PA STE URIZA TION. 



51 



the best. Unstable toxins and antitoxins, which are destroyed 

 easily by heat, are filtered through porcelain. 



The objection to this method of sterilization is that it is 

 effective for a time only. All these filters become clogged 

 with bacteria in the course of a very short time, and then the 

 bacteria are carried through en masse. 



Before a new filter is used, it should be sterilized by dry 

 heat. Filters should be cleansed thoroughly at least once a 

 week. Porcelain filters are scrubbed and then heated in 

 the flame of a blowpipe or Bunsen burner until all the con- 

 tained organic matter is consumed. As the organic matter 



FIG. 16. 



Wire test-tube basket (see p. 37;. 



is charred the filter turns black, but it gradually regains its 

 white color as the organic matter is consumed. Sand, pow- 

 dered glass, etc., are replaced with new material as often as 

 is necessary. 



PASTEURIZATION : This is partial sterilization at a com- 

 paratively low temperature with an exposure of two or three 

 hours. The temperature should be high enough to destroy 

 not only the saprophytic bacteria, but also the pathogenic 

 bacteria, especially the tubercle bacillus and the Bacillus 

 typhosus. Pasteurization is used principally for the steril- 

 ization of milk. 



The milk, contained in a bottle, is placed in hot water at a 



