and Hemihedrism of Leucophane. 359 



The fourth edge is not present, owing to the crystal being 

 broken at this part. 



The British Museum also contains a crystal which I have 

 not examined ; but the figure of the crystal published in 

 Tschermak's Mineral. Mittheil. (1871) shows that the crystal 

 is modified differently on the four edges of the base. This 

 crystal therefore seems to present the same hemihedrism as 

 that noticed above. Without pronouncing definitely on the 

 hemihedrism, I think that there is sufficient probability to justify 

 me in calling attention to the fact, and to encourage an exa- 

 mination as to whether other crystals present the same hemi- 

 hedrism. 



Instead of considering leucophane to be hemihedral, one 

 might suppose that it belonged to the oblique system rather 

 than to the prismatic. On this supposition the base would 

 become the plane of symmetry <7 1= =0 1 0, the bisectrix of the 

 optic axis would be the horizontal diagonal of the base, the 

 faces 5 1 = 1 1 2 and &*=1 1 1 would become m=110 and 

 e 4 = 1 4 respectively. But this supposition would only be 

 admissible if the hemihedrism to which I have called atten- 

 tion were a hemihedrism with parallel faces ; for in the case 

 of a hemihedrism with inclined faces one would not find the 

 four vertical faces necessary for forming the prism. But the 

 twinned specimen of which I have spoken above shows very 

 clearly a hemihedrism with inclined faces. Moreover, if leu- 

 cophane crystallized in the oblique system one would probably 

 observe a crossed dispersion in the polarizing microscope, 

 whereas the symmetry of the dispersion of the colours is most 

 perfect. This does not constitute a proof, but renders it ex- 

 tremely probable that the system is prismatic. 



I have likewise proved that the trace of the plane of the 

 optic axes on the face p — 1, parallel to the cleavage, coin- 

 cides with the diagonal of this face ; for if a twinned plate be 

 examined with the ordinary microscope between two Nicols, 

 the line of separation of the two crystals is observed to be 

 situated very approximately at 45° to the trace of the plane 

 of the optic axes of each of the two crystals, the plane of the 

 optic axes of one crystal being at 90° to that of the other. 



This coincidence, which is necessary in the prismatic system, 

 but not in the oblique, gives a fresh argument in favour of the 

 system being prismatic. 



We may therefore conclude that the greater part of the 

 crystals of leucophane are twinned and most probably hemi- 

 hedral. It will be interesting to obtain evidence whether 

 other crystals besides those mentioned above present the same 

 hemihedrism. 



