4 
Bartow—A Mechanical Cause of Homogeneity of Crystals. 559 
: Case of holohedrism of the monoclinic system. 
The arrangement of alternate layers in a stack of equal spheres 
whose disposition is described in p. 534, is depicted in figs. 3 and 
14. Such a stack can be partitioned into octahedral grouplets 
containing six spheres each, as indicated in the figure, each sphere 
being in contact with four others of the same grouplet, and the 
centres of the grouplets forming a triangular-prismatic space-lattice. 
Suppose now that, instead of employing equal spheres, we take 
spheres of two sizes, and form two sets of octahedral grouplets, one 
composed of the smaller spheres, the other of the larger, and pack 
the grouplets as closely together 
as possible, the general plan of 
the grouping of the spheres of 
two sizes on plane being of the 
nature indicated by the letters 
A, B, in the figure.? 
If the relative magnitude 
of the spheres of two kinds be 
such that the centres of one 
kind of similar groups in the 
same layer approximate closely 
to a square arrangement, the 
centres of the other kind of 
group falling therefore at the. 
middle points of the squares, 
it is evident that a very close-packed arrangement is obtained 
when the centres of the large grouplets of one layer lie about 
over the centres of the small grouplets of the succeeding layer. 
There will, however, be some slight racking or shifting over of the 
assemblage owing to the spheres employed being of two sizes. 
Thesymmetry of such an arrangement ismonoclinic. The type of 
homogeneous structure presented is numbered 63a, in my list.’ 
The generic symmetry is that of class 3 in Sohncke’s list of Krystall- 
Klassen. The two kinds are present in equal numerical proportions. 
* Probably a linking of the two kinds of balls to one another must be postulated to 
prevent closest-packing leading to the formation of two distinct assemblages one of each 
kind. 
* Zeitschr. fiir Kryst., 23, p. 48. 
SCIEN. PROC. R.D.S., VOL. VIII., PART VI. Tk 
