TERMINOLOGY. 



when complete, is bounded by twenty four isosceles triangular 

 planes, and is represented in Fig. 124. 



Fig. J23. Fig. 124. 



Fig. 125. 



It is produced from the regular Octahedron, also, from the modifi- 

 cation of its solid angles by four planes resting on the primary edges; 

 also, from the rhombic Dodecahedron by the replacement of its acute 

 solid angles by four planes resting on the primary planes. 



The replacement of the edges of the Octahedron by two planes, 

 leads to the erection of a trihedral pyramid on each primary plane, 

 as represented in Fig. 63. When the 

 primary planes P become extinct, we 

 have another isosceles triangular solid, 

 under twenty four faces, as in Fig. 125. 



This form is produced in like mariner 

 from the Cube, by the replacement of 

 its solid angles by three planes resting 

 on the edges of the Cube ; and from the 

 rhombic Dodecahedron, by the modifi- 

 cation of its obtuse solid angles by three 

 planes resting on the primary planes. 



Another new form,, still more fre- 

 quently met with among crystals, is 

 produced from the Cube, by the re- 

 placement of its solid angles by three 

 planes resting on the primary planes. 

 (Fig. 59.) The new form is denomina- 

 ted the Trapezohedron, and is contained 

 under twenty four equal trapezoids.* 

 Fig. 126. 



* A trapezoid is a four aided figure whose opposite edges are unequal, 

 and in which, if lines be drawn through the opposite angles, they will 

 intersect each other at right angles. 



