168 TERMINOLOGY. . 145. 



two forms, contains P + co (4.), the faces of the rhombohe- 

 dron retain their rhombic figure ; and this may be con- 

 sidered as a proof of the existence of the above mentioned 

 relations. This, however, follows immediately from the 

 derivation (. 117.). Ex. R (s) and P (P, z) in rhombohe- 

 dral Quartz. Vol. II. Fig. 145. 



2. Let n' be = n + 1. The combination is R + n. 

 P + n + 1. The faces of the rhombohedron appear instead 

 of the alternating terminal edges of the pyramid, and the 

 edges of combination are parallel to each other, to the al- 

 ternating terminal edges of the pyramid, and to the inclined 

 diagonals of the rhomhohedron. Ex. R (P) and P + I (/) 

 in rhombohedral Corundum. Vol. II. Fig. 121. From 

 this parallelism again follows the equal inclination upon 

 the axis of the alternating terminal edges in the pyramid, 

 and of the diagonals in the rhombohedron, and from this 

 the relations existing among the axes of the two forms. 



3. n or n' = co produces horizontal edges of combina- 

 tion (i. 3.). 



4. Let n' be == + co , the combination R + n. P + ce. 

 The regular six-sided prism P + co is the limit of the 

 series of isosceles six-sided pyramids (. 118.), and its faces 

 appear with parallel edges of combination in the place of the 

 lateral edges of R, + n, or the faces of the rhombohedron 

 terminate the regular six-sided prism, their figure being that 

 of a rhomb. Ex. R + 1 (r) and P + co (*) in rhombohe- 

 dral Emerald-malachite. Vol. II. Fig. 118. This rhombic 

 figure of the faces, or the situation of the edges in general, 

 affords the means of distinguishing a combination of R + n 

 and P + co from a combination of R + n and It + co 

 (i. 4.). The difference depends upon the position of P + 03 

 in respect to R + n, and P + n (. 118.). 



5. Let n be = + co . The combination is R + n. P + n'. 

 The faces of R + co appear as rhombs in place of the late- 

 ral angles of P -f n'. This figure by itself is a sufficient 

 character for distinguishing the prism R + co in this com- 

 bination from P + co . 



Q. If n and n' at the same time are = + ce, the com- 



