MEDICAL BACTERIOLOGY 



and the burner under the autoclave lighted. As the autoclave becomes hot the 

 contained air expands and escapes through the open valve, then steam forms, 

 eventually all the air is expelled and steam begins to escape. When steam comes 

 out of the exhaust valve, it is closed, then the pressure within rises until it 

 registers 15 pounds on the dial. As soon as the pressure reaches 15 pounds the 

 time is noted and exactly 20 minutes later the flame beneath the autoclave is 

 turned out. Gradually the pressure falls as the autoclave cools. It is not 



opened until the needle on the dial falls to 

 the zero mark. Then the contents of the 

 autoclave are removed, sterile. An exposure 

 to steam under 15 pounds pressure for 20 

 minutes, kills spores as well as bacteria; it 

 achieves complete sterility. 



This is the method of choice for steriliz- 

 ing water, salt solution, bouillon, agar, milk, 

 camphorated oil, mercury salicylate, mercury 

 sozoiodolate and other substances which are 

 not injured by so high a temperature, i2oC. 

 When an autoclave is not available and a 

 substance suitable for autoclave sterilization 

 must be sterilized at once, make brine by 

 adding common salt to watef, place in a 

 water bath or boiler, immerse the substance 

 to be sterilized and boil from 30 to 60 min- 

 utes. The addition of salt to water elevates 

 its boiling point close to the temperature of 

 the autoclave at 15 pounds pressure. 



Steam sterilization without pressure is 

 usually done in an "Arnold" or similar 

 sterilizer, an apparatus having a square 

 metal pan about 4 inches deep, in the 

 center of which is an upright cylinder about 

 inches high, that opens into a large 



5. AUTOCLAVE. 



and 

 This 



6 inches in diameter 



cubical compartment. This upper compartment, in which sterilization 

 is affected, has double walls and an inner and an outer door. The 

 apparatus is so constructed that when the pan is filled with water, a 

 flame placed beneath it and steam generated, the steam escapes through 

 the cylinder in the center of the pan into the sterilizing compartment 

 above. Anything that can be sterilized in an autoclave without injury, may be 

 sterilized in the "Arnold" steam sterilizer by steaming it i hour each day for 

 3 consecutive days. This method of sterilizing by short exposure each day, for 

 a number of days, is called intermittent, discontinuous or fractional steriliza- 

 tion. It is based on the theory that steam at iooC. will kill all bacteria in 

 i hour, but not spores, the spores surviving the first hour of sterilization, de- 

 velop, in the following hours, into bacteria, and as bacteria, are killed when 



