WATER 63 



steadily; the rate at which molecules leave the water maintains a 

 constant value. Hence the rate at which vapor molecules enter 

 the water must eventually equal that at which other water mole- 

 cules leave the liquid. At this point, occasion for visible changes 

 ceases and the mercury comes to rest. We are bound to think, 

 however, of the exchange as still going on, since nothing has oc- 

 curred to stop it. The condition is not one of rest but of rapid 

 and equal exchange. Such, described in terms of molecules, is 

 the state of affairs which is characteristic of a condition of equi- 

 librium. The condition is dynamic, and not static. 



Equilibrium. This term is used so often in chemistry, and is 

 used in so unfamiliar a sense, that the reader should consider 

 attentively what it implies. Three things are characteristic of a 

 state of equilibrium : 



1. There are always two opposing tendencies which, when 

 equilibrium is reached, balance each other. In the foregoing 

 instance, one of these is the hail of molecules leaving the liquid, 

 which is constant throughout the experiment. It represents the 

 vapor tension of the liquid. The other is the hail of returning 

 molecules, which, at first, increases steadily as the concentration 

 of the vapor becomes greater. This is the vapor pressure of the 

 vapor. These have the effect of opposing pressures and, when the 

 latter becomes equal to the former, equilibrium is established. 

 In all cases of equilibrium we shall symbolize the two opposing 

 tendencies by two arrows, thus: 



Water (liq.) < Water (vapor). 



2. Although their effects thus neutralize each other at equilib- 

 rium, both tendencies are still in full operation. In the case in 

 point, the opposing hails of molecules are still at work, but neither 

 can effect any visible change in the system. Equilibrium is a 

 state, not of rest, but of balanced activities. 



3. (and this is the chief mark of equilibrium.) A slight change 



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