4 DEFINITIONS. J9 



The terra decrement has been adopted to express 

 these omitted rows of molecules, because, in conse- 

 quence of such omissions, the primary form on which 

 the diminished plates are successively laid, appears 

 to decrease as it were, on the edge or angle on which 

 such omissions take place. 



Decrements are said to begin at, or to set out from, 

 the particular edge or angle at which the omission of 

 molecules first takes place ; and to proceed along that 

 plane on which the defective plate of molecules is 

 conceived to be superimposed. And they are said to 

 take place either in breadth or in height. 



Decrements in breadth are those which result from 

 the reduction of the superficial area of the super* 

 imposed plate, by the abstraction of rows of molecules 

 from its edges or angles. 



Decrements in height relate to the thickness of the 

 plate from which the abstraction of rows of molecules 

 takes place. 



Fig. 41. 





Let c d fig. 41, represent an edge of a primary 

 form, and let a b represent an edge of a double plate 

 of molecules, from which one row has been abstracted ; 

 the decrement, or omitted portion of this superimposed 

 plate, would be stated to consist of one row in breadth, 

 or one row omitted upon the terminal surface of the 

 primary crystal, and two rows in height, signifying 

 that the omitted row belonged to a double plate of 

 molecules \abcd would be the position of the new 

 plane produced by this decrement. 



