Latent Heats of Fusion. 443 



Theoretical Considerations. 



Although the thermal properties or! the solid state may 

 now be regarded as fairly satisfactorily established, the 

 liquid state is still in many respects obscure. This un- 

 certainty is reflected in the difficulty which at present exists 

 of forming any definite conception of the molecular changes 

 which accompany fusion, and it is not surprising that the 

 relations between the energy-changes on fusion and the 

 molecular properties of the materials are almost entirely 

 empirical. In the case o£ evaporation, the well-known rule 

 of Trouton, ML/T= const. = 21 (M= molecular weight) is 

 known to hold fairly accurately for non-associated liquids. 



P. W. Robertson (Trans. Cbem. Soc. lxxxi. p. 1233, 

 1902) suggested the expression ML/T v/V=const., where L 

 is the latent heat of fusion, T the melting-point (absolute), 

 and V the molecular volume. In the case of monatomic 

 metals (M>40) the constant is 1*13 ; inorganic salts gave a 

 mean constant of 2'0 ; organic compounds, 2"2-3"0. It is 

 known that the boiling-point is a *' corresponding temper- 

 ature iS in the theory of corresponding states (see Partington, 

 4 Thermodynamics.' p. 234) ; if T c and T m are the critical 

 temperature and melting-point of a substance, then, according 

 to F. W. Clarke (Amer. Chem. J. xviii. p. 618, 1896), 

 T c /T m = 2, so that fusion is also a "corresponding state." 

 E. Mathias (Le point critique, p. 59, 1904), on the assumption 

 of the law 7 of the rectilinear diameter, also deduces that T OT 

 is a corresponding temperature, and the same assumption is 

 made by V. Kourbatow (J. Chim. Phys. vi. p. 339, 1908). 

 P. Walden (Zeitschr. Elektrochem. xiv. p. 713, 1908) assumes 

 that the relation M/T = const, holds for the fusion of non- 

 associated substances, the value of the constant for 35 organic 

 substances being 13*5 (12"1 to 14*2). For other substances 

 the " degree of association, " x } is assumed to be given by 



13*5 /-rp-' Benzophenone, betol 7 and anthracene, for 



example, are assumed to be normal ; formic and acetic acids, 

 phenol, and benzene have association factors less than 2. 



E. Baud (Comptes rendus, 152, p. 1480, 1911) proposes 

 the equation lii=k(vi — v s ), where v t and v s are the specific 

 volumes of the liquid and solid at the melting-point. Accord- 

 ing to G. Tammann (Krystallisieren unci Schmelzen, p. 41, 

 1903), the constant k is the absolute melting point T m . 

 J. Narbutt (Zeitschr. .Elektrochem. xxv. pp. 51, 57, 1919) 

 finds large discrepancies in the application of Tammann's 

 formula ; Walden's rule gives fairly satisfactory results if 



