MONOCLINIC DOUBLE SELENATES OF THE NICKEL GROUP. 
227 
Cleavage. —A cleavage parallel to the orthoprism r'{20l} is well developed in the 
crystals of all four salts of the nickel double selenate group. In addition, the 
ammonium salt exhibits a feeble facility for cleavage parallel to the symmetry plane 
b {010}. This second direction of cleavage has also been observed more perfectly 
developed on the four double sulphates containing ammonium as the R-base and as 
M-metals, magnesium, nickel, manganese, and copper. It would appear to be connected 
with some obviously probable slight difference of structure, due to the different and 
more complicated chemical nature of the ammonium radicle NH 4 as compared with a 
simple ,alkali metallic atom. It is a fact which may prove later of value in elucidating 
the exact details of this slight difference of structure, and one which will require to 
be remembered in considering the meaning of the congruency with the rubidium salts 
referred to in the preceding paragraph. 
Orientation of the Optical Ellipsoid. —This comparison is best effected by taking 
the position, in the crystals of the four salts, of that one of the two median lines 
CE 
lying in the symmetry plane which is not far removed from the vertical morphological 
axis c, and which is the a axis of the ellipsoid in all four salts. This principal axial 
direction of the optical ellipsoid is the second median line in all the salts but caesium 
nickel selenate ; in this latter salt it is by exception the first median line. Such a 
comparison is made in the following table, which is also illustrated by fig. 9. 
Inclinations of a- Extinctions of the Ni Double Selenates in front of Axis c. 
(lM.L. in CsNi salt and 2M.L. in other three salts.) 
AmNi selenate 2° O' 
KNi „ 6° 59' 
RbNi selenate 
CsNi 
9° 59' 
16° 51'. 
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