.324 



MR. CLIVE CUTHBERTSON ON TITK 



elements, He, Ne, A, Kr, and X, are of peculiar importance in the study of optical 

 constants. They are all elements. They are all gases at the ordinary temperature. 

 And they are all members of the same chemical family and of zero valency. In all, 

 too, the molecule is monatomic, so that whatever complication may be caused by the 

 expenditure of radiant energy on internal work in the molecule may be presumed to 

 be absent. Any relation, therefore, which exists between the refractivities of the 

 elements may be expected to show itself more clearly in this chemical group than in 

 any other. 



The refractive indices of all five gases have beey measured by Sir WILLIAM 

 HAMSAY and Dr. TRAVERS, and are given in their paper published in the ' Philosophical 

 Transactions.'* 



In a letter, published in ' Nature' of October 16th, 1902, I drew attention to the 

 fact that these figures bear to each other a curiously simple relation. Within narrow 

 limits of divergence, easily accounted for by the difficulty experienced in purifying 

 the gases, and the smallness of the quantities with which the experiments were 

 made, the refractivities are in the proportion of 1, 2, 8, 12, and 20 ; or, more simply, 

 of $, 3-, 2, 3, and 5, as shown in the following table : 



TABLE I. 



In the calculation given in column 6 the value for argon is taken as the standard, 

 since it is the most easily purified. The discrepancies are not large, and it is remark- 

 able that in three cases out of four they take the direction which admixture with a 

 neighbouring element would cause them to do. Thus, it is difficult to separate He 

 and Ne, and we find that the refractivity of the former is too high and of the latter 

 too low. Similarly, the number obtained for X is too low, as would l^e the case if the 

 specimen contained any Kr. 



It is highly improbable that the ratios of these five numbers should approach so 

 nearly to integers by mere accident ; and the question naturally arises whether any 

 similar relation can be traced between the refractivities of the members of the other 

 chemical groups. 



* 'Phil. Trans.,' A, vol. 197, p. 81, 1901. 



