420 
DR. A. E. H. TUTTON ON THE 
Molecular Volume.- — ^ ^ = 233'21. 
d 3’048 
Molecidar Distance Ratios (topic axial ratios).— 
x : f : oo = 6-3847 : 87366 : 4\3499. 
Optics. 
Nature and Orientation of Optical Ellipsoid. —The symmetry plane b {010} is 
the plane of the optic axes. The double refraction is positive, so that the first 
median line corresponds to y and the second median line to a. Extinction determina¬ 
tions with two section-plates ground parallel to the symmetry plane afforded the 
following positions for one of the extinction directions which lies nearly normal to 
the basal plane c {001} :— 
Extinction Direction in the Symmetry Plane. 
Plate 1 .... 4° 47', Plate 2. . . . 5° 16', 
Mean . . 5° 2' with normal to c { 001 } . 
The direction is in front of the normal, further away than the latter from the 
vertical axis c; it is the second median line and corresponds to the refractive index a. 
As the angle between the normal to c {001} and the vertical axis is 16° 2' (ac = 73° 58'), 
the angle between the vertical axis c and the second median line is 21° 4' in front of 
the axis. The first median line lies 5° 2' below the inclined axis a, and thus lies in 
9-0 
the acute crystallographic axial angle ac, while the second median line lies in the 
obtuse angle /3 between the crystal axes a and c lying in the symmetry plane. The 
conditions are graphically shown in fig. 7. 
