MONOCLINIC DOUBLE SELENATES OF THE NICKEL GROUP. 
^25 
Angular Changes for Double Selenates containing Nickel. 
Replacement. 
Average change. 
Maximum change. 
K by Rb. 
23 
t o / 
57 = 0 57 
K by Cs. 
47 
119 = 1 59 
K by NH 4 . 
45 
110 = 1 50 
The average and maximum changes of angle are twice as great when potassium is 
replaced by csesium as when potassium is replaced by rubidium, corresponding to the 
changes of atomic weight (K^Rb = 46 and K^Cs = 93). When potassium is 
replaced by ammonium the changes (average and maximum) are very nearly the same 
as occur when potassium is replaced by csesium. That is to say, the average change 
in the crystal angles, and also the maximum amount of that change, in the cases of 
the metallic interchanges, are directly proportional to the change in the atomic 
weight of the alkali metal ; while the introduction of the radicle ammonium instead 
of potassium produces about as much average or maximum change of angle as when 
potassium is replaced by csesium, the actual amounts being very slightly less, to the 
extent of 2' in the average change (out of a total of 47') and 9' in the maximum 
change (out of a total of 1° 59'). 
From the above it will be clear that the dilferences in the crystal angles are of the 
same sign when potassium is replaced by rubidium on the one hand and by csesium on 
the other, that is, there is a progressive change, there being only a single exception 
(in the case of an angle showing only a very small change) out of all the 36 cases 
compared ; the amounts have been shown to be exactly proportional on the average to 
the two increments of atomic weight. In the case of the ammonium salt, 33 out of the 
36 angles compared show' differences on replacing potassium by ammonium in the 
same direction as when rubidium or csesium are introduced instead of potassium, the 
only three exceptions being in cases where the differences are very minute. In 32 of 
the 33 cases the changes are greater than when rubidium is introduced, and in 13 
cases they exceed the effect produced by introducing csesium for potassium. The 
maximum change of angle for the ammonium replacement, however, has been shown 
to fall somewhat short of that for the csesium replacement, and thus the crystal 
angles for the ammonium salt all fall within the limits of the changes for the three 
metallic salts, emphasising the true character of the isomorphism of all the four 
salts. 
Volume Constants .—The densities, molecular volumes and topic axial ratios of the 
four salts are compared in the next table. 
2 i 2 
