160 TERMINOLOGY. . 14)5. 



which they produce. The signs of the simple forms follow 

 each other in the designation of the compound one in a cer- 

 tain order, so as to let those precede which refer to forms 

 whose faces are more inclined to the axis, and those follow 

 whose faces are less inclined or parallel to the same line. 

 The signs of two different forms are separated by a full 

 stop. The sign of a combination of R and R + 1 will 

 therefore be 



R. R + 1. 



An example of the designation of a compound form con. 

 taining a greater number of simple ones will be found in 

 . 148. 



From the preceding observations it is evident, that the 

 designation contains every thing required for calculating 

 the combinations. 



OF THE COMBINATIONS OF THE DIFFERENT SYSTEMS OF 

 CRYSTALLISATION. 



. 145. RHOMBOHEDRAL COMBINATIONS. 



A combination of the rhombohedral system is 

 more particularly said to possess a Rhombohedral 

 Character, if each of the simple forms contained in 

 it appears only in one position, but with the whole 

 number of their faces. 



Hence a combination of rhombohedrons, six-sided pyra- 

 mids, and prisms, exhibits a rhombohedral character, if these 

 forms are found in only one position, and if those which 

 cannot assume any other position, as the isosceles six-sided 

 pyramid, and the six-sided and the twelve-sided prisms, 

 appear with the number of faces peculiar to them. In all 

 other cases the combinations receive a particular denomina- 

 tion, provided the changes which take place in respect to 

 the enlargement of some of their faces, are not owing to 



