A second class of self luminous substances are those in which this peculiarity is permanent and which are sometimes called pyrophors (fire bearers). Their phosphorescence consists in the diffusion of a pale light without manifestation of fire, the same as the well known appearance of phosphorus their prototype in the dark. In some of these substances the phosphorescence is caused by slow combustion; in others, such as some living and dead animals, and vegetable membranes, as well as some mineral substances, it depends upon causes not within our control, and therefore omitted from our selections; in others, again, it is produced by the illumination of the sun's rays by or during heating and other causes. Most of those which show a phosphorescence, after having been exposed to the sunlight, do the same upon being heated. The explanation of these peculiarities is beyond our limits, and . more properly belongs to philosophy.

To produce phosphorescence by means of the sunlight it is only required to expose the substance for some minutes to the direct rays of the sun, and then exhibit it at once in the darkest place that can be selected. In some instances this state of illumination continues for several days; in others, it passes off after a few hours or even minutes. The latter peculiarity is dependent upon certain circumstances attending the preparation of the substances and upon the manner in which they are preserved, which should be in all cases complete seclusion from the air in well-stopped glass vessels.

The color of the light exhibited by these substances varies with their nature, and the degree of heat to which they have been exposed. A certain scale of light and color may, therefore, be produced by grouping together different substances or samples of the same substance, each having been heated at a different temperature.

Sulphuret Of Barium

Finely powdered sulphate of baryta, which must be free from iron, is formed into balls with mucilage of gum tragacanth; the balls are dried at a moderate temperature, then placed in common crucibles, and kept at a red heat for an hour. They are then allowed to cool slowly, and while still warm are transferred into flasks with well-fit-ting glass stoppers, which may be further secured with wax or tallow. The phosphorescence of this substance is much improved when the baryta has been mixed with three per cent of magnesia.

Sulphuret Of Strontium

Sulphate of strontia, treated in the same manner as the above, produces the same effects.

Sulphuret Of Calcium

Oyster shells are calcined to perfect whiteness in a crucible, freed by dusting from the ashes, and then piled inside down in a crucible, with a layer of flowers of sulphur between each two. The crucible is then covered and exposed to a dull red beat for half an hour or more.

Sulpuuret Of Calcium And Antimony

Three parts of calcined oyster shells, treated with a mixture of 10 parts of flowers of sulphur and one of oxide of zinc, produce the same effects.

Chloride Of Calcium

Fuse chloride of calcium in a crucible, and pour it out on a bright iron surface or slab; break it into pieces when cold, and transfer it into well-stopped flasks. If no chloride of calcium is at hand, calcine a mixture of 7 parts of burnt lime, slacked to dry powder, with 15 parts of salt ammoniac.

Nitrate Of Lime

Make a solution of chalk or marble in nitric acid, evaporate to dryness, and fuse in a porcelain crucible.

These substances may bo made self-luminous also by other means besides the sunlight; for instance, by heating or pounding. A succession of experiments may be performed with them, after their phosphorescence from insulation has ceased, by first warming them in the hand, then in a water bath, and afterwards on a stone plate, each successive increase of temperature producing a fresh exhibition of light

Quinia And Sulphate Of Quinia

These two substances shine with remarkable intensity when slightly heated. Spread either of them over a sheet of paper, and warm it by holding it over a lamp. When this is withdrawn, a light will commence to spread from the edges to the centre, continuing sometimes for several minutes.

Boracic Acid

When boracic acid is fused, and is then allowed to cool, it will spontaneously, or rather by concentration, break into pieces, and along the cracks caused thereby a bright light will appear, strong enough to be visible even in daylight. Sulphate of potash, fused with cream of tartar and salts, shows the same phenomenon.

Phosphorus

As this is one of the most dangerous substances to experiment on, we shall restrict ourselves to a few experiments which are accompanied by no particular danger, but are still to be performed with great care.

In the first chapter of this work we have given all the precautions necessary, in handling phosphorus, to avoid danger.

Phosphuretted oil is the best means of exhibiting the luminous properties of this substance. The phosphorus dissolves in olive oil to some extent when melted. A small dry piece, of the size of a pea, being placed in a test-tube with oil, is held in warm water, and when melted shaken until it no longer dissolves, and then poured off clear into a small vial which is to be well-stopped, and preserved in a dark place. When this vial is opened in the dark, its contents will shine with the same light as phosphorus itself. The light disappears 15 upon closing the vial, but appears again on reopening it, and is intensified by shaking. It loses this property, however, after some time, when frequently opened. Characters written on paper with fresh phosphuretted oil appear in the dark with a bright glare.

Phosphuretted ether is prepared by digesting finely divided phosphorus in a closed vial with sulphuric ether for some weeks, with frequent shakings. A piece of sugar moistened with this ether and dropped into a vessel with tepid water will make the surface of the latter appear quite luminous in the dark.

The luminosity of phosphorus depends, for the greater part, upon its property of burning in contact with the air, by which a slight amount of light and heat is evolved, analogous to the strongly marked appearance of light and heat when it bursts violently into flame. Yet some of these phenomena, such as arc evidently connected with any combustion, cannot thus be explained. Phosphorus is luminous, also, during its evaporation or sublimation, as is also sulphur. Most of these phenomena we are unable to deduce from any single one of these causes, but originate more generally in both.

Sulphur

Roll sulphur, drawn over a warm brick, shows its marks in the dark with a bright light.