18 



DEFINITIONS. 



In these additions, the molecules appear to arrange 

 themselves so as to form laminae, or plates, which 

 successively, either partially, or wholly, cover each 

 other. 



These plates are theoretically supposed to be either 

 single, that is, of the thickness of single molecules, or 

 to be double, treble, &c. that is of the thickness of two, 

 three, or more molecules. 



Fig. 38. 



Fig. 38 represents a single plate of molecules, 



Fig. 39. 



X^x / / y >r ,.' 



Fig. 39 represents a double plate. 

 Fig. 40. 



When such additions envelope the whole of a 

 smaller crystal, its original form is preserved through 

 every increase of size. 



Fig. 40 represents a right rectangular prism which 

 has increased in magnitude without change of figure. 



When the additions do not cover the whole surface 

 of a primary form, but there are rows of molecules 

 omitted on the edges, or angles of the superimposed 

 plates, such omission is called a decrement. 



