FILLING THE TUBES. 105 



drained from 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 the hot-air sterilizer at a temperature of about 

 1 50 C. A very good rule 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 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, 

 pyroligneous, 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. 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 



