FILLING THE TUBES. 139 



material should be carefully removed. When the tubes 

 are quite clean they may be rinsed in a warm solution 

 of commercial hydrochloric acid of the strength of 

 about 1 per cent. This is to remove the alkali. They 

 are then to be thoroughly rinsed in clear, running water, 

 and stood top down until the water has drained from 

 them. When dry they are to be plugged with raw 

 cotton ; this 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. 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 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 guide 

 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 necessitates recleans- 

 ing. 



FILLING THE TUBES. When the tubes are cold 

 they may be filled. This is best accomplished by the 



