployment as dry heat 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 steam, on the other hand, the objects to be steril- 

 ized are frequently of such a nature that a prolonged 

 application of the heat would materially injure them. 

 For this reason 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 history 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 devel- 

 opment. This increased power of resistance is seen to 

 exist when these organisms are in the spore or resting 

 stage, to which reference has already been made. 

 When in the vegetative or growing stage, most of 

 these organism are killed in a short time by a relatively 

 low temperature, whereas, when conditions have arisen 

 which favor the production of spores, these spores are 

 seen to be capable of resisting very much higher tem- 

 peratures for an appreciably longer time. These differ- 



