STERILIZATION BY HEAT 85 



of the principles underlying the intermittent method of 

 sterilization. 



Discontinued Sterilization at Low Temperatures. The pro- 

 cess of discontinued sterilization at low temperatures is 

 based upon exactly the same principle, but differs in two 

 respects from the foregoing in the manner by which it is 

 practised, viz., it requires a greater number of exposures 

 for its accomplishment, and the temperature at which it 

 is conducted is not above 68-70 C. It is employed for 

 the sterilization of easily decomposable materials, which 

 would be rendered useless by steam, but which are unal- 

 tered by the temperature employed, and for certain albu- 

 minous culture media that it is desirable to retain in a 

 fluid condition during sterilization, but which would be 

 coagulated if exposed to higher temperatures. This pro- 

 cess 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 days, the interval of twenty-four 

 hours between the exposures admitting of the germination 

 of spores into mature cells. During this interval the sub- 

 stances under treatment are kept at about 25-30 C. The 

 temperature employed in this process suffices to destroy, 

 in about one hour, the vitality of almost all organisms in 

 the vegetative stage. Formerly blood serum was always 

 sterilized by the intermittent method at a low temperature. 



Direct Sterilization. Sterilization by steam is also prac- 

 tised by what may be called the direct method that is to 

 say, both the mature organisms and the spores which may 

 be present in the material to be sterilized are destroyed 

 by a single exposure to the steam. In this method steam 

 at its ordinary temperature and pressure live steam or 

 streaming steam, as it is called is employed just as in the 



