244 TERMINOLOGY. . 179. 



neous. Two individuals not homogeneous, even though 

 they were supposed to possess the same form, and to join 

 according to some rule, can never produce a twin-crystal ; 

 they cannot be considered as compound, but must be classed 

 among the mixed (. 14.) minerals. 



The forms of the single individuals of a twin-crystal 

 must therefore contain members of one and the same series 

 of crystallisation : They may be simple forms or combina- 

 tions. Commonly they either are or both contain the same 

 members, so that in most cases the forms of the two indi- 

 viduals may be considered merely as parts of one and the 

 same crystalline form, part of which only has assumed an 

 extraordinary yet determined situation. 



This has been the foundation of another method employ- 

 ed for explaining the form of regularly composed individuals. 

 A plane is imagined to bisect in a determined situation the 

 form of the simple mineral, and one of the halves to be 

 turned in this plane through a certain number of degrees, 

 while the other remains in its former situation. The num- 

 ber of degrees is equal to half the circumference, or = 180, 

 hence twin-crystals considered in this point of view, have 

 been called Hemitrope Crystals. 



The change in the situation, which is produced in the 

 parts of the combined individuals, occasions, under certain 

 circumstances, the production of angles greater than 180, 

 which are not to be met with in the forms of simple indivi- 

 duals as considered above. These angles are said to be re- 

 entering or re-entrant ; and they are commonly taken for a 

 character of a twin-crystal, or of a hemitrope one. 



Angles of this kind, however, greater than 180, may 

 arise if two individuals of a species of the same or of dif- 

 ferent forms, are joined in a parallel position ; as, for in- 

 stance, in rhombohedral Quartz, where it sometimes happens 

 that two crystals of the common form have all their faces pa- 

 rallel, and the axis of the one in the prolongation of the axis 

 of the other. The product, nevertheless, must not be con- 

 sidered as a twin-crystal, or as a hemitrope one. The two 

 apparent individuals in a composition of this kind, form parts 



