24 BACTERIOLOGY FOR NURSES 



If the soapsuds is not effective and the glass has 

 become cloudy with use, a cleaning mixture may be 

 used which contains — 



Potassium dichromate 60 parts 



Water 300 parts 



Concentrated sulphuric acid .... 460 parts 



the sulphuric acid being added slowly with con- 

 stant stirring. Especial care must be given to re- 

 move all traces of the acid by generous rinsing. 



Plugging Culture Tubes and Flasks. — The plugging 

 of culture tubes and flasks is done by inserting a suit- 

 able quantity of good cotton into the neck of the tube 

 or flask about three quarters of an inch with a glass 

 rod. The plug should be rolled smoothly, but not so 

 tightly as to entirely exclude the passage of air. If not 

 rolled smoothly, bacteria will gain access to the tube in 

 the crevices between the plug and the glass. Neither 

 should the plug be so loose that parts of the cotton 

 will separate when the plug is removed, nor so large 

 that the entrance end rubs the outer rim of the tube 

 when it is inserted. These details may seem exceed- 

 ingly trivial, but the observance of these minutiae is 

 what distinguishes good technique from haphazard 

 methods which fail utterly to accomplish the desired 

 results. The plugged culture tubes are then placed 



