ARTICLES 



253 



the attraction is sufficiently strong to hold them in stable non- 

 ionisable union. The other ionisable groups attached to them 

 he assumes to be outside this space held in a less firm union 

 by the resultant of all the forces operating, not only those of 

 the central atom but of the closely associated groups. The 

 inner space which no other group or atom can enter without 

 displacing some atom or molecule already there is sometimes 

 spoken of as the inner sphere, or zone, or first sphere of attrac- 

 tion, in contradistinction to the outer sphere, or zone in which 

 those atoms not forming part of the complex are assumed to 

 find their place. 



Granting that such complexes exist, the simplest assump- 

 tion regarding their structure, if the associated groups or atoms 

 are similar, is that they are symmetrically disposed upon the 

 surface of a sphere at the centre of which the grouping atom 

 is situated. This sphere would be somewhat deformed if the 

 groups were different, but would in either case constitute a 

 sort of shell surrounding the inner atom. Six similar particles 

 symmetrically distributed upon the surface of a sphere are in 

 the positions of the angular points numbered 1-6 of the regular 

 octahedron of Fig. 1. This arrangement can be conveniently 

 set up in type in the form of Fig. 2, which will be used through- 

 out this paper for the various octahedral formulae required. 



Fig. i. 



If the atoms, groups or molecules are not identical, the rela- 

 tive positions will be the same, and all the spatial relationships 

 will be unaltered though the octahedron may be irregular. 

 To complete the formulation of such compounds it is 



