Bartow—A Mechanical Cause of Homogeneity of Crystals. 653: 
many cases be a hindrance to tranquillity, there will probably be a 
tendency to form distinct layers of each kind, the growing portion 
at any point remaining disturbed, and therefore unsolidified, till 
the more or less fortuitous movements taking place have resulted 
in perfecting the uniformity of the layer. 
So long as the proportions of the different assemblages present 
in the unsolidified mass are unaltered, and the conditions remain 
the same, an average uniformity of composition, or general 
homogeneity of the growing mixed solid should result; if, on 
the other hand, the proportions, or the conditions, or both change 
gradually, a gradual change of average composition may be 
expected to occur. 
Where the fitting together of the corresponding planes of 
balls of the different assemblages produces less closeness of pack- 
ing than the fitting together of the planes of balls of one of 
the assemblages taken alone, it is evident that retardation of 
solidification will tend, by giving additional time for the carrying 
out of the arranging process, to diminish the number of separate 
fragments of the two assemblages and increase their size, and 
indeed ultimately to cause them to mass themselves separately. 
The facts with regard to isomorphous crystals bear a close 
resemblance to the properties of isomorphous assemblages which 
have just been deduced. . 
Thus in the large majority of cases isomorphs have a similar 
chemical constitution’ and a large proportion of common consti- 
tuents. And the constituents in which they differ have in general 
similar properties and appear to exercise a similar influence on the 
common constituents.” 
The isomorphism of crystals is seldom, if ever, absolute, so 
that when accurate measurements of corresponding angles are 
made differences are revealed, the amounts of which are ordinarily 
1 Fock’s ‘‘Einleitung in die chemische Krystallographie,”’ p. 65; or Enlarged 
English translation of same work (Pope), p. 98. 
2 ©omp. Mischerlich’s definition ‘‘that only those substances are to be designated 
isomorphous which possess, in addition to an analogous chemical composition, a similar 
crystal form, and which crystallize together in variable proportions.” 
The equality of molecular volume of isomorphous compounds so commonly observed. 
will naturally be traced to the similar influence exerted by the allied though different 
constituents. 
