DISINFECTION BY HEAT 111 



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 

 conduction, and amounting to a very small fraction of that 

 exerted by condensation. The disinfectant value 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 sufScient 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 escap- 

 ing disinfection. The extent to which this risk is of 

 practical importance varies with the design of the stove, 

 and has not at present been accurately determined for the 

 types of disinfector s 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 pro- 

 posal of the use of superheated steam for disinfection was 

 made by Koch, who, in 1881, suggested raising steam from 

 saline solutions of boiling-points above 100° C. Disinfectors 

 were made on the Continent, working respectively with 

 solutions of salt and of calcium chloride, but were found 

 unsatisfactory. 



Steam is used either confined under pressure or as a 

 current with or without pressure. The advantage of some 

 amount of pressure of saturated steam, however small, is 

 that it gives a real control over the temperature of steam, 

 which in a well-designed disinfector is practically uniform 



