250 ON THE USE OF SYMBOLS. 



The planes belonging to class &, which may be said 



to modify the edges partially, may be thus denoted. 



j_ 



B B' B" p P B '". 



The index not being repeated below, and on both sides 

 of the letters B, #c. affords an indication that the planes 

 are single upon each edge ; in which respect only, 

 does class k differ from classy. 



2. Let us suppose some angles or edges of a crystal 

 to be modified, while others, which are similar, re- 

 main entire. 



From what has preceded, it will be apparent, that 

 the character representing these differences may be 

 generally conferred on the symbol, by introducing 

 into it the letters which denote the unmodified edges 

 or angles, and by substituting cyphers in appropriate 

 positions near those letters, for the indices of the 

 symmetrical modifying planes. 



.An example derived from the defective modifica- 

 tions of the cube, will sufficiently illustrate the cha- 

 racters of these particular forms of symbols ; and ia 

 the following tables they will be further explained 

 in reference to the different classes of primary forms 

 in which irregular secondary forms occur. 



The angles of the lower plane of the cube, cor- 

 responding with those marked A and A' of the upper 

 plane, are, as it has been already stated, supposed 

 to be denoted by a and a 7 . The planes belonging to 

 class h may be represented by the following symbol. 



p o p o 



P A P A /0 p a' p V. 



This implies the occurrence of the modifying planes 

 on the alternate solid angles only. 



If, as it sometimes happens, one of the terminal 

 solid angles of a rhomboid is replaced by a tangent 



