38 PHARMACEUTICAL BACTERIOLOGY. 



thick, as it will then not permit enough cotton to enter the opening n or 

 yet too thin, as it will then be forced through the cotton. The plug must 

 not be too tight, as that would interfere with subsequent manipulations nor, 

 yet too loose, for obvious reasons. Enough cotton should project above the 

 opening to permit of ready grasping between the fingers in the later 

 operations. 



Plugging may also be done with fingers alone, but this is tedious and non- 

 professional. A far better method is to use a pair of fairly large blunt- 



FIG. 14. A hot air sterilizer. These sterilizers are double-walled, on stand, with per- 

 forations at top for thermometers. Ordinary baking ovens which can be secured from 

 hardware dealers will serve the purpose. 



pointed pincers. Remove the cotton from the roll by means of the pincers 

 and insert it into the test-tube with the pincers. 



Whatever method is used, remove the amount of cotton required to plug 

 one tube or flask at one time. Do not attempt to plug with several small 

 pieces. If an excess of cotton projects above the opening, pluck it away 

 with the fingers; do not cut it away with scissors. Plug the tubes as 

 uniformly as possible. 



3. Filling Test-tubes with Culture Media. 



The rule is to pour the culture media hot, although this is not absolutely 

 essential. For example, if the media are liquid in the cool or cold state, as 

 bouillon, serum, milk, etc., they may be poured cold. A good rule is to pour 

 a desired amount of the media just as soon as they are prepared, whether 

 they are still hot or merely warm or cold. Of course, gelatin and agar 

 media must be poured hot or must be liquefied before they can be poured. 



