43^ Mr. Greville on the 
Although the rhomboidal parallelopiped of 86° and 94 0 is 
the primitive form of the Corundum crystal, yet it is rare to 
meet with that substance under this perfect and determined 
form ; and, in most mineral substances, it is more rare to meet 
with their primitive crystals than their different modifications. 
Amongst Mr. Greville’s numerous specimens of Corundum, 
I have met with only one which has this primitive form, and it 
is doubtful whether even this may not be a fragment. 
The Corundum crystal presents another modification, under 
which the regular hexaedral prism, instead of having three al- 
ternate solid angles at each of its ends, (on which solid angles 
are placed isosceles triangular planes, forming a solid angle of 
122 0 34', with the planes at the extremities upon which they 
are inclined,) has also its angles supplied by isosceles triangu- 
lar planes; but these planes, instead of 122 0 34b form solid 
angles of 160 0 42', with the said planes on the extremities. 
(See fig. 11. and 12.) These new planes, which constitute a 
new modification of the primitive form of Corundum, are the 
result of a different order in the decrease of the laminae; which, 
in the primitive form, are deposited on the planes of its pri- 
mitive rhomboid by single rows of crystalline molecules, and 
increase the planes which terminate the hexaedron : whereas, 
in this second modification, the decrease of molecules is by two 
rows, which gives a more obtuse inclination, and forms new 
planes. The surface is usually striated, parallel to the sides of 
the planes which terminate this crystal ; an appearance always 
announcing imperfection in the crystallization, arising either 
from a change in the order of decrease or increase, or from a less 
perfect union of the crystalline lamina*. A section would show 
gradual risings or steps, as appears in fig. 14. which is a section 
