Luminosity or phosphorescence is a property enjoyed by some organic and inorganic substances of emitting light without heat. These substances are more numerous than is generally supposed.

To commence with organic phosphor escence.

Flowers of a bright yellow or red colour, as the pot marigold (Calendula officinalis), the great Indian cress (Tro-pwotum majus), Lilium bulbosum, and the Papaver orientate, are especially noticeable as having this property, which is observable on fine summer evenings a little after sunset. The luminosity in these flowers, however, is only of very short duration; but some plants, as the pocan or Virginian poke (Phytolacca dicmdra),shine with a faintly luminous continuous light, which is sometimes of a bluish-green and sometimes of a yellowish-green light; also the freshly-drawn juice of the Cipe de Cananum, a Brazilian plant, which will often shine for several hours.

Cryptogamic growths are also in some cases phosphorescent, often to be noticed in vaults where dead bodies are deposited, and also in the Rhxzomorpha phosphorens, often found in the mines of Hesse, which exhibits a brilliant light when broken. The Rhizomorpha subter-ranea and certain other underground plants are also luminous; most woods, too, can be rendered phosphorescent if sufficiently decomposed and dry. This will account for the faint flickering light often to be noticed in copses and woods during autumn. The animal world abounds in living phosphori, belonging both to the land and water. The light has, it is asserted by some writers, been seen hovering about the heads and ears of horses, and even in a few rare instances of some children. The cause of this has given rise to much speculation, and some attribute it to electricity, and others to the slow combustion of phosphuretted hydrogen; the latter explanation seems most probable, though it cannot be said that either theory is incontrovertible; in these instances, however, the light is only of accidental occurrence.

There are, however, animals in whom phosphorescence forms an essential part of their vita; economy; such, for example, is the common glowworm (Lampyris noctiluca) and congeners, whose brilliant little gems of yellow light so plentifully besprinkle the hedgerows in June. With regard to this last insect, it is interesting to remark that while the male is a coleopterous insect, the female is apterous. The light of this little insect is far outshone in tropical countries by the dazzling lantern-fly (Falgora lan-tcrnaria) of South America, of the order Hemiptera, which is often nearly 4 in. long, whilst its luminous organ is a kind of proboscis about 1 in. long. Two or three, it is said, of these remarkable creatures are sufficient to light up a good-sized room. The firefly (JSlater noctilucus) is an interesting insect, together with its two congeners, Elatrr phosphorens and Mater ignitus. Another interesting phosphorescent insect often overlooked is the electric centipede (Scolopendra electrica). This, though very common in Britain, is rarely noticed, as it seldom leaves its hole during the night, but when it does it leaves a palpable luminous track behind, which continues phosphorescent some little time.

The cause of the light of the glowworm and similar insects has given rise to much conjecture and various theories. All doubt on the subject, however, seems now - thanks to Dr. Phipson -to be pretty well cleared up. Dr. Phipson has shown that in all cases of phosphorescent animals, both sea and land, the luminous principle is a fluid organic substance, which he calls "noctilucine," and which is totally distinct from any solution of phosphorus in oil or other medium, and which will continue to shine after being extracted from the animal; it is a fluid organic substance containing nitrogen, and is produced in luminous animals by special organs, as the animal may require it. This noctilucine, it must be understood, is produced by both aquatic and land animals, and thus readily accounts for the luminous appearance of the sea, caused by numerous wonderful animalcula, and other inhabitants of the watery world belonging to the mollusca, vermes, crustacean, and other curious zoophytes. This latter phenomenon has long been misunderstood, and was thought at one time to be due, in some inexplicable way, to electricity, and later was attributed to putrescent particles of organic matter in the water.

Many have asserted that, when the sea is phosphorescent, it is indicative of a coming storm. Baird emphatically denies this assertion, but on the contrary conceives that the animalculae, etc, causing the luminosity, retire from the surface on change of weather. Dr. Maculloch noticed in the course of his investigations that the sea did not phosphoresce when of a blue colour, and also that the light was most brilliant when the wind was in the . and S.E. The Nereis noctiluca, discovered by Vianelli, is an inhabitant of every sea, and is an active agent in the production of phosphorescence, and it is the multitudes of the minute Noctiluca miliaris which causes the oft-remarked luminosity of the English Channel. The first phosphorescent aquatic animal of which we find record is one mentioned in Pliny's writings - the Photos dactylus, which caused the ancients much wonder, as it was said to render luminous the mouths of those who partook of it. It is a species of chonchiferous mollusca, protected by a testaceous shell.

To Sir J. Banks we owe the knowledge of two other interesting luminous sea animals - viz., the Cancer fulgens and Medusa pellucens, both of which were discovered on the coast of Brazil.

The luminous properties of minerals have received from men of science a good deal of attention, and have led in consequence to the elucidation of many interesting facts. Margraaf has ascertained that all the earthy sulphates, when calcined, exhibit this property, and that the metals, metallic ores, and agates are not phosphorescent; the former portion of the discovery had already been partly made known by a shoemaker of Bologna named Vincenzo Cascariola, the inventor of the famous Bolognian stone, which is prepared by strongly heating heavy spar (sulphate of baryta) with gum tragacanth. Most of the' minerals having phosphorescent properties only exhibit them after insolation or exposure to the sun. Of these the following are good examples: the diamond and several other precious stones; the Bolognian stone, mentioned abqve; Canton phosphorus, prepared from water-worn oyster shells, calcined with sulphur; it appears on the mass as a white coating, which is scraped off, and should be kept in stoppered bottles. Dr. Fry, in 1874, fount similar properties in gypsum, marble, and chalk; and Baldwin, of Misnia, in 1677, discovered that the residue of chalk in nitric acid was similarly affected.