Mr, Greville on the 
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talline laminae. It ; s indeed much less difficult to separate a 
portion of the crystal at these angles, than at any other part ; 
and, in following the natural direction of the faces, with a little 
patience and dexterity, all the crystalline laminae may be de- 
tached, and progressively increase the size of the triangular face. 
This operation, however, cannot be done indiscriminately on 
all the solid angles of the crystals, but only on the alternate 
ones at the same extremity, and in a contrary direction to each 
other. As to the other angles, they may be broken, but it is 
impossible to detach them. When, instead of the solid angles 
of a hexaedral prism, small triangular planes are met with, 
(which frequently happens, whether caused by violence or 
otherwise,) they are always placed in the direction above- 
mentioned. 
If, by following this indication of nature, we continue to 
detach the crystalline laminae, we shall at last cause the form 
of the hexaedral prism to disappear totally, and, in place of it, 
arhomboidal parallelopiped will be obtained, (fig. 2.) of which 
the plane angles at the rhombs will be 86° and 94 0 ; the solid 
angles at the summit* will measure 84° 31'; and that taken at 
the re-union of the bases will be 95 0 2c/. 
We can split this parallelopiped only in a direction parallel 
to its faces ; it will still consequently preserve the same form, 
which is that of the nucleus of this substance, and its primitive 
form. 
It is, therefore, by a modification of the rhomboidal parallelo- 
* For greater clearness, this rhomboidal parallelopiped may be considered as being 
formed by the junction of two triedral pyramids, base to base; and the two solid angles 
(each of which is formed by the re-union of three of the acute angles on the planes of 
the rhomb) will then be considered as the summits of these pyramids. 
