300 Formation of Compound or Twin Crystals. 



eral axes is represented in the figure. The other is at right angles 

 with the one given, (r n in C,) and is pointed towards the observer. 

 In the rnolecide C, its north pole, as the mark indicates, is in front. 



If the polarity is reversed, at the moment C is about to unite it- 

 self, m being thus rendered a pole of the same kind with n, as in 

 A' B', will repel the latter, and attract the nearest north of a simi- 

 lar axis, which is S. A revolution of 90° must hence take place. 

 But during this time the unlike poles t and n, (of like axes, the ver- 

 tical in the prism,) are also acting on one another and tending to- 

 wards an union. Consequently, combination will not take place at 

 either pole, but at the point of equilibrium of attraction between 

 them, that is, the point of contact in the crystal will be between m 

 and t, and in the molecule between p and ?i as in Fig. 10, A'^ B''. 



A similar explanation might be given of the postgenital mediax- 

 al compositions. But as it proceeds directly from the above, it 

 seems unnecessary to consider the subject farther. Fig. 12, may 

 be considered for illustration, an instance of either mediaxal or in- 

 teraxal composition, it depending on which are selected as primary 

 planes. The planes e being the primary, it is an instance of com- 

 position of the latter kind ; but if M are the primary faces, the com- 

 position is mediaxal, or on an angle. 



It is certainly quite amusing to follow the molecules of crystals 

 guided by their instinctive faculty, polar attraction, — and it may be 

 influenced, at times by other powers,— in the construction of those 

 beautiful specimens of architecture, the crystalline solids ; and equal- 

 ly interesting the fact, and we might say surprising, were it not in 

 accordance with the usual simplicity of the operations of nature, that 

 all their varied forms may proceed from the mutual action of solids 

 of a single kind, by varying the situation and length of the axes of 

 these solids, and the force of attraction in the direction of these axes. 

 That such may he the fact, cannot be doubted ; whether it is in re- 

 ality so, the opening facts of science will soon disclose, if they are 

 not already sufficient for a decision. 



