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 temperdture for 24 hours, and 
