VIII. On the Refractive Indices of tke Element.?. 



By CLIVE CUTHBERTSON. 

 Communicate^ by Professor F. T. TROUTON, F.R.S. 



Received October 18, Read November 24, 1904. 



THE relation between the constitution of the molecules of compounds and their power 

 to retard the velocity of light is a subject which has attracted many observers, and 

 has led to results which are interesting, both from their general regularity and 

 occasional discrepancy. It is singular, therefore, that so few attempts have been 

 made to investigate that portion of the subject which should, theoretically, have 

 occupied a prior place, the study of the refractive indices of the elements in the 

 gaseous state and their relation to one another. 



No doubt this defect in our information is due to the difficulty of obtaining all but 

 a very few elements in such a state that their index of refraction can be measured 

 directly. Till recently, the only elements in which this hud been done were H, N, 

 O, Cl, Br, I, Hg, S, P, and As ; and the numbers obtained in these instances did 

 not seem to bear any relation to each other, or to any known constant. Thus, the 

 refractivities* of Cl, Br, and I increase with the increase in their atomic weights, 

 though not proportionately. But O, with an atomic weight sixteen times that of 

 H, only retards light twice as much, while the index of N is actually greater than 

 that of 0. Discouraged by these anomalous results, investigators have generally 

 contented themselves with calculating values for the refractivities of the elements 

 from the effects they produce in compounds, and have aimed at obtaining an empirical 

 constant for each element which would satisfy an additive law in compounds. The 

 numbers so obtained are found to vary with the constitutional formula of the com- 

 pound in question, and the variations are sufficiently regular to afford assistance in 

 analysing the constitution of complex molecules ; but even the best results sometimes 

 fail to fit in with the facts in a way which proves that our empirical formulae are still 

 far from the truth. 



In 1901, however, the data at disposal received a notable enlargement by the 

 discovery of argon and the other members of the family of inert gases. These 



* In the following paper the word " refractivity " is equivalent to the expression (/* - 1) 10*. 

 (379.) 2x2 2.3.05 



