DECREMENTS. 



71 



which arc parallel to each other. And if we refer to 

 the tables of modifications of that form, which will be 

 found in a subsequent section, we shall observe it is 

 only the parallel edges which are affected by the 

 modifications 6, c and d. 



Fig. 104. 





Let us now suppose a modification belonging to 

 the class c to have taken place on a right rectangular 

 prism, and let us suppose the secondary crystal pro- 

 duced by this modification represented by fig. 104. 

 The upper part of this figure shews the manner in 

 which the new planes are conceived to be produced, 

 by the continual abstraction of single rows of mole- 

 cules on both the edges a 6, and c, of each of the 

 superimposed plates, until the last plate consists of 

 only one row, forming the new edge of the secondary 

 crystal. 



Fig. 105. 



The square prism has all its terminal edges similar^ 

 and 'all its terminal angles also similar, and con- 



