Dispersion in Relation to the Electron Theory. 465 



Hence the sum of such values for the atoms present in 

 a molecule gives the maximum number of electrons capable 

 of producing affinity ; and this number should be taken for 

 v in place of the normal valencies. The numbers in paren- 

 thesis given under v in the tables are calculated in this way. 



It is to be expected that the electrons which produce 

 the attraction between the atoms in a given compound, 

 and which according to Thomson's theory are transferred 

 from one atom to the other, will be those subjected to 

 the weakest controlling forces and will be, therefore, the 

 long-period electrons which give rise to the dispersion 

 and magnetic rotation. Various considerations support this 

 view. When the number of bonds increases relatively to 

 the number of atoms, as in unsaturated and aromatic com- 

 pounds, the dispersive power undergoes a marked exaltation. 

 The numerical values obtained above for the upper limit to 

 the number of electrons of longest period are too small to be 

 associated with any but the active bonds. In quartz, where 

 the dispersion constants are probably more accurately known 

 than in the case of any other substance, we have j9i = 4, 

 i. e. one electron for each bond in the molecule. In elements, 

 however/ which show high positive contravalency, particu- 

 larly those of high atomic weight, the electrons not associated 

 with the bonds may influence the dispersion owing to weak 

 controlling forces. Thus the six contravalency electrons of 

 sulphur are probably responsible for a large part of the 

 dispersion and refraction in carbon bisulphide, and the same 

 is probably true of nitrogen in the highly refractive amines. 

 The point of view assumed in the present theory will perhaps 

 be rendered more definite if we make a distinction between 

 the various electrons which may influence the optical 

 properties as follows. 



Class I. The recent application of photometrical measure- 

 ments to absorption phenomena has shown that the character- 

 istic absorption bands in many cases must be attributed to 

 electrons whose number in the unit of volume is only a small 

 fraction of the number of molecules. Henri * finds the 

 absorption in acetone is produced by 1 resonator in 36 mole- 

 cules ; Baly and Tryhorn f find the number of molecules 

 operating in the production of the less refrangible aniline 

 band is 1 in 30 ; Kcenigsberger and Kilchlingt deduce for 

 the bromine band at '412 fi only 1 in 60. Hallo, from 

 observations on the magnetic rotation in sodium vapour, 



* Phys. Zeit. (14) p. 515 (1913). 



f Trans. Chem. Soc. (107) p. 1121 (1915). 



X Ann. d. Phys. (28) p. 889 (1909). 



Phil. Mag. S. 6. Vol. 31. No. 185. May 1916. 2 I 



