. 50. OF SIMPLE FORMS IN PARTICULAR. 41 



only one, or because all tetrahedrons are similar ; a form 

 contained under six faces, is the Hexahedron ; a form con- 

 tained under eight faces, is the Octahedron ; a form con- 

 tained under twelve faces, is a Dodecahedron^ because there 

 are several.varieties, or because not all dodecahedrons are 

 similar ; a form contained under twenty-four faces is an 

 Icositetrahedron ; and a form contained under forty-eight 

 faces, a Tetracontaoctahcdron. 



The denominations which denote the different kinds and 

 varieties of simple forms, according to their peculiar pro 

 perties, are formed from these names by composition, or by 

 the addition of adjectives. 



II. OF SIMPLE FORMS IN PARTICULAR. 



CONSIDERATION OF SIMPLE FORMS, AND SOME OF THEItt 

 GEOMETRICAL RELATIONS. 



. 50. THE RHOMBOHEDRON. 



The rhombohedron. Fig. 7., is a form contained 

 under six equal and similar rhombic faces ; or the 

 rhombohedron is contained under six equal and si- 

 milar rhombs. 



1. Any six rhombs, which are equal and similar to each 

 other, limit one, and if the obtuse angle of their figure is 

 less than 120, two rhombohedrons. 



2. All rhombohedrons belong to the same kind of forms. 



3. The solid angles A, X, produced by equal plane angles 

 and equal edges (. 33.) of the rhombohedron, are termed 

 its Apices. 



4. The straight line AX through the apices, is the Axis of 

 the rhombohedron. The Rhombohedron has only one axis, 

 and this is of the first kind (. 40.). Since all forms con- 

 nected with the rhombohedron possess axes of this kind, 

 these in future will be designated by the denomination of 

 rhombohcdral Axes. 



