CHAPTEE XY. 



Various experiments in sterilization by steam and by hot air. 



Place in one of the openings in the cover of the 

 steam sterilizer an accurate thermometer; when the 

 steam has been streaming for a minute or two the ther- 

 mometer will register 100° C. Wrap in a bundle of 

 towels or rags or pack tightly in cotton a maximum 

 (s^lf-registering) 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 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 ster- 

 ilizing properties that steam has at the same temperature. 

 It is necessary, therefore, that this air should be ex- 

 pelled from the meshes of the material and its place 

 taken by the steam before sterilization is complete. This 

 is insured by allowing the steam to stream through the 

 substances a few minutes before beginning to calculate 

 the time of exposure. 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 time that is required for the expulsion 



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