22 DEFINITIONS. 



height and three in breadth ; for if either number 

 were a multiple of the other, as would be the case if 

 the supposed decrement took place by two rows in 

 height and four in breadth, or three in height and six 

 in breadth, the new plane thus produced would be 

 perfectly similar to that which would result from a 

 decrement by one row in height and two in breadth, 

 and would therefore belong to the planes produced by 

 simple decrements. 



Fig. 47. 



Fig. 47 shews a mixed decrement on an edge of the 

 primary form by two rows in breadth and three in 

 height, and the lines abed mark the position of the 

 new plane produced by this decrement. 



It has been found convenient to express mixed 

 decrements by fractions, of which the numerator, or 

 upper Jigurt, denotes the number of molecules in 

 breadth, and the denominator, or lower figure, the 

 number in height, abstracted from the edge or angle 

 of the superimposed plates; thus, a decrement by | 

 would imply a decrement by three molecules in 

 breadth and four in height. 



Intermediary decrements affect only the solid angles 

 of crystals, and may be conceived to consist in the 

 abstraction of rows of compound molecules from the 

 successively superimposed plates, each compound mo- 

 lecule containing unequal numbers of single molecule* 



