BACTERIOLOGICAL TECHNIC 



II 



certain chemicals, or by filtration. Chemicals are 

 used chiefly for sterilizing the skin, surgical instru^ 

 ments, and cultures which have been accidentally 

 spilt. Filters for sterilization are made of some porous 

 material, either infusorial earth or unglazed porcelain. 

 Substances which may be injured by heat are steriUzed 

 in this manner. Positive or 

 negative pressure is necessary 

 for this kind of sterilization 

 (Fig. 3)- 



Sterilization by dry heat. 

 — Sterilization by dry heat is 

 appUcable to the steriliza- 

 tion of most glassware. This 

 method of sterilization is car- 

 ried out by means of hot-air 

 sterilizers (Figs. 4 and 5). 

 These hot-air steriUzers are 

 boxes with double walls of sheet iron. The bottom 

 shelf should always be covered with a piece of asbes- 

 tos, to prevent heating the apparatus too rapidly. 

 The temperature is maintained at 160° or more for one 

 hour. The flame enters a hole provided at .the bottom 

 of the box. Care should be taken to avoid the possi- 

 bility of the flame becoming liuninous, otherwise the 

 glassware will be covered with soot. 



Culture media and all substances liable to be in- 

 jured by heat of 160° C. or over must be sterilized by 

 the application of moist heat. Experience has taught 

 that hot steam has greater germicidal powers than air 

 of the same temperature. Hot steam, therefore, is the 

 most common means of sterilizing culture media. 



Fig. 4 

 Koch's Hot-Air Sterilizer 



