STERILIZATION. 67 



has been heated with a rose-burner for twenty to thirty-five 

 minutes. 



The sterilizer invented by Koch is still largely in use. 

 It is a tall cylindrical tin vessel covered with asbestos or 

 felt. The lower portion is filled with water; on the side 

 is a water-gauge indicating the height of the water, in order 

 that one may observe when there is danger of the sterilizer 

 boiling dry. Over the top there is a tight-fitting cover. 

 The steam is generated by a Bunsen burner standing under- 

 neath. A perforated shelf placed some distance above the 

 surface of the water is for the reception of the tubes and 

 flasks that are to be sterilized. 



The sterilization of blood-serum sometimes has to be per- 

 formed in a specially devised sterilizer, when a clear, fluid 

 medium is desired. In this case the serum is heated for an 

 hour on each of six consecutive days to a temperature of 

 only 58 C. To obtain a transparent but solid medium the 

 serum is kept at a temperature of 75 C. for an hour on 

 each of four consecutive days. The process must be con- 

 ducted carefully to avoid clouding of the serum. 



Pasteurization. The name pasteurization has been ap- 

 plied to the partial sterilization of substances at a compara- 

 tively low temperature. It is employed particularly for 

 milk. The temperature used (70 to 75 C. for 20 to 30 

 minutes) is sufficient to destroy all ordinary pathogenic 

 bacteria ; for example, the bacilli of tuberculosis and typhoid 

 fever. Furthermore, the great majority of the saprophytic 

 bacteria are destroyed, and milk which has been pasteurized 

 will remain unchanged for several days, if kept cool. Its 

 application is principally in the feeding of infants when 

 ordinary milk has been found to produce undesirable results. 

 Freeman 1 has invented a pail of special form for the pas- 

 teurization of milk in bottles. This pail is filled with hot 



1 Medical Record, July 2, 1892, and August 4, 1894. 



