Formation of Compound or Twin Crystals. 299 



d These are the principal varieties of the doubly compound crys- 

 tals belonging to Right Rhombic Prisms. The formation of all is 

 found to be referrible to the principles already laid down. Thus 

 we see that all those accidents, if they may be so called, which 

 would naturally result to molecules endowed with axes of attraction, 

 do actually occur, and precisely as they might have been predicted, 

 had the nature of these molecules been previously known. And 

 with a perfect acquaintance with the operations of other agents be- 

 sides Crystallogenic attraction, the occurrence of doubly geniculated 

 crystals, which we are now about to consider, could have been pre- 

 dicted with the same certainty. 



In accounting for these forms, in my former communication, it was 

 supposed that they resulted from a reversion of the original polarity 

 in the molecules of the crystal. The causes of this reversion are 

 probably agents that are not unknown to us. Heat will have this 

 effect on crystals of Tourmaline, their polarity varying with the tem- 

 perature. Electricity is equally an efficient agent in producing sim- 

 ilar results. 



* 



A reference to a figure will prove the capability of this hypothe- 

 sis to account for these occurrences. Let AB, (Fig. 9,) represent 

 a line of molecules in a crystal which is now in the process of form- 

 ation. The poles may be situated as there marked, (the marked 

 poles are north.) v The particle C is supposed to be on the point of 

 obeying its axes of attraction, by uniting the point n with m. PkX 

 this moment the poles of the crystal are reversed, and consequently 

 their situation is, as in A'B'. The molecule C that was about to 

 join itself, finds now a repellent pole opposing it, m and n being both 

 north. A change of the position of the molecule, will therefore en- 

 sue, and the nearest south, o, will be brought in contact with tn 9 

 which is so represented in A" B". The same would take place in 

 every line of molecules, and in opposite parts of the crystal, and 

 thus give rise to two geniculations. These are instances of adaxal 

 composition. A horizontal section of a crystal of Arragonite thus 

 compounded is represented in Fig. 11. Fig. 13, PI. I, exhibits 

 nearly the form of the crystal. 



At other times the composition is interaxal, the situation of the 

 molecules of the crystals in these instances may be represented by 

 Fig. 10, A B, in which C is again the next particle that is to be 

 added. These molecules are those of a Right Square Prism, which 

 form is peculiarly subject to these accidents. Only one of the lat- 



