236 ON THE USE OI 1 SYMBOLS. 



As it sometimes, however, occurs that all the simi- 

 lar edges or angles of crystals are not similarly 

 modified, it will riot be sufficient in all cases to indi- 

 cate the decrement which has taken place on one 

 edge or angle, but our representative symbol should 

 also indicate the absence of the modifying plane from 

 some other edge or angle, where according to the 

 law of symmetry, it might be expected to appear. 



This necessity of distinguishing the symmetrical 

 modifications of crystals, from those which are not 

 so, will render the symbols rather more complicated 

 than they would be otherwise. 



The new theory of molecules which has been in- 

 troduced into this treatise, will render it necessary to 

 vary the character of some of the symbols employed 

 by the Abbe Haiiy in reference to the tetrahedron, to 

 all the classes of octahedrons, and to the rhombic 

 dodecahedron ; and as these changes will occasion 

 some other slight deviations from his system of no- 

 tation, it will conduce to perspicuity if we consider 

 the application of the symbols to each of the classes 

 of the primary forms separately. This will be done 

 in a table subjoined to this section, where the order 

 of the primary forms will correspond with that 

 adopted in the tables of modifications. 



The general nature of this system of notation will 

 be best illustrated by its application to one of the 

 least regular of the primary forms. 



Let us suppose that we are about to represent a 

 secondary crystal belonging to the class of doubly 

 oblique prisms, according to the theory of decre- 

 ments, and by means of the symbolic letters already 

 alluded to, the primary form being known, and the 

 law of decrement by which the secondary plane has 

 been produced, having been also ascertained. 



