98 BACTERIOLOGY. 



them. When dry they are to be plugged with raw 

 cotton. The 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 

 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 with cotton are 

 now to be placed upright in a wire basket and heated 

 for one hour in ths 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, and 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 Pig. 19. 

 The liquefied medium is poured into this funnel, which 

 has been carefully washed, and by pressing the pinch- 

 cock with which the funnel is provided, the desired 



