56 BACTERIOLOGY. 



practised in two respects, viz., it requires a greater 

 number of exposures for its accomplishment, and the 

 temperature at which it is conducted is not raised above 

 68°-70° C. It is employed for the sterilization of easily- 

 decomposable materials, which would be rendered use- 

 less by steam, but which remain intact at the tempera- 

 ture employed, and for certain albuminous culture 

 media that it is desirable to retain in a fluid condition 

 during sterilization, but which would be coagulated if 

 exposed to high temperatures. 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 days, the interval of twenty-four hours 

 between the exposures admitting of the germination 

 of spores into mature cells. During this interval the 

 substances under treatment are kept at about 25°-30" 

 C. The temperature employed in this process suffices 

 to destroy the vitality of almost all organisms in the 

 vegetative stage in about one hour. Until recently 

 blood-serum was always sterilized by the intermittent 

 method at low temperature. 



Sterilization by steam is also practised 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 ordi- 

 nary temperature and pressure — live steam or streaming 

 steam, as it is called — is employed just as in the first 

 method described, but it is allowed to act for a much 

 longer time, usually not less than an hour; or* steam 

 under pressure, and consequently of a higher tempera- 

 ture, is now frequently employed. By tlie latter pro- 

 cedure a single exposure of fifteen minutes is sufficient 



