BACTERIOLOGICAL TECHNIQUE. 223 
cent. carbolic acid solution for at least thirty minutes 
and then rinsed again with sterile water. New tubes 
and flasks sometimes require to be washed in a 2 per 
cent. solution of nitric acid, so as to remove any free 
alkali which may be present. They are finally thor- 
oughly rinsed in pure water. Old tubes, flasks, and 
other glassware are boiled for about thirty minutes ina 
5 per cent. solution of washing soda and then thoroughly 
rinsed off with water until perfectly clean. If neces- 
Dry heat sterilizer. 
sary, any dirt clinging to the insides of the flasks 
and tubes can be removed by bristle brushes or suit- 
able swabs. After the tubes and flasks have been 
thoroughly cleaned they are plugged loosely with ordi- 
nary cotton batting, or, if that is not at hand, the more 
expensive absorbent cotton. The tubes and flasks with 
their cotton plugs and all other glassware are sterilized 
by dry heat at 150° C. for one hour (Fig. 26). 
The sterile tubes and flasks are filled with the media, 
when small quantities are used, by means of a glass 
funnel. The main precaution to be observed is not to 
let the media soil the neck of the tubes and flasks, as 
