Bartow—A Mechanical Cause of Homogeneity of Crystals. 619 
same number of different derivatives 7f two enantiomorphs count as 
one form only, which they may do if they occur together in the 
same assemblage of groups and are not separable. The simple 
octahedral arrangement marked 67 is too symmetrical for the 
requirements of the case, as in it but two kinds of double sub- 
stitution are possible. 
Usually substitution of a different ball for one of several of 
the same sort will cause equilibrium to be found in a homogeneous 
assemblage of a different kind, commonly of a different type; 
there may, however, be cases in which the influence of the balls 
common to both assemblages is so predominant that the kind of 
internal symmetry is the same in both. This would almost 
certainly be the case if the balls changed were inoperative ones.’ 
We should expect that in most cases more or less similarity of 
arrangement of the unremoved balls would exist and would 
produce some resemblance between the assemblages thus re- 
lated.’ 
The similarity of crystallographic form displayed by some 
compounds and their derivatives, e.g., some benzene derivatives, 
may be compared with this.* | 
With regard to those homogeneous assemblages which consist 
both of groups and of loose balls, or of two or more kinds of groups, 
it is evident that for such to be stable, the different kinds of groups 
must, as to their configuration and their repulsions, be sufficiently 
adapted to one another to produce very close-packing ; otherwise 
the assemblages would not be likely to be examples of stable 
equilibrium. We shall, therefore, in the case of assemblages of 
this nature have a close fitting together of different groups which 
does not lead to a linking together of those groups capable of 
surviving in the liquid condition* with which we have been 
dealing. 
And for comparison with this, we may cite the loose chemical 
ties found in crystal bodies which break up as they dissolve, 7.e., 
in those in which so-called water of crystallization enters into the 
composition of the crystal, and in most double salts.* 
1 See p. 547. 2 Comp. p. 647. 3 Comp. 662 below. 4 See p. 583. 
* Compare Fock’s ‘‘ Chemical Crystallography,’’ Pope’s translation, pp. 29 and 36. 
