598 Mr. Clive Cuthbertson on the 



made for these changes. But reasons can be advanced for 

 assigning to them less importance than is given to the change 

 in the chlorine molecule. In the first place, the ref ractivity 

 of hydrogen is less than one-fifth of that of chlorine, so that 

 small changes in its ref ractivity due to similar causes would 

 probably be in the same proportion. The same argument 

 applies to hydrochloric acid, whose refractivity is about one- 

 half that of chlorine. Again, neither hydrogen nor hydro- 

 chloric acid shows any absorption in the ultra-violet as chlorine 

 does, so that, on the proposed hypothesis, the changes in 

 their refraction and dispersion, due to the breaking up and 

 formation of the molecules respectively, may also be expected 

 to be small. Finally, the interatomic effects due to hydrogen 

 and hydrochloric acid occur on different sides of the equation 

 and tend to balance each other. 



But the best test of the legitimacy of the hypothesis is to 

 be found in the numerical results which are obtained from it. 



The effect of the preceding analysis has been to split up 

 the expression for the refractivity of chlorine into two parts, 

 one of which, it is suggested, is due to the atomic and the 

 other to the interatomic frequencies of the chlorine molecule. 

 The values of the atomic refractivity are obtained by de- 

 ducting the experimental values for -J(H 2 ) from those for 

 HC1; while those for the interatomic refractivity of Cl 2 

 are found by deducting twice the resulting figure for (Cli) 

 from the experimental values for (Cl 2 ). 



The first calculation gives eight values for the atomic 



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refractivity, which are fitted by the curve given by 



v 3-8787 xlO 27 m 



while those for the interatomic refractivity are given by 



, ^ -07302 xlO 2 - . m 



(^-^Cli-ci,- -2935 x 102* _ n 2- • ' ' w 



The relations of the refraction, dispersion, and absorption of 

 bromine and hydrobromic acid and iodine and hydriodic 

 acid are exactly similar to those of chlorine and hydrochloric 

 acid. 



Adopting the same procedure as that given above we obtain 

 for the atomic refractivities of bromine and iodine: — 



4-35(56 xlO 27 



lM-l)Br J - 82 ^ xl0 27_ n 2^ • • ■ W 



■n 5-1548 xlO 27 f . 



^~ 1 \~6325xl0 2 '-^ 2 ' • • • W 



