PLUGGING TEST-TUBES. 25 



tube between those of the left, and gradually screw 

 the plug of wool into its mouth for a distance of about 

 2.5 cm., leaving about the same length of wool pro- 

 jecting. 



The plug must be firm and fit the tube or flask fairly 

 tightly, sufficiently tightly in fact to bear the weight 



FIG. 24. Plugging test-tubes: a, cylinder of wool being rolled; &, cylinder 

 of wool being doubled; c, cylinder of wool being inserted in tube. 



of the glass plus the amount of medium the vessel is 

 intended to contain, but not so tightly as to prevent 

 it from being easily removed by a screwing motion 

 when grasped between the fourth, or third and fourth 

 fingers, and the palm of the hand. 



For a large flask a similar but larger strip of wool 

 must be taken; the method of making and inserting 

 the plug is identical. 



