MONOCLINIC DOUBLE SELENATES OF THE IRON GROUP. 
437 
Axial Ratios of the Optical Velocity Ellipsoid. 
a : t : c 
a : fc 
c 
KFe selenate 
. . . 1 '0058 : 1 : (T9894 
1 '0058 : 1 
0-9894 
RbFe ,, 
. . . U0044 : 1 : CT9916 
1 '0032 : 0'9988 
0-9905 
NH 4 Fe „ 
. . . 1'0042 : 1 : 0V934 
0-9978 : 0'9936 
0-9871 
CsFe ,, 
. . . U0030 : 1 : 0'9960 
0-9919 : 0-9889 
0-9849 
Axial Ratios of the Optical Ellipsoid .—The values of these ratios are given in the 
two preceding tables. Besides the ratios calculated with the /3 value for each salt as 
unity, a second set is given (as in the cases of the previous groups studied) in which 
the (3 value for the potassium salt is taken as unity for all the four salts of the group, 
thus enabling the progress of the axial directional change in the dimensions of the 
optical ellipsoid to be followed on the passage from one salt to another. The left-hand 
series show a regular progression with the atomic weight and number of the alkali 
metal. The right-hand series emphasise the accelerating nature of the change, 
modified by the effect of the diminution of double refraction in causing the set-back 
of the y values, which is apparent in the case of rubidium ferrous selenate. 
The position of the ammonium salt is seen to be intermediate between the rubidium 
and csesium salts, as regards both series of ratios, which may be considered as yet 
another confirmation that ammonium ferrous sulphate is correctly regarded as truly 
isomorphous with potassium, rubidium, and csesium ferrous selenates. 
Molecular Optical Constants .—These constants are as valuable with regard to the 
optics of the group as were the molecular volumes and topic axial ratios in indicating 
the true relations of the salts in regard to their morphology. They are given in the 
next three tables. The conclusions derivable from them are the following. 
The whole of the molecular optical constants of the rubidium salt are intermediate 
between those of the potassium and csesium salts of the group. The important 
molecular refraction, whether calculated by the formula of Lorenz or by. that of 
Gladstone and Dale, increases with increase of atomic weight and atomic number 
of the interchangeable alkali metals, and at an accelerating rate, the replacements 
respectively of potassium by rubidium and rubidium by csesium being accompanied by 
increases in mean molecular refraction of 5'46 and 9'89 Gladstone units, which are 
in the proportion of 3 : 5. The result is unaffected by temperature change, as the effect 
of the latter is similar on both the density and refractive power, both diminishing on 
raising the temperature. 
The molecular refraction of the ammonium salt is very close indeed to that of the 
rubidium salt, being very slightly higher to the extent of one Lorenz or two 
Gladstone units; the mean molecular refraction by Gladstone’s formula is 1‘57 
higher. This result is analogous to that regarding the structural dimensions, the 
ammonium salt being shown to be practically isostructural with the rubidium salt. 
3 M 2 
