STERILIZATION 47 



posure insures sterility. The exposure of fifteen min- 

 utes is considered to begin when the steam is up and 

 the thermometer registers 100. For sterilization of 

 objects not injured by pressure the boiler or autoclave 

 is used. By this means as much as two extra atmos- 

 phere pressures can be run up, which will be equivalent 

 to 34.5 C. or 74 F. above the boiling point. After 

 starting up steam the apparatus should never be tightly 

 closed at the safety valve until all air is expelled. This 

 method is particularly adapted to the sterilization of 

 dressings and infected cast-off clothing. Hot air is 

 suitable for dried glassware and articles injured by 

 moisture. It is less efficient than moist heat. This 

 is due to the fact that organic substances are less 

 easily coagulated in a dried condition. Spores are 

 more resistant also, as, for example, the anthrax 

 spore, which requires an exposure of three hours at 

 140 C. or 284 F. dry heat. Hot dry air penetrates 

 less easily than hot moisture. Burning is best of all 

 methods. 



The two thermometric scales are explained as follows: 



F = Fahrenheit, the ordinary scale used in this country. Water 

 just at the freezing point registers 32 F., while just at the boil- 

 ing point registers 212 F. The zero has no relation to physical 

 changes. 



C = Centigrade, the French system. Water just at the freezing 

 point is C., and just at boiling point is 100 C. 



The 100 in the Centigrade scale is equal to the 180 in the 

 Fahrenheit scale, between 32 and 212. 



To change one system to the other proceed as follows : 



From Fahrenheit to Centigrade: Given degree F. 32 -i-9 X5 = 

 same degree in Centigrade scale. Example: 50 F. 32 = 18 -j- 9 = 

 2X5 = 10. Therefore 50 F. = 10 C. 



From Centigrade to Fahrenheit : Given degree C. -J- 5 X 9 +32 = 

 same degree in Fahrenheit scale. Example: 10 C.^5 

 18+32 = 50 F. 



