CONSTITUTION AND TEMPERATURE ON MAGNETIC SUSCEPTIBILITY. 



93 



is large compared with the other term of equation (14). Taking the value of the 

 local molecular field, H c = a' c l = 10 7 , we find since '= 10 5 , that I, the aggregate of 

 the local intensity of magnetization per unit volume, is of the order 100. This is 

 comparable with the saturation intensity of ferro-magnetic substances. If p c = I, 



which is the amount of potential energy associated with 1 gr. of the crystalline 

 medium in virtue of its molecular grouping. The thermal equivalent of this will be 

 of the order 10 9 /4 . 10 7 = 25 gr. calories, which represents the heat energy required to 

 destroy the crystalline structure, i.e., the latent heat of fusion. This is of the right 

 order of magnitude for many diamagnetic substances organic compounds and 

 elements.* The above reasoning applies only to the order of magnitude of the latent 

 heat. It is obvious that until we know the disposition of the molecules within the 

 crystalline structure the value of a' c is somewhat vague. But the experimental fact 

 that the latent heat of transformation of iron from the ferro-magnetic to the para- 

 magnetic state is of the same order of magnitude as the latent heat of fusion of many 

 diamagnetic crystalline substances is powerful evidence that the local forces and 

 local polarization which we have assigned to diamagnetic crystalline structures are 

 enormous, comparable, in fact, as the above and preceding calculations have shown, 

 with the intense forces and polarization of ferro-magnetic substances. 



In the crystalline state we must regard the molecules as orientated into definite 

 positions with respect to their neighbours by these large intermolecular forces. If at 

 the higher temperatures the molecules undergo rotational vibrations about their mean 

 positions, then it would be expected that the value of I 2 will be somewhat lessened 

 by these vibrations, and we should therefore expect that a small fraction of the 

 energy associated with the grouping would be dissipated as the temperature is raised 

 towards the fusion point. The effect this would have on the variation of the specific 



* The following values of the latent heat for some diamagnetic substances with which we are directly 

 concerned are taken from 'Recueil des Constantes Physiques,' Paris, 1913, pp. 323-4 : 



Benzene 30 



Xylene 39 



Chlorobenzene .... 30 



Bromobenzene .... 20 



Aniline 21 



Acetophenone 33 



Benzophenone 23 



Phenylhydrazine .... 36 



Pyridine 22 



Nitrobenzene 22 



Naphthalene 35 



Naphthylamine .... 22 



Acetic acid 44 



Carbon tetrachloride . . 4 



Bismuth 13 



Cadmium 14 



Lead 5 



Silver 22 



Tin . . 14 



Zinc 28 



Gallium 19 



Iron (ferro-magnetic) . . 59 



