7 
two rhombic, a rhombohedral and a regular form. In polarized 
light the crystals behave as altogether isotropic. If gradually — 
cooled, however, a change suddenly takes place at about 127°, and 
they become double-refracting, and asa less solubility is proper 
to the new form than to the regular one, the change of form is 
accompanied by increase of size in cases where the crystals are 
found in a solution. From the rudimentary form and the optical 
behaviour it can be concluded that the crystals obtained in this 
way are Frankenheim’s rhombohedra. 
Ifthe solution is further cooled, at about 87° needle-shaped 
rhombic crystals are produced. The regular situation of these with 
relation to the rhombohedral ones, as the latter are spontaneously 
transformed, furnishes an exceptionally good example of the 
phenomenon that when a transformation from one modification into 
another takes place, the newly-formed has in general a regular 
situation with respect to the older modification. Thus in the 
present instance the crystals are so placed that the vertical axis of 
the rhombic form, that which corresponds to the greatest extension, 
either has the same direction as that of a secondary axis of the 
rhombohedral form, or is at right angles to it, while the macro- or 
brachy-diagonal,! has the same direction as the principal axis of 
the rhombohedron.’ 
The readiness with which an assemblage of mutually-repellent 
particles, or spheres,’ will respond to a change of conditions past a 
critical point, and commence approximating to a different type of 
arrangement will, it is manifest, depend not only on what it 
comes in contact with, but will also partly depend on the amount 
of disturbance to which it is subjected.* And for comparison with 
this may be mentioned Lehmann’s remark with regard to the 
transformation from one solid modification to another. ‘ When a 
modification is heated while isolated it is sometimes possible to 
raise the temperature far above the point (normal critical point 
of temperature) at which, when in contact with a crystal of the 
580 Scientific Proceedings, Royal Dublin Society. 
1 It is impossible to determine which, as the prism of the rhombic crystal is nearly 
right-angled. 
? 0. Lehmann ‘Ueber physikalische Isomerie’’; Zeitschr. £. Kryst., 1., p. 107; 
Comp. Pope’s translation of Fock’s ‘‘Chemische Krystallographie,”’ p. 158. 
3 See p. 529, especially note 2. 4 Comp. No. 7 of Data, p. 688. 
