"FRACTIONAL" STERILIZATION. 41 



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 at any time of long 

 duration. 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 pressure, 

 for fifteen minutes on each of three successive days, and 

 during the intervening days to retain them at a temper- 

 ature of about 25-30 C. At the end of this time all 

 living organisms which were present will have been de- 

 stroyed, 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 steriliza- 

 tion 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 proper conditions for 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 in two respects, viz., it requires a longer time for 

 its accomplishment, and the temperature at which it is 

 conducted is not raised above 68-70 C. It is em- 

 ployed for the sterilization of easily decomposable mate- 

 rials, which would be rendered useless by the tempera- 

 ture of steam, but which remain intact at the temperature 

 employed. This process requires that the material to be 

 sterilized should be subjected to a temperature of 68- 

 70 C. for one hour on each of six successive clays, an 



