228 
I)R. A. E. H. TUTTON ON THE 
The other principal axial direction of the ellipsoid lying in the symmetry plane 
shows, of course, a similar progression for the four salts, as it is always 90° from the 
direction compared in the table. The third rectangular axis of the ellipsoid is 
identical in position with the morphological symmetry axis b, and is, therefore, 
identical and immovable for all four salts, in accordance with monoclinic symmetry. 
Thus the optical ellipsoid rotates about the symmetry axis when one alkali base is 
replaced by another. It is so situated in the ammonium salt that one axis is nearly 
identical with the vertical crystal axis c ; it rotates from this position further away 
from c when ammonium is replaced by potassium, still further when the latter is 
replaced by rubidium, and yet further when rubidium is replaced by ctesium. Its 
position for the rubidium salt is thus intermediate between the positions for the 
potassium and caesium salts, the rotations for the two replacements of K by Rb 
and Rb by Cs being respectively 3° O' and 6° 52'. The rotation is thus in the 
order of the atomic weights of the three alkali metals, the amount of the rotation 
increasing at a higher rate than in mere simple proportion to the change in atomic 
weight. 
The Optic Axial Angles .—In making a comparison of these it has to be remembered 
that the first median line is differently situated in the caesium salt; whereas for the 
potassium, rubidium and ammonium salts it is that principal axis of the optical 
ellipsoid lying near the inclined axis a , for the caesium salt it is that axis of the 
ellipsoid which lies near the vertical axis c, the one 90° away near axis a being by 
exception in caesium nickel selenate the second median line. We must, therefore, use 
the obtuse optic axial angle, the bisectrix of which lies near axis a and which is 
therefore comparable to the acute optic axial angles of the other three salts, the 
bisectrices of which lie also near axis a. With this understood, the following table 
affords the comparison. 
Optic Axial Angles 2V a of the Nickel Group of Double Selenates. 
KNi selenate. 
RbNi selenate. 
AmNi selenate. 
CsNi selenate. 
Li. 
o / 
72 45 
82 23 
86 19 
97 19 
C. 
72 45 
82 22 
86 19 
97 17 
Na. 
72 48 
82 13 
86 21 
96 52 
T1. 
72 51 
82 7 
86 24 
96 32 
Cd. 
72 53 
82 3 
86 26 
96 23 
F. 
72 56 
81 58 
86 29 
96 17 
The optic axial angle is thus observed to increase with the rise in atomic weight 
of the alkali metal, the angle for the rubidium salt being intermediate between that 
for the potassium and caesium salts, and the rate being again an accelerating one. 
