Hackktt — TV/r Ionic Theories of Magneto-optic Rotation. 19 



Taking for the yellow line of sodium, we find the average 

 71 A 



value of - — ^ is '038 for the same list of substances as before. 

 n- - 1 



Inserting the value of A for this wave-length, regarding and X- as 

 expressed in 10"^ cm., 



eX^ 



= ord. 2-3 X 10\ 



m 



In order to find what approximation this gives for an ultra-violet 

 absorption-band, which in the case of carbon disulphide and creosote 

 is seen to be due to electrons, we take -019 the average value of the 

 wave-length of the ultra-violet absoi-ption-bands in the table of 

 tUspersion constants, as the value of A- in units of 10~* cm. 



We get then 



e 2-3 X 10^ , , ^ 



— = ord. = ord. 1-2 x lO'. 



The experimental values of - lie between 1-86 x 10" and '64 x 10" : 



7?l 



and the preceding calculation from magnetic rotation has given the 

 values : — 





Carbon 

 Disulphide. 



Creosote. 



Diamagnetic 

 Substances. 



e 

 m 



•53 X 10' 



•77 X 10' 



order 10' 



These results show that the rotation observed has the magnitude 

 which would arise from the influence of the Hall-effect on the motion 

 of electrons. It is concluded, therefore, that the theory based on the 

 Hall-effect supplies a satisfactory explanation of the phenomenon of 

 magneto-optic rotation, and must be accepted in preference to the 

 theory of rotating ions which only accounts for a negligible part of 

 the rotation, unless impossible values are assigned to the radii of the 

 ionic orbits. 



£ 2 



