CONSTITUTION AND TEMPEEATURE ON MAGNETIC SUSCEPTIBILITY. 101 



between the molecules of the crystalline structure. In addition it is important from 

 our point of view to notice that these large fields are required by RITZ and 

 HUMPHREYS in order to explain certain spectral phenomena, and therefore they 

 are required apart from the magnetic nature of the atoms or molecules under 

 consideration. 



[*Let us now compare the theory which has been developed above and in Parts I. 

 and II. with the theory of ferro-magnetism given by WEISS. The intense magnetic 

 properties of iron below the critical temperature are attributed to the mutual effects 

 exerted by the molecules of iron. WEISS has shown that if we regard these effects 

 as purely magnetic, then the large increase of specific susceptibility due to the 

 transformation from the paramagnetic to the ferro-magnetic state may be represented 

 by a molecular field Nl, Avhere N is a constant of the order 10 4 and I is the intensity 

 of magnetization for a field strength H. For saturated iron Nl has the value 

 10 7 gauss (approximately). This internal field is called into action when an external 

 field H is applied, and its presence determines the abnormally large susceptibility of 

 iron below the critical temperature. 



In a similar way it has been shown that the change of specific diamagnetic 

 susceptibility when a diamagnetic substance crystallizes can be represented by a 

 mean magnetic molecular field which is proportional to the intensity of magnetization 

 (diamagnetic moment per unit volume) and whose constant of proportionality is 

 comparable with N. On account of the zero moment possessed by a diamagnetic 

 molecule initially, this mean molecular field is small relative to Nl, but reasons 

 have been given in prevkms work which show that the local value of this 

 field is comparable with the ferro-magnetic molecular field. It is this local field which 

 distorts the electron orbits on crystallization and gives rise to a small change of 

 diamagnetic susceptibility. This and other considerations have shown that the 

 intensity of the molecular field in diamagnetic substances is of the order 10 7 gauss 

 locally. 



Up to the present the representation of this field as a magnetic field has not been 

 justified except in so far as we have seen that it is in harmony with the important 

 work of RITZ, whose molecular magnets have moments not inconsistent with that 

 associated with the magneton, t and whose magnetic field intensity is comparable 

 with the ferro-magnetic molecular field of WEISS and the molecular field within 

 diamagnetic crystalline substances. It is important for our purpose to observe that 

 the intra-atomic fields of RITZ are required to represent the distribution of lines in 

 spectral series, whether the substance be ferro-, para- or diamagnetic. 



Now very good evidence that molecular magnets do exist in ferro-magnetic 

 substances has been obtained. The fields due to these must contribute to the 

 molecular field (Nl). Taking the moment of the magneton as 16'5 x 10~ 22 , we find for 



* [Added November 12, 1914.] 



t P. ZEEMAN, 'Researches in Magneto-optics,' p. 178. 



