42 BACTERIOLOGY. 



interval of twenty-four hours being allowed between the 

 exposures to this temperature for 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 sufBces to 

 destroy the vitality of almost all organisms in the vege- 

 tative stage in about one hour. Blood-serum is 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 

 ordinary temperature and pressure — live steam or stream- 

 ing 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 one hour. Or, steam 

 under pressure, and consequently of a higher tempera- 

 ture, is now frequently employed. In this method a 

 single exposure of fifteen minutes is sufficient for the 

 destruction of 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 equivalent to 

 an approximate temperature of 122° C. to which the 

 organisms are exposed. 



The objection to both of these methods of direct 

 sterilization by steam is that many substances which it 

 is desirable to retain in as near their normal condition 

 as possible are materially altered by this energetic form 

 of treatment. Gelatin is not only rendered cloudy, but 

 loses the power of gelatinzlng. Many of the other 

 media contain always a fine precipitate after this method ; 

 in fact, for most of the media which we employ, the 



