140 
MR. A. E. OXLEY ON THE INFLUENCE OF MOLECULAR 
If N be the number of molecules in one gramme of the substance we may write the 
specific susceptibility 
X = 
NAM, 
H, 
Ne 2 A 
4 m 
1 + 
a 
AH. 
H. 
( 7 ) 
and it is clear that the value of x is modified by crystallization, for the polarisation 
term aV varies with the state of the substance. If a = 0, (7) reduces to the Langevin 
expression for diamagnetic susceptibility in which the mutual influences of the 
molecules are neglected. 
The variation of x on crystallization may be written 
— =(a c -a/)-^P £ .( 8 ) 
X M * * * § 
where a c and cq are the electric polarisation constants for the crystals and liquid 
respectively ; a l is of the order l/3 for most liquids. # It now remains for us to examine 
the amount of this change and to do this we must determine a c . The value of the 
polarisation constant for any substance is positive and may be very large compared 
with the value 1 /3 which it usually possesses for the liquid state; but its exact 
determination in any particular case is difficult since it depends upon the actual 
distribution of the molecules, about which we know very little. 
It appears, however, that we can deduce an approximate value of the magnitude of 
a c from Chaudier’s researches on the change of magnetic rotatory power with change 
of state. This deduction leads to a reasonable value. 
It has been shown by Becquerel, Bichat, and later by Chaudierj that the 
magnetic rotatory power of the liquid and vapour states is not generally the same, and 
it is reasonable to suppose that a similar but more pronounced effect will be found in 
the passage from the liquid to the crystalline state, but the phenomenon will usually 
be complicated in the latter case owing to the appearance of double refraction. The 
researches of Gouy| and Chauvin§ show that in the case of Iceland spar the effects of 
double refraction and magnetic rotation are simply superposed. Now Chaut>ier|| has 
established that the magnetic rotatory power disappears completely during the 
crystallization of aniline, benzene, and nitrobenzene (all of which have been investigated 
in Part I.). If this is a true discontinuity of the magnetic rotatory power, apart from 
any effect due to double refraction, then we can use this fact to obtain an estimate of 
* Loc. tit., p. 138, or Larmor, ‘ Phil. Trans. Roy. Soc.,’ 1897, A, p. 233. 
t ‘ Comptes Rendus,’ vol. 156, p. 1008, 1913. 
| ‘ Journ. de Phys.,’ ser. II., vol. 4, p. 149, 1885. 
§ ‘Journ. de Phys.,’ ser. II., vol. 9, p. 5, 1890. 
|| ‘Comptes Rendus,’ vol. 156, p. 1529, 1913. 
