MONOCLINIC DOUBLE SELENATES OF THE NICKEL GROUP. 
21 9 
Cleavage. —The cleavage common to the series, parallel to r'{ 20 l}, is very well 
developed in this salt. There is also a feeble and very imperfect cleavage parallel to 
b { 010 } ; it is not so distinct, however, as in several other ammonium salts of the series 
(ammonium magnesium, ammonium nickel, ammonium manganese, and ammonium 
copper sulphates). 
Volume. 
Relative Density. —Four determinations by the immersion method yielded the 
following values 
I. Density for 18° "3/4° . 
, . 2-2422 
For 20 /4' 
. . 2-2418 
II. 
18°-7/4° . 
. . 2-2445 
„ 20°/4" . 
. . 2-2442 
HI. 
18°-9/4° . , 
. . 2-2443 
„ 2074“ . 
. . 2-2441 
IV. 
I8°-9/4° . 
. . 2-2435 
„ 2074” . 
. . 2-2433 
Mean . . . 2’2434 
Accepted value for 20°/4°, 2'243. 
The value 2'228 was obtained by Tops0e. 
Molecular Volume. — ^ ^ = 216'53. 
d 2'243 
Molecular Distance Ratios (topic axial ratios ).— 
x : f ® = 6'2520 : 8’4543 : 4-2678. 
Optics. 
Nature and Orientation of the Optical Ellipsoid. —The optic axes lie in the 
symmetry plane b {01 0 }. The sign of the double refraction is positive. Extinction 
determinations with two section-plates ground parallel to the symmetry plane yielded 
the following results :—- 
Extinction Direction in the Symmetry Plane. 
Plate 1 .... 14° 18' Plate 2 .... 14° 16', 
Mean . . . 14° 17', behind the normal to c { 001 }. 
This direction is the second median line, and corresponds to the refractive index a, 
the other extinction direction at right angles in the symmetry plane being the first 
median line and corresponding to the index y. As the morphological axial angle 
ac — 7 3° 43', and therefore the angle between the normal to c {001} and the vertical 
axis c — 16° 17', the second median line lies 2° O' in front of the vertical axis c, in the 
obtuse angle ac of the morphological axes a and c. The first median line also lies in 
