. 148. OF COMBINATIONS. 179 



. 148. DEVELOPEMENT OF RHOMBOHEDRAL AND 

 DI-RHOMBOHED11AL COMBINATIONS. 



The developement of the combinations containing 

 three or more simple forms, is founded upon the 

 knowledge of binary combinations. 



A few examples will be the best means of instructing 

 the beginner how to proceed in similar cases. The example 

 chosen for rhombohedral combinations, shews at once the 

 sufficiency of the few particular cases mentioned in . 145. 

 147- of binary combinations for the developement of such 

 as consist of a greater number of simple forms. 



The 59th figure represents a rhombohedral combination 

 (. 145.), consisting of four rhombohedrons, two scalene six- 

 sided pyramids, and a regular six-sided prism. Its inde- 

 terminate designation is 



11 + n. R + n T . R + mi. R -I- n TH . (P + n)<*. 

 a c e d b 



(P + nV)>'. R + co. 

 / g 



The only form immediately determined in this combina- 

 tion, is R + cc . The edges of combination between the 

 faces of this form and those of the rhombohedrons R + n r , 

 R -|- n 11 and R 4- n are horizontal (. 145. i. 4.), whereas 

 P + 03, if it were contained in the combination, would 

 produce edges of combination parallel to the lateral or 

 terminal edges of these rhombohedrons (. 145. iii. 4.). 



Among the rhombohedrons, one must be selected and 

 fixed upon as the fundamental form, and the letter n in 

 its sign therefore, must be made = 0. The figure repre- 

 sents a crystal of rhombohedral Lime-haloide, which mine- 

 ral is cleavable (. 162.), parallel to the faces of the rhom- 

 bohedron here designated by R -f- n 1 = 105 5'. According 

 to this we determine the rhombohedron R + n T to be the 



