Refraction and Dispersion of Gaseous Compounds. 597 



curve which expresses such a term would rise rapidly, indi- 

 cating an absorption band in the near ultra-violet, as is the 

 case in chlorine, and the disappearance o£ this term would 

 be accompanied bj the disappearance o£ the absorption band, 

 as we find is the case when hydrochloric acid is formed. 



The inference that the forces which determine this free 

 frequency have their seat in more than one atom is based on 

 the fact that when the link which binds the atoms of chlorine 

 together is broken the absorption band disappears. 



in the figure these changes are exhibited graphically. 



o rj ~^ increasing 



Plotting the refractivities against squares of frequencies, let 

 Cli denote the refractivity of gaseous chlorine divided by 

 two, H x that of hydrogen divided by two, Hj + Cli the 

 additive curve, and HC1 the experimental curve for hydrogen 

 chloride. What has to be accounted for is the disappearance 

 of the portion indicated by vertical shading between Hj + 

 Cli and HC1. Consider this portion as due to a distinct free 

 frequency, and lay it down on the base-line as shown at X. 

 It is evident that the curvature is great, since the ratio of 

 the ordinates at the red and violet ends is greater than when 

 the curve is bodily shifted higher, and it is natural to 

 associate it with the absorption band which occurs in the 

 near ultra-violet in chlorine. 



It may be objected that no reason has been shown why 

 the whole of the observed changes should be attributed to 

 the chlorine molecule, and no allowance made for the dis- 

 appearance of the interatomic refractivity of the hydrogen 

 molecule or the appearance of a new interatomic effect for 

 the molecule of hydrochloric acid. The criticism is just ; 

 and it must be admitted that some allowance ought to be 



