the Corundum Stone, and its Varieties, &c. 263 
of perfect red corundum, or oriental ruby, is almost exactly 
similar to that represented in Fig. 22, and indeed only differs 
from it by the prism being dodecaedral, as in Fig. 32. 
Eighth Modification. I am also acquainted with this modifica- 
tion only by a s single crystal. This crystal, which is a sapphire 
of a beautiful deep blue colour, is likewise in Mr. Greville’s 
collection. Its form is a simple hexaedral pyramid, which is 
almost complete, and has 24 0 for the measure of the solid angle 
of its summit Each of its six edges are replaced by a very 
narrow plane, which is equally inclined upon the two faces that 
are adjacent to it. This renders the pyramid dodecaedral, with 
broad and narrow faces alternately, as in Fig. 33. Three of 
these new planes appear to me to be occasioned by a decrease, 
which has taken place at the obtuse plane angles that rest 
upon the base of the rhomboid, but which differs from those 
which occasion pyramidal modifications, and is of such a nature 
that (the new planes to which it gives rise being in pairs, and 
on the same level, ) each of the solid angles of the base is re- 
placed only by a single plane. The three others appear to 
me to be caused by a decrease at the acute plane angles that 
rest upon the base ; but this decrease differs from that of the 
seventh modification, in being more rapid, and in having the 
planes to which it gives rise inclined upon the axis of the crys- 
tal. The three latter planes have the following peculiarity, viz. 
their inclination is exactly equal to that of the three others ; so 
that, if the two modifications which are united together in this 
crystal were complete and separate, they would produce two 
acute rhomboids, perfectly similar to each other. 
Mm2 
