24 APPLIED BACTEEIOLOGY 



it can condense. ' Superheated ' steam has about the same 

 germicidal action as heated dry air at the same tempera- 

 ture. This is shown by the experiments of Esmarch, 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. Saturated steam under pressure, either con- 

 fined or as a current, is now recognised as the only trust- 

 worthy form of steam for the purposes of general disinfection. 



From 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° F.) 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 110° to 115° C. ( = 230° to 

 239° F.) for at least fifteen minutes. 



Fractional Sterilisation. — This is a method of rendering 

 culture and other media germ-free by exposing them to a 

 temperature of not more than 60° to 70° C. ( = 140° to 

 158° F.) for several times in succession, the operation 

 extending over at least three days. 



By the first heating the adult bacteria are killed, the 

 spores only remaining alive ; the liquid is then kept at 

 about 20° to 25° C. ( = 68° to 77° F.) for about twelve to 

 twenty hours, to allow the spores to germinate, and then 

 again heated. All the spores that have developed into 

 full-grown bacteria are thus killed, and in case some of the 

 spores should not have developed, the process is repeated 

 again on one or more successive days. 



But, as has been pointed out by Miquel, absolutely 

 certain results are not to be attained by this method, as 

 some spores take days, or even weeks, to germinate. There 

 is, therefore, always the chance that some such spores may 



