ARTICLES 



59: 



large local molecular field can also be applied with success 

 to account for the anomalous cases of the Faraday effect, 

 the reversed action of the local molecular field producing in 

 certain cases a change in the sign of the rotation (Part 4). 



Our conception of a diamagnetic atom (or molecule) is 

 represented diagrammatically in fig. 2. Taken as a unit, each 

 atom (or molecule) a, b, etc., is a balanced system possessing 

 zero magnetic moment. Of course a and b may each possess 

 a large number of electrons rotating in small orbits scattered 

 over the " surface " and interior (see fig. 3, p. 597) which would 

 determine forces in other directions than those indicated by 

 the straight arrows in fig. 2, but as all the electrons in a dia- 

 magnetic molecule must have a total zero moment, the above 



Fig. 2. 



Diagrammatic representation of the electromagnetic attractive force between two units, A and 

 B, of the crystalline structure. The black dots indicate the diffuse nuclei, consisting of positive 

 charges and a distribution of electrons rotating in small circles at varying distances. The circular 

 currents shown are positive ; the actual direction of rotation of the electron is opposite to that 

 indicated by the arrow on each small circle. 



diagrammatic scheme fulfils our purpose. The general case 

 of an asymmetric diamagnetic atom, whose localised surface 

 electrons may have a close bearing on directed valencies, is 

 referred to below (p. 597). In the crystalline structure, a and 

 b are attracted to one another and to the surrounding atoms 

 or molecules, locally, as indicated by the arrows between the 

 attracting orbits, and it will be observed that the local mole- 

 cular field is of an alternating character, the distance over 

 which it is unidirectional being comparable with the distance 

 between the atoms or molecules. We should naturally expect, 

 on this view, that the molecular field in between the atoms 

 of a diamagnetic medium would be comparable with that in 

 ferro-magnetic media, since the periods of the electron orbits 

 and the number of electrons are comparable in the two cases. 



