18 
DE. A. E. H. TUTTON ON THE 
CcBsium Cobalt Sulphate , Cs 2 Co (S0 4 ) 2 . 6H 2 0. 
I. 
Density for 16°7/4° 
. . . 2T468 
For 20°/4° . 
. . 2-8459 
■ II. 
„ 16°7/4° 
. . . 2-8444 
„ 20°/4° . 
. . 2-8435 
III. 
„ l7"7/4” 
. . . 2-8439 
„ 20°/4° . 
. . 2-8432 
IV. 
,, 18°T/4° 
. . . 2-8436 
„ 20”/4° . 
. . 2-8431 
Mean . 
. . 2-8439 
The value accepted for 20°/4° 
is 2'844. 
The former pyknometer values for potassium, rubidium, and ceesium cobalt sulphates 
were 2'212, 2"557, and 2'837 respectively. The new values all being higher are 
certainly nearer the truth. 
The next table affords a comparison of the volume constants of all four cobalt- 
containing salts. 
Volume Constants of the Cobalt Group. 
Salt. 
Molecular ' 
weight. 
Specific 
gravity. 
Molecular 
volume. 
Topic axial ratios. 
KCo 
sulphate . . 
434-21 
2-219 
195-68 
X = l w 
6-0405 : 8-1583 : 4-1093 
EbCo 
526-31 
2-567 
205•03 
6-1494 : 8-3201 : 4-1692 
NH,Co 
5) 
392-37 
1-901 
206-40 
6-1860 : 8-3753 : 4‘1667 
CsCo 
J5 
620-31 
2-844 
218-11 
6-2386 : 8'5814 : 4‘2632 
It shows that the position of the ammonium salt as regards both molecular volume 
and topic axial ratios is close to that of the rubidium salt ; the molecular volume and 
the ratios x and f are slightly higher, and the ratio w slightly lower. 
Optics. 
Orientation of the Optical Ellipsoid. —-The symmetry plane b {010} is the plane of 
the optic axes. The double refraction is positive, the first median line corresponding 
to the refractive index y, and the second median line to a. Two section plates 
parallel to the symmetry plane afforded the following extinction angles :•— 
Inclination of 2M.L. from normal to c {001}. 
Plate I. . . . 10° 38' Plate II. . . . 10° 48' 
Mean . . . 10° 43'. 
The direction is behind the normal, towards axis c. 
Murmann and Potter obtained 11° O' for this extinction angle, and Senarmont 
gives it as 12' O'. 
