110 BACTERIOLOGY. 



dry they are to be plugged with raw cotton. The plug- 

 ging 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 regu- 

 larly all around. 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 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 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. 



The cotton used for this purpose should be the ordi- 

 nary 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, 

 empyreumatic, oily deposit, which renders them unfit 

 for use until they have been 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. 22. The liquefied medium is poured into this 

 funnel, which has been carefully washed, and by 

 pressing the pinchcock with which the funnel is pro- 

 vided the desired amount of material (5-10 c.c.) 



