MONOCLINIC DOUBLE SELENATES OF THE IRON GROUP. 
433 
Volume Constants of the Iron Group of Double Selenates. 
Salt. 
Molecular 
weight. 
Specific 
gravity. 
»• 
Molecular 
volume. 
Topic axial ratios. 
KFe selenate . . 
524-72 
2-494 
210-39 
X 
6-2230 
i p : oj 
8-3085 : 4-1908 
RbFe ,, . . 
616-82 
2-800 
220-29 
6-3109 
8•5006 : 4■2503 
CsFe ,, . . 
710-82 
3-048 
233-21 
6-3847 
8-7366 : 4-3499 
NH 4 Fe „ . . 
482-88 
2-191 
220-39 
6-3212 
8-5043 : 4'2684 
The density increases with the atomic weight of the alkali metal, and is greater by 
6 to 5 for the K by Rb replacement than for the Kb by Cs interchange. The 
ammonium salt is the lightest member of the group, corresponding to the low 
molecular weight of the radicle NH 4 . 
The molecular volumes of the three alkali metallic salts progress at an accelerating 
rate with the rise in atomic weight or atomic number of the alkali metal. When 
potassium is replaced by rubidium the increase is to the extent of 9'9 units, but 
when rubidium is replaced by caesium the increase amounts to 12‘92 units. The 
interchange of caesium for potassium is accompanied by an increase of 22'82 units. 
The molecular volume of ammonium ferrous selenate is almost exactly identical with 
that of rubidium ferrous selenate, being O'l unit higher, the replacement of potassium 
by ammonium being accompanied by an increase of exactly 10 units. 
The topic axial ratios follow similar rules, being the directional expression of the 
volume along the directions of the crystallographic axes, and proportionately to the 
lengths of those axes. The values of corresponding ratios are always intermediate in 
the case of the rubidium salt, between the values for the potassium and caesium salts; 
a regular extension of the edges of the unit cell of the structural space-lattice thus 
occurs as potassium is replaced by rubidium and the latter in turn by caesium. The 
lengths of the cell edges for the ammonium salt are almost the same as, very slightly 
greater than, those for rubidium ferrous selenate. 
This striking fact of the congruency of the ammonium and rubidium salts—-which 
is in keeping with all the author’s previous results for the other groups of double 
selenates yet investigated, for the whole of the groups of double sulphates of this 
monoclinic series, and for the two rhombic groups of simple alkali sulphates and 
selenates—affords yet another confirmation of those former results, and further 
substantiates the important conclusions which were drawn therefrom in a memoir* 
read to the Royal Society in November, 1916. 
Cleavage .—All four salts exhibit the development of cleavage parallel to r'{201}, 
the cleavage common to the whole series. Some of the ammonium salts of the series 
* ‘Roy. Soc. Proc.,’ A, vol. 93, p. 72 (1917). 
