458 , Mr. S. S. Richardson on Magnetic Rotary 



Yoigt's theory of magnetic rotation and birefringence, 

 which attributes the two phenomena to essentially the same 

 cause, fails to account quantitatively for the latter property 

 in liquids, the observed value being some 10 3 times as great 

 as that calculated. Cotton and Mouton & Langevin * have 

 shown that the property of magnetic birefringence is satis- 

 factorily explained by a molecular orientation set up by the 

 magnetic field, — a theory which also accounts for the rapid 

 decrease in the effect with rise of temperature. Now such 

 orientation may bring into play intermolecular forces, either 

 electrical or magnetic, and influence the periods of vibration 

 of the resonators. As a first approximation the variation 

 in the magnetic rotation so produced may be represented 

 by a virtual change in the dispersion constants, which 

 therefore become (a + a£H 2 ). The experiments of Cotton 

 and Mouton have shown that among organic liquids the 

 magnetic double refraction is exhibited most strongly by 

 the unsaturated and aromatic compounds, saturated and 

 aliphatic substances showing scarcely any trace of the effect. 

 It is therefore reasonable to expect that the value of f will 

 be negligible for liquids of the latter class. Yoigt's theory 

 suffices to explain the birefringence in metallic vapours, and 

 f may be taken as zero for substances in the gaseous state. 

 We may also regard it as inoperative in solids, for the applied 

 field is always very small in comparison with the intense 

 intermolecular fields brought into play during crystallization. 

 From the change in diamagnetic susceptibility during the 

 solidification, Oxley finds the latter equivalent to magnetic 

 fields of the order 10 7 gauss. It may be added that the 

 hypothesis of intermolecular fields is not essential to the 

 theory. The effective free periods of an assemblage of 

 seolotropic molecules when these are partially orientated by 

 the field may be expected to differ from the effective periods 

 of the same molecules when their indiscriminate motions 

 render the medium as a whole isotropic. Thus, apart from 

 the change ^H/2mC directly imposed by the field, the change 

 in the effective period will influence the rotation, which will 

 therefore differ from that calculated in terms of the dispersion 

 constants of the isotropic medium, and this influence we may 

 also attempt for the present to express by f. As we have 

 seen above, it is probably only in unsaturated compounds 

 that it may be great enough to be measurable, and in general 

 therefore it can be omitted. 



In the formulae e/m has been taken to have different 

 values for electrons of different frequencies. The velocity 

 of the vibrating electron is so small in comparison with that 

 * Le Radium, Sept. 1910. 



