PRIMARY FORMS. 



. 46. THE RHOMBOID.* 



The Rhomboid is a solid, contained under six equal 

 rhombic planes. 



faces are unequal. Fig. 37, illustrates this kind of the doubly oblique 

 Prism : the vertical projection is perpendicular, so that the transverse 

 plane xyv z is represented by a straight line. We observe in this in- 

 stance, that those portions of the lateral edges which are above this plane 

 are all unequal. The inclination of iaeo upon the two lateral planes 



Fig;. 37. 



Fig. 38. 



erso and amre is different, and less in either case, than the inclination 

 of that base to the prismatic axis. This form is found in the Axinite and 

 in Sulphate of Copper. In the other case, arising out of the different 

 method in which the bases are disposed, the position is such, that either 

 of them, as ae oi, Fig. 38, forms with two opposite parallel lateral faces, 

 aerm and i o s n, two angles, obtuse and acute, equal to those which it 

 forms with the prismatic axis, or, what is the same thing, with the edges. 

 We may recognise this position in the figure by the equality of the upper 

 portions of the adjacent edges, which takes place two and two, a x=e z, 

 i y=o v. In the particular instance represented by the figure, and which 

 belongs to Feldspar, the section x y v z of the prism is rectangular ; then 

 the face erso, and its parallel, are perpendicular to the base. 



* Weiss has proposed the name Rhombohedron for this solid, in analo- 

 gy with Tetrahedron and Octahedron. 



3* 



