ON THE USE OP SYMBOLS, 



243 



of the cube, tetrahedron, and rhomboid, and rest, as 

 it is said, upon the planes of those primary forms, are 

 distinguished from those which are said to rest upon 

 their edges. But in reference to the theory of decre- 

 ments, both these sets of planes are similar in character, 

 and result from simple or mixed decrements on an angle 

 of the primary form. 



The planes which are said to rest upon the primary 

 planes, are produced by decrements in which the 

 number of molecules abstracted in breadth, is greater 

 than the number in height, while those which are said 

 to rest upon the edges, result from decrements where- 

 in the number in height exceeds the number in breadth. 

 The numbers or fractions expressing ihejirst of these 

 sets will be always greater than unity, as 2, 3, 4, -f, f , 

 &c. ; those expressive of the latter set, will be al-? 

 ways less than unity, as , -f, f, &c. ; and the planes 

 in this latter case are conceived to be carried, as it 

 were, over the solid angle, and made to replace a 

 portion of the adjacent edge. 



Let us now suppose a cubic crystal, modified on 

 the angles by three planes belonging to class b of the 

 modifications of that form ; and let us suppose that 

 the modifying planes result from a decrement by two 

 rows in breadth on the angles of the cube. The sym 



bol denoting these planes would be 2 A% and if this 



