596 Mr. Olive Cuthbertson on the 



mode of vibration is roughly '8 or *9 of the whole refractivity 

 measured in the visible. The absorption connected with it 

 is that which lies in the Schumann region. The atomic 

 refractivity, being due to vibrations governed by forces 

 which have their seat exclusively in a single atom, may be 

 assumed to remain nearly constant. The latter, or inter- 

 atomic portion of the refractivity, is the result of the ex- 

 istence of free periods in the visible or neighbouring portions 

 of the ultra-violet or infra-red, due to forces which have 

 their seat in more than one atom of the same molecule. The 

 absorption due to these vibrations is that which is observed 

 in the spectrum of these regions. The interatomic refractivity 

 of an atom of an element varies with the atom with which it 

 is combined, and with the nature of the linkage. 



(2) When two elements combine to form a compound the 

 observed changes of refraction, dispersion, and absorption 

 are due to the disappearance of the interatomic portions of 

 their refractivities and the appearance of new interatomic 

 free frequencies. 



An example will make the hypothesis clearer. Gaseous 

 chlorine is characterized by high refractivity, moderately 

 high dispersive power, and strong continuous absorption in 

 the ultra-violet. Hydrogen has low refractive and dispersive 

 power and exhibits no trace of absorption up to the limits of 

 the Schumann region. The resulting compound (HC1) has 

 a refractivity 2*8 per cent, less than the sum of the refrac- 

 tivities of -J(Cl2)H-i(H 2 ), a dispersive power intermediate 

 between those of chlorine and hydrogen, and an absence of 

 absorption, so far as is known, in the ultra-violet. 



In the first place it appears highly probable that both the 

 chlorine and the h}^drogen atom carry into the compound 

 the main portion of their refractivity. The additive rule 

 fails, no doubt, to go further than a first approximation, even 

 in the most favourable instances; but no one who studies 

 the figures for gases or the work of Gladstone and Dale on 

 solids and liquids can avoid the conviction that this rule 

 must be the starting point of the explanation. 



We have, then, to account for a small change in the sum 

 of the refractive powers of the two elements, coupled with a 

 considerable change in the dispersive power, which is indi- 

 cated by the increase of ?? 2 from 9629*4 x 10 27 in Cl 2 to 

 10697 X 10' 27 in HC1, and the disappearance of an absorption 

 band. This could be done by supposing that the refractivity 

 of one of the elements (01 2 ) contains a term of which the 

 absolute value is small but the slope steep, and that this 

 term disappears when the molecule is broken up. The 



