PLUGGING THE TUBES AND FLASKS 5 
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 
