SECTION XIV. 



ON THE USE OF SYMBOLS FOR 



DESCRIBING THE SECONDARY FORMS 



OF CRYSTALS. 



IN Section 11, p. 102, it vyas stated that certain 

 letters had J)een appropriated by the Abbe Haiiy, as 

 symbols, to designate the similar and dissimilar edges, 

 angles, and planes of each of the classes of primary 

 forms. And in the tables of modifications, these let- 

 ters are placed on the figures of the primary forms, 

 to denote in each its similar, and dissimilar, edges, 

 angles, and planes. 



The order in vyhich they are placed on the figures 

 is obviously that of the alphabet ; and they are ar- 

 ranged also according to the ordinary method of 

 writing, beginning at the upper part of the figure, 

 and then proceeding from left to right until the seve- 

 ral parts of the crystal are marked by the appropriate 

 letters. 



This will be very apparent, if we refer to the pri- 

 mary form of the doubly oblique prism. 



The letters P M T are retained to designate the 

 primary planes of crystals, although the term primir 

 tive, from which those letters were derived, is not 

 used in this treatise. 



The letter A is, by the Abbe Hauy, placed, gene- 

 rally, on an obtuse angle of the primary form ; but 

 according to the positions in which the primary forms 



