CHAPTER XIX 

 EQUILIBRIA [AND THEIR DISPLACEMENT 



THE idea of a statistical or mobile equilibrium we 

 have met in considering a system of two phases, e.g. a 

 liquid and its vapor. When the system is in equilib- 

 rium there are just as many molecules of the liquid 

 evaporating from the surface each instant as there are 

 molecules of vapor condensing into the liquid. If the 

 temperature is raised evaporation increases until the 

 vapor density corresponds to the new temperature. 

 As the density of vapor molecules increases there is a 

 corresponding increase in the number of them which 

 strike back into the liquid surface. When the rate at 

 which molecules condense has increased sufficiently 

 to be equal to the rate at which they evaporate a new 

 equilibrium is reached. The proportions of liquid 

 and vapor have meanwhile been altered, in this case 

 the vapor gaining at the expense of the liquid. 



The term "equilibrium" must of course be under- 

 stood in a statistical sense. The balance is between the 

 rates at which two different molecular reactions occur. 

 It is the results of these reactions which we consider as 

 opposing. In effect we use the word " equilibrium" 

 much as if the amounts of liquid and vapor formed 

 opposite ends of a lever, the position of the fulcrum rep- 

 resenting the condition of equilibrium between the two 



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