558 Scientific Proceedings, Royal Dublin Society. 
A case of holohedral rhombic symmetry. 
Place a number of equal spheres in the same plane in triangular 
order in contact with one another. On this layer, place a second — 
layer composed of rather larger spheres half as numerous; the ~ 
most even distribution of these which brings them over the spaces 
between the spheres of the first layer is indicated in fig. 13, the 
centres of the first layer being indicated by the plain points, those 
of the second layer lying over the points marked with an asterisk. 
The latter centres are nearly equidistant, and slight relative 
movements would make them so. Consequently if an appropriate 
relation subsists between the radii of spheres of two sizes and the 
two kinds are linked into groups in some way, they will probably 
pack closest when the centres 
of alternate layers have ap- 
proximately the relative situa- 
tions shown in the figure. 
Neither of the two kinds of 
layers will, however, have pre- 
cisely the regular triangular 
arrangement, and the assem- 
blage, as a whole, will display 
rhombic symmetry. The type 
of homogeneous arrangement 
is numbered 53a, in my list.’ 
The generic symmetry is that Fig. 13. 
of class 6 in Sohncke’s list of 
Krystallklassen. The two kinds of balls are present in the 
numerical proportions 1 : 2. The positions in the structure oc-— 
cupied by the more-numerous are of two different kinds; all 
the ball centres form singular-point systems the points of which 
are on digonal axes which lie in planes of symmetry. It is inter- 
esting to notice how little removed the assemblage is from being 
hexagonal—one among many instances of a low type of symmetri- 
cal arrangement possessing features more or less similar to those 
of some higher type. 
oe SEO, EE ee eee 
1 Zeitschr. fur Kryst., 23, p. 47. 
