178 TERMINOLOGY. . 147. 



The mode in which the rhombohcdrons themselves, and 

 the limits of their series, produce hemi-rhombohedral com- 

 binations, is particularly remarkable, and very distinct from 

 what we have seen till now. Here all the faces belonging 

 to one apex enter into the combination, whilst all those con- 

 tiguous to the opposite apex disappear. The results are 

 combinations of a dissimilar configuration in their opposite 

 terminations. The crystallisations of rhombohedral Tourma- 

 line give the most generally known examples of this pecu- 

 liarity, which, however, is likewise frequently met with 

 in rhombohedral Ruby-blende. It is evident that the six- 

 sided prism R + CD, if subject to this modification, will 

 only shew half the number of its faces ; and that the 

 three-sided prism occurring in these two species, can only be 

 explained upon the supposition, that it is a rhombohedron 

 of an infinite axis, three faces of which belong to one of 

 the 'apices of the combination, and are enlarged, while those 

 belonging to the other apex disappear. 



The designation of hemi-rhombohedral and hemi-di-rhom- 

 bohedral forms, depends upon the same principle as that 



of the halves in the tessular system (. 129 133.). The 



number 2 is added to the sign of the entire, simple form, 

 as a divisor ; and by the signs + and , or r and 1, is in- 

 dicated the parallel and the transverse position, or the dif- 

 ference between right and left. 



The following signs refer to the figures 51 58, com- 

 prehending the different forms of a di-rhombohedral, hemi- 

 rhombohedral, or hemi-di-rhombohedral character : 

 n)">)Fig. 51.; 



