iJ22 



The field produces the change of resistance through its effect on 

 the molecules and not on the "free" electrons (when these indeed 

 exist) and it is natural to suppose that the field has also an orientating 

 influence on diamagnetic molecules. From the magnetic double refraction 

 Cotton and Mouton have also concluded, that for diamagnetic sub- 

 stances too the field has a directing influence ^). One can readily imagine, 

 that anisotropic molecules have, in general, a greater magnetic moment 

 than isotropic molecules; therefore, if the magnetic moment is already 

 present before the field is excited, tiie field will have a stronger 

 directing influence on anisotropic molecules. Similarly if we suppose, 

 as is generall}' done, that diamagnetisni is an induced phenomenon, 

 we may assume that a bigger moment is produced in an anisotropic 

 molecule than in an isotropic one. We may therefore conclude that 

 the crystal system must have an influence on the phenomenon. 



Now we find this to be really the case. All substances, which 

 show a large or rather large resistance change, belong to the hexa- 

 gonal system, while those which have a much smaller effect belong 

 to the regular system. 



Finally we may still i-emark, that those molecules, which have 

 a large susceptibility and which are besides anisotropic will undergo 

 the greatest influence of the magnetic field. And as the resistance 

 change is caused by a directing effect, there must be a connection 

 between susceptibility and resistance change. That this connection 

 realiy exists is proved by the experiments. It is from this connexion 

 that a large effect for graphite could be predicted *). 



When we put the diamagnetic metals in a series in the order of 

 tiie values of (/?' — R)/R, beginning with the largest value, we have first: 



1) A. Cotton and H. Mouton, Journal de Physique (5) 1 p. 40, f 911. P. Langevin, 

 le Radium 7 p. U9, 1910. 



-) D. E. PvOBERTS, loC. Cit. 



