42 CRYSTALLOGRAPHY. 



that is, in the clinodiagonal zone ; and this is a consequence of 

 the right angle which axis b makes with both axis c and axis cu 

 The plane i-i is called the orthojnnacoid, it being parallel to the 

 orthodiagonal ; and the plane i-i, the clinojnnacoidy it being 

 parallel to the clinodiagonal. 



Vertical rhombic prisms have the same relations to the lateral 

 axes as in the trimetric system. Domes, or horizontal rhombic 

 prisms, occur in the orthodiagonal zone, because the vertical 

 axis c and the orthodiagonal b make right angles with one 



O CO 



another. In fig. 6 the planes 1-2, 2-1 belong to two such 

 domes. They are called cli?wdomes, because parallel to the 

 clinodiagonal. 



In the clinodiagonal zone, on the contrary, the planes above 

 and below the basal plane differ, as already stated, and hence 

 there can be no orthodomes ; they are hemiorthodomes. Thus, 

 in fig. 6, -|~i, \-i are planes of hemiorthodomes above i-i, and 



— |-i is a plane of another of different angle below i-i. The 

 plane, and its diagonally opposite, make the hemiorthodome. 



The octahedral planes above the plane of the lateral axes also 

 differ from those below. Thus, in figs. 5 and 6, the planes 1, 1 

 are, in their inclinations, different planes from the planes — 1, 



— 1 ; so in all cases. Thus there can be no monoclinic octahedrons 

 — only hemiuctahedrons. An oblique octahedron is made up of 

 two sets of planes ; that is, planes of two hemioctahedrons. 

 Such an octahedron ma}' be modelled and figured, but it will 

 consist of two sets of planes : one set including the two above 

 the basal section in front and their diagonally opposites behind 



(fig. 9), and the other set including the two below the basal sec- 

 tion and their diagonally opposites (fig. 10). 



A hemioctahedron, since it consists of only four planes, ia 

 really an obliquely placed rhombic prism, and very frequently 

 they are so lengthened as to be actual prisms. 



