74 M:. A. E. Tutton. A (.' CryttdOograpMeal 



Volume. 



Relative Density. Four determinations with independent material 

 afforded the following results : 



Weight of Sp. gr. 



saltempltm-.l. at 20 y /4. 



5-2958 3-1148 



5-5387 3-1175 



5-1525 3-1126 



5-5317 3-1164 



Mean 3-1153 



Molecular Volume. ^ = 72 *' = 232-40. 



Distance Ratios. Combination of this molecular volume with the 

 axial angle and axial ratios already given, affords the following dis- 

 tance ratios : 



X : ^ : w = 6-3860 : 8'7311 : 4-3402. 



Optics. 



Orientation of Axes of Optical Ellipsoid. The plane of the optic axes 

 (bi-normals) is again the plane of symmetry. The sign of the double 

 refraction is also still positive, as for the potassium and rubidium salts. 



Stauroscopic observations carried out with two section-plates ground 

 parallel to the symmetry plane gave the following extinction angles 

 with reference to the normal to the basal plane : 



Section 1 6 7' 



Section 2 5 26 



Mean 5 46 



The direction is in front of the normal, nearer to the inclined morpho- 

 logical axis a. This axis of the optical indicatrix is the second median 

 line. The first median line is accordingly situated in the acute angle 

 of the morphological axes at, and is inclined 5 46' to the axis a. The 

 second median line lies in the obtuse angle ac } and is inclined 21 5?' 

 to the vertical axis c. 



11 iff active Indices. The results of the refractive index determina- 

 tions with six prisms, ground on crystals from different crops, are 

 presented in the accompanying table. 



The intermediate refractive index of caesium zinc selenate, corrected 

 to a vacuum, is expressed to near F, for any wave-length A, by the 

 following formula : 



i.*i AT _,_ 704 232 2 877 60 

 ft - 1 ol87 + -_ 



