THE DESTRUCTION OF BACTERIA 85 



of fluids, such as blood serum, by immersion in a water-bath at a 

 temperature varying above 55 C., according to circumstances. Ex- 

 posures at such low temperatures may be repeated on five or six 

 consecutive days, usually for an hour each day. 



The use of steam under pressure is the most powerful method 

 of heat-disinfection which we possess. It is applicable to the 

 sterilization of fomites, clothing, or any objects of a size suitable 

 to be contained in the apparatus at hand, and which are not injured 

 by moisture. In laboratories this method is employed for the 

 sterilization of infected apparatus, such as flasks, test tubes, Petri 

 plates, etc., containing cultures. The device most commonly used 

 in laboratories is the so-called autoclave, of which a variety of 

 models may be obtained, both stationary and portable. The prin- 

 ciple governing the construction of all of these is the same. The 

 apparatus usually consists of a gun-metal cylinder supplied with a 

 lid, which can be tightly closed by screws or nuts, and supplied 

 with a thermometer, a safety-valve, and a steam pressure gauge. 

 In the simpler autoclaves, water may be directly filled into the lower 

 part of the cylinder, and the objects to be sterilized supported upon 

 a perforated diaphragm. In this case the heat is directly applied 

 by means of a gas flame. In the more elaborate stationary devices, 

 steam may be let in by piping it from the regular supply used for 

 heating purposes. Exposure to steam under fifteen pounds pressure 

 (fifteen in addition to the usual atmospheric pressure of fifteen 

 pounds to the square inch) for fifteen to twenty minutes, is sufficient 

 to kill all forms of bacterial life, including spores. 



In applying autoclave sterilization practically, attention must be 

 paid to certain technical details, neglect of which would result in 

 failure of sterilization. It is necessary always to permit all air to 

 escape from the autoclave before closing the vent. If this is not 

 done, a poorly conducting air-jacket may be left about the objects 

 to be sterilized, and these may not be heated to the temperature 

 indicated by the pressure. It is also necessary to allow the reduction 

 of pressure, after sterilization, to take place slowly. Any sudden 

 relief of pressure, such as would be produced by opening the air- 

 vent while the pressure gauge is still above zero, will usually result 

 in a sudden ebullition of fluid and a removal of stoppers from flasks. 



The temperature attained by the application of various degrees 

 of pressure is expressed in the following table : 



