184 BACTERIOLOGY 



exist. The plug should fit neither too tightly nor too loosely, 

 but should be just firmly enough in position to sustain the 

 weight of the tube into which it is placed when held up by 

 the portion which projects from and overhangs the mouth 

 of the tube. The tubes thus plugged are now to be placed 

 upright in a wire basket and heated for one hour in the hot- 

 air sterilizer at a temperature of about 150° C. A very good 

 guide for this process of sterilization is to observe the tubes 

 from time to time, and as soon as the cotton has become a 

 very light-brown color, not deeper than a dark^ cream tint, 

 to consider sterilization complete. The tubes are then 

 removed and allowed to cool. 



Fig. 21 

 a 



Brushes for cleaning test-tubes. 



The cotton used for this purpose should be the ordinary 

 cotton batting of the shops, and not absorbent cotton; 

 the latter becomes too tightly packed, and is, moreover, 

 much too expensive for this purpose. 



Care should be taken not to burn the cotton, otherwise 

 the tubes will become coated with a dark-colored, empyreu- 

 matic, oily deposit, which necessitates recleansing. 



Filling the Tubes. — ^When the tubes are cold they may be 

 filled. This is best accomplished by the use of a separating 

 funnel, such as is shown in Fig. 22. The liquefied medium 

 is poured into this funnel, which has been carefully washed, 



