XX CONTENTS. 



. 133. The Tetrahedral Trigonal-icositetrahedron, the 

 Trigrammic Tetragonal-icositetrahedron, and 

 the Pentagonal-icositetrahedron , 140 



. 134. The Tetrahedral Pentagonal-dodecahedrons 142 



General Ideas of Simple Forms. 



. 135. System of Crystallisation . . 145 



. 136. Series of Crystallisation . . 146 



. 137. The System of Crystallisation determined from a 



single Form . . .147 



III. OF COMBINATIONS. 

 Of Combinations in general. 



. 138. Definition . . . 148 



. 139. First Law of Combination . .150 



. 140. Second Law of Combination .' 151 



. 141. Symmetry of Combinations . . 152 



. 142. Edges of Combination . . 153 



. 143. Developement of Combinations . 153 



. 144. Magnitude of the Edges of Combination 158 



Of the Combinations of the different Systems of Crystallisation. 



. 145. llhombohedral Combinations 160 



. 146. Di-rhombohedral Combinations . '173 

 . 147. Hemi-rhombohedral and Hemi-di-rhombohedral 



Combinations . . 174 

 . 148. Developement of llhombohedral and Di-rhombo- 

 hedral Combinations . .179 

 . 149. Pyramidal Combinations . . 183 

 . 150. Hemi-pyramidal Combinations . 191 

 . 151. Developement of Pyramidal Combinations 193 

 , 152. Prismatic Combinations . . 195 

 , 153. Hemi-prismatic Combinations . .199 

 . 154. Tetarto-prismatic Combinations . 201 

 . 155. Developement of Prismatic Combinations 203 

 . 156. Tessular Combinations . 205 

 .157- Semi-tessular Combinations . 207 



