274 MR. A. E. TUTTON ON A COMPARATHM: CRYSTALLOGRAPHICAL STUDY 
Comparison of the Three Magnesium Salts. 
The Morphological Angles are compared in the accompan)dng table. The results 
deducible are as follo\¥S :—■ 
The axial angle /3 of rubidium magnesium selenate is nearly the mean of the axial 
angles of potassium magnesium and cmsium magnesium selenates. 
With only one exception all the morphological angles of the rubidium salt are 
intermediate in value lietween those of the potassium and caesium salts. The 
exception, the angle ho’, only escapes follovdng the rule by 2', an amount Muthin the 
limits of experimental error. 
The change in the exterior angles on passing from one salt to another of the 
trijilet is not, as a rule, directly proportional to the change in the atomic weight of 
the alkali metal. The primary prism zone shows the greatest divergence from direct 
proportion, the change in ap being as 1 to 2. 
Comparison of the Axial Ratios. 
For potassium magnesium selenate. . a :h : c — 0'7485 : 1 : 0‘5031 
,, rubidium ,, ,, .. a : h : c = 07424 : 1 : ObOll 
,, ctesium ,, ,, . . « ; : c = 0‘7314 : 1 : 0’4960 
From this talde it is clear that the morphological axial ratios of rubidium magne¬ 
sium selenate are intermediate between those of potassium magnesium and caesium 
mao’iiesium selenates. 
o 
The general Ilahit of the crystals of the three salts exhibits very markedly the 
ju’ogression of type which has been established throughout the double sulphate 
series, and in the case of the zinc group of double selenates. 
The potassium salt is characterised l)y a stout piismatic habit, the prism zone 
forming the prism, and by relatively large end basal plane faces. The caesium salt is 
characterised l)y another prismatic hal)lt, the prism faces lieing those of the clinodome 
pjOllj, and tlie faces of the basal plane are generally reduced to a strip. The 
ruljidium salt has been observed to exhibit every gradation between these two quite 
dilfereiit habits, and a very large proportion of the crystals examined were of a 
clearly intermediate type, showing moderate-sized faces of the basal plane, and the 
prism and clinodome faces more or less equally developed. The three accompanying 
figures, representing typical crystals of the three salts, will render this progression of 
habit perfectly })lain. 
Fig. 3. 
CsMg selenate. 
