630 Scientific Proceedings, Royal Dublin Society. 
of outline. The general features of the allotment of the sub- ' 
stance of the crystal to the different individuals are to some — 
extent dependent on the relative predominance of particular 
external forms. The crystals when in the less symmetrical of the 
two dimorphous states exhibit strong double refraction. The 
change of structural symmetry is indicated both by the optical pro- 
perties, and also by the nature of the figures etched on the crystal — 
faces.2 The boundaries between the individuals, which vanish 
when the change to the regular form takes place, do not reappear 
in the old places when the crystal cools again, but commonly 
occupy quite different situations.* The change of volume pro- 
duced by the passage from one type of symmetry to the other is. 
very inconsiderable, but there appears to be some slight strain in- 
duced by it which causes the crystals to become brittle, and pyro- 
electric properties are acquired. The optic axes of the less-symme- 
trical form are inclined at 90° to one another, and are always 
normal to the direction proper for some cube face, whether that 
face be actually present or not. The bisecting line between the 
axes! is always vertical to some direction possible for a dodecahe- 
dron face, whether the face be present or not.° 
Again, potassium ferrocyanide® possibly furnishes an example 
of the kind of dimorphous change just explained 72 which the alter- 
ation takes place in the initial stage of crystallization, while accretion 
subsequent to it prevents reversibility, and another example of this 
kind may be presented by the hydrated crystals of trans-7-camphotri- 
carboxylic acid recently obtained by Kipping,’ and whose optical 
properties have been ascertained by Pope.* 
For suppose that a homogeneous assemblage of either tetrago- 
nal or hexagonal symmetry which does not possess planes of symmetry 
through the principal axis, undergoes a dimorphous change to a 
1 Brauns, loc. cit., p. 91. Sometimes one or two of the individuals are of insignifi- 
cant size, or even quite wanting. 
2 Baumhauer, ‘‘ Bemerkungen iiber den Boracit.” Zeitschr. f. Kryst., X., p. 461. 
3 Klein, ‘‘ Géttingen Nachrichten,” 1881, Nr. 8. Mallard, ‘‘ De l’action de la 
chaleur sur les cristaux de boracite.’’? Bull. min., V., p. 144. 
4 Since the axes are inclined at 90° this applies to either of the two bisecting lines. 
which appear to be distinguishable optically as positive and negative. 
5 Brauns, loc. cit., p. 104. 6 Brauns, loc. cit., p. 58. 
7 Trans. Chem. Soc., 1896, p. 951. 8 Tbid., p. 978. 
