162 Critical Energy Increment and Troutons Rule. 



support to this belief ; the extra energy which is supplied 

 during evaporation is thus to be ascribed as that necessary 

 for the decomposition of the associated molecules into 

 single ones prior to or during evaporation. The constant 

 Of Trouton and the observed data indicate that in many 

 cases this energy of dissociation may be as much as half 

 of the energy necessary for vaporization of the normal 

 liquid, or 



ass. 



In a few cases an identical relationship obtains at the 

 melting-point, as is evident from the following values : — 



Substance. M.P.°. L calc. L obs. 



Cl a 170 1,670 1,628 



Br 2 265-7 2,600 2,590 



I 2 386 3,800 3,000 



NH 3 198 1,950 1,890 



SbCl 3 346 3,400 3,000 



SbBi-3 367-2 3,600 3,500 



H,P0 4 291 2,870 3,007 



HOOOH 265-5 2,020 2,600 



CH3COOH 278 2,920 2,600 



O.H, . CH 2 COOH ... 348 3,430 3,500 



Butyric 273 2,690 2,500 



Benzol 275 2,700 2,300 



Nitrobenzol 277 2,900 2,700 



Sulphur at the melting-point would appear to exist chiefly 

 in the form S 12 , since .the latent heat of fusion calculated 

 per atom is 299 and the calculated value M.P. = 388K° is 

 3,800. 



In the case of metals and of dissociated salts the values 

 calculated from Trouton's law are much higher than the 

 observed values, indicating a change in the mechanism 

 of fusion (Lindemann, Phys. Zeit. xi. p. 609, 1910 ; Honda, 

 Phys. Rev. xi. pp. 12, 425, 1918). 



Summary. 



The critical energy increment is shown to be an average 

 value. The nature of the equilibrium between the molecules 

 of any one species in their various stages of activation is 

 assumed to follow the energy curve of the circumambient 

 radiation. Only in the case of "black" molecules is the 

 curve complete. At the critical temperature the maximum 



