MONOCLINIC DOUBLE SULPHATES CONTAINING AMMONIUM. 
21 
Axial Ratios of the Optical Ellipsoid .—These are given in the next table, both for 
the ammonium salt now described and for the three alkali-metal salts previously 
described. The comparison shows that the dimensions of the optical ellipsoid (either 
form) of the ammonium salt are also intermediate between those for the rubidium and 
Caesium salts, and much nearer to those for the rubidium salt. 
Axial Ratios of the Optical Indicatrix. 
X 
P 
: 7 
a 
: P 
7 
KCo sulphate . 
. (T9961 
1 
1*0094 
0-9961 
: 1 
1*0094 
RbCo ,, 
. 0-9962 
1 
1-0066 
0-9996 
U0034 
1*0100 
NH 4 Co „ 
. 0'9966 
1 
1-0053 
1-0025 
1*0059 
1*0113 
CsCo 
. 0-9981 
U0031 
1-0129 
1*0148 
1*0180 
Axial Ratios of the Optical Yelocity Ellipsoid. 
a 
b 
c 
a 
b 
c 
KCo sulphate . 
. 1'0039 
1 
0-9907 
1-0039 
1 
0-9907 
RbCo ,, . . 
. 1-0038 : 
1 : 
0-9935 
1*0004 
0-9966 
0-9901 
NH 4 Co „ . . 
. 1*0034 : 
1 : 
0*9947 
0-9975 
0'9941 
0-9889 
CsCo „ . . 
. 1-0019 : 
1 : 
0-9969 
0-9872 
0-9854 
0*9823 
Molecular Optical Constants .—These are given in the next three tables for all 
four salts of the cobalt group. The newly determined densities were employed in 
recalculating the values for the three alkali-metal salts. The specific refraction and 
. dispersion of the ammonium salt are considerably higher than those of the three salts 
containing the alkali metals. The molecular dispersion of the ammonium salt is 
slightly higher than that of the caesium salt. The molecular refraction, either of 
Lorenz or Gladstone and Dale, is invariably just higher than that of the rubidium 
salt, a fact which is very clearly demonstrated by the mean molecular refraction given 
in the last column of the third table. 
Table of Specific Refraction and Dispersion (Lorenz). 
Sulphate. 
Specific refractio 
n 2 - 1 
Specific dispersion. 
u G - n c . 
(n 2 + 2) cl 
For ray C (Ha). 
For ray Hy near G. 
x. 
P- 
7- 
x. 
P- 
7- 
x. 
* 
y- 
AmCo . . . 
0-1515 
0-1528 
0-1549 
0-1552 
0'1565 
0-1586 
0-0037 
0-0037 
0-0037 
KCo. . . . 
0-1277 
0-1290 
0-1320 
0-1304 
0-1317 
0-1319 
0-0027 
0-0027 
0-0029 
RbCo . . . 
0-1144 
0-1125 
0-1144 
0-1137 
0-1148 
0-1168 
0-0023 
0-0023 
0-0024 
CsCo. . . . 
0-1040 
0-1045 
0-1053 
0-1062 
0-1067 
0-1075 
0-0022 
0-0022 
0-0022 
