2rt8 AfR. A. E. TRTTOX ON A COMPARATIVE CRYSTAELOCtR.VPHICAL STUDY 
Fig. 4 ill the doiihle sulphate memoir, representing csesium magnesium sulphate, is 
eipially applicable to the commonest type of caesium magnesium selenate. It is 
reproduced in fig. 3 (p. 274) for the purpose of the comparison of the habits of the 
tliree mao-nesium selenates. 
There is a good cleavage parallel r' )201}. 
Volume. 
Jielative Density .—The following four determinations 
material were made :— 
with separate quantities of 
MTight of salt 
Sp. gr. at 
employed. 
2074 ’. 
5-8081 
2-9385 
5-79G1 
2-9391 
6-2G59 
2-9389 
5-9848 
2-9387 
Mean, 2'9388 
Molecular Volume. — M/d = G83 -p 2'9388 = 232’4i. 
Distance llatios .—Tlie following distance ratios are afforded bv 
molecular volume with the axial angle and ratios already given ;— 
combination of the 
X ■■ ijj ■■ CO = G-3918 ; 87390 : 4-3345. 
(Jptics. 
Caesium magnesium selenate exhibits extraordinary optical characters, Inclnding 
crossed axial })lane dispersion of the optic axes and great sensitiveness of the optic 
axial angle to cliange of temperature, together with corresponding apparently abnormal 
refraction ])henomena. In this res})ect it is surprisingly similar to caesium mag¬ 
nesium sulphate, the change of selenium for sulphur simply advancing all the optical 
constants without materlallv altering their mutual relations. Moreover, the abnor- 
malities will be shown to be the direct result of the progression in optical properties 
which has so far throughout l:)oth sulphate and selenate series been found to 
accompany progress in the atomic weight of the alkali metal. 
Orientation o f Axes of Optical Ellipsoid. —The extinction angle in the symmetry 
2 >lane, with respect to the normal to the basal plane, was determined with the aid 
of two sections parallel to the symmetry plane as usual, and afforded the following 
results :— 
Section 1, 20^ 35': Section 2, 20'^ 25'. Mean, 20° 30'. 
The direction is in front of the normal to (001), that is, nearer to the morpho¬ 
logical axis a. This direction is tlie second median line for all wave-lengths (d’ light 
