Cleaning the Glassware 131 



pull off a piece of cotton of sufficient size to fit into 

 the opening of the article to be plugged without dan- 

 ger of falling out or of sticking. Put the pipettes into 

 glass tubing which has been cut into lengths which 

 will hold the pipettes, then plug firmly both ends of 

 the tube or case. After plugging, the articles are 

 ready for sterilizing. Metal boxes holding a number 

 of pipettes may be used in place of glass tubes; and 

 when much plating is to be done, there is economy 

 of work and time in the practice, but there is greater 

 possibility of the pipettes becoming contaminated. 

 Place them with the petri dishes and glass bottles in 

 the hot-air sterilizer or oven. Tip the stoppers of the 

 glass bottles back, to prevent exploding. In many 

 laboratories, dilution bottles without glass stoppers are 

 used, plugs of cotton being substituted. This custom 

 permits the use of cheaper bottles. These should be 

 kept at a temperature of 150 C. for one hour, or 

 until the cotton plugs are slightly browned. 



Water. It is convenient to sterilize water in the 

 liter flasks. Fill the flasks three -fourths full and 

 plug them with cotton. If an autoclave is available, 

 sterilize water for one hour at 15 pounds pressure. 

 Otherwise, place the flask in boiling water, or steam 

 and hold for one and a half to two hours. 



Bouillon, gelatin and agar. Media can be steril- 

 ized in an autoclave for 20 minutes at 10 pounds pres- 

 sure, or the intermittent method may be used. In 

 sterlizing by this method, the flasks or tubes of media 

 are steamed or placed in boiling water for 20 minutes; 

 then left at room temperature for 24 hours, and 



