SPECIFIC HEAT OF LIQUIDS 65 



which with the 0-015 given above for the increase in kinetic 

 energy of molecular movement makes 0-03, a number in 

 fair agreement with the actual specific heat 0-032 of 

 mercury at that temperature 1 . 



Whilst, therefore, in the case of mercury, which consists 

 of free atoms, the specific heat may be calculated imme- 

 diately from the latent heat of evaporation, in the case of 

 bodies with polyatomic molecules the specific heat in the 

 form of gas or vapour at constant volume is needed 

 a quantity that cannot be calculated, but can be obtained 

 by experiment. Thus, e.g. for ethyl ether between 25 

 and 111 this is 0-4, and as the heat of evaporation is 90 

 and the coefficient of expansion 0-0017, we g e ^ ^ or the 

 specific heat of liquid ether 



0-4 + 90x0-0017 = 0-553, 

 which again agrees approximately with the facts. 



3. Difference between Specific Heat of Liquid and Vapour. 



If we combine the relation of p. 64 



L = aD t , 



which holds for temperatures well under the critical point, 

 with the law of rectilinear diameter, according to which at 

 such temperatures the density of the liquid falls off linearly 

 with the temperature, it follows that the latent heat of 

 evaporation must do so too. It follows, however, from 

 Trouton's rule that this diminution must be the same for 

 molecular quantities of different substances, since at the 

 absolute zero and at any other corresponding temperatures 

 the molecular latent heat is proportional to the critical 

 temperature. We have thus for any pair of bodies 



or aTlc Q : a p k Q = k : 



Jj a TI.. -L/Q A Li 



so that ~ - or w = const. 



1 In the above calculation it is assumed as a first approximation that a 

 is independent of the temperature : the actual values for the two parts of 

 the specific heat probably differ a good deal from those calculated. 



E 



