27G MR. A. E. TUTTON ON A COMPARATIVE CRVSTALLOGRAPHICAL STUDY 
Comparison of the Ilelative Densities. 
Potassium map'nesinm selenate . . 2’3630 1 aot-- 
^ )■ Dm. O'31/o 
Iviibidium ,, ,, . .^2'6805 
Csesimn ,, ,, . . 2’938 8 
The density increases with the atomic weight of the alkali metal, and is greater for 
the replacement of potassium by rubidium than for rubidium by caesium in the 
proportion of 5 : 4. This proportion is the same as for the corresponding magnesium 
double sulphates, hut the actual amounts of the differences are smaller ; the differ¬ 
ence between potassium magnesium sulphate and its rubidium analogue is 0'354, and 
between the latter and caesium magnesium sulphate 0’288. 
Diff. 0-2583 
Comparison of the Molecular Volumes. 
Potassium mao’nesium 
o 
selenate . 
. 209-73 ] 
1 Diff: 
9-63 
Iluhidium ,, 
95 • 
. 219-36 j 
. 232-41 J 
. Diff: 
13-05 
CJfesium ,, 
99 
The molecular volumes show 2 )rogresslon with the atomic weight of the alkali 
metal, but the replacement of I'uhidium by csesium is accompanied by the greater 
change. The proj^ortion is similar to that found for the double sulphates ; the 
differences observed between the magnesium double sulphates were 9-33 and 13-19. 
Comparison of the Distance Patios. 
X- 
Diff. 
Diff. 
cu. 
Diff 
KMg selenate. 
. 6-2233 
> 768 
8-3144 . 
> 1717 
4-1829 . 
> 695 
RbMg „ . . 
6-3001 = 
> 917 
8-4861 = 
> 2529 
4-2524 = 
. 821 
CsMg „ . . 
. 6-3918 J 
8-7390 -J 
4-3345 J 
1685 
4246 
1516 
The sinpdiffed ratios, taking \{j for KMg selenate as unity, are as follow :— 
X- 
Diff. 
P- 
Diff 
to. 
Diff. 
KXIg selenate. . 
0 - 7 485 - 
!> 92 
1-0000 . 
. 207 
0-5031 , 
83 
Rl.Mg „ . . 
. 0-7577 = 
^ 111 
1-0207 = 
t 304 
0-5114 = 
99 
CsMg „ . . 
. 0-7688 -J 
1-0511 J 
0-5213 -J 
203 
511 
182 
These ratios (topic axes) indicate that there is an extension of the distance 
separating the structural units in all three axial directions, the maxinnun being along 
