Bartow—A Mechanical Cause of Homogeneity of Crystals. 647 - 
to. It is, however, unlikely that we have before us the precise 
arrangement prevailing in this body, because a division into unit 
groups of the assemblage in question would necessarily be an 
arbitrary one on account of the very specialized situations occupied 
by the balls. 
The direction adopted for the shear agrees better with the 
cleavage direction of Iceland spar than does the one it is usually 
supposed to take. 
If, instead of independent balls we have groups of balls, and 
the groups have some simple rhombohedral arrangement, there is 
still no need to suppose rotation of the groups with respect to the 
moving planes in which they he, provided each of the latter is a plane 
of symmetry of each of the individual groups which it intersects. 
Many peculiarities of crystal growth and of crystal-twinning 
and grouping are no doubt traceable to the partial application or 
distribution of the external forces or conditions of various kinds 
prevalent during the process of crystallization, e. g., there is some- 
times a tendency for a crystal to grow faster in some particular 
direction when, so far as symmetry is concerned, there are other 
similarly related directions equally available. All such cases are 
outside the scope of this investigation, for there are no analogies 
to peculiarities of this kind in the interactions of the parts of a 
homogeneous assémblage, these conforming strictly to the sym- 
metry. . 
Formation of isomorphous assemblages and their intermixture—Re- 
semblances to isomorphous, isogonous, and nixed crystals, and to 
crystalloids,® also to some kinds of diffusion. 
A few words will now be said as to the nature of the likeness 
of their parts and their conditions which will cause two homo- 
geneous assemblages, composed wholly or in part of different 
constituents, whether groups are present or not, when in equili- 
brium—(a) to have corresponding angles between planes of centres 
1 In the usually received explanation of the nature of the artificial twinning of Ice- 
land spar, rotation of the molecules is resorted to; this is unnecessary, all that is 
requisite is change of orientation of their situation relatively to the assemblage, and this, 
in the case referred to, is accomplished by the simple shear. 
2See note 1, p. 620. 
