42 PROCEEDINGS OF SECTION A. 



This table shews that ^A varies inversely as n''^ 



If, then, we regard n as measuring the actual volume of the 

 molecule we hnd that the parameter A varies inversely as the 

 surface of the molecule. The parameter equivalent of cZ is about 

 5 or 6 ; of N in the amines, -8 in the nitrides and nitrates, -6 ; 

 while in the amides it varies with the molecular weight of the 

 radical to which the N atom is attached. The organic bromides 

 and iodides are also exceptional, as the introduction of more 

 carbon atoms raises the value of ^ A a little instead of lowering 

 it a little, but the want of symmetry in the union with such 

 massive atoms as those of Br and I will account for the 

 exceptional results. 



The law known as Waterston's law can be obtained immediately 

 from the law of the inverse fourth power, if we make the 

 assumption that the latent heat of evaporation is proportional to 

 the change of the mutual potential energy of the molecules in 

 passing from the liquid to the gaseous state. The potential 

 energy in the gaseous state is negligible in comparison to that in 

 the liquid, so that we can consider the change of potential energy 



2 A 1 J- 



as^.Aplog.^ 



Let X be the latent heat of the liquid, then X varies as 



A p log. •- 



and we have shewn that a varies as Ap'^ ?w^ 



mXpt , L 

 .-. ^ a log. - = constant. 



a 7)13 



a 



This is Waterston's law ; but the roughness of the assumption 

 made above is shewn by the roughness of the approximation of 



the experimental values of m Xp-^am'-^ to constancy, as they range 

 from 4.4 to 6.9, even when such cases as those of the alcohols 

 and fatty acids are excluded, on account of their departure from 

 Eotvos's law. 



2.— MOLECULAR REFRACTION. 

 By William Sutherland, M.A., B.Sc. 



Newton was led by the emission theory of light with the 

 hypothesis of an attraction between material molecules and the 

 corpuscles of light to expect that (vr-l)/d or the excess of the square 

 of the index of ref inaction of a substance over unity, divided by its 



