CHAPTER XV. 



Various experiments In sterilization by steam and by bot air. 



Place in one of the openings in the cover of tlie steam 

 sterilizer an accurate thermometer ; when the steam has 

 been streaming for a minute or two the thermometer will 

 register 100° C; wrap in a bundle of towels or rags or 

 pack tightly in cotton a maximum thermometer ; let this 

 thermometer be in the centre of a bundle large enough 

 to quite fill the chamber of the sterilizer. At the end 

 of a few minutes' exposure to the streaming steam 

 remove it ; it will be found to indicate a temperature 

 of 100° C. 



Closer study of the penetration of steam has taught 

 us, however, that the temperature which is found at the 

 centre of such a mass may sometimes be that of the 

 air in the meshes of the material, and not that of 

 steam, and for this reason the sterilization at that 

 point may not be complete, because hot air at 100° C. 

 has not the sterilizing properties that steam at the 

 same temperature possesses. It it necessary, there- 

 fore, that this air should be expelled from the meshes 

 of the material and its place taken by the steam be- 

 fore sterilization is complete. This is insured by allow- 

 ing the steam to stream through the substances a few 

 minutes before beginning to calculate the time of ex- 

 posure. There is as yet no absolutely sure means of 

 saying that the temperature at the centre of the mass is 

 that of hot air or of steam, so that the exact length of 



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