66 



ESSENTIALS OF BACTERIOLOGY 



As modified, and in use in most laboratories, fifteen minutes, 

 sterilization in steam, at 100 C., in the Arnold sterilizer on 

 three successive days, has been found sufficient, while one 

 sterilization in the autoclave at 120 C. for fifteen minutes 

 will serve in most cases, especially if the medium is for imme- 

 diate use, and does not contain gelatin or sugar. 



Cotton Plugs or Corks. All the glass vessels (test-tubes, 

 flasks, etc.) must be closed with cotton plugs, cotton-wool, 



1 



I 



Fig. 17. Wire cage. Fig. 18. Cotton-plugged test-tubes. 



or a good quality of non-absorbent cotton) , the cotton being 

 easily sterilized and preventing the entrance of germs from 

 the air. 



Tin-foil may be used to cover the cotton, or caps made of 

 india-rubber. 



Test-tubes. New test-tubes are washed with hydro- 

 chloric acid and water to neutralize the alkalinity often pres- 

 ent in fresh glass, or in chromic acid cleaning mixture one 

 hour. (Potassium dichrorrtate, 6; water, 30; sulphuric acid, 

 46.) They are then well washed and rubbed with a brush, 



