CONSTITUTION AND TEMPERATURE ON MAGNETIC SUSCEPTIBILITY. 89 



the mean molecular field, for the relationship of the molecules to one another in the 

 crystalline structure determines their common magnitude. Hence H c . = 10 7 = 10 8 . 1 

 and I = 100. 



We can now use these values to form an estimate of the potential energy 

 associated with a diamagnetic crystalline medium in virtue of the local molecular 

 field and the local molecular polarization. 



But before passing on to this, it will be interesting to compare the above deductions 

 with regard to the local molecular forcive in diamagnetic crystalline media with those 

 of WEISS which are concerned with the forcive in ferro-magnetic media. 



Imagine a spherical cavity, large compared with molecular dimensions but lying 

 wholly within the same crystal of (l) a diamagnetic ; (2) a ferro-magnetic medium. On 

 account of the structure which has been assigned to the diamagnetic molecule, the 

 force at the centre of the cavity (when there is no external field) will be zero in case (l). 

 In (2) the force will be ?$ . I where I is the spontaneous intensity of magnetization. 

 Now let us move our point of observation from the centre of the cavity out towards 

 the surface. When the point approaches within a range comparable with molecular 

 dimensions, in case (2), the forcive increases, and when the point is at a distance from 

 the wall equal to that which separates two molecules of the crystalline structure the 

 forcive is represented by N I where N is the constant of W Kiss's molecular field, of the 

 order 10 4 . If we move our point of observation in case (l) (the diamagnetic medium) 

 from the centre of the cavity to the surface, then as the point approaches to within a 

 distance comparable with molecular dimensions the local force is due almost entirely 

 to the molecule which is nearest to the point. This molecule maintains a definite 

 orientation with respect to the point, and when the latter is so close to the molecule as 

 to be almost on its surface the polarization is comparable with the saturation intensity 

 in iron. This is the interpretation of the large values of H c and Nl which are each 

 of the order 10 7 gauss. As H = a c ' . I, we may suppose that the large coefficients N 

 and aj determine the enormous magnitude of the forcives quite close to a molecule in 

 the respective crystalline structxires. These local forcives we could hardly hope to 

 calculate directly for we do not know the proximity of the molecules in the structure 

 or the law of force which holds at such a close range. The local molecular field of a 

 diamagnetic crystalline substance alternates as we pass from molecule to molecule of 

 the structure and is therefore localized. If we could take a crevasse between two 

 molecules of the structure then the induction across it would give us a measure of H,,. 

 Similarly in the ferro-magnetic case a crevasse of such small dimensions would give us 

 a measure of WEISS'S field Nl. If we take a crevasse in the ferro-magnetic medium, 

 which is large compared with molecular dimensions, the force in the gap is H + 47rl and 

 this is small compared with N I . The difference between these forces must be 

 attributed to the localization of the intense fields associated with the iron atom. We 

 then get continuity of magnetic induction while the intense field is still capable of 

 modifying the structure of a neighbouring molecule. 



VOL. ccxv. A. N 



