72 DECREMENTS. 



sequently when one of those edges or angles is affected 

 by any decrement, they will generally all be so. 



Fig. 105 exhibits modification c, of the square 

 prism, the upper part of the figure shewing the man- 

 ner in which the secondary planes are conceived to 

 be generated, by the continual abstraction of single 

 rows of molecules from each edge of each of the suc- 

 cessively superimposed plates. 



Fig. 106. 



Fig. 106 exhibits the effect of a decrement by one 

 row of molecules on the angle of a square prism, 

 producing a secondary form belonging to the class a 

 of the modifications of that figure. See tables. 



The cube has its three adjacent edges similar, and 

 consequently they are all affected equally by decre- 



