70 TEXT-BOOK OF BACTERIOLOGY. 



If such an apparatus be heated, it is certain that as soon as the 

 water fairly boils and the full steam development has commenced 

 the thermometer will rise to 100° C. and will continue at that point. 

 An exposure of fluids for from half an hour to an hour, reckoning' 

 from the moment of full steam development, will, as already said, 

 suffice in the great majority of cases to effectually sterilize them. 



Of course we can also sterilize by this means all substances 

 which bear the high temperature without damage — for instance, 

 India-rubber stoppers, paper filters, etc. 



The peculiar efficacy of steam, the undoubted superiority of wet 

 over dry heat, is a very striking circumstance. It is explained by 

 the fact that the tough covering of the spore, the very touchstone 

 of all disinfecting processes, swells and softens on coming into con- 

 tact with the moisture, and thus becomes permeable, as has been 

 proved by the experiments of Esmarch. 



He found that if the steam which has been developed by boiling 

 water under atmospheric pressure is afterward superheated by 

 passing over hot metal plates, its germ-killing power diminishes. 

 This is because the steam is dryer, further removed from its con- 

 densation point, less disposed to set water free; it has a longer 

 way to go, must lose more heat than steam of 100° C, before it can 

 deposit moisture and efl'ect the softening of the spore envelopes. 

 It is a different case, naturallj', when the increase of temperature 

 goes hand in hand with the increase of pressure which results from 

 increased tension. In that case the steam remains close to its con- 

 densation point, and is capable of losing water, of becoming water, 

 at the moment when it meets with the bacterial spores intended to 

 be sterilized. 



But the heat of freely-escaping steam cannot be employed when 

 dealing with substances which cannot support the boiling temper- 

 ature. Strongly albuminous fluids, for instance, cannot be heated 

 to 100° C, since their albumin curdles and the solution is essen- 

 tially altered in its composition. For such cases a method intro- 

 duced by Tyndall and afterward considerably improved by Koch, 

 called " discontinuous sterilization," is employed. 



Most bacteria, in their usual forms, cannot survive a tempera- 

 ture of about 60° C, while their spores are in no way affected by 

 this heat. If a culture fluid be warmed, therefore, for some time to 

 60° C, as a general rule only the spores remain alive. But when 

 the temperature diminishes these very soon begin to germinate, 

 the bacilli lose their protective sheath, and if on the following day 

 heat is again employed to 60° C, the newly-formed rods are de- 

 stroyed; if this process is often repeated, it is certain that all the 



