. 179. OF COMPOUND MINERALS. 243 



from something which is foreign to the mineral. Compo- 

 sitions of this kind cannot be called regular in the sense 

 of the word now explained. 



. 179. REGULAR COMPOSITION. TWIN-CRYSTALS. 



The regular composition of two homogeneous 

 individuals, joined in one crystalline form, has 

 been designated by the name of a Twin-Crystal. 



It is unnecessary to consider by themselves regular com- 

 positions of three, four, five, or more individuals, because, 

 although compositions of that kind, according to several 

 laws, should take place in nature, yet they always can be 

 reduced to, and explained by the regular composition of two 

 individuals. 



The property peculiar to the twin-crystals consists in 

 the close and exact connexion of the Face of Compo- 

 sition (. 177')? dr that in which the individuals join, with 

 the series of crystallisation of the species. The face of 

 composition is either parallel to the face of a form' belong, 

 ing to this series, or it is perpendicular to a certain edge. 

 The situation of the two individuals themselves is obtain- 

 ed, if we first suppose both to be in the parallel position, 

 and then turn one of them round a certain line, likewise of 

 a determined direction, under an angle of ISO , while the 

 other remains unmoved. This line is termed the Axis of 

 Revolution. It is either perpendicular to the face of com- 

 position, or it coincides with this face, while it is parallel 

 to a crystallographical axis of the individual. The angle 

 of 180 is the Angle of Revolution. 



These properties are necessary to, and characteristic 

 of, the twin-crystals, which are very easily distinguished 

 from any other compositions occurring in nature, in which 

 the junction of two or more individuals takes place in other 

 faces and in other directions, the aggregation of the indi- 

 viduals being accidental. 



It^is moreover necessary that the individuals be homoge- 



