STERILIZATION OF CULTURE MEDIA. 57 



has been made either by sterilizing a pure culture by means of heat, 

 or by separating the bacteria from the culture liquid by filtration. 

 Some of these toxic products of bacterial growth are destroyed by a 

 comparatively low temperature ; the method of sterilization by fil- 

 tration is therefore very important in researches relating to the 

 composition and pathogenic power of these soluble products. Pas- 

 teur, in his earlier experiments, used plaster of Paris as a filter, and 



Fig. 30. 



subsequently resorted to the use of unglazed porcelain, through 

 which a liquid may be forced by pressure, but which does not per- 

 mit of the passage of suspended particles, however small. 



As the porcelain filter is the most reliable and convenient for 

 accomplishing the object in view, we shall not describe other methods 

 of filtration which have been proposed and successfully used. The 

 porcelain used is a very fine paste, manufactured at Sevres, which is 

 moulded into cylinders (bougies) of the form proposed by Chamber- 

 land and baked at a high temperature. 



