PLUGGING THE TUBES AND FLASKS 



EXERCISE II 



PLUGGING THE TUBES AND FLASKS AND STERILIZING 

 THE GLASSWARE 



6. After the tubes and flasks are cleaned they must be 

 plugged. The plugged tubes and flasks and the Petri dishes, 

 all of which are to be used for holding culture media or in 

 making cultures, must be sterilized before they can be used. 

 The plugs should be neatly made and of the proper length 

 and firmness. The best quality of absorbent cotton is ordi- 

 narily used for this purpose, although common cotton is 

 employed in some laboratories. If the latter is used, it should 

 be first heated to a very slight browning in the hot-air ster- 

 ilizer. This drives off the oil and kills the spores which it 

 might contain. Glassware is sterilized with dry steam or by 

 means of dry heat, i.e. in the hot-air sterilizer. (See methods 

 for sterilizing apparatus and instruments in text-books.) 



7. Work for this exercise. Plug all the tubes and flasks 

 with absorbent cotton and sterilize them, together with the 

 Petri dishes. 



After they are sterilized, store them in the locker until they 

 are needed. The Petri dishes must not be opened until they 

 are used. 



If the periods are short this exercise and the following one 

 may be worked together. 



8. Plugging the tubes and flasks. For this purpose absorb- 

 ent cotton is used. Rolls of it are cut in short segments 

 of from 5 to 7 cm. in length. A piece of this narrow strip 

 of sufficient length to give cotton enough for the plug is 

 torn off. The quantity varies, of course, with the size of 

 the mouth of the tube or flask, but a little experience will 

 enable one to estimate the quantity quite accurately. The 

 edges of the piece of cotton torn off are turned in and it 



