SYSTEMS OF CRYSTALLIZATION. 239 



a crystalline plane to the axis ;" and again, " An 

 axis is any line governing the figure, about which 

 all parts are similarly disposed, and with reference 

 to which they correspond mutually." This he soon 

 followed out by examination of some difficult cases, 

 as Felspar and Epidote. In the Memoirs of the 

 Berlin Academy 5 , for 1814-5, he published An 

 Exhibition of the natural Divisions of Systems of 

 Crystallization, In this Memoir, his divisions are 

 as follows : The regular system, the four-mem- 

 ber ed, the two-and-tivo-membered, the three-and- 

 tliree-membered, and some others of inferior degrees 

 of symmetry. These divisions are by Mohs (Out- 

 lines of Mineralogy, 1822,) termed the tessular, 

 pyramidal, prismatic, and rhombohedral systems 

 respectively. Hausmann, in his Investigations con- 

 cerning the Forms of inanimate Nature^, makes a 

 nearly corresponding arrangement ; the isometric, 

 monodimetric, trimetric, and monotrimetric ; and 

 one or other of these sets of terms have been adopted 

 by most succeeding writers. 



In order to make the distinctions more appa- 

 rent, I have purposely omitted to speak of the 

 systems which arise when the prismatic system 

 loses some part of its symmetry ; when it has only 

 half or a quarter its complete number of faces ; 

 or, according to Mohs's phraseology, when it is 

 hemihedral or tetartohedral. Such systems are 



5 Edinb. Phil. Trans. 1823, vols. xv. and xvi. pp. 290 336. 

 u Gottingen, 1821. 



