CONSTITUTION AND TEMPERATURE ON MAGNETIC SUSCEPTIBILITY. 81 



polarization due to the applied field and s is the coefficient of the corrective 

 polarization the effect of the replaced molecules. 



Within a crystalline structure, however, this method of averages applied to the 

 molecules within the cavity is not valid, for each molecule is orientated so as to occupy 

 some definite position with respect to its neighbours. In such a case the local force, 

 due to a molecule which is close to the point under consideration, is uucompensated, 

 and we have to resort to indirect deduction from experimental facts to obtain a 

 measure of the polarization effect due to the molecules immediately surrounding the 

 point. It is to this uncompensated effect that we have attributed the rigidity of the 

 crystalline medium.* In order to avoid the complexity introduced by the elasticity 

 of crystalline media LARMOR confined his investigations to fluids ; but, as we are 

 attempting to ascribe to these very elastic forces a nature which permits of their 

 interpretation in terms of a local molecular field binding the molecules together, the 

 treatment for crystalline substances is the same in kind as that for fluid substances. 



The rigidity of gels at low temperatures is of a different nature and is probably 

 due to an interlocking of the molecules (arranged at random) whose thermal agitation 

 is sufficiently reduced. This is in accordance with the work of TAM.MANN! who found 

 that at very low temperatures the power and velocity of crystallization were very 

 small. 



(3) THE MEAN AND LOCAL MOLECULAR FIELDS OF A DIAMAGNETIC 



CRYSTALLINE SUBSTANCE. 



In a former paperj the author has shown that the change of specific susceptibility, 

 which accompanies the transition from the liquid to the crystalline state for many 

 substances, can be satisfactorily interpreted in terms of a mean molecular field which 

 operates to an appreciable extent only in the crystalline state. The nature of this 

 molecular field need not be definitely specified, and it will suffice for our purpose to 

 regard it as a magnetic field limited to such a magnitude as to produce in the 

 molecules of the substance a distortion, or polarization, such as is equivalent to that 

 actually produced by the mutual forces of the molecules of the crystalline structure. 



In Part II. of the work referred to, the mean molecular field was represented 

 magnetically by the term a c . AH, where a c is a coefficient (the constant of the 



* 'Roy. Soc. Phil. Trans.,' A, vol. 214, p. 143, 1914. 



t WHETHAM, ' Theory of Solution,' p. 44, and references to TAMMANN'S work there given. 



I 'Roy. Soc. Phil. Trans.,' A, vol. 214, p. 109, 1914. This paper contains Parts I. and II. of the present 

 work, and, for brevity, reference is made to these in what follows. 



A force of electrostatic nature will modify the electron orbits, and therefore the susceptibility, an 

 effect which we may represent magnetically. If the revolving electrons are controlled by molecular 

 magnets (as on RITZ'S theory), a magnetic field would be the true interpretation of the molecular field. 

 Probably both types of molecular field co-exist (vide infra, p. 22, for a discussion of the nature of the 

 molecular field). 



VOL. CCXV. A. M 



