414 
DR. A. E. H. TUTTON ON THE 
n'{l2\] were also shown, but the reflections of the signal were too faint for accurate 
allocation to the spider-lines. 
The faces of the forms p {110} and c {001} on the crystals of this salt were more 
than usually affected by striation, and only two crops were obtained sufficiently free 
from the defect for the purpose of the measurements. Even the 10 apparently 
excellent crystals selected from these crops have afforded results which show in the 
cases of certain angles greater differences between the calculated and observed angles 
than usual, compared with the salts of the series previously measured. The differences, 
however, are not sufficiently great to affect any of the conclusions drawn from the 
comparison of the results with those for the other members of the iron group of 
double selenates. 
A table of angles is given on the previous page. 
Cleavage. —There is a good cleavage developed parallel to ? y {20l}. 
Relative Density .—Six determinations by the 
immersion method 
gave the following 
results:— 
I. Density for 16°"6/4° 
. 2-8034 
For 2074° . 
. . 2-8025 
II. „ 16°-2/4° . . 
. 2-7986 
„ 2074 o . 
. . 2-7975 
III. * „ 16°-4/4° . . 
. 2-8018 
„ 2074° . 
. . 2-8008 
IV. „ 16°T/4° . . 
. 27985 
„ 2074 “ . 
. . 2-7974 
V. „ 16°T/4° . . 
. 2-8031 
„ 2074° . 
. . 2-8020 
VI. „ 16°-2/4° . . 
. 2-8002 
„ 2074 o . 
. . 2-7991 
Mean . 
. . 27999 
The value accepted for 20°/4° is 2"800. 
Molecular Volume .-— ^ ^ 
d 2-800 
= 220-29. 
Molecular Distance Ratios (topic axial ratios ).—• 
x :^:w= 6‘3109 : 8-5006 : 4-2503. 
Optics. 
Nature and Orientation of the Optical Ellipsoid. —The plane of the optic axes is 
the symmetry plane b {010}. The double refraction is positive, the first median line 
corresponding to the refractive index y and the second median line to a. Extinction 
determinations with two plates parallel to the symmetry plane gave the following 
positions for one of the extinction directions with respect to the normal to c {001}. 
Extinction Direction in the Symmetry Plane. 
Plate 1 1°44 / , Plate 2 ... . 0° 56', 
Mean ; . 1° 20' with normal to c {001}. 
