Law of Molecular Force. 319 



CH3I C2H5I CsH;! C3H,I(Iso) C3H5I CHgl 

 150 159 171 170 170 176 



178 168 



In the case of bromides containing one atom of Br we see 

 that the value of kK is stationary, or increases a little with 

 increasino- molecular weio-ht, while in the case of the iodides 

 the same phenomenon is more pronounced, the value nicreases 

 with addition of CHo instead of diminishing, as it does in 

 compounds containing C, H, 0, N, and CI. The exceptional 

 character of these results is not surprising when M^e consider 

 the large masses of the Br and I atoms. The refraction-equi- 

 valents of the organic bromides and iodides are quite regular; 

 so that there is no ground for thinking that the volume of a 

 Br or of an I atom varies according to the number of carbon 

 atoms with which it is united ; but the surface of a molecule 

 containing Br or I united to an organic radical may perhaps 

 no longer be considered to be proportional to the two-thirds 

 power of the molecule on account of the pronounced want of 

 symmetry in structure due to the preponderant atomic mass 

 of Br and I. With this in view, and remembering that if Br 

 and I parameter-equivalents are large, as the preceding table 

 show^s that they must be, then 228/n^ would vary slowly with 

 the slight variation in n produced by the introduction into 

 the molecule of a few C atoms ; the effect, therefore, of dys- 

 symmetry in the building up of the organic bromides and 

 iodides would not require to be very great to explain the ex- 

 ceptional character of kK in them. 



In conclusion, we will see what light the law of the inverse 

 fourth power throws on Waterston^s law. If for a moment 

 we ignore thermodynamical considerations and regard the 

 evaporation of a liquid from the purely mechanical point of 

 view as a change of a system of molecules attracting according 

 to the inverse fourth power from a configuration where the 

 density is p to one in which it is cr, then, as shown in my former 

 paper (Phil. Mag. July 1887), the mutual potential energy 

 of the molecules in the two configurations is ^irAp log L/a 

 and 'lirKcr log L/a respectively. If, then, for the moment we 

 consider the latent heat-vaporization as equivalent to this 

 change of potential energy and call the latent heat X,, we have 



X.=27rA{p — a} log—. 



Now, except in the neighbourhood of the critical point, we 

 can neglect cr, the density of the vapour, in comparison with p, 



