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trol of a definite number and a definite quantity — the number 6 and 
the angle of 60° determining their form and limiting their variations. 
Among the various figures which I observed on the occasion referred 
to, we may perhaps select, as the archetypal combination, a 6 -rayed 
star (fig. 1), having for its centre or nucleus a tabular crystal in 
the form of a regular hexagon, each of whose angles supports one of 
the six rays of the star. Each of these rays would seem to be a very 
much elongated hexagonal prism, and every one of them gives off 
from each side, and in the common plane of the figure, secondary 
arms, which spring from it with a pinnate arrangement, and at an 
angle of 60°. These secondary arms in the figure under considera- 
tion are also elongated hexagonal prisms ; in the greater number of 
instances, however (figs. 2—5), the secondary arms are wedge-shaped, 
and probably the result of more complex conditions than those under 
which the type form is produced. The axis of symmetry of each of 
these wedge-shaped arms is inclined to the ray at an angle of 60°, 
but I cannot say at what angle the sides of the arms are inclined 
to one another. The central hexagon in the figure, from which 
the sketch was taken, was marked with elegant concentric striae. 
It is here worth noting, that the figure I have assumed as the 
type consists exclusively (if we except the termination of the arms) 
of a combination of the two limiting forms of the Rhombohedron m R ; 
namely, that in which m becomes practically 0, and that in which 
it becomes infinite, producing in the former case the tabular crystal 
OR, which constitutes the centre of the figure, and in the latter 
case the prismatic crystals a R, which constitute the rays and their 
branches. 
Assuming then the form now described as the type, there would 
seem to be but little difficulty in deducing from it most of the 
peculiarities presented by the other figures which I have had an 
opportunity of observing. In fig. 2, the central hexagon has dis- 
appeared, and the rays meet at a point in the centre of the star. 
In fig. 3, a tabular hexagonal crystal is developed diagonally in 
the course of each ray at a uniform distance from the centre, and in 
the common plane of the star. 
Fig. 4 is easily derived from fig. 3 ; for in order to produce it, 
we have only to suppose all the six hexagons to be simultaneously 
prolonged in the direction of the ray, until they meet in the centre 
of the figure, when, in consequence of mutual and symmetrical 
