386 Professor G. D. Liveing on Crystallisation. [May 15, 



Explanation of the Plate. 



Fig. 4 shows half of a regular octahedron formed of a pile of spherical balls, 

 and Fig. 5 shows part of a face of a dodecahedron produced by truncating one 

 edge of Fig. 4. In this it is seen that in the plane of the dodecahedral face each 

 ball is touched by only two others. 



Fig. 6 shows the triangular pyramid formed of oblate spheroids, which 

 becomes one corner of a cube when the ratio of the diameters of the spheroids 

 is 2 : 1, but one corner of a rhombohedron if the ratio is greater or less. 



Figs. 7, 8. and 9 represent lialves of octahedra formed of prolate spheroids. 

 In Fig. 7 the axes are perpendicular to the base, and the octahedron has 

 pyramidal symmetry. In Fig. 8 the axes are parallel to one edge of the base, 

 and the octahedron has right prismatic symmetry. In Fig. 9 the axes are in 

 planes parallel to one edge of the base, but inclined to that edge, and the 

 octahedron is oblique. 



[G. D. L.] 



