82 BACTERIOLOGICAL ANALYSIS LESSONS 



II. CLEANING OF TEST-TUBES WHICH HAVE ALREADY 



BEEN USED. 



(1) Eemove the cotton-wool plugs. 



(2) Steam the tubes in the steriliser or autoclave for 

 thirty minutes, in order to dissolve the gelatine or agar- 

 agar, and to disinfect the tubes. 



(3) Pour away the contents of the tubes, and place the 

 empty tubes, upright, in a large sauce-pan containing a weak 

 solution of caustic soda. 



(4) Fill each tube with the same caustic soda solution, 

 boil for an hour, and then allow the sauce-pan to cool. 



(5) Pour away the caustic soda solution, and wash the 

 tubes three or four times in water. 



(6) Now clean each tube with weak nitric acid, using a 

 glass rod to which a little cotton-wool has been fastened. 



(7) Wash the tubes repeatedly in water. 



(8) Rinse them with a little absolute alcohol, and allow 

 them to drain. 



(9) When dry, they are ready for sterilisation. 



Very old tubes in which the culture media have dried 

 up must be discarded, since no amount of cleaning 

 will make them fit for use. 



III. CLEANING OF FLASKS AND BEAKERS. 



(1) Immediately after use rinse out the flask or beaker 

 with hot water. 



(2) With a piece of bent wire, to which a little cotton- 

 wool is tied, and which is charged with sapolio, clean the 



