246 TERMINOLOGY. . 179. 



This must be referred to the case explained above in the 

 examples of octahedral Fluor-haloide, and rhombohedral 

 Quartz. 



Turn now one of these octahedrons round the common 

 rhombohedral axis through an angle of 180, this axis being 

 considered as the axis of revolution, while the other re- 

 mains unmoved ; the face of composition being perpendi- 

 cular to the axis of revolution. A twin-crystal will now 

 be formed, because the individuals can no more be consi- 

 dered the one as the continuation of the other, since their 

 respective homologous parts have assumed a different, yet 

 determined situation towards each other, which is the pe- 

 culiar character of a twin-crystal. The faces of the one 

 produce re-entering angles with those of the other, equal 

 to double the edge of the octahedron, or = 218 56' 32" 

 = 360 141 3' 28". 



The same result is obtained, if we bisect an octahedron 

 by a plane through its centre, parallel to two of its faces, 

 or perpendicular to one of its rhombohedral axes, and allow 

 one of the halves to make a revolution of 180 round that 

 rhombohedral axis, upon which the section is perpendicular, 

 while the other half remains unmoved. The plane of the 

 section is the face of composition itself. 



In the preceding case, the term twin-crystal is more ap- 

 propriate to the first, that of a hemitrope crystal, more to 

 the last mode of explanation. The first supposes that every 

 twin-crystal consists of two different individuals, which re- 

 quire to be joined in a certain determined situation, in 

 order to produce the compound crystal ; the other sup- 

 poses only one individual, in which the situation of some 

 of its parts has undergone a regular change by an opera- 

 tion which can never have been employed by nature. This 

 is the reason why the first, and the name twin-crystal re- 

 ferring to it, has been preferred in the present work, to that 

 of a hemitrope crystal, particularly since it is applicable 

 to many cases, where the latter term would produce an 

 erroneous idea. The expression regular composition being 

 more general than either of them, is very often useful in its 

 application. 



