FORM OF MINERALS.	55
	the crystals are bounded on the opposite ends of their chief axis by faces belonging to distinct forms, and hence only the upper or under half of each form is produced, or the crystal, as the name implies, is half-formed. Figure 67 rep­resents a common variety of tourmaline, bounded on the
	upper end by the planes of the rhombohedrons R and—2R, and on the lower end by the basal pinacoid. In fig. 68 of electric calamine the upper extremity shows the basis k, two brachydomes o and p, and two macrodomes m and I; while on the lower end it is bounded by the faces P of the primary form. This appearance becomes more interesting from the fact that most hemimorphic crystals acquire polar electricity from heat,—that is, exhibit opposite kinds of electricity at opposite ends of the crystal. The faces of crystals are very frequently rendered im­perfect by striae, or minute linear and parallel elevations and depressions. These arise in the oscillatory combination of two crystal forms, alternately prevailing through small spaces. The striae, therefore, are in reality the edges of combined forms. They are very common on quartz, sliorl, and some other minerals; and frequently indicate combina­tions where only a simple form would otherwise appear to exist. The cubes and pentagonal dodecahedrons of iron