the Corundum Stone , and its Varieties , &c. 259 
of the solid angle of the summit, a decrease much more rapid 
than that we have already mentioned, when speaking of the 
first modification. This decrease replaces the solid angle by 
three new planes ; which planes, if they were to become of 
such extent as to cause the primitive faces of the rhomboid to 
disappear, would occasion a secondary obtuse rhomboid, that 
would have considerable analogy, in the measure of its angles, 
with that rhomboid of calcareous spar which is called lenti- 
cular; that is to say, the solid angle of its summit would 
measure about 139 0 ; and the plane angles of its rhombs 114 0 
and 66 °. I have not yet met with this rhomboid perfectly 
formed ; but it exists, or at least one of its halves, in a very 
well defined state, at the summit of a simple pyramid, eight or 
nine lines in height, the solid angle of which summit measures 
12 0 ; it is represented in Fig. 27. The great number of striae, 
parallel to the small diagonals of the primitive rhombic planes, 
with which the faces of the secondary rhomboid are covered, 
prevent me from being perfectly certain respecting the accuracy 
of the measures I have just stated ; but, if they are not strictly 
exact, they must at least be very nearly so. The crystal I have 
just described is from the coast of Malabar, and is in Mr. 
Greville's collection. The planes of the secondary rhomboid 
are slightly chatoyant . 
Fifth Modification. Another mode of decrease, of a similar 
kind, but still more rapid, sometimes takes place at the same 
solid angle of the summit of the primitive rhomboid. The 
triedral pyramid which replaces this angle, is then much less 
elevated than in the preceding modification. When it is com- 
plete, that is to say, when there remains no trace of the planes 
of the primitive rhomboid, the crystal becomes changed into a, 
1 . 
