432 
DR. A. E. H. TUTTON ON THE 
Angulak Changes for Double Selenates containing Ferrous Iron. 
Replacement. 
Average change. 
Maximum change. 
K by Rb. 
27 
/ o / 
69 = 1 9 
K by Cs. 
55 
139 = 2 19 
K by NH 4 . 
54 
139 = 2 19 
In 37 of the 38 angles compared the value for the rubidium salt is intermediate 
between the values for the potassium and caesium salts. In the case of the single 
exception the change is very small. 
The average and maximum changes of angle are exactly twice as large when 
potassium is replaced by caesium as when potassium is replaced by rubidium, corre¬ 
sponding precisely to the change in atomic weight (K ^ Cs = 93 and K ~ Rb = 46), 
or atomic number (K ~ Cs = 36 and K ~ Rb =18). Thus the average change of 
angle, and also the maximum amount of angular alteration observed, are directly 
proportional to the change in the atomic weight or number. 
As regards the position of the ammonium salt, in 35 of the 38 compared angles the 
changes for the K by NH 4 replacement are in the same direction as for the metallic 
replacements K by Rb and K by Cs, and the other three are cases where the changes 
are very minute. 
In 34 of these 35 cases the changes for the K by NH, replacement are greater 
than when potassium is replaced by rubidium, and in 15 of them the changes are 
somewhat greater than those which occur when potassium is replaced by caesium. 
When all the K by NH 4 changes are added together (as regards amounts, irrespective 
of sign), and the mean taken, in the same manner as was done for the metallic replace¬ 
ments, this average change for the ammonium replacement of potassium is found to 
be practically identical with the average change which occurs when potassium is 
replaced by caesium; the maximum amount of change observed (for K by NH 4 ) is 
also identical with that which occurs for the K by Cs replacement. Thus the angular 
effect of replacing potassium by ammonium is of the same order as that of the 
weightier metallic interchange, a fact in full keeping with the supposition of the 
isomorphism. 
Volume Constants .—The densities, molecular volumes and topic axial ratios are 
compared in the next table. 
