20 BACTERIOLOGY. 



Frac- will kill bacteria in thirty minutes, but not their spores. 



steriii- In order to destroy the spores fractional sterilization is 

 employed. This is done by sterilizing for thirty 

 minutes on three successive days. By the first exposure 

 the bacteria are killed, but the spores that may be 

 present are not. The dressings are now allowed to 

 stand for twenty-four hours at roomi temperature in 

 order to allow the spores to develop into bacteria, 

 when another exposure of thirty minutes is made. 

 This is repeated at the end of another twenty-four 

 hours. At the end of the third exposure it is presumed 

 that all spores have developed into bacteria, and that 

 all bacteria have been destroyed by steam. Live steam 

 is also used for killing bacteria in milk, and will be 

 considered later, 

 zafion^by -^7 ^ ar tne miOS t effective method of sterilizing by 



unde? heat is the use of steam under pressure. The action of 

 the steam is intensified and its penetrating power in- 

 creased by the pressure. The instrument used is called 

 an autoclave. It consists of a double-walled cylinder 

 or globe made of metal, with a steam gauge and vent 

 at the top. The materials to be sterilized are placed 

 in the inner chamber, the door closed, and the steam 

 allowed to enter the outer jacket. The vent at the top 

 is left open until the steam has forced out all the air, 

 as air interferes with the action of the steam. After 

 closing the vent the steam is forced into the inner 

 jacket until the gauge shows a pressure of 15 pounds, 

 or one atmosphere, and allowed to< remain so for 

 fifteen to twenty minutes. This exposure will kill all 



