250 MAJOE A. E. OXLEY ON THE INFLUENCE OF MOLECULAR 



susceptibility of matter in bulk depends upon a transition from the liquid to the 

 crystalline state. When thus considering diamagnetic matter in bulk, the large local 

 forcive which has been shown to bind the molecules of the crystalline structure 

 together need not be considered, since for matter in bulk its effects are cut out by the 

 mutual compensation of molecular forcives. It is only when we enquire into the 

 molecular structure of the crystalline medium, or to changes in this structure, that we 

 pass to the inner limit where the principles of LARMOR and CAUCHY for the fluid state 

 no longer apply. LARMOR remarks : " The result of the integration still however gave 

 us a valid estimate of the effect of the material system as a whole, when we bore in 

 mind that the infinite or rather undetermined term entering at the inner limit really 

 represents the part of the result which depends solely upon the local molecular 

 configuration ; a part whose actual magnitude could be determined only when that 

 configuration is exactly assigned or known " (loc. cit., p. 125). 



It is with this " infinite or rather undetermined term which depends 



solely upon the local molecular configuration " that these researches are mainly 

 concerned. It has been called the local molecular field of the crystalline medium 

 (Part TIL, p. 83). 



(2) ON THE ENERGY AND ULTIMATE TENSILE STRENGTH ASSOCIATED WITH 



CRYSTALLINE MEDIA OR GELS. 



The large intrinsic potential energy associated with a crystalline medium has been 

 discussed in para. 5, Part III., pp. 90-95. It now remains for us to examine the 

 accompanying stresses to see how far the elastic properties of material media may be 

 interpreted in terms of these intrinsic forcives. Consider first the case of a liquid 

 which is gradually cooled in liquid air so that it passes into a glass-hard transparent 

 gel when it arrives at the temperature of the liquid air. 



It has been suggested (Part III., p. 81 ) that the appearance of rigidity in the gel 

 is due to an interlocking of the irregularly shaped molecules (arranged at random) 

 whose thermal agitation is sufficiently reduced. On account of this random orientation 

 of the interlocked molecules the gel will be isotropic. At such a low temperature, 

 also, the molecular motions will be highly constrained so that a particular molecule 

 will present practically the same aspect to the surrounding molecules over a long 

 period. If this is the case, then the local molecular forcive between this and a 

 neighbouring molecule will act in a definite direction and will not be rapidly changing 

 its direction as would be the case with the same molecules at a considerably higher 

 temperature (in the ordinary liquid state). It is clear therefore that between the 

 molecules of the gel at low temperature we shall have a large local forcive in operation, 

 due to the interaction of the magnetic systems or revolving electrons within each 

 molecule, but the direction of the action of this forcive between any pair of molecules 

 will be one of random distribution, as we pass from pair to pair of molecules, although 

 at any given point it is fixed in direction. 



