''FRACTIONAL" STERILIZATION. 53 



tain spores that might have been present resist this 

 treatment, providing the sterilization has not been con- 

 tinued for too long a time. If now the sterilization is 

 discontinued, and the material which presents conditions 

 favorable to the germination of the spores is allowed to 

 stand for a time, usually for about twenty-four hours, 

 at a temperature of from 30°— 35° C, those spores which 

 resisted the action of the steam will in the course of this 

 interval germinate into the less resistant vegetative cells. 

 A second short exposure to the steam kills these forms 

 in turn, and by a repetition of this process all organisms 

 which were present may be destroyed without the appli- 

 cation of the steam having been of long duration at 

 any time. In this process the usual plan is to subject 

 the materials to be sterilized to the action of steam, 

 under the normal conditions of temperature and pres- 

 sure, for fifteen minutes on each of three successive 

 days, and during the intervals to retain them at a 

 temperature of about 25°-30° C. At the end of this 

 time all living organisms which were present will 

 have been destroyed, and unless opportunity is given 

 for the access of new organisms from without, the 

 substances thus treated remain sterile. 



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 

 the 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- 



