50 METHODS OF STUDYING MICROORGANISMS 



Simple boiling should be undertaken whenever prac- 

 ticable, and immersion for five minutes in boiling 

 water will destroy the vegetative forms of all bacteria. 

 For spores, however, at least of the disease-producing 

 kind, two hours is necessary. It is advisable to add 1 

 per cent, of sodium carbonate to the water. This 

 assists in killing of spores, and metal objects are not 

 so apt to rust. This simple boiling for ten minutes 

 is sufficient for dry cleaned syringes, trays, dishes, 

 and surgical instruments in the absence of infective 

 material known to contain spores. Sterilization in 

 live steam is the most practical method of killing bac- 

 teria, as it can be carried out in the kitchen. In the 

 laboratory it is done by the Arnold sterilizer (Fig. 19). 

 It is the custom to employ what is called fractional 

 sterilization. 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 insures sterility. The exposure of fifteen min- 

 utes is considered to begin when the steam is up and 

 the thermometer registers 100. The foregoing method 

 is practicable for dressings and rubber gloves. For 

 sterilization of objects not injured by pressure the 

 boiler or autoclave is used. The principle of this 

 apparatus is that steam is admitted into the steriliz- 

 ing 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 appa- 



