38 MOLECULAR MOTION AND ITS ENERGY 20 



in general move in straight lines, except when two approach 

 very near to each other. In consequence of this linear 

 motion it must often happen that a molecule of vapour 

 strikes against the surface of the liquid ; in this case, under 

 favourable conditions, it may be held there by cohesion. 



In the liquid, too, the molecules are not at rest, but are 

 in as brisk motion as in the vapour, but not in straight lines. 

 In consequence of this motion it may happen that a liquid 

 particle gets out of the range of the forces of cohesion and 

 passes again into the vapour. 



There is, therefore, a continuous interchange of molecules 

 between liquid and vapour, and, since there is equilibrium, as 

 many molecules must on the average pass from the vapour 

 into the liquid as from the liquid into the vapour. 



On considering that this equilibrium between liquid and 

 vapour extends also to temperature, we see that not only as 

 much mass but also as much energy necessarily passes from 

 the liquid to the vapour as from the vapour to the liquid. 

 The same number of molecules therefore carry over the 

 same amount of energy from the one state of aggregation to 

 the other, and this is only possible if the energy of a 

 molecule is as great in the liquid as in the vapour. 



In this theorem we have to remember that we are not 

 dealing with kinetic energy only, but in the case of liquid 

 molecules with the sum of their kinetic and potential 

 energy. 



This equilibrium will only be maintained, however, when 

 the vapour has a certain density, so that there is a sufficient 

 number of molecules to bombard the surface of the liquid. 

 Such a vapour is called a saturated vapour. 



If liquid is introduced into a vessel in which there is at 

 first no vapour of this substance, vapour at once begins to 

 form in consequence of the heat present. Molecules detach 

 themselves from the liquid surface and move about as 

 molecules of vapour in the free space above. Such a 

 separation of a molecule from the liquid left behind takes 

 place the more easily the greater the energy which the 

 escaping molecule possesses. With the molecules that 

 have darted off, therefore, the liquid loses a sum of energy 



