326 BACTERIA 



I. From the earliest days of bacteriology heat has held a 

 prominent place as a disinfector. But it is only in com- 

 paratively recent times that it has been fully established 

 that moist heat is the only really efficient form of heat dis- 

 infection. Boiling at atmospheric pressure (loo^ C.) 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 

 (ioo° C.) may not effectually remove the spores of all bacilli. 

 Besides, boiling is not applicable to furniture, mattresses, 

 and such-like frequently infected objects. For many of 

 these hot-air ovens were used in the early days. But it was 

 found that such disinfection was no disinfection at all, for 

 not only did it leave many organisms and spores untouched, 

 but 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 of some form of moist heat, viz., 

 steam. 



Here it will be necessary to digress for a moment into 

 some of the characters of steam. When water is heated 

 certain molecular changes take place, and at a certain tem- 

 perature (ioo° C, 212° F.) the water becomes steam, or 

 vapour y and on very little cooling will condense. 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 boiling-point is substantially below 

 its actual temperature. But we know that the temperature 

 at which it boils depends upon the pressure to which it 

 is subjected (Regnault's law). Hence in reality *' 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 



