MONOCLINIC DOUBLE SELENATES OF THE NICKEL GROUP. 
213 
of practically all the measured angles the agreement between the observed and cal¬ 
culated values lies within two minutes, as will be seen from the table. The habit 
defined above is essentially characteristic of the salt, the vast majority of the crystals in 
all the 30 crops grown consisting of crystals of this type. Usually also only the forms 
g{01l} and c {001}, forming the prism, and smaller faces of p (l 10}, forming the 
ends, are visible. Closer inspection, however, reveals small faces of r' {201}, still 
more minute ones of o' {ill}, and occasionally traces of the primary orthopinakoid 
a {100}, at the ends ; while the sharper long edges of the prism q {Oil} are blunted 
by long but very narrow faces, often mere lines, of the clinopinakoid b {010}. The 
signal images from the q andp faces were wonderfully sharply defined and perfect on 
all the crystals measured, and on most of these crystals, selected for their perfection, 
the other faces just referred to were somewhat more prominently developed than 
usual, and gave excellent reflections. 
Some larger crystals were obtained in several of the crops which were somewhat 
tabular parallel to c{00l}, owing to the faces of the basal plane being more 
extensively developed ; the faces of r'{201} were also frequently relatively larger on 
these crystals. 
Cleavage .—A good cleavage is developed parallel to r'{20l}. There is no facility 
for cleavage parallel to any other direction. 
Volume. 
Relative Density .—Five determinations by the immersion method gave the following 
results :— 
I. Density for 16°"2/4° 
. 3-1137 For 20°/4° . . 
. 3-1125 
II. „ 16°-3/4° . . 
. 3-1130 „ 2074° . . 
. 3-1118 
o 
CO 
o 
CO 
i— l 
HH 
HH 
HH 
. 3-1186 „ 2074 o . . 
. 3-1175 
IV. „ 17°-0/4° . . 
. 3-1149 ,, 2074 °. . . 
. 3-1140 
V. „ 17-1/4" . . 
. 31134. „ 2074 o . . 
. 3-1125 
Mean . . 
. 3-1137 
The value accepted for 20 /4° is 3T14. 
The specific gravity of caesium 
nickel selenate does not appear 
to have been 
previously determined. 
Molecular Volume. M - 713 ’ 62 
- 229-17. 
d 3-114 
Molecular Distance Ratios (topic 
ixial ratios ).— 
x : V- : to = 6‘3317 : 8-6878 : F3378. 
