64 BACTERIOLOGY 



known to be produced under these circumstances. They rapidly 

 decompose. 



Cold appears to be fatal to some pathogenic forms, and a con- 

 siderable percentage of the bacteria in a culture are usually killed 

 by freezing. Cultures cannot be completely sterilized even by 

 exposure to the temperature of liquid air. Cold is therefore not 

 to be regarded as an efficient germicide, although it may com- 

 pletely check the growth of bacteria. 



Heat is the most important of the physical means and 

 doubtless the most important of all means of destroying 

 bacteria. Its value as a purifying agent was recognized among 

 the ancients. Heat is applied under conditions insuring 

 the presence of liquid water, so-called moist heat, and in 

 the absence of water, so-called dry heat or hot- air sterilization. 

 The most reliable methods of sterilization by dry heat are those 

 which accomplish the combustion or destructive distillation of 

 organic matter in general. Actual combustion of clothing and 

 bedding, and even of houses has been resorted to in the past 

 as a method of disinfection. Heating to redness in the naked 

 flame is the routine method of sterilizing our platinum wire, and 

 glass articles, such as capillary pipettes, cover-glasses and slides 

 are commonly sterilized in the flame. Flaming may even be 

 employed for sterilization of surgical instruments in an emergency, 

 although such treatment quickly destroys steel instruments. 

 Sterilization of large objects and of combustible material by dry 

 heat is generally accomplished in an oven or hot-air sterilizer. 

 The common laboratory sterilizers are boxes of sheet iron with 

 double walls, with air space between to allow the hot gases from 

 the flame completely to surround the inner compartment. The 

 door, which occupies one full side, is usually double. A tubula- 

 tion through the top allows a thermometer to be inserted into the 

 interior so that the temperature may be read off at any time. 

 Even the best hot-air sterilizers fail to give an even temperature 

 all over the interior, so that the thermometer bulb at one corner 

 cannot be implicitly relied upon to record the temperature of 



