36 CRYSTALLOGRAPHY. 



oid (figs. 25 or 26), and also forms in which only half of the 

 sixteen planes of the double eight-sided pyramid, or half the 

 eight planes of an eight-sided prism — those alternate in position 



—are present (tigs. 27, 28). In fig. 27 the absent planes are 

 those of half the pairs of planes ; and in fig. 28 they induce 

 one of each of the pairs, as will be seen on comparing these 

 figures with fig. 21. 



4. Cleavage. — In this system cleavage may occur parallel to the 

 sides of either of the square prisms ; parallel to the basal pi me ; 

 parallel to the faces of a square octahedron ; or in two of these 

 directions at the same time. Cleavage parallel to the base and 

 that parallel to a prism are never equal, so that such prisma 

 need not be confounded with distorted cubes. 



5. Irregularities in Crystals. — The square prisms are very 

 often rectangular instead of square, and so with the octahedrons. 

 But, as elsewhere among crystals, the angles remain constant. 

 "When forms are thus distorted, the four prismatic planes will have 

 like lustre and surface markings, and thus show that the faces 

 are normally equal and the lateral axes therefore equal. If 

 the plane truncating the edge of a prism makes an angle of 

 precisely 135° with the faces of the prism, this is proof that 

 the prism is square, or that the lateral axes are equal, since the 

 angle between a diagonal of a square and one of its sides is 45°, 

 and 135° is the supplement of 45°. 



6. Distinctions. — The dimetric prisms have the base different 

 in lustre from the sides, and planes on the basal edges different 

 Id angle from those on the lateral, and thus they differ from 

 isometric forms. The lateral edges may be truncated, and the 

 new plane will have an angle of 135° with those of the prism, 

 in which they differ from trimetric forms, while like isometric. 

 The extremities of the prism, if it have any planes besides the 

 basal, will have them in fours or eights, each of the four, or 

 of the eight, inclined to the base at the same angle. When 

 there is any cleavage parallel to the vertical axis, it is alike 



