OF THE DOUBLE SELENATES OF THE SEFJES EAI(SeOj.,GH,0. 
203 
A typical crystal of this salt is represented later in fig. 2 (p. 274). 
The common cleavage of the series is well marked in this salt, namely, parallel 
r'(201[. 
Volume. 
Relative Density. —The following results were obtained from four determinations 
with independent material :— 
AVeight of salt 
employed. 
Sp. gr. at 
2074 °. 
6-4573 
2-6803 4 
6-1688 
2-6808 1 
6-9637 
y 
2-6806 1 
6-7175 
2-6803 J 
Mean, 2-6805 
Molecular Volume.—RL/d = 588 A- 2-6805 = 219-36. 
Distance Ratios. —-The following are the values of these ratios, obtained by coml)i- 
nation of the aliove molecular volume with the axial ratios and axial angle already 
given :— 
X -. i// : w = 6-3001 ; 8-4861 : 4-2524. 
Optics. 
Orientation- of Axes of Optical Ellipsoid. —The plane of the optic axes (bi-normals) 
is the symmetry plane. 
The sign of the doidjle refraction is positive. 
The extinction angle was found to l;)e as follows, with the aid of two excellent 
section plates ground parallel to the plane of symmetry :— 
Section 1 , . 1° 3'; Section 2 . . 1° 18'; Mean . . 1° 10'. 
The direction is in front of the normal, that is, nearer to the inclined axis a. This 
extinction direction is the second median line. The first median line lies in the 
acute morphological axial angle ac, and is inclined 1° 10' to tlie axis a. The second 
median line is inclined 16° 24' to the vertical axis c. 
Refractive Indices. —Six prisms, ground on separate crystals derived from four 
dilferent crops, were employed in the determinations, which afforded the following 
results:— 
