variety of the Iodide of Potassium. 223 



original surface, and hence the gradual conversion of the 

 replacing triangles into rhombs, which effacing the terminal 

 plane formed the pyramidal summit already noticed. 



The prisms are straight, with square bases. The replacing 

 triangular or rhomboidal planes form with the lateral edges, 

 to which it is inclined, an angle of 150°, and with the terminal 

 plane an angle of 120° : its angles with the vertical and 

 horizontal axes of the crystal being therefore 60° and 30°, 

 giving the ratio of the axes therefore as 1 : 1'73. The angle 

 formed by two adjacent rhomboidal planes of the pyramid 

 was found to be 105°, and that of the summit formed by two 

 opposite rhomboidal planes was 60°. The angle across the 

 summit, measured on the edges of the rhomboidal planes, was 

 80°, and that of a I'homboidal plane, on the adjacent side, 

 was 14'0°. The angles of the rhomboidal plane were 60° 

 and 120°. 



In these measurements I could not obtain greater accuracy 

 than within a degree, from the circumstance that the repla- 

 cing surfaces were not, in reality, planes, but portions of sphe- 

 rical or at least curved surfaces of great radius, so that the 

 adjacent edge had different inclinations to different portions 

 of the rhombic surface. In addition to this peculiarity, 

 other marks of a complex or macled structure were very 

 evident. The smaller crystals, although equally well marked 

 as to form and replacements with the larger, differed from 

 them in being wholly clean and transparent. The larger ones, 

 on the contrary, consisted of three distinct portions, the ex- 

 ternal being a hollow shell of perfectly transparent material, 

 the next being a core of opake white substance, apparently 

 porous and granular, as if formed of a congeries of minute 

 crystals independent of the case in which they were con- 

 tained, whilst in the centre there was to be seen a delicate 

 but well-defined transparent rectangular cross, the arms of 

 which generally penetrated quite through the opake sub- 

 stance and united with the external transparent shell. 



A section of such a crystal had in fact the 

 appearance represented in the little sketch, the 

 white opake portion being shown shaded. 



These crystals possessed single refraction. 

 They had no action on polarized light transmitted 

 along their axis; and hence, although with a pyramid be- 

 longing to the square prismatic system, they belonged really 

 to the regular system, by a congeries of minute crystals (pro- 

 bably cubes) of which they must be formed. In solulbility 

 they were the same as the common iodide of potassium, with 

 which their composition likewise identified them. From their 



