Formation of Compound or Twin Crystals. 291 



This is shown by the rectangular figure described about the mo- 

 lecule. 



In the other case, in which the molecules are in contact in a point 

 of equihbrium between three poles, these poles of one molecule are 

 adjacent to three opposite poles of the other, and to their combined 

 attraction is owing the union of the molecules. Such is the source 

 of those twin crystals in which the face of composition is parallel to 

 a plane on a solid angle of a prism. 



These are two common species of twin crystal, and they result 

 from the nature of Crystallogenic attraction. In the action of 

 particles on one another just assuming their axes of attraction and 

 commencing to obey their influences, it is an occurrence to be ex- 

 pected that two should combine elsewhere than at their poles, pro- 

 vided their mutual attractions remain balanced ; and as was said of 

 a similar occurrence, did it not occasionally take place, it might be 

 justly concluded that some other arrangement of the attracting pow- 

 er beside that here supposed to exist, was engaged in the formation 

 of crystals. 



Instances of composition between two axes, or on an edge, are of 

 common occurrence in Hornblende, Feldspar, Staurotide, Pyrox- 

 ene, Iron Pyrites, &;c. Fig. 14, represents a crystal of Pyroxene, 

 in which the union has taken place parallel to an edge between M 

 and M, the acute edge of the prism. This kind of composition is 

 sometimes repeated on all the similar edges of a crystal as in Iron 

 Pyrites. 



Geniculated crystals of Rutile and Manganite are examples of 

 Twin Crystals of the third kind. Fig. 15, is a sketch of a crystal 

 of the latter mineral compounded parallel to a plane on an angle, 

 (a). Tin ore affords other examples ; also native gold and silver, 

 he. In Rhombohedral minerals, composition usually, although not 

 always, takes place on the vertical sohd angle. Fig. 16, is a twin 

 of Calc Spar of this kind. In the nucleus the north poles of one 

 molecule are near and opposite the south poles of the other. The 

 lateral edges of the Rhombohedron, have in this instance been bev- 

 eled, so that the present form is a scalene dodecahedron with the 

 upper half turned around apparently 60° on the vertical axis. A 

 repetition of this kind of composition, also, takes place at times on 

 all the similar solid angles. 



Distinguishing terms being convenient for designating these three 

 species of composition, which are the only kinds that occur in simple 



