1209 
Thus as far as experience goes, the phenomena observed in crystals 
of the regular system seem to be in all cases in full agreement 
with the postulations of the theory. In this connection it may here 
be definitely stated, that the conclusions made by Haca and JarGer!) 
some time ago, from their experiments with boracite above and 
below 400° C., are now seen to be completely justified. For boracite 
at room-temperature, — if it were really regular, — would be of 
hexacis-tetrahedrical symmetry; and thus its ROnraunogram would 
possess just the same symmetry as that of the sphalerite; i.e. that 
the image would be identical with the pattern of boracite, which 
was obtained above 400° C., — because this corresponds to the 
holohedrical symmetry of the regular system. But the image obtained 
by the authors at room-temperature, now only shows the presence 
of two perpendicular planes of symmetry and a binary axis: there- 
fore it van only correspond either to a rhombic structure, or to a 
dyacisdodecahedrical, or to a tetrahedrical-pentagonedodecahedrical 
crystal. The last mentioned two symmetry-groups however must be 
excluded definitely because of the characteristic development of the 
boundary forms of the boracite; thus the symmetry of the RÖNTGEN- 
pattern at room-temperature can only correspond to a rhombic arran- 
gement of the molecules, the optical behaviour (biaxial) of the 
composing lamellae being in full agreement with this supposition. 
The internal change of symmetry of the boracite, if heated above 
400° C., seems therefore to be incontestably proved by the authors 
in this experiment. 
§ 4. If now we leave for the present out of consideration the 
cases of the composite pseudo-symmetrical (mimetic) crystals hitherto 
studied, it seems really, as if in a// cases, where regular or uniaxial 
crystals were studied, the results of the experiments were in full 
agreement with the conclusions which necessarily follow from the 
uow adopted theory of the said phenomena. 
However, the case of the rhombic curdierite is in flagrant contra- 
diction with it: for from the theory it follows immediately, that 
crystals of all three classes of the rhombic system must give Röntgen- 
ograms whose symmetry corresponds to that of the holohedrical forms. 
Thus plates parallel to the three pinacoidal faces: {LOO}, {O10} 
and {001}, must always give patterns which are symmetrical with 
respect to two perpendicular planes of symmetry; their intersection, 
i.e. the line perpendicular to the photographic plate, must therefore 
in all cases be a binary axis. 
: 1) H. Haca and I, M. Jaeger, These Proceed. 16. 792. (1914). 
