STERILIZATION BY HEAT 45 



in the Arnold sterilizer for an hour on each of three con- 

 secutive days. This makes certain the destruction of 



Fig. 21. Instrument sterilizer. 



all spores. Surgical dressings may also be sterilized in 

 this form of sterilizer. 



(5) The destructive action of steam can be enor- 

 mously increased by employing it under pressure. For 

 this a special form of apparatus is required. It is im- 

 possible to heat water, in an open vessel or in an Arnold 

 sterilizer, to more than 212 F. As soon, however, as 

 we heat the water in a tightly closed vessel, which will 

 not allow the steam to escape, we can raise the tempera- 

 ture beyond this. The higher the pressure of the steam, 

 the higher the temperature. It is mainly because of this 

 fact that the use of steam under pressure is so much 

 more effective in sterilization than the use of live 

 steam not under pressure. Another reason is the 



