STERILIZATION 47 



It is the custom to employ what is called fractional sterili- 

 zation. This method is the exposure of the material to be 

 disinfected to the temperature of 100 C. or 212 F., which 

 is the temperature reached by the steam in the inner chamber, 

 for fifteen minutes on three successive days. On the first 

 occasion vegetative forms are killed and the spores remaining 

 are permitted to pass into the vegetative state overnight. 

 On the second occasion these will then be killed. A third 

 exposure ensures sterility. The exposure of fifteen minutes is 

 considered to begin when the steam is up and the thermom- 

 eter registers 100. The foregoing method is practicable 

 for dressings and rubber gloves but the newer types of oper- 

 ation room autoclaves, those with air-drying appliances, are 

 to be preferred whenever practicable. For sterilization of 

 objects not injured by pressure the boiler or autoclave 

 should be used, as it destroys spores with certainty. The 

 principle of this apparatus is that steam is admitted into the 

 sterilizing chamber, the air having been expelled by heating 

 of the walls and displacement by the entering steam. When 

 no air is present the pressure within the apparatus rises and 

 steam penetrates all permeable objects. When the steam 

 escapes and air enters, moisture is absorbed and the objects 

 become dry. By this means as much as two extra atmos- 

 pheric pressures can be run up, which will be equivalent to 

 34.5 C. or 74 F. above the boiling-point. After starting 

 up steam the apparatus should never be tightly closed at the 

 safety-valve until all air is expelled. This method is particu- 

 larly adapted to the sterilization of dressings and infected 

 cast-off clothing. Hot air is suitable for dried glassware and 

 articles injured by moisture, and can be used for domestic 

 sterilization by exposing the articles in the household oven. 

 It is less efficient than moist heat. This is due to the fact 

 that organic substances are less easily coagulated in a dried 

 condition. Spores are more resistant also, as, for example, 

 the anthrax spore, which requires an exposure of three hours 

 at 140 C. or 284 F. dry heat. Hot, dry air penetrates less 



