8 2 ME. A. E. OXLEY ON THE INFLUENCE OF MOLECULAR 



molecular field) defining the extent of the polarization of the molecules in the 

 crystalline state and depending upon the definite space lattice which the molecules 



assume. = '-, where H is the applied field, defines the distortion produced in an 

 H 



electron orbit by the forces exerted by surrounding molecules when the substance is 

 in the liquid state. AH is the polarization (proportional to the applied field H) due 

 to this distortion. As we do not know the actual disposition or the proximity of the 

 molecules in the structure, the difficulty in attempting to calculate a e is insuperable 

 and we are compelled to try the more indirect method afforded by experiment. A 

 rough estimate of the value of a c has been obtained from CHAUDIEE'S observations on 

 the change of magnetic rotatory power when aniline, benzene and nitrobenzene 

 crystallize, the order of magnitude deduced being at least 10 2 .* The corresponding 

 value of a, for the liquid state, deduced from theoretical considerations, is of the order 

 ? { .t Thus in the liquid state the mean molecular field is represented by ^ . AH. The 

 distortion of the molecules in the liquid state of the substance is therefore insignificant 

 compared with that for the crystalline state and, neglecting a t in comparison with a e , 



it was shown that 



........ (1) 



where H is the intensity of the applied magnetic field, Xc an d xi are the specific 

 susceptibilities of the crystalline and liquid states respectively. Thus we may ignore 

 the mutual influences of the molecules of the crystalline state providing we supply a 

 mean molecular field whose magnetic equivalent is a e . AH. Equation (l) shows that 

 the actual change of x on crystallization (a few per cent, with aromatic compounds) 

 demands that AH/H shall be of the order 5 . 10~ 4 ; which implies a small and 

 reasonable distortion of the molecules of a liquid due to the surrounding molecules. 

 It should be noted that this is a maximum value of the distortion for the liquid state. 

 The value will be smaller if, as is probable, a c is of a higher order than 10 2 . At 

 present no physical interpretation has been put upon the large value of a c , but we see 

 that it is representative of the local part of the forcive in the crystalline state just as 

 the smaller constant a t is representative of the local part of the forcive in the liquid 

 state. 



On the theory of magnetism developed by LANGEVIN, a diamagnetic molecule 

 contains oppositely spinning systems of electrons which counterbalance one another 

 externally so that the molecule possesses no initial magnetic moment 4 When an 

 external magnetic field is applied the period of one system of electrons is lengthened, 

 that of the other system is shortened, and the molecule becomes slightly polarized or 

 distorted. This small differential effect, which is compatible with the Lorentz 



* Part II., p. 141. 



t LARMOE, 'Roy. Soc. Phil. Trans.,' A, vol. 190, p. 233, 1897. 



J This null initial magnetic moment corresponds to the null electric moment possessed by a molecule of 

 a dielectric which is not subjected to an external electrostatic field. 



