600 



Mr, Clive Cuthbertson on the 



The parallelism between these two sets o£ figures is 

 unmistakable, and seems to afford strong evidence that 

 this method of obtaining expressions for the refractivity of 

 the halogen atoms is on the right track. 



But it is possible to carry the comparison a step further. 

 In the paper in the Phil. Mag. just referred to it was shown 

 that, in the case of the four inert gases whose critical tem- 

 perature has been determined, there is a constant ratio, 

 between the squares of the numerator of the dispersion 

 formulae (which, on Drude's theory, are proportional to the 

 number of electrons in the atom which influence dispersion) 

 and the critical temperatures. It, therefore, occurred to me 

 to compare the critical temperatures of the halogens with 

 those of the inert gases. Unfortunately, the determinations 

 for bromine and iodine rest on the authority of only one 

 observer; but, assuming their accuracy, the proportionality 

 is evident. 



Comparison of the critical temperatures of the 

 halogens and inert gases. 



Element. To 



Absolute. 



Chlorine... 420 



Argon 1556 



Bromine... 575 "1 



Krypton... 210*5 j 



Iodine ... 785 



Xenon ... 289"6 



Eatio. 

 2-70 

 2-73 

 2-71 



Observer. 



Knietsch (419), Ladenberg (421). 

 Ramsay and Travers. 



Nadescbdine. 

 E. &T. 



Eadice. 



Patterson, Cripps & Wbytlaw-Gray. 



It follows that there is a constant ratio between the squares 

 of the numerators of the expressions for the refractivities of 

 the halogen atoms and their critical temperatures. 



Turning now to the expressions assumed to be connected 

 with the interatomic free frequencies (2), (5), (6), we can 

 test the usefulness of the hypothesis by comparing the 

 position of the calculated free frequency with that of the 

 centre of the absorption band. The coincidences are not so 

 striking as in the case just discussed, but, considering that 

 we are here dealing with figures which are the differences 

 between two nearly equal quantities and that the calculation 

 takes no account of the influence of absorption on the value 

 of the free frequency, the agreement is not unsatisfactory. 



It will be observed that, in each case, the calculated free 

 frequency is greater than the observed, and this is also true 



and of nitrous oxide, nitric oxide, and 

 far as can be judged without having 

 centre of the absorption-band of the 

 latter element. It seems probable that the dispersion curve 



in the case of ozone 

 even of oxygen, so 

 observed the actual 



