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 molecides, it will be rotated considerably locally, hut 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 liy 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 Isevo-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, vdiereby, 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 MoutonI (see Part HI., 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 
the applied field H, wave-length A, and 
(r) with the Zeeman coefficient = 
refractive index //, viz.. 
Ittw/ 
r = 
iTT 
ZHx 
where c is the velocity of light. 
0/X 
(1) 
* Langevin, ‘ Le Radium,’ vol. 7, p. 251, 1910. 
t ‘ Ann. de Chim. et de Phys.,’ ser. viii., vol. 19, p. 155, 1910. 
X Schuster, ‘ Theory of Optics,’ p. 307. 
