Luminous barium sulphide or " Bolognan phosphorus " may be obtained by heating precipitated barium sulphate with powdered charcoal or common gum. The product should be sealed up in glass tubes, or kept in a tightly corked bottle. When exposed to the rays of the sun, or to an incandescent gas-light, and then taken into a dark room, the sulphide shines with a brilliant orange-coloured glow for a considerable time. Calcium sulphide possessing similar properties was sometimes known as " Canton's phosphorus." The compound was originally prepared by calcining a mixture of crushed oyster shells and sulphur, but good products are now obtained by heating finely powdered lime with sulphur and starch, and a trace of bismuth nitrate. The colour of the light is generally violet, but by substituting lead acetate for the bismuth salt, a yellowish-green may be obtained. Products containing traces of uranium, manganese or zinc give bluish, orange or greenish phosphorescence ; and a particular crystalline zinc sulphide shines brilliantly green.

Other bodies besides the sulphides are slightly luminous after exposure to light rich in ultra-violet rays. Among these may be mentioned some natural fluorides, rock salt and other minerals ; and amber, gums and resins. Fused calcium chloride is known as " Homberg's phosphorus," and fused calcium nitrate as " Baldwin's phosphorus." The luminescence of some of these lasts only for a few seconds. It is best observed by remaining in a dark room and exposing the body to a ray of sunlight by means of a hole in a shutter over the window. If the eyes are kept closed until the light is again obscured the luminescence will be very distinct.

Fluorspar Crystals

There are many substances which ordinarily do not phosphoresce after insulation but become luminous when broken into small fragments and thrown upon a heated surface. The most remarkable of these is the natural fluoride of calcium, fluorspar, occasionally known as " Blue John."

This mineral occurs both massive and in beautiful cubic crystals, usually of a pale bluish-violet colour and strongly fluorescent. It is a common vein mineral associated with metallic ores such as gelena, zinc blende, and barytes.

When thrown upon the surface of heated mercury or on to an iron shovel just below visible red-heat, fluorspar decrepitates and immediately emits a brilliant phosphoric light, which is generally violet. Some specimens possess the property to a greater extent than others. Certain pale-yellow crystals will give the phenomenon when thrown into hot water, and a green variety called chlrophane becomes luminous when held in the hand. Very perfect and transparent crystals will not become phosphorescent until their surface has been roughened, or until they have been fractured. And recently it has been shown that colourless crystals can be tinted deep blue by the β and γ rays of radium. On gently warming these they show a beautiful green luminescence which fades and changes to the pale violet light characteristic of most fluorspar.

Crystals of fluorspar may be colourless or show an extensive range of colour varying through violet, blue, green, yellow, orange and pink. Fine specimens are found at Weardale in Durham, near Castleton in Derbyshire, and in Cumberland. The cause of these colours has long been a subject of discussion. They are destroyed when the crystals are raised to a higher temperature, and in some instances change of colour is effected by exposure to sunlight, when certain greens are turned to purple. No doubt the remarkable phenomenon of fluorescence is closely associated with thesechanges. By transmitted light a crystal may appear pinkish or greenish, but when viewed in reflected light it exhibits a rich plum-blue with a velvety softness diffused immediately beneath the surface.

Thermoluminescence

One may experiment upon a number of minerals for luminescence by reducing them to a moderately fine powder, and sprinkling small portions on a thick iron plate heated to a temperature just below visible redness. The operation must be conducted in a dark room, and any light from the source of heat should be carefully obscured. Some specimens appear luminous for only a few seconds, while others keep up the emission of light for a considerable time.

The intensity and duration, and the colour of the light produced depend upon the nature of the mineral, and is independent of the colour of the mineral. The light emitted by fluorspar is generally bluish-violet or yellowish-green, but the variety chlorophane gives a bright green luminescence. Pink crystals have given a green light, and purple crystals a yellowish light. Harmotome, one of the zeolites, shines with a greenish-yellow glow ; topaz gives a bluish or bright blue colour ; and rubellite a pinkish or sometimes vivid-scarlet phosphorescence. Varieties of calcareous spar glow with a golden yellow tone and some specimens of diamond shine with a brilliant white phosphoric light. Anatase is entirely different from other minerals, as its phosphorescence appears suddenly like a flame and dies whilst emitting white flashes.

Crushed Sugar And Fire Pebbles

Some minerals and numerous chemical products glow when they are struck or broken into pieces, excited by friction or cleavage, or whilst they are crystallising from solution or after fusion. When a lamina of mica, for instance, is divided by cleavage a feeble emission of light is perceived at the moment the separation of the two plates occurs. Some varieties of felspar and other cleavable minerals are found to do the same.

When lumps of sugar are broken in the dark a very remarkable display is observed. Each fissure produced by the shock of the pestle gives birth to a streak of faint blue light which lasts for an instant, and when a quantity of sugar is rapidly ground down in a mortar, the whole mass appears as though on fire. This phenomenon is very striking if transparent felspar is used in place of sugar, or when crystals or uranium nitrate are rapidly shaken in a glass tube. Certain varieties of blendet the native sulphide of zinc, emit a fine golden-yellow light by percussion, or on being scratched with a knife. And it is fairly well known that the pebbles of milk quartz found along the seashore give a bluish light on being struck or rubbed briskly together. This emission of light will occur under water, and invariably it is accompanied by the production of a peculiar sulphurous odour.

There are other examples of mystic light which may be mentioned. When boracic acid is melted in a crucible and allowed to cool, it cracks and splits as the temperature of the mass falls, and at the same time throws out brilliant flashes of light. Vanadic acid acts very similarly and after fusion glows with a red light during the whole of the time crystallisation proceeds. Stibnite, the native sulphide of antimony, becomes luminous when heated to a dull red heat and gives a very brilliant incandescence.

Electroluminescence

Strong phosphorescence is exhibited by some gem-stones, such as the diamond and the ruby, when they are exposed to kathode rays in a vacuum. Electric discharges transmitted along the surface of certain bodies will produce the same effect. Sulphate of barium will give a bright yellowish-green, acetate of potash a brilliant green, and rock crystal a dull red light which gradually fades to white.

Phosphorescence is generally looked upon as an unexplained and mysterious production of light. When a phosphorescent body is exposed to the more-active rays of the sun it seems that the energy of those rays, that is their vibration, is stored up in some form of strain. In the dark there is probably a more or less recovery of this vibration and phosphorescent light may be produced. That is the theory of phosphorescence after insulation, as displayed by the luminous sulphides. The cause of other types of mystic light, such as the brilliant twinkling observed when sodium fluoride crystallises from solution, are at present unexplained.

The luminous properties of the Bologna Stone, discovered by Cascariolo, and its related compounds, have already been recognised and have found application as luminous paints for watch-dials, prismatic compasses and gun-sights, and in various scientific instruments.

(English Mechanics.)