ON THE OPTICAL PHENOMENA OF CRYSTALS. 223 



of crystals resembling two opposite sectors of a circle 

 combined. This form may be traced in different crys- 

 tals, from its commencement when the angle of the 

 sector is small, through all degrees of increase, until at 

 length the opposite sectors unite and form a complete 

 circle. 



2. The crystals are frequently of a very irregular elon- 

 gated shape, which does not approximate either to a 

 prismatic or a cylindrical form. This stem, as it may be 

 termed, subdivides itself at both extremities into an 

 immense multitude of diverging fibres, giving it the ap- 

 pearance of a bundle <Jf elastic filaments firmly held to- 

 gether in the central part, but with its extremities left at 

 liberty to diverge*. 



3. Another variety resembles in the same way irregu- 

 lar steins or branches, which, however, instead of being 

 subdivided, are abruptly truncated at both extremities 

 perpendicularly to the general line of their direction. 



4. Sometimes, on the contrary, the ramification is 

 much more developed, and then resembles two plumes 

 united by a common stem. 



5. Crystals of regular geometric form. These appear 

 to require the presence of combined water. 



Whichever of these formations occurs, it is for the 



* This appearance is iiot very uncommon in the crystallization of other 

 substances, though I believe it has not yet been described. The divergence 

 of the filaments suggests the idea of electrical repulsion as being at least its 

 primary cause. 



