Refractivity and Atomic Interaction. 113 



to the influence of those four nearest neighbours, and there 

 would certainly be no cogent reason for declaring the 

 diamond-value as the atomic refractivity N . But, in 

 absence of better knowledge, let us tentatively assume tltat 

 the diamond -refractivity is the true atomic refractivity of 

 carbon*, i.e. that, in general, iVo=iV"diam., and therefore, 

 for the D-line, 



N =2-lll (B) 



It is interesting, at any rate, to see what values follow 

 from this assumption, and from (A), for the distances R 1 

 and R 2 . Now, resolving (28) with respect to R x , R^ we 

 have 



, _ /sr_|N y.3 r, _ / n"-$n,,\ 

 '-\w-%) ' k~\®"-%) 



where "the singular distance" B/ is as in (26 a). Inserting 

 ZV ft , A 7 ', A 7 " from (A), (B), we find, first of all, 



Rj =0-9052 R' ; R 2 = 0-9443 R', 



i. e. both distances smaller than the singular distance, for 

 the D-line, and R X <R 2 . Next, to obtain both distances in 

 absolute measure, we have, by (26 a) and (27), 



R' 3 = 2aN =9-76 . 2-111 . 10~ 24 cm. 3 , 



that is, R' = 2'742 . 10" 8 , or, in round figures, R' = 2'74 A.U. 

 Ultimately, therefore, we should have, for the interatomic 

 distances in C - G and in C = C, respectively, 



R a = 2-481, R 2 = 2-588 A.U. 



These seem quite reasonable figures, being of the order of 

 " molecular dimensions." Also the circumstance that R x is 

 somewhat smaller than R 2 seems to harmonize with the 

 phenomenal correlatives of a simple and a double bond. 

 In short, the result would be that in C — C the atoms are 

 somewhat closer packed than in = C. And o the incon- 

 spicous increase of their distance by AR = 0*107 A.U. would 

 be sufficient to account for the huge extra- term |=, that is, 

 in the case of sodium light, for AA T =1'733. 



* This Tvould amount to assuming that the disturbing actions of 

 the four neighbours compensate one another, and such an assumption 

 harmonizes with the optical isotropy of diamond. 



Phil. May. S. 6. Vol. 33. No. 193. Jan. 1917. T 



