THE METHODS OF STERILIZATION. 



81 



sure of the destruction of all spores which may have resisted the 

 two previous applications. It has been demonstrated that 

 this sterilization is very effective and complete, and substances 

 which have undergone it may be kept indefinitely bacteria- 

 free thereafter. 



5. Another method of fractional sterilization is occasionally 

 used for certain media which cannot withstand the tempera- 

 ture of boiling water without deteriorating. It consists in 



FIG. 40. 



Steam sterilizer, pattern of Koch. (Abbott.) 



submitting the medium to a temperature of 68 to 70 C. for 

 from two to three hours during seven consecutive days. This 

 method is necessarily yery imperfect, and though successful 

 in those cases in which no spores have to be destroyed and 

 where none of the bacteria contained in the medium are of 

 the pyogenic variety, it cannot but have a very limited 

 application, and media so sterilized should not be used for 

 6 M. B. 



