1354 
of the now established general theoretical interpretation of the sym- 
metry of the diffraction-phenomenon studied; and this will no longer 
be possible after all proofs, which were gathered hitherto in favour 
of its correctness. Moreover, — even then it would be hardly pos- 
sible to understand, why the symmetry of these RÖNTGEN-patterns 
appears to be so remarkably /igh. Thus we are for the present 
rather strongly compelled to accept the supposition of a polysynthe- 
tie twin-structure of all scheedite-individuals. 
IV. Tetragonal-trapezohedrical Class. 
a. Nickelselenate: NiSeO, + 6H,O. This compound was prepared 
from pure, cobaltfree colloidal nickeloxide, by solving it with an aqueous 
solution of pure selenie acid and evaporating the filtered solution in 
vacno. The beautiful green crystals represent pyramidal crystals with 
the forms: {142}, {111, OOI, {203}, {101} and {100}, and an axial 
ratio: a: ¢ = 1: 1,8365. The obtained RörtceNpatterns are reprodu- 
ced in figs. 6, 7, and 8 on Plate II, while in figs. 11 and 12 on 
Plate IV their stereographical projections are shown. The image 
obtained with a plate cut parallel to (OOL), — this being 0,72 mm. 
thick, — possesses evidently a quaternary axis and four planes of 
symmetry passing through it, while the images parallel to (100) and 
(110), (a =1,11 mm. in both cases) manifest two planes of symme- 
try perpendicular to each other, their intersection thus being a binary 
axis. The higher symmetry of the three patterns is therefore also 
in this case in full agreement with the theoretical expectations. 
b. Also to obtain a new case of Röntgenpatterns of isomor- 
phously related substances, we investigated moreover the analogous 
Nickelsulphate: NiSO,-+ 6H,O. At room-temperature we always 
obtained from aqueous solutions the rhombic-bisphenoidical sulphate 
with 7H,O, whose Röntgen-pattern parallel to {001} appeared 
generally similar to the corresponding one of zine-sulphate. Between 
30° and 40° C. the tetragonal form is obtained, between 50° and 
70° C., however, the monoclinic form of the hexahydrate, which mono- 
clinic form e.g. in the ease of the corresponding, isodimorphous 
cobalt-salt appears to be the more stable form even at ordinary 
temperatures. The tetragonal modification is deposited from solutions 
also at room-temperatures, if an excess of free sulphuric acid is 
added before *). The erystals of NiSO,+6H,O have an axial 
ratio: a:¢=1:1,9061; they are perfectly cleavable parallel to {001}. 
1) It may remarked here however, that in this case too, first monoclinic crystals 
are deposited, changing subsequently into the more stable tetragonal modification. 
Here thus the so-called rule of the reaction-steps (GAY Lussac, OSTWALD) is again 
illustrated in a rather striking way. 
