STERILIZATION BY HEAT. 55 



the thermophilic (possibly facultative thermophilic) va- 

 riety, and show little tendency to develop into the vegeta- 

 tive stage between the heatings, germinating perhaps so 

 slowly at the temperature under which they find them- 

 selves as not to completely leave the spore stage before 

 another exposure to the steam, but manifesting after a 

 time properties of life in the media that is thought to be 

 sterile arid which has been placed aside for subsequent use. 

 This is a mere hypothesis, however, and is as yet entirely 

 .wanting in experimental proof. 



Fortunately, these undesirable experiences are rare, 

 but that they do occur, and result in no small degree of 

 annoyance, is an experience that has probably been had 

 by most bacteriologists. 



It must be borne in mind that this method of sterili- 

 zation is only applicable in those cases which present 

 conditions favorable to the germination of the spores 

 into mature vegetative cells. Dry substances or organic 

 materials in which decomposition is far advanced, where 

 conditions of nutrition favorable to the germination of 

 spores are not present, cannot be successfully sterilized 

 by the intermittent method. 



The process of fractional sterilization at low tempera- 

 tures is based upon exactly the same principle, but dif- 

 fers from the foregoing in the method by which it is 

 practised in two respects, viz., it requires a greater num- 

 ber of exposures for its accomplishment, and the tem- 

 perature at which it is conducted is not raised above 

 68-70 C. It is employed for the sterilization of easily 

 decomposable materials, which would be rendered use- 

 less by the temperature of steam, but which remain 

 intact at the temperature employed, and for certain 

 albuminous culture media that it is desirable to retain 



