436 DISINFECTION 



ing the germs for the time being. If this latter is of no moment, 

 as in rags or carcases, cremation or burning is the simplest and most 

 thorough treatment. But with mattresses and beddings, bedclothes 

 and garments, as well as with the human body, it is obvious that as 

 a rule something short of burning is required. 



Disinfection by Heat 



From the earliest days of bacteriology heat has held a prominent 

 place as a means of disinfection. But it is only in comparatively 

 recent times that it has been fully established that moist heat is the 

 only really efficient form of heat disinfection. Boiling at atmo- 

 spheric pressure (100° C. or 212° F.) is the oldest form of moist heat 

 disinfection, and because of the simplicity of its application it has 

 gained a large degree of popularity. But it must not be forgotten 

 that mere boiling (100° 0.) may not effectually remove the spores of 

 all bacilli, and obviously boiling is not applicable to furniture, 

 mattresses, and similar objects. For such objects hot-air ovens were 

 used in former days. But it was found that such dry heat disinfec- 

 tion (150° C. for. an hour) injured articles of clothing, etc., and yet 

 left many organisms and spores untouched, as the degree of 

 temperature was rarely, if ever, uniform throughout the substance 

 being treated. The failures following in the track of these methods 

 were an indication of the need for some form of moist heat, viz., 

 steam. 



When water is heated certain molecular changes take place, and 

 at a certain temperature (100° C, 212° F.) the water becomes steam, 

 or vapour, and on very little cooling, or on coming into contact with 

 cooler bodies, will condense and give off its latent heat. But if the 

 vapour is heated, it will become practically a gas, and will not 

 condense until it has lost the whole of the heat, i.e. the heat of 

 making water into vapour plus the heat of making vapour into gas, 

 A gas proper is, then, the vapour of a liquid of which the boUing 

 point is substantially below the actual temperature of the gas. 

 But we know that the temperature at which it boils depends on the 

 pressure to which it is subjected (Eegnault's law). Hence in reahty 

 "steam at any temperature whatever may be a vapour proper, 

 provided the pressure is such as prevents the liquid from boiling 

 below that temperature." In such a condition of vapour it is termed 

 saturated steam, or steam at or near its condensation point. Steam 

 at any pressure is "saturated," when it is at the boiling-point of 

 water for that pressure. But if it is at that same pressure further 

 heated, it becomes practically a gas, and is called superheated steam, 

 or steam heated above its natural condensation point. The former 

 can condense without cooling ; the latter cannot so condense at the 



