STERILIZATION 



35 



empty flasks, jars, bottles, tubes and pipettes, are commonly sterilized in a hot- 

 air sterilizer. They can, if necessary, be sterilized by boiling or steaming, but 

 ordinarily, hot air is preferable. 



Culture media and pharmaceutical preparations as a whole cannot be 

 sterilized by hot air without destruction. 



Hot-air sterilizers are constructed to maintain an even distribution of heat 

 throughout, and are equipped with thermometers. It is a safe rule, especially 

 when sterilizing thick glass and inflammable material, to put the articles in the 

 sterilizer before it has been heated, to elevate the temperature gradually. After 

 sterilization allow the sterilizer and its contents to cool to room temperature 

 before opening the sterilizer. 



FIG. 4. LAUTENSCHLAGER HOT-AIR STERILIZER. 



Hot air lacks the penetrating power of steam and at the same temperature, 

 hot air is less germicidal than steam. Exposure to a temperature of i5oC. 

 for i hour or 2ooC. for 30 minutes in the hot-air sterilizer is necessary to affect 

 the destruction of bacteria. Spores frequently withstand this and to destroy 

 them a temperature of i5oC. must be maintained for several hours or 2ooC. 

 for 2 hours. 



The quickest and most sure method of sterilization is accomplished by means 

 of an autoclave. An autoclave is a vertical or horizontal cylinder, constructed 

 like a boiler. There is a gas burner beneath it for heating, and a water jacket 

 several inches deep at the bottom. A perforated, false bottom, rises above the 

 water. An air-tight door or lid on one end opens to permit loading and un- 

 loading. This lid is fastened in place with a number of thumb screws. It is 

 equipped with a dial which registers pounds pressure, and two valves, an exhaust 

 valve. and a safety valve. The safety valve is set to blow off steam at 1 5 pounds 

 pressure. To operate one must first see that sufficient water is in the autoclave 

 (several inches); the substance to be sterilized is then placed in the autoclave 

 and the lid fastened tight with the thumb screws. The exhaust valve is opened 



