M:. A K. Tiittoii. A Comparative Cryf'i//<>t/i'i'}>}iical 



The " ;ni(l l> faces were usually both well defined; the hcmi-pyramid 

 0' was occasionally \\cli drvdnprd, lut more usually small, while small 

 p' faces were only observed on two crystals. The general type exhi- 

 bited p,r,r and q as the main faces, r always large, and ?' occasionally 

 large but generally rather smellier than the q faces. 



Topsoe and Christiansen (loc. cit., p. 77) give for the ratio of the 

 and the axial angle, a : b : c = 0'7441 : 1 : 0*5075, and ft = 

 7~> 46'. The measurements were made by Topsoe in the year 1870. 



There is an excellent cleavage parallel to the faces of r'{201}, as 

 stated by Topsoe and Christiansen. 



Volume. 



Relative Density. The following four independent determinations 

 were made : 



Weight of salt Sp. gr. at 



employed. 2074. 



5-5358 2-5535 



6-1058 . 2-5544 



5-1597 2-5524 



5-0657 2-5546 



Mean 2-5537 



Molecular Volume.^- = -fff^ = 210'13. 

 a 2'5537 



Distance Ratios. Combination of the axial ratios and axial angle 

 previously given, with the molecular volume, affords the following 

 distance ratios : 



X : ^ : (o = 6-1941 : 8'3054 : 4'2133. 



Optics. 



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

 (optic binomials) is the plane of symmetry. The sign of the double 

 refraction is positive. 



Two section-plates ground parallel to the symmetry plane afforded 

 the following extinction angles, relative to the normal to the basal 

 plane : 



Section 1 5 30' 



2... 4 36 



Mean 5 3' 



The direction is behind the normal, nearer to the vertical axis. 



This axis of the optical ellipsoid (indicatrix) is the second median 



