188 MICROSCOPIC AND CULTURAL STUDY OF BACTERIA 



Bacterial cultivation is usually carried out in glass vessels test- 

 tubes, flasks, fermentation tubes, and Petri dishes because glass is 

 transparent and permits an unobstructed view of the reactions taking 

 place within. It is obvious that glassware employed in bacterial 

 laboratories must be chemically and bacteriologically clean. 



Preparation of Glassware. The method to be employed in the clean- 

 ing of glassware depends somewhat on the purpose for which it is 



FIG. 12. Petri dish. 



used. New glassware frequently contains alkali, which is readily 

 neutralized by diluted acid, hydrochloric or sulphuric. Glassware 

 that has contained cultures of bacteria is first sterilized in the auto- 

 clave to remove all danger of infection, then immersed in a strong 

 solution of soap-powder and soap-suds maintained at a boiling tem- 

 perature for half an hour. The adherent media is removed with a 

 brush or swab; a final thorough rinsing in clear water removes all 



FIG. 13. Fermentation tubes various types. 



traces of soap. Very dirty glassware or glassware in which chemical 

 determinations are to be made should be cleaned in chromic acid 

 solution, which is prepared by adding a saturated aqueous solution 

 of potassium bichromate to a 1 to 3 dilution of sulphuric acid. Twenty- 

 four hours' exposure to chromic acid removes all traces of organic 

 matter, as a rule. Following the acid bath the glassware is thoroughly 

 rinsed in clear water and dried. 



