STERILIZATION BY HEAT. 69 



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, 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. 



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 for not less than an hour ; or steam 

 under pressure, and consequently of a higher tempera- 

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

 cedure a single exposure of fifteen minutes is sufficient 

 for the destruction of practically all bacilli and their 

 spores, providing the pressure of the steam is not less 

 than one atmosphere over and above that of normal ; 

 this is approximately equivalent to a temperature of 

 122° C. to which the organisms are exposed. 



The objection that has been urged to both of these 

 methods, particularly that in which steam under press- 

 ure is employed, is that the properties of the media 

 are altered. Gelatin is said to become cloudy and lose 

 the property of solidifying ; in bouillon and agar-agar 



