MONOCLINIC DOUBLE SULPHATES CONTAINING AMMONIUM. 
7 
Rubidium Nickel Sulphate, Rb 2 Ni(S0 4 ) 2 . 6H 2 0. 
I. 
Density for l7°'8/4° 
. . . 2-5857 
For 20°/4° . . 
. 2-5851 
II. 
„ 17 0, 2/4° 
. . . 2-5854 
„ 20°/4° . , 
. 2-5847 
III. 
„ 17 0, 5/4° 
. . . 2-5867 
„ 20 o /4° . . 
. 2-5861 
IV. 
„ 17°’6/4° 
. . . 2-5886 
„ 2074° . . 
. 2-5880 
Mean . 
. 2-5860 
The value accepted for 20°/4° 
is 2-586. 
Ccesiwn Nickel Sidphate, Cs 2 Ni 
(S0,) 3 . 6H.O. 
I. 
Density for 19°'l/4° 
. . . 2-8720 
For 2074 o . . 
. 2-8717 
II. 
„ 18°'0/4° 
. . . 2-8753 
„ 2074 o . . 
. 2-8747 
III. 
„ 19°-2/4° 
. . . 2-8706 
„ 2074 o . . 
. 2-8704 
IV. 
„ 19°-6/4° 
. . . 2-8730 
„ 2074 o . . 
. 2-8729 
Mean . 
. 2-8724 
The value accepted for 20°/4° 
is 2-872. 
The former pyknometer values obtained for the three salts were respectively 
2'233, 2"583, and 2'865. The higher values now obtained are doubtless very near 
the truth, the error of the pyknometer method being on the side of lowness, due to 
inevitable mother-liquor inclusions, whereas the immersion method affords the result 
for the most cavity-free crystal. 
In the next table the volume constants of the four nickel-containing salts are 
compared. It shows that the molecular volume and topic axial ratios of the 
ammonium salt are close to those of the rubidium salt; the molecular volume is half 
a unit higher, the y and f ratios are also slightly higher and the w ratio very slightly 
lower. 
Volume Constants of the Nickel Group. 
Salt. 
Molecular 
weight. 
Specific 
gravity. 
Molecular 
volume. 
Topic axial ratios. 
KNi 
sulphate . . 
433-96 
2'237 
193-99 
X 
6-0170 
8-1542 
CO 
4-0934 
PbNi 
J? 
526-06 
2-586 
203-43 
6-1065 
8-3081 
4-1723 
NHfSfi 
392-12 
1-923 
203-91 
6-1426 
8-3312 
4-1656 
CsNi 
n 
620-06 
2-872 
215-90 
6-2097 
8-5416 
4-2572 
Optics. 
Orientation of the Optical Ellipsoid .—The plane of the optic axes is the symmetry 
plane b {010}. The double refraction is of positive sign, so that the first median line 
corresponds to the refractive index y and the second median line to a. 
