TERMINOLOGY. 



primary form a Cube ; if a dodecahedron with scalene triangular 

 faces, it can come only from the Rhomboid. 



Where one of these new forms is capable of being derived from 

 several primary forms, the student should seek directly for the 

 cleavages, which will at once settle the question. 



2. Crystals not possessed of Cleavage. 



Among crystals of this kind, which are not numerous, the student 

 will be liable to experience occasional embarrassments. In general, 

 he should endeavor to procure several forms of the same substance ; 



to each other; and to the right rhombic Prism, if these planes incline to 

 each other alternately, at an acute and an obtuse angle. Again, we will 

 suppose similar oblique planes to belong only to the opposite terminal 

 edges, and the vertical planes to be at right angles to each other ; the 

 primary form, in this case, is a right rectangular Prisrn. 



Let us suppose a crystal to be contained within any series of vertical 

 planes, and to be terminated, not by a horizontal plane, but by a single 

 oblique plane ; it will belong to the oblique rhombic Prism, the doubly 

 oblique Prism, or the Rhomboid. 



If, instead of one oblique plane, there be four, inclining to each other 

 at equal angles, the crystal may belong to the right square Prism, or to 

 the Octahedron with a square base, if there be four oblique planes, 

 each of which inclines on two adjacent planes, at unequal angles, the 

 crystal will belong either to the right rectangular Prism or right rhom- 

 bic Prism, or to the Octahedron with rectangular or rhombic bases. 



If the series of vertical planes consist of twelve, eighteen or twenty 

 four, and if there be a single horizontal plane, the crystal will probably 

 belong to the hexagonal Prism. If there should be six, nine, twelve, 

 or some other multiple of three, and three oblique planes, the primary 

 form is the Rhomboid. 



It is by thus noticing the symmetrical arrangement of the vertical 

 planes, or of the oblique planes, if there be any, that we are able to re- 

 fer a complicated crystal to one of the primary solids. 



Crystals belonging to the doubly oblique Prism are among the most 

 difficult to be understood; and the student, in examining them, as well 

 as very irregular crystals of other forms, will generally apply directly to 

 the cleavage, the knowledge of which, though it be but in one direction, 

 will often be sufficient to enable him to distinguish the primary planes. 



