2 LABORATORY BACTERIOLOGY 



tubes thoroughly in hot water and drain them, using a drain- 

 age board. [It is sometimes desirable to use the cleaning 

 mixture. 1 In this case, after the tubes are washed with soap 

 and water, they are stood in a glass jar (aquarium) and filled 

 with the cleaning mixture. After it has acted for from 10 to 

 20 minutes, pour it back from the tubes into the bottle origi- 

 nally containing it. The tubes are then to be thoroughly rinsed 

 and dried as before.] 



(b) Fermentation tubes. Treat these with the cleaning mix- 

 ture in the same manner as the test tubes. This is necessary 

 because the brush cannot be used. 



(V) Flasks. Wash the flasks thoroughly with soap and 

 water. Then fill them with the cleaning mixture and allow 

 it to act for at least 10 minutes, after which it is to be poured 

 back. Rinse the flasks thoroughly in the same manner as the 

 test tubes and drain them. When dry the outside should be 

 wiped with a damp cloth. 



(d) Petri dishes and reagent bottles. Thoroughly wash the 

 Petri dishes and reagent bottles in hot soapsuds, after which 

 rinse them separately in tap water and drain. The cleaning 

 mixture need not be used. After they are dry the two parts 

 of the Petri dish should be put together. 



(e) Cover glasses and slides. Drop the cover glasses singly 

 into a glass jar containing cleaning mixture and allow them to 

 remain there for 24 hours or longer. Pour off the cleaning 

 mixture and rinse the cover glasses in boiled water until all the 

 color disappears ; then cover them with alcohol until needed, 

 when they can be wiped with a soft linen cloth or with lens 



1 Formula for chromic acid cleaning mixture. Dissolve 80 grams of 

 potassium dichromate (K 2 Cr 2 O 7 ) in 300 cc. of warm water; when all of 

 the K 2 Cr 2 O 7 is dissolved and the solution cooled, add it slowly, with 

 constant stirring, to 460 cc. of concentrated sulphuric acid. Store the 

 mixture in a glass-stoppered bottle. The liquid will be quite thick with 

 small crystals. When the crystals are used up the liquid should be 

 discarded. 



