DR. A. E. H. TUTTON ON THE 
208 
Volume. 
Relative Density .—Four determinations by the immersion method yielded the 
following values :— 
I. 
Density 
for 16°'9/4° 
. . 2-8579 
For 20°/4° . 
. . 2-8570 
II. 
33 
l7°-3/4° . . 
. . 2-8577 
„ 20°/4° . 
. . 2-8569 
III. 
3 3 
17°-3/4° . 
. . 2-8551 
„ 20°/4° . 
. . 2-8543 
IV. 
33 
18°'9/4° . 
. . 2-8556 
„ 20°/4° . 
. . 2-8553 
Mean . 
. . 2-8559 
Accepted value for 20 c /4°, 2'856. This appears to be the first determination of the 
specific gravity of rubidium nickel selenate. 
Molecular Volume. — ^ - = 216'96. 
d 2’856 
Molecular Distance Ratios (topic axial ratios ).— 
X : A : « = 6-2533 : 8'4561 : A2542. 
Optics. 
Orientation and Nature of Optical Ellipsoid. —The plane of the optic axes is the 
symmetry plane b {010}. The double refraction is of positive sign, the first median 
line corresponding to the refractive index A and the second median line to a. 
Extinction determinations with two section-plates parallel to the symmetry plane 
afforded the following indications of the direction of one of the two principal axes of 
the optical ellipsoid (the median lines) lying in that plane. 
Extinction Direction in the Symmetry Plane. 
Plate 1 .... 5° 34} Plate 2 . . . . 5° 8', 
Mean . . . 5° 21' with normal to c {001}. 
