48 * 
Mr . Greville on the 
When the lamina of the Corundum crystal have, during 
their superposition on the planes of the primitive rhomboidal 
paralleiopiped, experienced a progressive decrease at one of 
their acute angles, and along the sides of the other, at the same 
time, and in the same proportion, it is easy to conceive that 
the height of the hexaedral prism must be the same as that of 
the rhomboidal paralleiopiped, upon which it has been formed. 
The height BC (fig. 1.) must therefore bear the same propor- 
tion to the line AB, drawn through the middle of the two op- 
posite sides of the planes on the extremities, as the whole 
height EF, of the rhomboidal paralleiopiped, (fig. 2.) bears to 
the small diagonal GH, from one of the rhombs ; that is, 
nearly as 6 , 45 : 5. 
But, although this exact proportion appears in a very great 
number of Corundum crystals, yet we meet with some whose 
lengths are more or less considerable; and this is owing to 
different circumstances which have existed at the time of their 
crystallization. We may conceive, for instance, that if, before 
the progressive decrease of the crystalline laminae, in the man- 
ner abovementioned, the increase of the rhomboidal paralle- 
lepiped had taken place by a superposition of laminae, in 
which the rows of crystalline molecules experienced a pro- 
gressive decrease along the edges of the acute angle of the 
base only, (fig. 6 .) and that (the sides of the prism having al- 
ready acquired a certain length) the succeeding crystalline 
laminae had experienced a decrease at the acute angle of the 
summit,, the same regular hexaedral prism would have resulted 
from this process ; but the proportion between the height and 
the line drawn from two of the opposite sides of the planes on 
the extremities, would have been much greater than that of 
