trahedron,
also derived from the same primary form. It occurs in iron pyrites and
cobaltine. There are two other tetrahedral forms, the pentagonal
dodecahedron (fig. 14), and the pentagonal icositetrahedron (fig. 15),
both bounded by irregular pentagons, but not yet observed in nature.
Combinations.—These
forms of the tesserai system (and this is true also of the five other
systems of crystallization) not only occur singly, but often two,
three, or more are united in the same crystal, forming what are named
combinations. In this case it is evident that no one of the individual
forms can be completely developed, because the faces of one form must
partially interfere with the faces of the other forms. A combination
therefore implies that the faces of one form shall appear symmetrically
disposed between the faces of other forms, and consequently in the room
of certain of their edges and angles. These edges and angles are thus,
