52 BACTERIOLOGY. 



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 prolonged application of heat might 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 to heat which 

 the organisms to be destroyed are known to possess at 

 different stages of their development. During the life- 

 cycle of many of the bacilli there is a stage in which 

 the resistance of the organism to the action of both 

 chemical and thermal agents is much greater than at 

 other stages of their development. This increased 

 power of resistance appear.s when these organisms are 

 in the spore or resting stage, to which reference has 

 already been made. When in the vegetative or grow- 

 ing stage most bacteria are killed in a short time by a 

 relatively low temperature, whereas, under conditions 

 which favor the production of spores, the spores are 

 seen to be capable of resisting very much higher tem- 

 peratures for an appreciably longer time; indeed, spores 

 of certain bacilli have been encountered that retain the 

 power of germinating after an exposure of from five to 

 six hours to the temperature of boiling water. Such 

 powers of resistance have never been observed in the 

 vegetative stage of development. These differences in 

 resistance to heat which the spore-forming organisms 

 possess at their different stages of development are 

 taken advantage of in the process of sterilization by 



