Bartow—A Mechanical Cause of Homogeneity of Crystals. 643 
twin individuals in a nucleus, whatever its development afterwards, 
is at first a principal plane of each individual, and the networks 
_ of centres found in planes parallel to it display the same angles 
sameways orientated in both individuals... A kind of twinning 
is, however, conceivable in which this is not the case. 
For, if after a twinned nucleus has been produced by a change 
of external conditions in the way above explained, this change 
immediately proceeds further, so that the individuals forming the 
twin seek to pass by distortion to a yet lower kind of symmetry, 
and the accomplishment of this further change for one individual 
would involve a distortion in the separating plane different in direction 
from the distortion in this plane which would be involved by the corre- 
sponding change in the other individual meeting it at this plane, the 
following results must be looked for when the twinned nucleus is 
partly solidified :— . 
A condition of strain at the separating plane and in its immediate 
vicinity where the individuals will be mutually restrained from 
compliance with the demands of the further change of conditions, 
but, especially in cases where the plane is very small, a rapid 
approximation to the newly-acquired symmetry, and consequent 
falling off in strain as we pass away from this plane. 
As a consequence of this, two individuals of a twinned nucleus 
- will be more or less contorted near the place of contact, and, where 
the symmetry with respect to the plane is of a low order, will be 
also bent. Where there is no bending, a principal plane direction 
of each individual will still remain parallel to the separating plane, 
but the angles of the structure of one individual in this plane-. 
direction will cease to be identical in orientation with those of the 
structure of the other individual. Where there is bending in 
addition, no principal plane direction of either individual will 
continue parallel with the separating plane. 
Notwithstanding the contortion of the nucleus thus brought 
about, layers subsequently deposited may be expected to be laid 
comformably with the homogeneous symmetry reached by the free 
ends of the individuals, any irregularity of growth caused by the 
contortion rapidly dying out; the growing individuals will thus 
extend themselves to meet one another unconformably, so far as 
internal structure is concerned, as in the case of individual 
