13 



§ 3- Formula for chromic acid cleaning mixture. Dis- 

 solve 80 grams of potassium dichromate (KjCr^O,) in 300 

 c.c. of warm water, when all of the Kj Cr^ O, is dissolved 

 and the solution cooled add it slowly, with constant stirring, 

 to 460 c.c. concentrated sulphuric acid with constant stirring, 

 store the mixture in a glass-stoppered bottle. 



§ 4. A method for cleaning cover-glasses for flagella 

 stain. For this work the ordinary method of cleaning 

 cover-glasses is not sufficient. The following treatment was 

 recommended to me by Dr. Erwin F. Smith. After being 

 cleaned by the ordinary method boil the cover-glasses in an 

 agate cup or glass beaker in a 10% solution of caustic soda 

 for five minutes. After cooling rinse the cover-glasses 

 thoroughly in distilled water after which place them in a 

 beaker and cover with a 1% solution of hydrochloric acid, 

 heat to the boiling point, and allow to stand for several 

 hours (over night or longer). Then pour the acid off and 

 rinse the cover-glasses several times in distilled water and 

 finally in alcohol. Wipe them out of alcohol as they are 

 needed. 



EXERCISE II. 



PLUGGING THE TUBES AND FLASKS AND STERILIZING THE 

 GLASSWARE. 



§ 5. Explanatory note. After the tubes and flasks are 

 cleaned they must be plugged. The plugged tubes and 

 flasks and Petri dishes, all of which are to be used for hold- 

 ing culture media or in making cultures, must be sterilized 

 before they can be used. The plugs should be neatly made 

 and of the proper length and firmness. Absorbent cotton is 

 ordinarily used for this purpose, although common cotton is 

 employed in some laboratories. Glassware is sterilized by 

 means of dry heat, i. e., in the hot air sterilizer. (See 

 methods for sterilizing apparatus and instruments in text- 

 books. ) 



