MONOCLINIC DOUBLE SULPHATES CONTAINING AMMONIUM. 
39 
Caesium Copper Sulphate, Cs 2 Cu (S0 4 ) 2 . 6H 2 0. 
I. 
Density for 19°'8/4° 
. 2-8605 
For 20 /4 . 
. 2-8604 
II. 
„ 17 0, 2/4° . . 
. 2-8570 
„ 20°/4° . . 
. 2/8562 
III. 
„ 17 0, 2/4° . . 
. 2-8591 
„ 20°/4° . . 
. 2-8583 
IV. 
„ 17°'3/4° . . 
. 2-8586 
„ 20°/4° . . 
. 2-8578 
Mean . 
. 2-8582 
Accepted value for 20°/4°, 2'8-58. 
The former pyknometer values for potassium, rubidium, and caesium copper sulphates 
were 2'224, 2'570, and 2‘854 respectively. There is every-reason for confidence that 
the new values now given are nearer the truth. 
The next table presents the whole of the volume constants of the four copper- 
containing salts in comparative form. It shows that the molecular volume of the 
ammonium salt is almost identical with that of the rubidium salt, and in this case 
very slightly lower. As regards topic axial ratios, the x and \fr values are slightly 
lower, and the w value somewhat higher, than the corresponding values for the 
rubidium salt. 
Volume Constants of the Copper Group. 
Salt. 
Molecular 
weight. 
Specific 
gravity. 
Molecular 
volume. 
Topic axial ratios. 
KCu 
sulphate . 
438-76 
2-233 
196-49 
X 
6-0709 
\p : u) 
8-1053 : 4-1240 
RbCu 
55 
530-86 
2-574 
206-24 
6-2017 
8-2800 : 4-1640 
NH 4 Cu 
55 
396-92 
1-926 
206-08 
6-1786 
8-2790 : 4-1942 
CsCu 
55 • 
624-86 
2-858 
218-64 
6-3332 
8-5249 : 4-2164 
Optics. 
Orientation of the Optical Ellipsoid. —The plane of the optic axes is the symmetry 
plane b {010}, as usual throughout the series; but the double refraction is by 
exception of negative sign, the first median line corresponding to the refractive index 
and axis of indicatrix a, and the second median line to y. A pair of section-plates 
ground parallel to the symmetry plane 6{010} afforded the following extinction 
angles:—• 
Inclination of 1st M.L. from normal to c {001}. 
Plate 1 . . . 2° 26' Plate 2 ... 2° 50' 
Mean ... 2° 38'. 
The direction is (by exception in the series) in front of the normal, away from the 
axis c. Murmann and Rotter observed the same fact and found the angle from the 
normal 2° 33'. 
