139 



Fig. 1 20. 

 Section parallel to (111). 



lips of spheres shown in the octahedral section of fig. //pb, are 

 1 rrnt i v> of symmetry of the unlimited system. The ternary axes are 

 rpendicular to the planes of 

 -t rlosely packing of spheres 

 the whole system, and of 

 ese planes, which are parallel 

 those of the octahedron 

 lere are three consecutive 

 lifferently arranged, the 

 >urth being identical with the 

 rst and the seventh, the fifth 

 itli the second and the eighth, 

 tc. (fig. //?&). The projection 

 three consecutive layers 

 irallel to (111) is shown in 

 ig. 122, and can make clear 

 ic mutual marshalling of the 

 )heres in this direction if 



>mpared with the figure 121, which in its turn represents the 

 ost closely packed assemblage of equal spheres mentioned above 

 der the name of the hexagonal arrangement. 



The symmetry-elements of 

 this hexagonal assemblage are 

 those of the holohedral class 

 of the hexagonal system. The 

 system of spheres possesses 

 parallel planes of symmetry 

 passing through the centres of 

 the spheres of each of its most 

 closely packed triangularly ar- 

 ranged layers, (fig. 122). In 

 the centres of the cavitirs 

 of each triad of spheres, ternary 

 axes of rotation are present 

 Fig. I2I . perpendicular to the different 



Section parallel to (loo). layers just mentioned, in such 



a way that they pass simul- 

 taneously through a sphere-centre of an adjacent layer b and c\ \vhiU- 

 through the centres of the cavities of every group of six contiguous 

 spheres of two consecutive layers, senary screw-axes pass, of 



