METHODS FOR MICROSCOPIC STUDY OF- BACTERIA 209 
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. Pyrex 
glassware is most suitable. 
Fig. 12. — Petri dish. 
Preparation of Glassware.— The method to be employed in the 
cleaning of glassware depends somewhat on the purpose for which it 
is 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 one-half hour. The adherent media is removed with 
a brush or swab; a final thorough rinsing in clear water removes all 
traces of soap,' Very dirty glassware or glassware in which chemical 
Fig. 13. — Fermentation tubes — various types. 
determinations are to be made should be cleaned in chromic acid 
solution.^ Twenty-four hours' exposure to chromic acid removes all 
traces of organic matter, as a rule. Following the acid bath the glass- 
ware is thoroughly rinsed in clear water and dried. 
The cleaned glassware— test-tubes, flasks, or fermentation tubes- 
is then stoppered with non-absorbent cotton— cotton batting— which 
has a long staple or fiber. The cotton plugs must be carefully fitted — 
neither too loose, which would permit of the passage of adventitious 
microorganisms, nor too tight, for obvious reasons. The cotton plugs 
For preparation, see page 195. 
