2 



Proceedings of the Royal Irish Acadenuj. 



deductions of this theory, which is accordingly taken to hold for this 

 particular case, and, possibly, for all gases and vapours. It is not 

 quite certain whether a change of sign in the magnetic rotation has 

 been actually observed ; in the case of praseodymium chloride, Wood^ 

 under great difficulties, obtained indications of such a property. 

 The validity of the hypotheses in other cases has been usually tested 

 by their dispersion formulae ; but as these are equally satisfactory 

 within the range of the spectrum, no verdict can be given. Recently, 

 however, Ingersoll,- extending the range of examination of the rotatory 

 power of carbon disulphide into the infra-red, found that the Hall-efPect 

 formula was beginning to give more satisfactory results. 



While, therefore, there is a disposition to reject the hypothesis of 

 rotating ions as a general explanation of the phenomenon of magnetic 

 rotation, the experimental evidence on the subject is by no means 

 conclusive. It is proposed to show in this paper that : — 



The hypothesis of molecular currents, in conjunction with the 

 average values for the magnetic rotation and index of refraction in 

 diamagnetic substances, leads to values for the radii of the assumed 

 ionic orbits and magnetic susceptibility altogether at variance with 

 the actual or possible values of these magnitudes, so that this hypothesis 

 must be rejected in the case of diamagnetic substances in favour of the 

 hypothesis of the Hall-effect, which leads to a normal value for the 



ratio ^ of the electron, and so supports the view taken that the 



Hall-effect is the true explanation of magnetic rotation in diamagnetic 

 substances. 



The leading principle in the " ionic theory" is the explanation of 

 the interaction of light and matter by means of charged particles, or 

 ions, anchored to fixed positions in space, which can be disturbed from 

 their position of equilibrium in every direction, and with a restoring 

 force after displacement proportional to the displacement, but indepen- 

 dent of its direction. Each ion has its own period of vibration, and 

 will absorb vibrations of that period. From these assumptions formulae 

 can be deduced, expressing the optical properties of matter in terms of 

 the period, mass, charge, and number of these ions per unit volume. 

 It is, evidently, the disturbance caused by the external magnetic field 

 in the motion of the ions, which is the origin of the magneto-optic 

 rotation. As stated above, Drude has considered this disturbance from 



Phil. Mag., May, 1905. 



- Ibid., Jan., 190G. 



