DISINFECTION AND DISINFECTANTS 245 



Dry Heat depends for its penetration on conduction 

 and convection, which are slow processes, and does not 

 kill all organisms at a temperature which can be borne by 

 any ordinary fabric, except horsehair, even when the 

 organisms are exposed on the surface. It is, therefore, 

 inadmissible for disinfection of fabrics. The temperature 

 of a dry-heat disinfector must not rise above 120 C., or 

 woollen material will be damaged. Wide variations of 

 temperature occur within the disinfecting chamber, 

 owing to unequal diffusion of the gases and radiation from 

 the heated surfaces. 



Moist Heat. Steam at any temperature and pressure 

 which can condense without cooling is called ' saturated 

 steam,' and will wet the surface on which it condenses. 

 When moisture is present in a disinfector, steam con- 

 denses in the pores of fabrics, more steam is sucked in to 

 fill the place of that condensed, and this process continues 

 till the interior of the fabric becomes so hot that con- 

 densation ceases. 



When by contact with a 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, the steam is called ' superheated.' The dis- 

 infectant value of strictly superheated steam is about the 

 same as that of hot air. In practice, the extent of super- 

 heat present in a disinfector is usually not sufficient to 

 prevent, the steam from being rapidly reduced to satura- 

 tion, and acting as saturated steam. It is only in the 

 later stages of a disinfection that there is risk of the 

 objects being too hot to cool the steam to saturation, and 

 of organisms on the surface thus escaping disinfection. 

 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. 



If the coagulation theory of heat disinfection is correct, 

 the disinfectant effect of hot air or of superheated steam 

 will be considerably less than that of saturated steam or 

 hot water. The presence of air in steam has the effect of 

 delaying or preventing the condensation of the steam, and 

 should have a marked effect in reducing the efficiency of 

 the steam. The temperature of saturated steam increases 

 directly as the pressure, and on any theory, therefore, 



