Bartow—A Mechanical Cause of Homogeneity of Crystals. 631 
rhombic type, the principal axis being retained as the axis of the 
lower symmetry ; and that the change commences simultaneously 
and symmetrically in four or in six similar segments of the nucleus 
as the case may be, and does not effect any material change in the 
situations of the singular points of the assemblage, or in the general 
distribution of the parts. 
It is then evident that if, when the rhombic form is reached, 
the optic axial planes are parallel to the axis, symmetry will 
require them to be inclined to one another symmetrically at. equal 
angles, in the one case of 90°, in the other of 60°. 
Further, we see that there being no planes of symmetry between 
the segments, and their growing surfaces perpendicular to the axis 
being coincident, 7.e., not inclined to one another, the position of 
the boundary between them will not be fixed by considerations of 
symmetry, but merely by the relative rate of growth of the adjoin- 
ing individuals which happens to prevail from point to point. 
Consequently adjoining twin individuals must be expected to 
tooth into one another in a very fortuitous manner, the boundary 
between them in one layer of the growing common surface 
scarcely ever happening to be directly over the boundary in the 
underlying layer.! 
The relative orientation of parts and gradual passage from the 
optical effects proper to one segment to the optical effects proper 
to the differently-orientated adjoining segment, as the boundary 
between them is passed, which would be produced in the way 
suggested appear to be just such as those observed by Grailich 
and subsequently by Wyrouboff in the case of potassium ferrocy- 
anide, and by Pope in the case of trans-7-camphotricarboxylic acid. 
There is another way in which a dimorphous change may 
cause the passage from a higher to a lower type of symmetry and 
produce scarcely any perceptible alteration of external form, the 
dimorphous change being under some conditions reversible, under 
others non-reversible. 
1 Comp. p. 623, also Zeitschr. f. Kryst., 27, p. 474. 
Examples of intricate interlacing, probably due to dimorphous change in the way 
described, but where the symmetry is of a lower order, may be called to mind. Thus 
in the case of aragonite the individual crystals sometimes interpenetrate in such a way 
that parts of one individual are inclosed in another. Comp. p. 648. 
