78 BACTERIOLOGY. 



plugging with the cotton requires a little practice before it 

 can be properly done. The cotton should be introduced 

 into the mouths of the tubes in such a way that no cracks 

 or creases exist, but should fill them quite regularly all 

 around. The plugs should be neither too tight nor too 

 loose, the regular rule being that when in position the 

 plug should fit tight enough to just 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 with cotton 

 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 rule for this process of 

 sterilizing 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 down. 



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, aud is, 

 moreover, much too expensive for this purpose. 



Care should be taken not to burn the cotton, other- 

 wise the tubes will become coated with a dark-colored 

 oily deposit which renders them unfit for use, and they 

 will have to be cleaned again. 



Filling the Tubes.— When the tubes are cold 

 they may be filled. This is best accomplished by the 

 use of a spherical form of funnel, such as is shown in 

 Fig. 9. The liquefied medium is poured into this funnel, 

 which has been carefully washed, and by pressing the 

 pinch-cock with which the fuunel is provided, the 



