22 LABORATORY GUIDE IN BACTERIOLOGY 



medium. Place the material to be sterilized in the basket. 

 Fit the lid of the autoclav by joining corresponding 

 marks (usually a number or a letter) on the side of the 

 lid and margin of the main body. Then tighten the 

 thumbscrews by hand, always tightening those diamet- 

 rically opposite to each other at the same time. See that 

 the steam-valve is open, and then light the gas. Regu- 

 late the safety-valve to blow off at the required pressure, 

 if provided with the means of doing this. 



It is necessary to allow the steam to escape for about 

 one minute, in order to drive all air out of the cyhnder. 

 Then close the steam-valve. The pressure will begin to 

 increase, as indicated by the hand of the manometer. 

 When the desired temperature is reached, shut off the 

 gas from the outer ring of burners and regulate the gas 

 pressure of the inner rings, to maintain the proper tem- 

 perature. It is sufficient to expose the media to a tem- 

 perature of 120° for 5 minutes. After this has been 

 accomplished, shut off the gas and allow to cool to 

 normal pressure, or ioo° or below. The lid should not 

 be released before this, as a sudden diminution of pressure 

 causes the contents of the tubes suddenly to boil up and 

 push the stoppers out. 



The steam-valve should be opened when the auto- 

 clav has cooled to ioo° or below. This will guard 

 against the possibility of the top blowing off and injuring 

 the operator, if the gauge does not register properly. 

 On the other hand, the suction caused by the contrac- 

 tion of the steam may hold the lid down, so that it cannot 

 be removed until the pressure is equalized. 



When sterilizing media which contain carbohydrates 

 (glucose-agar, for instance), it is important not to allow 



