36 Mr. D. L. Hammick on Latent Heat 



The above examples show that from a knowledge of the- 

 surface energy of a liquid at a low temperature and the 

 critical data (from which a knowledge of d, the molecular 

 diameter, can be derived), it is possible to calculate the 

 latent heats at temperatures up to the critical temperature 

 with very fair accuracy by means of Bakker's equation. The 

 agreement between calculation and Young's "experimental^ 

 values for the latent heat is not so striking as that obtained 

 by Appleby and Chapman (Trans. Chem. Soc. cv. p. 734 

 (1914) by the use of a modification of Bakker's equation. 

 Appleby and Chapman's expression is, however, open to 

 criticism (cf. Sutton, Phil. Mag. [6] xxix. p. 593 (1915)) 

 that cannot be applied to the method used above. 



When the value of a at the boiling-point is compared with 

 the value at the critical temperature, it is found that the ratio 



-^ is approximately constant and equal to 1*4. This figure 



is supported b}' Traube's results (loc. cit. above), which show 

 that for a large number of liquids the ratio of the latent 

 heat at the boiling-point, calculated from Bakker's equation * 

 using the critical value a Cj to the experimental latent heat is 

 constant and equal to 1*4. 



Using this ratio, —^ = 1-4, it becomes possible to deduce 

 a c 



with precision the semi-empirical relationship between* 

 latent heat and absolute boiling-point known as Trouton's 

 Rule. From Bakker's equation at the boiling - point 



T/^-, where V 2 is the gramme molecular volume of the- 



\ V 2 ^ N 



vapour, being small J we have 



T~= :Ll ' 



Y l being gramme molecular volume of the liquid and Lj the- 

 internal molecular latent heat. Writing a T =l'4,a o and 



f>-7T>2T 2 



putting a c — p c , its value in terms of the critical data,. 



we get T _l-4x27R 2 17 ( ~ 



Ll ~ 64P77VT" "• ' ' ' ' ' W 



* Traute used Bakker's equation in its equivalent form 



RT, v x -b 

 M c v-2 - b 



