The cubic (tetrahedral) arrangement will be clear from fig. 121 a 

 and b. It has all symmetry-elements of the holohedral class of the 



a. Fig. 121. b. 



Cubic Assemblage of Equal Spheres. 



cubic system (K H ). The centres of the spheres, the points of 

 contact between the spheres, and the centres of the octahedral 

 groups of spheres shown in the octahedral section of fig i2ib, are 

 all centres of symmetry of the unlimited system. The ternary axes are 

 perpendicalur to the planes of most closely packing of spheres in the 

 whole system, and of these planes, which are parallel to those of the 

 octahedron, there are three 

 consecutive ones differently 

 arranged, the fourth being 

 identical with the first and 

 the seventh, the fifth with the 

 second and the eighth, etc. 

 (fig. i2ib.) The projection of 

 three consecutive layers pa- 

 rallel to (111) is shown in fig. 

 122, and can make clear the 

 mutual marshalling of the 

 spheres in this direction, if 

 compared with figure 123, 

 which in its turn represents 

 the most closely packed as- 

 semblage of equal spheres 



mentioned above under 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 sphe- 

 res possesses parallel planes of symmetry passing through the centres 



Fig. 122. 

 Section parallel to (111). 



