TERMINOLOGY. . 179- 



hedrons of the form R + n join in their own faces, so 

 that their axes include an angle. They are brought into 

 this situation by supposing the axis of revolution to be 

 perpendicular, the face of composition parallel, to one of 

 the faces of R + ri. Of this composition, the sign is 

 R, {!} The law of composition remaining the same, 

 we may substitute in the place of R + n, any one of the 

 forms R + n + 1, R -f n 1, R + co, &c. ; in short, every 

 member of the series of crystallisation derived from R + n, 

 and every combination which they possibly may produce. 

 On the other hand, we may likewise alter the situation of 

 the face in which the individuals join ; and, provided it 

 does not coincide with R co, the resultant twin-crystals 

 still belong to the present section. Nature produces 

 many examples of this law ; for instance, in rhombohedral 

 Lime-haloide, R 1 composed in the face of R, of which 

 the crystallographic sign is R 1,|||; the combina- 

 tion R cc. R + CD, composed in the face of R, Vol. II. 

 Fig. 132., or in the face of R 1, Vol. II. Fig. 133., the crys- 

 tallographic sign of the former being R co. R + co,{ ! J} ; 

 that of the latter, R co. 11 + co,{?iZll}; ft, j n t h e 



face of R 1, designated by R { - }. Vol. II. Fig. 130.; 

 (P) 3 , in the face of R + 1, which is designated by 



(P)S{1} ; &C . 



The present law is not confined to the rhombohedral 

 system. The pyramid P in pyramidal Copper-pyrites, if 

 composed in one of the faces of P, produces a twin -crystal, 

 very much resembling that of the octahedron in octahedral 

 Iron-ore, but differing from it, in as much as the axis 

 of revolution in pyramidal Copper-pyrites, is not at the 

 same time an axis of crystallisation. Its sign is P, { ? } . In 

 the common twin-crystals of pyramidal Tin-ore two in- 

 dividuals of the crystalline form P + 1, or P + 1. [P + co], 

 are joined in a face of P ; the axis of revolution is 

 perpendicular to this face, and is not an axis of crystallisa- 





