a
heated wire, we at once obtain excellent phosphorescence. Common
quartz, rubbed against a second piece of the same quartz in the dark,
becomes highly phosphorescent. Certain gems, also, when merely exposed
to light—sunlight for preference—then taken into a darkened room, will
glow for a short time. The diamond is one of the best examples of this
kind of phosphorescence, for if exposed to sunlight for a while, then
covered and rapidly taken into black darkness, it will emit a curious
phosphorescent glow for from one to ten seconds ; the purer the stone,
the longer, clearer and brighter the result.
