50 MINERALOGY. 
The axes in this system are at right angles to each other, but all of 
unequal length. The fundamental form is: 
1. The Rhombic Octahedron. Ifwe suppose the square base of the preceding 
system to become oblique, a rhomb will be produced. If we suppose the 
lateral axes to occupy the place of diagonals to this rhomb, one will connect 
the acute and the other the obtuse angles, and while at right angles to each 
other, will be of unequal lengths. The vertical axis, passing through the - 
point of intersection, and perpendicular to their plane, is either shorter than 
the lateral, in which case we have the obtuse rhombic octahedron 
(pl. 32, fig. 477), or else more elongated, producing the acute rhombic 
octahedron (pl. 32, fig. 47°). 
Since all three axes are unequal, it is a matter of indifference which we 
take as the principal or vertical axis. Having assumed one, those two 
corners alone have equal crystallographical value which lie on one ahd the 
same axis. Any two such corners may, for instance, be bevelled (jig. 48), 
without this modification needing to be extended to the other corners. The 
general shape of this form is that of the square octahedron, only the base 
here is rhomboidal. Thus the twelve edges of this octahedron are different 
in the whole of three different kinds. Derivative forms from this are : 
2. The rectangular prism (fig. 56). It is produced by passing planes 
through the rhombic octahedron at right angles to the axes. Supposing these 
to increase until they intersect and truncate the two extremities of the 
primary axis, we shall obtain this figure. Sharpening the lateral faces of this 
prism, we shall obtain an octahedron (fig. 47°), whose base is a rectangle. 
Figs. 44, 45, 57, and 58, represent modifications of this octahedron. 
3. The right rhombic prism, terminated by the faces of the rhombic 
octahedron (fig.69). Truncating the four horizontal basal edges of the 
rhombic octahedron by planes parallel to the vertical axis, and letting them 
enlarge until they intersect, we shall have this figure. It may be considered 
as the passage of the rhombic octahedron to the following. 
4. The right rhombic prism (fig. 50). This is produced by truncating 
the two solid angles of the vertical axis, and letting the new faces thus 
formed enlarge until they intersect the faces truncating the basal edges of 
the last figure, these also being supposed to be extended. The figure thus 
formed will be bounded by two rhombs and four rectangles. 
5. The irregular six-sided prism ( fig. 51) is formed from the preceding 
by truncating the two obtuse or the two acute lateral edges. 
6. The irregular eight-sided prism ( fig. 52) is produced when the acute 
and obtuse lateral edges are so replaced by faces as still to leave the original 
lateral faces. 7 
7. Figs. 53, 54, and 55, present some combinations of prisms and octahe- 
drons, belonging to this same system, and frequently observed in certain well 
known salts. Figs. 53 and 54 are erystals of saltpetre. Fig. 55 often 
occurs in sulphate of tin. 
IV. The Monoclinic System, Dana. Das zwei-und-eingliederige System, 
Weiss. Heminorthotype, Mohs. Monoclinohedric, Naumann. 
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