92 PATHOGENIC BACTERIA. 



having first shown that when the months of flasks and 

 tubes are closed with plugs of sterile cotton no germs 

 can filter through. This observation has been of ines- 

 timable value to every bacteriologist. Before sterilizing 



FIG. 8. Hot-air sterilizer. 



flasks and tubes' we plug them with ordinary raw cotton, 

 and are sure that afterward their interiors will remain 

 free from the access of germs until opened. Instruments 

 may be sterilized wrapped in cotton, to be opened only 

 when ready for use ; or instruments and rubber goods 

 sterilized by steam can subsequently be wrapped in 

 sterile cotton and kept for use. It is of the utmost 

 importance to carefully protect every sterilized object, 

 and to allow as little dust to collect upon it as possible, 

 in order that the object of the sterilization be not de- 

 feated. As the spores of moulds falling upon cotton 

 sometimes grow and allow their mycelia to work their 

 way through and drop into a culture-medium, Roux 



