The power in minerals, plants, and animals of producing light is apparently a consequence of these objects being under the direct influence, permanently, or for a time, of heat, light, or electricity, as some substances become phosphorescent after insolation, or exposure to the sun's rays; others, from heat: others, by having an electric current passed through them; and lastly, some give forth a phosphoric light of their own, without any appreciable warmth. Whatever may be the cause of this property, it is found to pervade all parts of creation: the atmosphere, the common stones by the wayside, the flowers in cottage gardens, and the humble insects or worms crawling at our feet, can shed around a faint glimmer of light. The earth itself is occasionally, if not always, self-luminous, as are other of the heavenly bodies. Venus, Jupiter, the moon, and comets, are conjectured to have a certain portion of phosphoric light, which is independent of and unborrowed from the sun. The luminosity of the earth is made evident to us on starless, moonless nights. We may not have thought of it, but still it is certain that light surrounds us from some source or other in varying quantities, on such nights as are above described; for our movements are very different, even when walking in the open air on the darkest nights, from what they would be in a cave, or when groping in a room with closed shutters. This phase of phosphorescence, and also that of faint flickering clouds against the horizon, is distinct from meteorological phosphorescence, which branch of the subject includes luminous rain, fog, dust, ignis-fatuus, northern and southern lights. A shower of dust which fell during an eruption of Vesuvius in 1794, had a faint luminosity in the dark, distinctly visible on the sails of vessels on which it had fallen. Many instances are recorded of rain producing sparks as it touched the ground, and Arago collected the authentic accounts of this phenomenon. In June, 1731, an ecclesiastic near Constance described the rain during a thunder-storm as falling like drops of red-hot liquid metal; and it is observable that most of these sparkling showers seem to have occurred during thunder-storms, or when the air was highly charged with electricity.
But complete mystery still surrounds the cause of luminous fogs and mists, [{771}] which are of rare occurrence. Of these there are few well founded accounts, and the most recent instance of one was, we believe, in 1859, continuing for a succession of nights. It lasted from then 18th to the 26th of November, and, in the absence of any moon, so illuminated the heavens as to render small objects distinctly visible in the sitting room of M. Wartenan of Geneva, whose description of it will be found in the Comptes Rendus of the Academy of Sciences, Paris, for December, 1859. It was not a wet fog, but a sort of dry mist, so impenetrable as to render invisible the banks of the river Leman, but at the same time diffusing sufficient phosphoric light to make small objects clear as on a moonlight night. This was also testified by persons travelling on foot from Geneva to Annemasse, between the hours of 10 and 12 P.M. Another famous instance was in 1783, when a dry fog, lasting for a month, covered the northern parts of America, and Europe from Sweden to Africa. It resembled moonlight through a veil of clouds, and was equally diffused on all sides, making objects visible at a distance of six hundred yards. Being, as it were, a deep mass of phosphoric vapor, reaching to the summit of the highest mountains, no storms of rain or wind seemed to affect it; but in Europe it was thought to emit an unpleasant sulphurous smell.
Another feature of meteorological phosphorescence is that of luminous appearances at sea, quite distinct from the luminosity of the ocean itself as produced by marine animalcula. Mrs. Somerville gives the following interesting description of one of these phosphoric phenomena: "Captain Bonnycastle, coming up the Gulf of St. Lawrence on the 7th of September, 1826, was roused by the mate of the vessel in great alarm from an unusual appearance. It was a starlight night, when suddenly the sky became overcast in the direction of the highland of Cornwallis country, and an instantaneous and intensely vivid light, resembling the aurora, shot out of the hitherto gloomy and dark sea on the lee-bow, which was so brilliant that it lighted every thing distinctly, even to the mast-head. The light spread over the whole sea between the two shores, and the waves, which before had been tranquil, now began to be agitated. Captain Bonnycastle describes the scene as that of a blazing sheet of awful and most brilliant light. A long and vivid line of light, superior in brightness to the parts of the sea not immediately near the vessel, showed the base of the high, frowning, and dark land abreast; the sky became lowering and more intensely obscure. Long tortuous lines of light showed immense numbers of very large fish darting about, as if in consternation. The sprit-sail-yard and mizzen-boom were lighted by the glare, as if gas-lights had been burning directly below them; and until just before daybreak, at four o'clock, the most minute objects were distinctly visible. Day broke very slowly, and the sun rose of a fiery and threatening aspect. Rain followed. Captain Bonnycastle caused a bucket of this fiery water to be drawn up: it was one mass of light when stirred by the hand, and not in sparks, as usual, but in actual coruscations. A portion of the water preserved its luminosity for seven nights. On the third night, scintillations of the sea reappeared; in the evening the sun went down very singularly, exhibiting in its descent a double sun; and when only a few degrees high, its spherical figure changed into that of a long cylinder, which reached the horizon. In the night the sea became nearly as luminous as before; but on the fifth night the appearance entirely ceased. Captain Bonnycastle does not think it proceeded from animalcula, but imagines it might be some compound of phosphorus, suddenly evolved, and disposed over the surface of the sea; perhaps from the exuviae or secretions of fish connected with the oceanic salts, muriate of soda and sulphate of magnesia."
Quite distinct from luminous mists is another species of phosphoric phenomenon in the shape of luminous bodies of considerable size and brilliancy. We find Arago saying, in 1838, "that great luminous meteors, similar to lightning in their nature, show themselves sometimes at the surface of the globe, even when the sky does not appear stormy." An instance of this is given by a Mr. Edwards, as having been seen by him when crossing Loch Scavig in a boat at night. In this instance, a light swept rapidly over the face of the water, resembling the light in a cabin window, but moving with great rapidity. It passed near the boat, and caused much consternation among the boatmen, who viewed it as something supernatural; but it was soon out of sight, following a curved course. A far more startling occurrence was seen by the ship Montague when "a few minutes before mid-day, and in perfectly serene weather, a large bluish globe of fire rolled up to the ship, the Montague, and exploded, shattering one of the masts. This globe of fire appeared as large as a millstone." This appearance does not seem to have had the swiftness of motion we should expect if it had been a species of globular lightning, but rather resembled a gigantic ignis-fatuus, which sometimes takes a globular form, and although generally attributed to the combustion of phosphuretted hydrogen gas, may and does arise from certain electrical conditions of the atmosphere. A remarkable ignis-fatuus is described by Dr. Shaw in his travels in the Holy Land. He observed it on Mount Ephraim, and it followed him for more than an hour. "Sometimes it appeared globular, at others it spread itself to such a degree as to involve the whole company in a pale inoffensive light; then it contracted itself, and suddenly disappeared, but in less than a minute would appear again; sometimes running swiftly along, it would expand itself over two or three acres of the adjacent mountains."
We will not dwell on other instances of ignis-fatuus, a phenomenon so common as to be known to all. But although this form of gas—phosphuretted hydrogen—has been long known as luminous, it is only since 1859 that gases in general have been discovered to possess phosphoric qualities when exposed to the sun's light. It is a remarkable fact, but one which has been proved, that, with the exception of metals, nearly all terrestrial bodies appear luminous when taken into the dark after insolation or exposure to the sun. They absorb so much light as to give it back again when removed from its influence, and this property is opposed to electricity, for we find that good conductors of that fluid are not liable to insolated phosphorescence. The first discovery of this property was made by Viscenzo Cascariolo, a shoemaker of Bologna, who, loving alchemy, and seeking gold, found in his ramble a heavy stone, from which he hoped and longed to produce the precious metal. Failing in this, he found what till then was unknown, that sulphuret of baryta would "absorb the sun's rays by day, to emit them by night." From him this substance has received the name of Bologna stone; and this first discovery has been followed by others, which prove that phosphoric light may be produced by heat, friction, cleavage, and many other forces beside insolation. Some diamonds shine in the dark after a few minutes' exposure to the sun; others cannot be made phosphorescent by heat if uncut, but when polished, or submitted to two or three electric discharges, easily become luminous. So slight a heat is required to call forth this light-giving property in some substances, that rare kinds of clorophane shine in a dark room from the mere warmth of the hand; and other substances are phosphorized by the slightest friction. Thus Dana says: "Merely the rapid motion of a feather across some specimens of sulphuret of zinc will often elicit light more or less intense from this metal."
Several simple and amusing experiments may be made to show the [{773}] phosphorescence of minerals. The power of cleavage to produce light is seen when sugar is broken in a mortar. If a sufficient quantity is ground rapidly in the dark, the whole will appear a mass of fire. If phosphuretted hydrogen is evolved by throwing phosphuret of calcium into water, each bubble as if rises will fire spontaneously on combining with the air. But the most elegant production of light is the result of an experiment by Professor Pontus in 1833: "He showed that a vivid spark is produced when water is made to freeze rapidly. A small glass, terminating in a short tube, is filled with water; the whole is covered with a sponge or cotton-wool imbibed with ether, and placed in an air pump. As soon as the experimenter begins to produce a vacuum, the ether evaporates, and the sponge or cotton-wool descends, the temperature of the water rises rapidly. But some instants before congelation takes place, a brilliant spark, perfectly visible in the daytime, is suddenly shot out of the little tube that terminates the glass globe."
Before passing on to the consideration of animal phosphorescence, let us glance at the luminosity of plants. This is found in many phanerogams and cryptogams. In the latter, it is well known, from being found frequently in mines, where the fungus mycelium is seen spreading its web-like growth, and diffusing a tranquil light, sufficiently strong to read by, as some have affirmed. The most beautiful instance of this is found in the mines in Hesse, where the galleries for supplying air are illumined with this soft phosphoric light. No example of phosphorescence among sea-weed has been known, but the delicate little moss Schistostega osmundacea is luminous. Among phanerogams, or ordinary plants, are many examples of phosphorescence. Several kinds of garden nasturtiums, sun-flowers, French and African marigolds, yellow lilies, and poppies, have been seen to emit either sparks or a steady light. By some it is thought that it is produced when the pollen flies off and is scattered over the petals, but it is invariably noticed on warm tranquil evenings, when there is electricity in the atmosphere. It is observed that nearly all the flowers proved to be phosphoric are of a yellow color, but the cause of this has not been ascertained. The leaves of an American plant (OEnothera macrocapa) have been seen, during a severe storm of thunder and lightning, to emit brilliant flashes of light, and this is, we believe, the only plant as yet discovered with phosphoric foliage. M. Martins of Montpellier has noticed that the juice of the Euphorbia phosphorea, when rubbed on paper, appears luminous in the dark, or when heated. But the most remarkable instance is that of the common potato emitting a brilliant light: Mr. Phipson states that a soldier of Strasburg thought that the barracks were, on one occasion, on fire, from the light which was found to proceed from a cellar full of potatoes. It is a question whether they were in a state of decomposition, and if so, it differs slightly from the luminosity of decaying wood, which is usually caused by the presence of phosphoric fungi.
To attempt to enumerate the animals of inferior organism which are phosphoric would be impossible, as almost every known zoophyte is possessed of this light-giving quality; and perhaps no branch of the subject has received so much attention as that which concerns animals, from the fact of the phosphorescence of dead animal matter and insects being phenomena of daily occurrence. On the former, very early observations were made. In 1592, Fabricius d'Acquapendente relates the astonishment of three Roman youths who found the remains of their Easter lamb shining like candles in the dark. Nearly a century later, Robert Boyle described the phosphorescence of a neck of veal "as a very splendid show," and in a paper in the Philosophical Transactions tried to [{774}] account for it. It is found that flesh will continue luminous about four days.
Among the insect-world there are numerous light-giving members. The common glowworm needs no description, and the lantern flies of the tropics are almost as well known. Tropical regions abound with these fire-flies, seventy kinds of which are found in South America and the southern states of the northern continent. Some of them emit the light from the abdomen, others from the head. The famous Fulgora lanternaria, or lantern-fly of Linnaeus, produces the light from the long transparent horn or proboscis curving upward from the head. The light of one of these is sufficiently bright to read a newspaper by, and two or three of them in a bottle is the common form of lamp. The natives also light their way on a dark night by tying one or two at the end of a stick. The Noctua psi, a little gray night-flying moth, is luminous, as also are some kinds of caterpillars; and the cricket and "daddy long-legs" have the same property attributed to them by some naturalists. The reader cannot fail to have noticed that there is no instance recorded of any larger animal producing phosphoric light. Invisible animalcula and insects are numerous, and of late years the common earthworm, or Lambricus, has been proved beyond doubt to have a phosphoric power; but beyond this, and the crawling centipede (Scolopendra), there is no animal with light-giving power. The gleaming light seen in the eyes of cats, dogs, and wild animals has been called phosphoric; but this is doubtful, and more nearly resembles some phase of reflected light. Humboldt, and later the natural historian, Reuger, speak of a monkey, Nyctipithecus trivirgatus as having eyes so brilliant as to illumine objects some inches off.