MONOCLINIC DOUBLE SELENATES OF THE IRON GROUP. 
435 
The position of the ellipsoid for the rubidium salt is thus intermediate between its 
positions for the potassium and caesium salts, and the amount of rotation is an 
accelerating one as the atomic weight and number of the alkali metal rises. 
Optic Axial Angles .—-These can be directly compared, as the double refraction is 
positive and the first median line is analogously disposed, subject to the rotation just 
described, in all four salts. The values are set out in the next table. 
Optic Axial Angles 2V„ of the Iron Group of Double Selenates. 
KFe selenate. 
RbFe selenate. 
AmFe selenate. 
CsFe selenate. 
Li. 
64 11 
73 35 
77 36 
82 58 
C. 
64 12 
73 34 
77 37 
82 56 
Na. 
64 18 
73 32 
77 44 
82 47 
T1. 
64 25 
73 30 
77 50 
82 33 
Cd. 
64 30 
73 28 
77 52 
82 25 
F. 
64 36 
73 26 
77 54 
82 20 
It will be apparent that the optic axial angle increases as the atomic weight 
and number of the alkali metal rises, and the value for the rubidium salt is about 
midway between the values for the potassium and caesium salts. The optic axial 
angle of ammonium ferrous selenate is slightly larger than that of rubidium ferrous 
selenate. 
Refractive Indices .—The refractive indices are compared in the table on next page, 
together with the mean refractive index for sodium light and the magnitude of the 
double refraction. 
It will be observed that the mean refractive index for the rubidium salt is 
intermediate between the values for the potassium and caesium salts, although the 
advance of this specific refractive constant in the case of the rubidium salt is not 
much beyond that of the potassium salt. The double refraction (difference between 
a and y indices) progressively diminishes (with acceleration) as the atomic weight and 
number of the alkali metal rises, and the result of this is curious on the y indices, these 
indices for the rubidium salt being brought lower than those for the potassium salt. 
Although the increasingly progressive diminution of the double refraction tends also 
to reduce the y indices of the caesium salt, the rapidly accelerating progressive growth 
of refracting power causes even these indices of the caesium salt, like the a and (3 
indices, to show a clear advance in the refractive indices of this salt. The real 
progression in the refractive power is shown, however, by the molecular refraction, 
which will presently be dealt with. 
The refractive indices of ammonium ferrous selenate are intermediate between those 
of rubidium and caesium ferrous selenates. 
VOL. CCXVIII.-A. 
3 M 
