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 in a 

 5 per cent, solution of washing soda and then thoroughly 

 rinsed off with water until perfectly clean. If neces- 



FIG. 26. 



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 



