120 



A MANUAL OF BACTERIOLOGY 



Before using the autoclave sufficient clean water is poured in to 

 cover the bottom to a depth of o cm. It is best to use distilled water 

 and to renew it frequently. If the water is dirty or contains media 

 escaping from broken vessels, it is liable to foam up and wet the 

 cotton plugs. 



The lid is securely clamped after the media are put in and the 

 gas burners lighted. The stopcock must remain open for the escape 

 of air, since the requisite temperature may not be attained unless 

 the autoclave contains nothing but steam. When the thermometer 

 registers 100 C. and steam issues rapidly from the stopcock, it may 

 be assumed that the air has been displaced, and the stopcock may 

 then be closed. The gas supply may then be reduced somewhat 

 and pressure allowed to run up to 10 or 15 pounds. The contents 

 of the sterilizer are held at this pressure for twenty to thirty min- 

 utes. The gas is then shut oft' and the sterilizer allowed to cool. The 

 stopcock may be gradually opened as the pressure falls, but caution 

 should be used, because, if the pressure is too suddenly reduced, the 

 superheated media may boil over and dislodge the plugs. If direct 

 steam is used instead of a gas burner, much time and expense will 

 be saved. 



The corresponding pressures and temperatures are shown in the 

 following table : 



Partial sterilization Pasteurization. For many industrial purposes 

 it is not necessary to obtain complete sterilization, provided certain 

 forms can be killed. This is readily accomplished by the use of tem- 

 peratures somewhat below the boiling point of water. The process 

 is named for Pasteur, who first suggested its use in the treatment of 

 grape must to overcome wine faults. 



In this country Pasteurization is extensively used in commercial 

 dairy work. In different establishments different temperatures are 

 used. As a usual thing the milk is heated to 60-65 C. for twenty 

 minutes. In what is known as the continuous process the milk is sub- 

 jected to a momentary heating of 85 C., and is then quickly cooled. 



