54 



TERMINOLOGY. 



tangent replacement of its lateral edges, or an octahedron by the 

 truncation of its terminal edges or solid angles. 



The right rectangular Prism will afford a rhombic prism by the 

 replacement of its lateral edges by single planes, or an Octahedron 

 with a rectangular base, by the truncation of its terminal edges. 



The right rhombic Prism may pass into an Octahedron with a 

 square base, through the replacement of its obtuse solid angles by 

 single planes, which intersect the terminal plane parallel to its 

 greater diagonal, (Fig. 88) ; portions of the primary lateral planes 

 M, M' still remain. Fig. 128. 



The same new solid, Fig. 129> may also result from a similar re- 

 placement of the acute solid angles, (Fig. 89) ; but it is reversed in, 

 its position, when compared with the former one. 



Fig. 128, 



Fig. 129. 



It passes into Octahedrons with rhombic bases, through the re- 

 placement of its terminal edges by single planes. 



Crystals of Heavy Spar illustrate these transitions. 



The right oblique angled Prism does not give rise to either of the 

 other primary forms, in the modifications it undergoes among crystals. 



The oblique rhombic Prism gives origin to an oblique, six sided 

 prism, by the tangent replacement of the acute edges of the prism, 

 (Fig. 95,) as in the crystals of Mica. 



The doubly oblique Prism does not pass by its modifications into 

 either of the geometrical solids. 



The regular six sided Prism, as has been already noticed, passes 

 into the Dodecahedron with isosceles triangular faces, (Fig. 118,) by 

 the replacement of its terminal edges by similar planes. (Fig. 116.) 



The Rhomboid passes into a Rhomboid more obtuse than the 

 primary, when its superior edges are replaced by tangent planes, 

 (Fig. 109.) 



The replacement of the lateral edges, (Fig. 108,) or angles, Fig, 

 ISO, by tangent planes, produces a regular six sidecj Prism, 





