223 
mriety of the Iodide of Potassium, 
original surface, and hence the gradual conversion of the 
replacing triangles into rhombs, which effacing the terminal 
plane form^ed & 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 : T73. 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 rhomboidal plane, on the adjacent side, 
was 140°. 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- 
texmal 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 solubility 
they were the same as the common iodide of potassium, with 
which their composition likewise identified them. From their 
