206 
DR. A. E. H. TUTTON ON THE 
Rubidium Nickel Selenate , Rb 2 Ni (Se0 4 ) 2 .6H 2 0. 
Morphology. 
The crystals of this salt do not appear to have been hitherto investigated. They 
are bright green in colour, resembling the potassium salt, but are somewhat richer in 
faces. About 20 crops were successively obtained, three of which were of special 
excellence for goniometrical purposes. 
Ten small crystals were measured, selected from the three crops above mentioned, 
the crystals of which were most free from striation. The individuals measured were 
especially perfect, as will be obvious from the close agreement between the observed 
and calculated values in the accompanying table of angles representing the results of 
the measurements. 
Crystal System. —Monoclinic. Class No. 5, holohedral-prismatic. 
Ratios of Axes .— 
a : b : c = 07395 : 1 : 0‘5031. 
Axial Angle. — (3 — 105° 20'. 
Forms Observed.-—-b {010}, c{001},jo{ll0}, q {Oil}, ?’'{201} , and (/{Til}. 
Habit. —More or less tabular parallel to c {001}, the commonest type being 
represented in fig. 3 ; occasionally the tables become so thick as to affect a short 
prismatic habit parallel to p {110}. 
The q {011} faces were relatively more prominent on the crystals of this salt, and 
the c{00l} faces correspondingly less so, than in the case of the crystals of the 
potassium salt. The c faces were generally, however, still to some extent the 
larger. The faces of the primary prism p{ll0} were invariably predominant. 
Several of the crops consisted of crystals showing good little faces of the clino- 
pinakoid b {010}, and very narrow faces of (/{Ill} on each side of the small faces 
of r'{20l}, which latter were shown by most of the crystals of all the crops 
prepared. 
Cleavage .—There is a good cleavage parallel to r'{20l}. No trace of any other 
cleavage was observed. 
