ACTION OF MOIST HEAT 23 



With regard to the resisting power of ' spores ' to moist 

 heat, those of most of the pathogenic bacteria are quickly 

 killed by a very short exposure to 100° C. ( = 212° F.). 



On the other hand, the spores of certain non-pathogenic 

 species resist the boiling- water temperature (100° C.= 

 212° P.) for hours. 



In the practical application of steam for disinfecting pur- 

 poses, it must be borne in mind that, although steam under 

 pressure is more effective than ordinary steam, it is not 

 necessary to give it the preference, in view of the fact that 

 all the pathogenic organisms and their spores are quickly 

 killed by a comparatively short exposure to the temperature 

 of boiling water. ' Superheated ' steam has about the same 

 germicidal action as heated dry air at the same tempera- 

 ture. This is shown by the experiments of Bsmarch, who 

 found that the spores of anthrax were killed by three 

 minutes' exposure to ordinary steam, but were not killed 

 by the same time in ' superheated ' steam at a temperature 

 of 140° C. 



Prom the above facts it will be seen that for any object 

 to be rendered germ-free, i.e., sterilised by dry heat, it must 

 be exposed to a temperature of 160° to 180° C. (=320° to 

 356° P.) for half an hour at least, if this object is to be 

 attained with certainty. 



If sterilisation is to be secured by the agency of moist 

 heat, the article must be heated to 60° to 70° C. ( = 140° to 

 158° P.). As already mentioned, most spores are killed by 

 simple boiling ; but there are, on the other hand, certain 

 resistant kinds which can support for a short time a 

 temperature of 110° to 115° C. ( = 230° to 239° P.). 



In order to sterilise culture media, it is necessary to heat 

 them to 116° C. for fifteen minutes under pressure, in vessels 

 of special construction. 



Fractional Sterilisation. — This is a method of rendering 



