CONSTITUTION AND TEMPERATURE ON MAGNETIC SUSCEPTIBILITY. 283 



an external field produces a minute diamagnetic polarity, which differential effect 

 determines the magnitude and sign of the rotation. When a polarised ray passes 

 over such molecules, it will be rotated considerably locally, but only to be rotated in 

 the opposite direction to an almost equal extent in a neighbouring molecule or in 

 another part of the same molecule. 



This will also be true if the diamagnetic medium is crystalline. Hence the final 

 rotation will be a relatively small differential effect compatible with the diamagnetic 

 effect of the medium in bulk. We shall omit the exceptional case of titanium chloride 

 for the present and pass on to consider the ferro-magnetic media iron, nickel and cobalt. 



In ferro-magnetic media, on account of the continuity of magnetic induction, there 

 is an enormous reverse local field, and if this acts over regions of the molecule 

 containing magnetically active electrons, a large rotation will be produced, which will 

 not be compensated in neighbouring molecules, when the ferro-magnetic material is 

 saturated. Hence we should expect a very large rotation to be produced by such 

 media. This has been confirmed experimentally, in the cases of iron, nickel and 

 cobalt, by KUNDT. 



Paramagnetic solutions lie in an intermediate category. The application of an 

 external magnetic field causes a certain amount of molecular orientation depending 

 on the temperature. Such orientation causes a reverse field over a part of the system, 

 but over the neighbouring molecules in combination with the one considered, the field 

 depends upon the difference between the reverse local field and the applied field, 

 which difference may be positive or negative. If, in such regions, there are electrons 

 capable of orientating the polarised beam, the rotation may be dextro- or Irevo-gyric, 

 according as the resultant field is opposite to, or in the same direction as, the applied field. 

 This will not explain the exceptional case of titanium chloride unless this molecule 

 possesses some peculiar dissymmetry, whereby, in spite of its diamagnetic nature, it 

 becomes orientated under the applied field. Such orientation is the basis of the 

 magnetic double refraction theory developed by LANGEVIN* and confirmed experi- 

 mentally in some respects by COTTON and MoUTONt (see Part III, p. 87). An 

 alternative explanation of the behaviour of the titanium chloride molecule may be 

 found in a rotation of the paramagnetic titanium atom relative to the compound 

 molecule. 



BECQUEREL has deduced an interesting relation} connecting the magnetic rotation 



/ \ 



(r) with the Zeeman coefficient ( Z = ), the applied field H, wave-length A and 



\ 47rW 



refractive index n, viz., 



r = 2ir . ^r: > (1) 



c 3X 



where c is the velocity of light. 



* LANGEVIN, 'Le Radium,' vol. 7, p. 251, 1910. 



t ' Ann. de Chim. et de Phys.,' ser. viii., vol. 19, p. 155, 1910. 



I SCHUSTER, ' Theory of Optics,' p. 307. 



