292 Formation of Compound or Twin Crystals. 



twin crystals, the following are proposed: 1st. Adaxal, the axes of 

 the two simple crystals being united : 2d. Interaxal, the union taking 

 place at a point of equilibrium of attraction between two axes of each 

 molecule : 3d. Mediaxal, the combination being effected at a cen- 

 tral point as to attraction between the poles of three axes. The 

 first is applied to instances of composition parallel to a face, the sec- 

 ond where it is parallel to a plane on an edge, the third where paral- 

 lel to a plane on an angle. 



Compound crystals sometimes occur whose composite character 

 has evidently been received subsequently to the commencement of, 

 yet during their formation. Such are the doubly geniculated crys- 

 tals of Rutile and other mineral species. To cause such forms is 

 required merely a reversion of the polarity of the crystal, (that is, of 

 its molecules,) by electrical influence or some other cause. 



Other twins occur which have been joined after a previous state 

 of separate existence. They are always imited by their homolo- 

 gous parts, and consequently owe their union to the attraction of op- 

 posite poles of similar axes in the molecules of the two crystals. 

 Crystals of Quartz are very subject to such accidental combinations. 

 It is not improbable that distant crystals may have an influence upon 

 one another to cause a similar direction of similar poles. 



In the preceding remarks, the statements have necessarily been 

 very general. To have entered minutely into all the particulars 

 and explained the formation of each twin crystal that occurs would 

 have required more time than is proper for me in this place to em- 

 ploy ; besides, this appears to be unnecessary, as with a little thought 

 any of these forms, although some are quite complex, may be 

 easily understood and referred to one of the above classes. It will 

 be observed that in the explanations given, no new principles have 

 been added to the theory first proposed. On the contrary, these 

 solids of so peculiar forms, have been shown to be a necessary re- 

 sult of the same simple law, that gives to the ordinary crystal its 

 geometrical regularity. This may then be added as an important 

 argument for the truth of the adopted theory, its ample ability to ex- 

 plain apparently anomalous occurrences. 



That the above explanation of the formation of twin crystals is 

 correct, will also appear from the simplicity with which, on the same 



