LAWS OF DECREMENT. 301 



After effecting our division of P-2H by , and of 



q n n 







, we should find 



r o o 



i b = p, 

 I c = q, 

 i a =. r, 



which would imply a decrement by p molecules in 

 the direction of i k, q molecules in that of i rf, and 

 r molecules in the direction of f h. 



If we suppose fig. 329 to be a doubly oblique prism, 

 the letter to denote the edge 



i h would be D, 

 ik . / . F, 

 id . . . H, 

 and the symbol of the plane a b c, would then be 



(Dr Ep Hq). 



It may be remarked here, that tangent planes are 

 generally the result of a decrement by 1 row of 

 molecules, whether they replace the angles or edges 

 of those classes of the primary forms in which they 

 occur. 



By this general method of proceeding we may, 

 when we know the inclination of the primary planes 

 to each other, and of the secondary plane on one or 

 more of the primary, discover the law of decrement 

 by which any secondary plane has been produced on 

 any of the classes of parallelepipeds ; and it may be 

 adapted also to all the other classes of primary 

 forms. 



We shall now apply it to the several classes of 

 those forms in succession, and the calculation will be 

 found to become much more simple in its application 

 to many of those classes. 



