284 The Refractive Indices of the Elements. 



exceed 1J per cent. Comparing these values for P 2 and S- 2 with those 

 of N> and 2 , it is shown that the simple relations found in the case of 

 the inert gases and the halogens also hold in the case of nitrogen and 

 phosphorus, oxygen and sulphur; and that an atom of phosphorus 

 retards light four times as much as an atom of nitrogen, an atom of 

 sulphur four times as much as an atom of oxygen. 



Efforts have also been made to measure the index of fluorine in the 

 gaseous state, but, owing to the experimental difficulties, success has 

 not yet been attained. 



It appears then, that, out of fourteen elements whose index of 

 refraction has been measured in the gaseous state, twelve conform to 

 the rule that in each chemical group the refractivities of the elements 

 are in the ratios of small integers. The other two, Hg and H, have no 

 allied elements with which they can be compared. 



It is pointed out that N, O, and Ne are each followed, in 

 their respective families, by an element whose refractivity is 

 four times as great, and that, consequently, there are reasons for 

 believing that the elements composing the series N, O, F, and Ne, and 

 P, S, Cl, and A are, in some sense, homologous. Comparing the 

 refractivities of the latter series we see that the power to retard light 

 appears to be closely connected with the valency, increasing as it 

 increases, in spite of the decrease in atomic weight, as shown in the 

 following table : 



Element. 



/ A _^ 



P. S. 01. A. 



Atomic weight 31 32 35 '5 40 



Kefractivity 299x4 275x4 192x4 141x4 



The series Ne, 0, N, show the same relation, and it is probable that 

 the refractivity of C is even higher than that of N. 



The refractivity of B, estimated from BCls and BBrs, is certainly 

 very great ; but whether it exceeds that of C there is not sufficient 

 jBvidence to determine. 



