MONOCLINIC DOUBLE SELENATES OF THE IKON CROUP. 
429 
Axial Ratios of the Optical Ellipsoid. —The calculated values for the two types 
of ellipsoid are as under :— 
Indicatrix. a : /3: y = 0‘9958 : 1 : 1'0066. 
Optical Velocity Ellipsoid . . a : 1) : C = 1'0042 : 1 : 0'9934. 
Molecular Optical Constants. —These are given in the next table, calculated by 
both the formulas of Lorenz and of Gladstone and Dale. 
Lorenz 
J J 
Gladstone 
Axis of optical indicatrix 
OL. 
A 
y • 
Specific refraction, f ; = n . . 19 
(n 2 + 2) a l Lt 
Molecular refraction, ~—- . ^ = m . 1 9 
n 2 + 2 d l C 
Specific dispersion, n G — n c . 
Molecular dispersion, ut G - m c . 
Molecular refraction, n ^ M . . . . C 
d 
0*1384 
0*1422 
66*81 
68*68 
0*0038 
1*87 
114*21 
0*1398 
0*1437 
67*50 
69*38 
0*0039 
1*88 
115*62 
0*1420 
0*1459 
68*59 
70*46 
0 * 0039 
1*87 
117*87 
Mean molecular refraction (Gladstone), 4(a + /? + y) = 115* 90. 
Comparison of Results. 
The successful completion of the investigation of the potassium salt of this ferrous 
iron group enables a full comparison to be made of the crystals of all the four salts 
of this group, on exactly the same lines as the four salts of each of the three groups 
(zinc, magnesium, and nickel) of double selenates already described have been 
compared. The fact that crystals of potassium ferrous selenate only form at 
temperatures very near to 0° C., and spontaneously decompose into an opaque 
porcelain-like substance, in less than half a day in moderately cold weather and almost 
immediately at the temperature of a warm day, together with the fact that the 
crystals of caesium ferrous selenate are magnificently transparent and relatively 
permanent, capable of preservation in closed vessels for years, afford striking proof 
of the progression in stability with the rise of atomic weight and atomic number of 
the alkali metal. 
Habit .—The progression in habit which has been observed in the other groups 
of both double selenates and double sulphates is very clearly exhibited by the 
ferrous iron group of double selenates. An inspection of figs. 1 and 2 (potassium salt), 
4 (rubidium salt), and 6 (caesium salt) will make clear the progressive diminution in the 
relative development of the faces of c {001}, and the relative growth of the faces of 
^ {Oil} from very small and subordinate in the potassium salt to predominatingly 
large in the caesium salt, through the intermediate stage of a moderate size in the 
rubidium salt. The importance of the faces of p {110} similarly diminishes more or 
3 L 2 
