STERILIZATION BY HEAT. 55 



isms," and represent the forms most resistant to the 

 influence of heat. We are not as yet sufficiently familiar 

 with all their peculiarities to warrant our speaking with 

 certainty as to a means of sterilizing media in which 

 they are present. It does not seem unl ikely that they are 

 of the thermophilic (possibly facultative thermophilic) 

 v^ariety, and they show little tendency to develop into 

 the vegetative stage between the heatings, germinating 

 perhaps so slowly at the temperature luider whicli they 

 find themselves as not to leave completely the spore 

 stage before another exposure to the steam, but mani- 

 festing after a time properties of life in the media that 

 are thought to be sterile and which have been placed 

 aside for subsequent use. This is a mere hypothesis, 

 however, and is as yet entirely wanting in experi- 

 mental 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 steril- 

 ization is only applicable in those cases which present 

 conditions favorable to the germination of the spores 

 into mature vegetative cells. Dry substances, such as 

 instruments, bandages, apparatus, etc., or organic ma- 

 terials in which decomposition is far advanced, where 

 conditions of nutrition favorable to the germination of 

 spores are not present, do not offer the conditions requi- 

 site for the successful operation of the principles under- 

 lying the intermittent method of sterilization. 



The process of fractional sterilization at low temper- 

 atures is based upon exactly the same principle, but 

 differs from the foregoing in the method by which it is 



