226 
DR. A. E. H, TUTTON ON THE 
Volume Constants of the Nickel Group of Double Selenates. 
Salt. 
Molecular 
weight. 
Specific 
gravity. 
Molecular 
volume. 
Topic axial ratios. 
X 
4* 
(jl) 
KNi 
selenate . . 
527-52 
2-559 
206-14 
6-1677 
8-2598 
4-1786 
RbNi 
619-62 
2 • 856 
216-96 
6-2533 
8-4561 
4-2542 
CsNi 
11 • 
713-62 
3-114 
229-17 
6-3317 
8-6878 
4-3378 
NH 4 Ni 
11 
485-68 
2-243 
216-53 
6-2520 
8-4543 
4-2678 
The density increases with the atomic weight of the alkali metal, and is greater for 
the replacement of potassium by rubidium than for the replacement of rubidium by 
caesium in the proportion of 6 to 5. The density of the ammonium salt is the lowest 
of the group, corresponding to the low molecular weight of the ammonium radicle 
NH 4 . 
The molecular volumes of the three alkali-metallic salts show progression with the 
atomic weight of the alkali metal; the replacement of rubidium by caesium is 
accompanied by the greater change, 12‘21 units as against 10'82 units for the 
replacement of potassium by rubidium, so that the increment of volume is an 
increasing one. The molecular volume of the ammonium salt is almost identical 
with that of the rubidium salt, a result similar to what has been observed in all the 
other groups of salts of the series yet investigated. 
The topic axial ratios of the rubidium salt are also intermediate between those for 
the potassium and caesium salts, there being a regular progressive increase in the 
directional (axial-edge) dimensions of the structural unit cell of the space-lattice, 
when potassium is replaced by rubidium and the latter in turn by caesium. The topic 
axial ratios of ammonium nickel selenate are almost identical with those of rubidium 
nickel selenate, the structural (space-lattice) unit cells of the rubidium and ammonium 
salts being thus almost identically congruent, that is similar in shape and symmetry 
and almost exactly equal in total volume and in the dimensions of their sides and 
edges. This result, similar to the results with all the other groups of double selenates 
yet investigated and of all the double sulphates and the simple alkali sulphates, 
thus forms yet another confirmation of one of the most remarkable facts brought to 
light by the author’s investigations, namely, the extraordinary congruency of the 
ammonium and rubidium salts of any group ; the replacement of two atoms of 
monad rubidium by eight atoms of monad hydrogen and two atoms of triad (at 
least, more probably pentad) nitrogen, actually occurs without any appreciable 
opening up of the structure. The bearing of this important fact on the theory of 
valency volumes of Barlow and Pope, with which it is in entire antagonism, is 
discussed in a separate communication (see Roy. Soc. Proc.). # 
* ‘ Roy. Soc. Proc.,’ A, vol. 93, p. 72 (1917). 
