Refraction and Dispersion of Gaseous Compounds. 593 



elements and simple compounds in the gaseous state between 

 W 6708 and 4800 ; and, though the field of investigation 

 is still somewhat restricted, sufficient data have now been 

 collected to warrant the formulation of an hypothesis which 

 seems to explain, in a qualitative way, the changes in re- 

 fraction, dispersion, and absorption which are observed when 

 gaseous compounds are formed from elements. 



In the experimental work the refractivity of each substance 

 was first determined for the green mercury line A, = 5461, 

 and reduced to a standard density, viz. that of hydrogen at 

 0° C. and 760 mm. multiplied by the ratio of the theoretical 

 molecular weight to that of hydrogen. The dispersion was 

 then found by measuring the refractivity at seven other 

 wave-lengths, \\ 6708, 6438, 5790, 5770, 5209, 5085, 4800, 

 relatively to the value obtained for A, 5461. 



The eight values of the refractivity were generally found 

 to fall on a smooth curve, which can be fitted by an 

 •equation of Selhneyer's form 



11 =fJ> — l (approx.) = J 



2 li y n^—n 2 



where n 2 is the square of the frequency of the free vibration 

 in the molecule (assuming there to be only one), and n 2 is 

 the square of the frequency of the light, and is equal to 



V 2 9xl0 2 V 



rr-s = ro — (in cms.;. 



A" A." 



In the following table (p. 594) are given the values of the 

 constants of this equation, calculated from the experimental 

 Tesults, for those elements and compounds which will be 

 referred to below. 



In this table column 4 gives, for convenience, the value 

 of X corresponding to that of n 2 in column 3. Column 5 

 gives the value of /u — 1 for infinite waves found by equating 

 it to C/n 2 . Column 6 shows, in the case of compounds, the 

 additive value of the refractivity, i. e. that obtained by adding 

 those of its constituents. In this calculation the refractivity 

 of a single atom of an element is taken as half that found for 

 biatomic molecules, and similarly for polyatomic molecules. 



The last column shows the difference between the additive 

 and experimental refractivities of compounds. 



The modern theory of dispersion indicates that an intimate 

 relation exists between dispersion and absorption. The re- 

 fractivity should be abnormally high on the less refrangible 



Phil. Mag. S. 6. Vol. 25. No. 148. April 1913. 2 S 



