Refraction and Dispersion of Gaseous Compounds. 595 



side of an absorption band, and abnormally low on the more 

 refrangible side. it may, therefore, be expected that the 

 changes of position of absorption bands in the spectra of 

 elements or compounds should corroborate the conclusions to 

 be drawn from the variations of refraction and dispersion. 



We are, thus, able to obtain three kinds of evidence from 

 which to infer the physical meaning of the changes which 

 take place. If, in the equation given above, C is altered by 

 the act of combination, the whole scale of the figures is pro- 

 portionally affected. If the free frequency is changed the 

 value of n 2 will differ from that which it would have had on 

 an additive hypothesis. If the absorption spectrum of the 

 compound differs from that of its constituents, we have a 

 clue to the identification of that portion of the refractivity 

 which is connected with it. 



The experimental results shown in the table above, and 

 the concurrent changes in absorption, so far as they are 

 known, may be summarized as follows: — 



A. (1) In hydrochloric, hydrobromic, hydriodic acids, 

 hydrogen disulphide, and sulphur dioxide, the refractivity of 

 the compound is less than the sum of the refractivities of its 

 constituents, and the dispersive power of the compound lies 

 between those of its constituents. 



(2) In the spectra of chlorine, bromine, iodine, and sulphur, 

 strong absorption bands occur in the violet and near ultra- 

 violet, which disappear in the compound. 



B. (1) In nitrous oxide, nitric oxide, ammonia, and ozone, 

 the refractivity of the compound is greater than the sum of 

 the refractivities of the constituents, and the dispersive power 

 is greater than that of either constituent. 



(2) In the specf ra of nitrous oxide, nitric oxide, and ozone, 

 absorption-bands appear in the ultra-violet which are not 

 present in the spectra of the elements. 



To explain these results I propose the following hypo- 

 thesis : — 



(I) That the refractivity of a gaseous element (except 

 those whose molecules are monatomic) or compound is mainly 

 made up of two portions which may be called the atomic 

 and the interatomic. The former is due to vibrations of parts 

 of the same atom relatively to one another (on the electro- 

 magnetic theory of electrons relatively to the rest of the 

 atom). The free frequency lies in the extreme violet 

 (X= 1000 A.U. approximately), and the dispersion curve is 

 very flat in the visible region. The refractivity due to this 



2S2 



