184 TERMINOLOGY. . 149. 



forms among each other; which is immediately evident 

 from the derivation. 



2. JLet n' be = n _ 2 r. The forms are in a parallel posi- 

 tion. The edges of combination therefore will be horizon- 

 tal. Ex. P (c) and P + 2 (ft) in pyramidal Garnet. Vol. II. 

 Fig. 96. They retain this situation, even though the com- 

 bined forms belong to different series, and their relative di- 

 mensions therefore in such a combination remain undeter- 

 mined. The reason why the faces of all parallel pyramids in- 

 tersect each other in horizontal edges, consists in the pa- 

 rallel situation of their respective lateral edges, or of the 

 sides of their horizontal projections. 



3. Let n or n' be = os. One of the forms under 

 these circumstances appears as a face perpendicular to the 

 axis of the other ; and the edges of combination, inde- 

 pendently of the dimensions of the forms, must be horizon- 

 tal, as is evident from 2. Ex. P oo () and -l^JL P _ 3 (6) 

 in pyramidal Lead-baryte. Vol. II. Fig. 92. 



4. Let n or n' be = + co ; one of the forms becomes a re- 

 gular four-sided prism. In a parallel position, the edges in 

 which the faces of the two forms meet, must be parallel to 

 the lateral edges of the finite form, and therefore horizon- 

 tal (2). Ex. P (P) and P + oo (J), or P + 1 (*) and [P + co] (g) 

 in pyramidal Tin-ore. Vol. II. Fig. 102. In a diago- 

 nal position they are parallel to the rhombic principal sec- 

 tion (. 53. 2.) of the finite member, and their situation 

 depends on its dimens'ons. Ex. P + 1 (*) and P + oo (Z) 

 in pyramidal Tin-ore. Vol. II. Fig. 102. If the faces of 

 the prism do not intersect eac|i other, they appear as 

 rhombs. The combinations P cc. P + oo, and P co. 

 [P + oo ], are right rectangular four-sided prisms. 



5. If both n and n' are == + or, and the forms in a dia- 

 gonal position, the faces of one of the prisms appear in the 

 place of the edges of the other ; their combination produces 

 an equiangular, and if the faces are of equal extent, a re- 

 gular eight-sided prism. Ex. P + co (Z) and [P -f ooj (g) 

 in pyramidal Tin-ore. Vol. II. Fig. 102. 



