MOLECULAR ARCHITECTURE 357 



atomic occupation of space and in addition furnishes a geometri- 

 cal interpretation of chemical valency. 



Before it was possible to develop such a conception as the 

 one under consideration, based directly as it is upon the atomic 

 theory, it was necessary to make certain precise assumptions 

 regarding the atoms themselves. It is necessary to assume that 

 each atom in a crystal structure occupies a distinct portion of 

 space ; simple as this assumption appears to be, it is yet of 

 great significance in that it recognises that the atoms retain their 

 individuality in the molecule. The idea should prove quite 

 acceptable to modern chemists and particularly to followers 

 of stereochemistry. Each atom in the crystal structure is re- 

 garded as the centre of attractive and repulsive forces, the 

 structure itself as the equilibrium arrangement of these centres 

 of force. On the basis of this assumption, it can be foreseen that 

 the structure will almost certainly be one in which the smallest 

 distance separating component atom centres is the maximum 

 compatible with the density of arrangement of the centres. 



The portion of space commanded by each atom will indeed be 

 as nearly spherical as possible under the conditions of distribu- 

 tion of the atom centres, since the attractive and repulsive forces 

 are supposed to be exerted equally in all directions from these 

 centres. The conditions prevailing in the structure may then 

 be simulated by representing each component atom by a 

 deformable but incompressible elastic sphere, the spheres being 

 arranged homogeneously and with the maximum density of 

 packing, the stacked mass being subjected to a general pressure 

 sufficient to eliminate interstitial space and thereby convert 

 each sphere into a polyhedron. Each polyhedron may then be 

 regarded as the " domain " or " sphere of influence " of the atom 

 of which it forms the habitat ; within its own domain the atom 

 exercises predominant influence. 



A crystal structure may now be considered as a closest- 

 packed homogeneous assemblage of atomic spheres of influence ; 

 or, as defined by Barlow and Pope, as the " homogeneous 

 structure derived by the symmetrical arrangement in space of 

 an indefinitely large number of spheres of atomic influence." 

 By homogeneously subdividing such a structure, a space unit 

 is eventually obtained ; this unit is the domain of the chemical 

 molecule. The question of the partitioning of an assemblage 

 into units is one of considerable import and may sometimes 



