Refraction and Dispersion of Gaseous Compounds. 603 



which gives a value of 52080 x 10~ 8 at A = 5461, instead of 

 52000 found experimentally. Hence the dispersion curve 

 of ozone has greater curvature than can possibly be explained 

 on the hypothesis of a single free frequency. At least two 

 are required. The same effect was observed in a less degree 

 in the case of chlorine. 



For the three nitrogen compounds, ammonia, nitrous oxide, 

 and nitric oxide, the refractivity of the compound is in excess 

 and the dispersion curve becomes much steeper, suggesting 

 the appearance of new free frequencies in the ultra-violet. 

 But in the case of nitrous oxide the absorption spectrum has, 

 apparently, not been examined. Nitric oxide lias a band 

 which begins about 3000 a.u. and increases to the limits 

 of observation (Kreusler). The calculated band falls at 

 X=1940. Ammonia is said by Soret to show an absorption 

 band in the ultra-violet, but Hartley denies this. 



The case of steam is a difficulty on the hypothesis now 

 advanced ; for though the dispersion curve has greater cur- 

 vature than those of hydrogen and oxygen, the refractivity 

 is notably less. This conjunction of changes is not impossible 

 under the hypothesis, but it is less easily explicable. 



In concluding this paper I wish to emphasize the fact that 

 quantitative verification of the hypothesis is not claimed. 

 The data are still too scanty, the range of observation too 

 short, the gaps, both in mathematical theory and in observa- 

 tion, too many to permit this. All that it is desired to 

 maintain is that the direction of the changes of refractivity, 

 dispersion, and absorption consequent on the formation of 

 •compounds are consistent with the hypothesis that in every 

 gaseous element (except those whose molecule is monatomic) 

 -and compound the refractivity is made up mainly of two 

 portions. Of these one is governed by a free frequency in 

 the far ultra-violet and is responsible for the bulk of the 

 refractive power. Since it is always associated with the 

 presence of an atom, it may be supposed to be due to forces 

 which have their seat within the atom, and are hardly at all 

 affected by neighbouring atoms. The other is due to a free 

 frequency governed by forces which have their seat in more 

 than one atom of the molecule, and which change with the 

 linkage. 



I think the numerical results obtained by the adoption of 

 this hypothesis in the case of the simplest series of compounds 

 known, the halogens and halogen acids, is strongly in its favour; 

 and that the agreement between the observed and calculated 

 positions of the absorption band in ozone is very remarkable. 



