52 TERMINOLOGY. . 62. 63. 



4. There are two known varieties of these dodecahe- 

 drons, whose dimensions are as follows : 



a. b. A. B. 



1. 117 2' 8". 31 28' f>6". 109 28' 16". 146 26' 33". 



2. 112 53' 7". 33 33' 261". 129 31' 16". 129 31' 16". 



5. Of the first variety of this form we have examples in 

 tetrahedral Copper-glance ; of the second variety, in dode- 

 eahedral Garnet-blende. 



. 62. TETRAGONAL-DODECAHEDRONS. 



The Tetragonal-dodecahedrons are contained 

 under equal and similar tetragons. 



1. There are two kinds of these forms. 



2. Of the faces of the one, two are always parallel to 

 each other, and they contain two pairs of equal angles. Of 

 the faces of the other, no two faces are parallel, and they 

 contain only one pair of equal angles, the remaining two 

 being also different betwixt themselves. All the edges of 

 the former are equal, while the latter possess two kinds of 

 different edges. 



3. From this last mentioned difference, the denomina- 

 tions of the two kinds are derived ; the first containing the 

 monogrammic, the second the digrammic Tetragonal-dodeca- 

 hedrons. 



. 63. THE MONOGRAMMIC TETRAGONAL-DODECA- 

 HEDRON. 



The faces of the monogrammic Tetragonal-do- 

 decahedron, or of the Dodecahedron, Fig. 3L, are 

 rhombs. 



* The small letters a, b, signify the plane angles of the 

 faces, and the large ones A, B, the angles of the incidence 

 at the edges, as referring to the figures. 



