CULTURES IN LIQUID MEDIA. 



61 



For larger quantities of liquid than can be held in an ordinary 

 test tube the small flasks with a flat bottom, known as Erlenmeyer 

 flasks, are very convenient (Fig. 33). 



In his earlier researches Pasteur used flasks and tubes of various 

 forms, which served a useful purpose, but have been displaced in his 

 laboratory by the simpler form of apparatus shown in Fig. 34. 

 This is a little flask having a cover which is ground to fit the neck. 

 This cover is drawn out above into a narrow tube which admits 

 oxygen to the flask through a cotton air filter. To obtain access 

 to the interior of the flask for the purpose of introducing bacteria 

 to start a culture, or to obtain material for microscopical examina- 

 tion, the cover is detached at the ground joint by a gentle twisting 

 motion. 



There is much less danger that a sterile culture liquid will become 



FIG. 33. 



FIG. 34. 



contaminated during the momentary removal of the cover from 

 one of these little flasks, or of the cotton plug from a test tube, than 

 is usually supposed. Abundant laboratory experience demonstrates 

 that such contamination by bacteria floating in the atmosphere rarely 

 occurs. The spores of mould fungi are commonly more abundant 

 in the air, but even these do not very frequently fall into the culture 

 liquid when the tube is opened to inoculate it with the bacteria it is 

 proposed to cultivate. This inoculation is best made with a platinum 

 wire, bent into a loop at the free extremity, and sealed fast into the 

 end of a glass rod (Fig. 35). This is sterilized in the flame of a 

 Bunsen burner or alcohol lamp by bringing the platinum wire to a 

 red heat and passing the end of the glass rod which carries it 

 through the flame several times. With this instrument we may 

 transfer a little drop from a culture to the sterile fluid in another 



