202 
DR. A. E. H. TUTTON ON THE 
The values of the principal angles measured by vom Rath are quoted in the last 
column of the table of angles. 
Cleavage .—There is a good cleavage parallel to r'{201}. The cleaved surfaces gave 
excellent signal images at exactly 180° from the r’ face left on each fragment. 
Volume. 
Relative Density .—Five determinations of specific gravity by the immersion 
method, using methylene iodide and benzene as the immersion liquid, gave the 
following results :— 
I. 
Density for 16°‘3/4° 
. . . 2'5578 For 2074° . . 
. 2-5569 
II. 
17°'l/4° 
. . . 2-5586 
2074° . . 
. 2-5579 
III. 
17°-6/4° 
. . . 2-5618 
20°/4° . . 
. 2-5612 
IV. 
,, l7°-9/4° 
. . . 2-5619 
2074° . . 
. 2'5614 
V. 
„ 18°-2/4° 
. . . 2-5591 
2074 o . . 
. 2-5586 
Mean . 
. 2-5592 
The value accepted for 20°/4° 
is therefore 2"559. 
Von Groth (‘ Chemische Krystallographie,’ vol. II, 
p. 545) quotes 
a value, 2"539, 
of Tops0e and Christiansen for the specific gravity of this salt ; the author is unable 
to find it in the 1874 memoir of these authors (which deals with the optics), but it 
is given by Tops0e in his dissertation of 1870. # 
Molecular Volume .— ^ = 206T4. 
d 2‘559 
Molecular Distance Ratios (topic axial ratios ).— 
x : yjr : « = 61677 : 8’2598 : 41786. 
Optics. 
Orientation and Nature of the Optical Ellipsoid. —The plane of the optic axes is the 
plane of symmetry b {0 L 0}. The sign of the double refraction is positive. The first 
median line, therefore, corresponds to the refractive index y and the second median 
line to the index a. 
The orientation of the two median lines in the plane of symmetry was determined 
with two section-plates ground parallel to that plane with the aid of the cutting and 
grinding goniometer. The following results were obtained. 
Extinction Direction in the Symmetry Plane. 
Plate 1 . . . . 7° 15', Plate 2 .... 7° 40', 
Mean . . . 7° 28' with normal to c { 001 }. 
The position of this extinction direction is behind the normal to c {001}, and it is 
* ‘ Krystallogr.-kem. Unders. over de selensure Salte,’ Copenhagen, 1870, p. 10. 
