114 BIOLOGY AND TECHNIQUE 



steps, however, are not essential, thorough washing in hot water after 

 the soapsuds or soda solution being usually sufficient to yield good 

 results. Other workers have recommended immersion of the glassware 

 after mechanical cleansing in five per cent to ten per cent potassium 

 bichromate solution in twenty-five per cent sulphuric acid. This is 

 followed by thorough washing in hot running water, and drying. 



Clean flasks and test tubes are then stoppered with cotton, which has 

 been found to be a convenient and efficient seal against the bacteria 

 of the air, catching them in the meshes of the fibers as in a filter. The 

 technique of the stoppering or plugging of glass receptacles is important. 



FIG. 15. FLORENCE FLASK. 



FIG. 16. ERLENMEYER FLASK. 



in that, when poorly plugged, sterility is not safeguarded, and the pur- 

 pose of culture study is defeated. 



In almost all laboratories in this country non-absorbent cotton or 

 " cotton batting " is used for the plug. In a few of the German labora- 

 tories the absorbent variety is employed. The disadvantages of the 

 latter, especially in the case of fluid media, are obvious. The plugs 

 should fit snugly, but not so tightly that force is necessary to remove 

 them. Care should be taken, furthermore, that no creases are left be- 

 tween the surface of the glass and the periphery of the plug; for these, 

 if present, may serve as channels for the entrance of bacteria. Fig. 

 18, accompanying, will illustrate some of the more common and un- 

 desirable defects in poorly made plugs. The plugging itself is carried 

 out by tearing a small piece of cotton, about 2X2 inches, from the roll, 



