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 Uquid 

 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 combus- 

 tion 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. Heat- 

 ing to redness in the naked flame is the routine method of steriliz- 

 ing 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 instru- 

 ments in an emergency, although such treatment quickly destroys 

 steel instruments. Sterilization -of large objects and of combus- 

 tible 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 com- 

 partment. The door, which occupies one full side, is usually 

 double. A tubulation 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 throughout the interior, so that the thermometer 

 bulb at one corner cannot be implicitly relied upon to record the 

 temperature of other parts. Ordinarily a temperature of 150° C. 



