134 BIOLOGY AND TECHNIQUE 



thorough mechanical cleansing is practiced, and the glassware may 

 be treated by acid and alkali followed by running water, as given 

 above. These last steps, however, are not essential, thorough wash- 

 ing 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 recep- 

 tacles is important, in that, when poorly 'plugged, sterility is not 

 safeguarded, and the purpose 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 

 laboratories, 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 between the surface of the glass and the periphery of the 

 plug; for these, if present, may serve as channels for the entrance 

 of bacteria. The plugging itself is carried out by tearing a small 

 piece of cotton, about 2X2 inches, from the roll, folding over one 

 of its corners, and, applying the smooth end of a glass rod to the 

 folded portion, gently pushing it into the mouth of the tube. 



After plugging and before media are introduced into the tubes 

 and flasks, these should be sterilized. This is best done in one of 

 the "hot-air sterilizers," by exposing the tubes for one hour to a 

 temperature of 150 C. If greater speed is desired exposure to 

 180 to 190 C. for half an hour is usually safe. If by mistake, 

 however, the temperature is allowed to rise above 200 C., a brown- 

 ing of the cotton plugs occurs and the glassware is apt to be stained 

 by the burning of the fat and other organic material derived from 

 the cotton. Petri dishes after cleansing are fitted together, and then 

 sterilized in the hot-air chamber at 150 C. for one hour. 



Glassware so prepared is ready for the reception of media. 



Ingredients of Culture Media. The food requirements of bac- 

 teria have been discussed in another section. From what has there 



