ON THE PRIMITIVE FORMS OF CRYSTALS. 57 



3. Rhomboidal Dodecahedron. 

 Garnet. 

 Blende. 



As the preceding table contains all the transpar- 

 ent crystals whose primitive forms have been deter- 

 mined, I have not been able to introduce into it a 

 variety of other crystals, the number of whose axes 

 I have carefully ascertained. Relying on the cor- 

 rectness of the general principle which is establish- 

 ed by the preceding comparison, the crystals which 

 I have examined may be referred to certain classes 

 of primitive forms, as shewn in the following table ; 

 and hence, the individual primitive form may be 

 easily deduced, from an examination of the secon- 

 dary crystals. 



1ST CLASS OF PRIMITIVE FORMS. 



Mica with Amianthus, and some other kinds of 

 Mica. See Biot, Mem. Inst. 1816, p. 275. 

 Read 22d June 1818, 



Idocrase*, 



Hydrate of magnesia. 

 Hydrate of strontites. 

 Arseniate of copper. 

 5 Arseniate of potash. 

 Muriate of lime. 



* Haiiy makes the primitive form of Idocrase a right prism 

 with a square base ; but as this form is incompatible with one 

 axis, it is more likely to be the same as that of Zircon. 



