﻿592 Mr. D. L. Hammick on Latent Heats 



The question now arises as to the connexion between the 

 value for the latent heat of expansion as given by (4) and the 

 latent heat of vaporization at ordinary temperatures. The 

 two latent heats would be equal, provided that no change in 

 the internal energy of the substance occurred during the 

 transition, at constant volume, from the liquid to the vapour 

 phase. In other words, the condition for equality is : 



( G v dT-{ c v dT = (5) 



Jo Jo 



(G v , c are the specific heats at constant volume 

 respectively.) 



If (5) does not hold, then the difference between the 

 latent heat of vaporization and X ex . of equation (4) will be 



C(C v -c v )dT=K. 



Again, if during the passage from the liquid to the 

 gaseous state a change in molecular aggregation occurs, 

 a further quantity of heat, li, representing heat of asso- 

 ciation, must be taken into consideration. Hence we 

 may put 



Vap. = /W + H 4- h (6) 



In the Table values of X ex ., calculated according to 

 equation (4) at the boiling-point, are compared with the 

 observed values of X vap ., the latent heat of vaporization. 

 Values of "a" are given as atmospheres pressure-f- (volume 

 of 1 gramme molecule of gas at N.T.P.) 2 (Guye and Frederich, 

 Arch. Sci. phys. et natur. Geneve, ix. p. 22, 1900). Specific 

 volumes and a, the coefficients of expansion, are taken from 

 Young (Sci. Proc. Roy. Dubl. Soc. xii. p. 414, 1910) and 

 Tyrer (Trans. Chem. Soc. 1914, p. 2534). The latent heats 

 of vaporization refer to 1 gramme of liquid, and are mainly 

 Young's values (loc. cit.). 



It will be seen from the Table that the values of A, ex . at 

 the boiling-point agree very well with the values of the 

 latent heat of vaporization. At the boiling-point, there- 

 fore, we have, in equation (6), 



H-hA = 0. 



For " normal " or unassociated liquids h = 0, and hence 



H= \ (C v -c v )dT = 0. 

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