The third physical property of light, PHOSPHORESCENCE, is the property possessed by certain gems and minerals of becoming phosphorescent on being rubbed, or on having their temperature raised by this or other means. It is difficult to say exactly whether this is due to the heat, the friction, or to electricity. Perhaps two or all of these may be the cause, for electricity is developed in some gems--such as the topaz--by heat, and heat by electricity, and phosphorescence developed by both. For example, if we rub together some pulverised fluorspar in the dark, or raise its temperature by the direct application of heat, such as from a hot or warm iron, or 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. CHAPTER VI. PHYSICAL PROPERTIES. D--COLOUR. Colour is one of the most wonderful effects in nature. It is an attribute of light and is not a part of the object which appears to be coloured; though all objects, by their chemical or physical composition, determine the number and variety of vibrations passed on or returned to the eye, thus fixing their own individual colours. We have also seen that if an _equal_ light-beam becomes obstructed in its passage by some substance which is denser than atmospheric air, it will become altered in its direction by refraction or reflection, and polarised, each side or pole having different properties. Polarised light cannot be made again to pass in a certain direction through the crystal which has polarised it; nor can it again be reflected at a particular angle; so that in double-refracting crystals, these two poles, or polarised beams, are different in colour, some stones being opaque to one beam but not to the other, whilst some are opaque to both. This curious phenomenon, with this brief, though somewhat technical explanation, shows the cause of many of the great charms in precious stones, for when viewed at one angle they appear of a definite colour, whilst at another angle they are just as decided in their colour, which is then entirely different; and as these angles change as the eye glances on various facets, the stone assumes a marvellous wealth of the most brilliant and intense colour of kaleidoscopic variety, even in a stone which may itself be absolutely clear or colourless to ordinary light.