DEFINITIONS. 



imaginary plate; the two molecules c, rf, as the second 

 row; the three molecules e,/J as the third row, and so 



on. 



Fig. 45. 



Fig. 45 shews a simple decrement by two rows in 

 height on the edg-e of the primary form. 



Fig. 46. 



Fig. 46 shews a simple decrement by two rows in 

 height on the angle of the primary form. 



It is observable in these figures, that each successive 

 plate is less by one row of molecules than the plate 

 on which it rests. It is by this continual recession of 

 the edges of the added plates, that the crystal appears 

 to decrease on its edges or angles, and that new planes 

 are produced. The edges of the new planes which 

 would be produced by the four preceding decrements, 

 are shewn by the lines a b c d, fig. 42 and 45, and 

 by the lines a b c, fig. 43 and 46.* 



A mixed decrement is one in which unequal numbers 

 of molecules are omitted in height and in breadth, 

 neither of the numbers being a multiple of the other ^ 

 such as three in height and two in breadth, or four in 



* The molecules of crystals are so minute, as to render those in- 

 equalities of surface imperceptible which are occasioned by decrements. 



