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DR. A. E. H. TUTTON OX THE 
less regularly from the potassium to the caesium salt, through the rubidium salt. 
The ammonium salt, shown in figs. 8 and 9, forms crystals of all types of development, 
but a great majority are of the intermediate type exhibited in fig. 8. Fig. 9 
illustrates a crystal showing faces of c {001} and q {011} very like those of a 
potassium salt of the series. In all four salts of the iron group there is a marked 
tendency for a larger development of the faces of r'{20l} than in the other groups of 
double selenates studied. A similar fact was observed with respect to the ferrous 
iron group of double sulphates. 
Crystal Elements and Angles .—The axial angles and axial ratios are compared in 
the following table. 
Comparison of the Axial Angles and Axial Ratios. 
Axial angle. 
Axial ratios. 
Potassium ferrous selenate . . 
Rubidium „ ,, . . 
Ammonium ,, ,, 
Caesium ,, ,, . . 
A 
103° 50' 
104° 57' 
106° 9' 
106° 2' 
a : b : c 
0-7490 : 1 :0’5044 
0-7424: 1 :0-5000 
0-7433 : 1 : 0‘5019 
0-7308 : 1 : 0-4979 
The differences between the morphological axial angles of the three alkali metallic 
salts are 1° 7' between the potassium and rubidium salts, 1° 5' between the rubidium 
and caesium salts, and 2° 12' between the potassium and caesium salts. Obviously, 
therefore, the axial angle (3 increases in direct proportion to the increase of the atomic 
weight and atomic number of the alkali metal. As the values of the two latter 
constants will be constantly required in the course of these comparisons, they are 
given below for reference. 
K. 
Rb-K. 
Rb. 
Cs - Rb. 
Cs. 
Cs - K. 
Atomic weights 
38-85 
46 
84-9 
47 
131 '9 
93 = 2x46-5 
Atomic numbers 
19 
18 
37 
18 
55 
36 = 2 x 18 
As rubidium stands precisely half-way between potassium and caesium as regards 
atomic weight and atomic number, so does the crystallographic (monoclinic) axial 
angle /3 stand half-way in the case of the rubidium salt. 
The axial angle of ammonium ferrous selenate is very nearly identical with (7' larger 
than) that of the caesium salt. 
The morphological axial ratios of the rubidium salt are intermediate between those 
for the potassium and caesium salts. Those of the ammonium salt are close to those 
of the rubidium salt, and thus partake of an intermediate character in the group. 
This fact is in complete harmony with the supposition of true isomorphism of the 
ammonium salt with the three alkali metallic salts of the group. 
