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 sterilizer 
to a temperature of from 135° to 150°C. and keep it there 
for one hour, not allowing it to rise above 150° C. Tum 
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. 
(4) Dry steam. Sterilize 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. 
