6 LABORATORY BACTERIOLOGY 



is rolled up to form a firm plug which should snugly fit the 

 neck of the tube or flask. It should be inserted into the tube 

 for about 2 cm. and the end should be nearly flat and smooth. 

 The projecting part should be about the same length and 

 of equal firmness. (For method of closing the tubes more 

 securely, see § 22.) 



9. Sterilizing glassware, (a) Hot air. Place all the test 

 tubes, flasks, and Petri dishes in the hot-air sterilizer, close the 

 door tightly, and light the gas. Heat the air in the sterihzer 

 to a temperature of from 135° to 150° C. and keep it there 

 for one hour, not allowing it to rise above 150° C. Turn 

 the gas off, and when the temperature of the air in the ster- 

 ilizer goes down to or below 45" C. the door may be opened 

 and the apparatus removed. 



(ti) Dry steam. SteriHze the fermentation tubes in the auto- 

 clave at 15 lbs. pressure for half an hour. This method pre- 

 vents a considerable amount of breakage, especially of the 

 stand of the tube, that often occurs when these tubes are 

 heated in the hot-air sterilizer. 



