Bartow—A Mechanical Cause of Homogeneity of Crystals. 629 
substances whose crystals undergo dimorphous change may be 
noted.’ 
(a) When the change takes place the position of the new 
modification bears some symmetrical relation to that of the old. 
(0) When but a slight change of volume is associated with the 
change, strains are set up which are partly relieved by fractures, 
and by the sliding of laminz on one another, complete destruction 
of form being thereby avoided. 
For comparison with the suggestion that the portion of an 
assemblage growing after a dimorphous change has taken place 
may be somewhat differently fitted together, we may cite the 
not uncommon phenomenon of the existence of optically normal 
nuclei (Kernen) in anomalous crystals.” 
For comparison with the reversible cases, there are certain sub 
stances, individual crystals of which present two different dimor- 
phous states as the conditions are changed. One of the most 
signal instances of this is that described by Mallard as occurring 
in the case of boracite.® 
A crystal of this substance at temperatures above 265° is found 
to belong both in form and structure to the cubic system, being of 
tetrahedral-cubic symmetry; but the same individual crystal, 
when the temperature falls below this point, although it preserves 
its external form, displays properties which show that its structure 
has passed to a lower symmetry. It does not make the change as 
a single individual, but ordinarily in six or twelve portions whose 
corresponding axes are symmetrically placed with respect to the 
faces of the crystal, adjoining individuals being, however, very 
much interlocked, and showing indeed no more regularity than 
suffices to mark out the definite number of differently orientated 
portions, or most of them, without giving to them any uniformity 
1 Brauns, “ Optischen Anomalien der Krystalle.’’? Leipzig, 1891, §§ 3 and 4, p. 87. 
2 Ben Saude, ‘‘ Die wahrscheinlichen Ursachen der anomalen Doppelbrechung der 
Krystalle.’’ Lissabon, 1896, p. 26. 
3 Mallard, ‘‘ De l’action de la chaleur sur les substances cristallines.’”’ Bull. min. 
V., pp. 216-219. Comp. Bull. min. VI., p. 122. 
Another instance is furnished by potassium chlorate. A homogeneous crystal of 
this substance when heated to about 245° twins polysynthetically. (See Madan in 
“‘Nature,’’ vol. xxxiv., p. 66, and Rayleigh, Philosophical Magazine, 1888, II., 
p. 260.) 
