16 GENERAL MICROBIOLOGY 



microorganisms present on its surface and in or on the cotton 

 or paper used respectively for plugging or wrapping. After 

 sterilization the cotton and paper serve to prevent micro- 

 organisms from entering and contaminating the sterile 

 utensils. 



Dry heat, though not as effective a germ destroyer as 

 moist heat, is more adaptable to the sterilization of empty 

 culture flasks, pipettes and other glassware. Hot-air steril- 

 ization not only accomplishes the sterilization of the glass- 

 ware, cotton plugs, etc., but " sets " the plugs so that they 

 may be handled with greater facility. 



All glassware must be absolutely clean and dry or contain 

 traces of alcohol only before preparing for sterilization; 

 otherwise sterilization cannot be accomplished. If consider- 

 able moisture is present in test tubes, flasks, etc., it will not 

 evaporate during the hot-air sterilization process, and it is 

 very evident that the temperature of such moist portions 

 of the glassware will not reach or at least- will not exceed 

 100 C. 



Directions. Test tubes and flasks are plugged with 

 cotton. The ordinary forceps are used for this purpose. 

 (A 'glass rod may also be used.) A small piece of cotton is 

 grasped on the edge with the forceps and inserted in the 

 mouth of the test tube. Plugs should project into test 

 tubes from 3 to 4 cms., and from 3 to 5 cms. into the 

 neck of flasks, according to the size of the flask. Only an 

 amount of cotton should project out of the mouth that is 

 sufficient to protect the outward turned portion (lip) of the 

 test tubes or flasks from dust. A " Christmas-tree " effect 

 is to be avoided. Plugs should not be so tight as to be 

 removed with difficulty, nor so loose as to offer no resistance 

 to removal. A little experience will suffice to demonstrate 

 the amount of cotton to use and the firmness with which 

 the plug should fit. 



Cotton plugs for test tubes, flasks, etc., may be rolled. 

 This kind of plug is more stable and may be used several 



