364 APPLIED BACTERIOLOGY 



Heat can be most effectively applied for the disinfection 

 of fabrics by causing steam to condense in their pores. 

 More steam is sucked in to fill the place of that which has 

 been condensed, and is in its turn condensed; and the 

 process goes on till the interior of the fabrics becomes so 

 hot that no more condensation takes place ; that is to say, 

 that the temperature of the entire contents of the vessel is 

 equal to that of the incoming steam. 



Steam at any temperature and pressure which can con- 

 dense without cooling is called 'saturated steam.' Thus, 

 steam from a kettle or in a boiler is saturated. When in 

 any way, such as, for instance, by contact with hotter 

 surface or by being derived from a saline solution, its 

 temperature is raised above that at which it can condense 

 under its existing pressure, it is called ' superheated.' The 

 process described in the last paragraph does not occur with 

 steam so long as it is superheated, its heating effect while 

 in that condition being due only to its being cooled by con- 

 duction, and amounting to a very small fraction of that 

 exerted by condensation. The disinfectant vahie of strictly 

 superheated steam is about the same as that of hot air. 

 In practice, the extent of superheat present in a disinfector 

 is usually not sufiicient to prevent the steam from being 

 rapidly reduced to saturation, and acting as saturated 

 steam. It is only in the latter stages of a disinfection that 

 the risk enters of the objects being too hot to cool the 

 steam to saturation, and of organisms on the surface thus 

 escaping disinfection. The extent to which this risk is of 

 practical importance varies with the design of the disin- 

 fector, and has not at present been accurately determined 

 for the types of disinfectors used in this country. A more 

 certain objection to the use of superheated steam is that its 

 temperature, not being determined solely by its pressure, 

 cannot be read off on a pressure gauge. The first proposal 



