84 MR. A. E. OXLEY ON THE INFLUENCE OF MOLECULAR 



The small value of a t possessed by fluid substances implies a mean molecular field 

 much smaller than that associated with a crystalline structure. But how much 

 larger the mean and local molecular fields of the crystalline state are compared with 

 those of the liquid state has not been ascertained at present. All that has been 

 shown in Parts I. and II. is that the minimum value of the mean molecular 

 field in some diamagnetic crystalline media is large compared with that in 

 liquids. We shall now proceed to obtain an estimate of the intensity of the local 

 molecular field in crystalline media from the small variation of x which occurs on 

 crystallization. 



(4) ON THE MAGNITUDE OF THE LOCAL MOLECULAR FIELD. 



If we apply a strong magnetic field to a diamagnetic liquid whose molecules possess 

 dissymmetry, LANGEVIN* has shown that the liquid becomes doubly refracting and 

 that a very small proportion of the unsymmetrical molecules suffer an orientation 

 under the action of the field. The amount of orientation which can be produced by 

 the largest magnetic field obtainable is not sufficient to affect the value of the suscepti- 

 bility appreciably.! When, however, the liquid crystallizes and all the molecules are 

 similarly orientated, we should expect that the susceptibility will be different in different 

 directions if the molecules are not symmetrical. But this cannot be the explanation 

 of the change of x observed in the experiments of Part I. (and in those carried out by 

 HONDA and OWEN). For in these experiments the crystals were small and their axes 

 would be distributed at random. In some, cases the crystals were observed to grow in 

 such a way. The explanation here put forward, and which lias already been advanced 

 in Parts I. and II., is that each molecule of the Substance is distorted when it forms part 

 of a crystalline structure by the local forces due to the neighbouring molecules. This 

 mutual action between the molecules is necessary to account for the rigidity of the 

 crystalline structure and for the existence of planes of cleavage (see p. 92, infra). 

 We can therefore find the intensity of the local molecular field by making it of such 

 magnitude as to produce a change of susceptibility of the same order of magnitude as 

 that actually observed in the crystallization experiments. 



We may write} 



AM Hre 



M 47TOT 



= -10- 9 H (3) 



where AM is the change of moment produced in an electron orbit of moment M by 

 applying a field H, T is the period of the electron, approximately 10~ 15 sec., and e/m 

 the ratio of the charge to the mass of an electron, 17 x 10 7 . The largest field which 



* 'Le Radium,' vol. 7, p. 249, 1910. 



t Loc. ctt., p. 252. 



t LANGEVIN, 'Ann. de Chim. et de Phys.,' s4r. VIII., vol. 5, p. 96, 1905. 



