. 140. OF COMBINATIONS. 151 



phy are therefore such as are produced by forms belonging 

 to the same Series of Crystallisation. 



. 140. SECOND LAW OF COMBINATION. 



The second Law of Combination is : That the 

 simple forms contained in the combination, must be 

 in such Positions towards each other as are peculiar 

 to them in the systems to which they belong. 



According to the preceding derivations, the simple forms 

 of every system are obtained in certain determined posi- 

 tions. In these positions, and only in these, they join in 

 combinations. Thus, in the rhombohedral system we have 

 the parallel position and the transverse position ; in the py- 

 ramidal system the parallel position and the diagonal posi- 

 tion, &c. In most cases, the combined forms assume those 

 positions, in which they have been derived. Thus, in the 

 rhombohedral system, the subsequent rhombohedrons R and 

 11 4- 1 are in a transverse position, the alternating rhombohe- 

 drons 11 and 11 -f 2 in a parallel position, in regard to each 

 other. Yet there are some exceptions in this respect. The 

 position of It, the fundamental form of the rhombohedral 

 system, is considered as the normal one, to which the position 

 of all other forms is referred ; yet this rhombohedron some- 

 times appears in a transverse position, whilst other forms, 

 though according to the derivation obtained in the trans- 

 verse position, nevertheless affect the parallel one in the 

 combinations. If thus one and the same simple form appears 

 in both positions at once in a combination, a remarkable re- 

 sult will be obtained, after the necessary enlargement of 

 their faces, all other faces having been made to disappear. 

 They produce a form contained under equal and similar 

 faces, which assumes the aspect of a simple one (. 138.), 

 though it is really compound, as results from the process 

 by which it has been obtained. 



Nature confirms this second law relative to the position 



