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. ] 
(6) 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. 
(¢) 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. 
(@) 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. 
(¢) 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 (K2Cr2O7) in 300 cc. of warm water; when all of 
the KgCreOz 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. 
