to be emitted for thirty or forty minutes. It must, however, be said in fairness, that even in this experiment an advocate in favor of the hypothesis might ma ntain that the continuance of light is due to some extent, if not wholly, to the residual oxygen still remaining in the air-tubes of the insect after removal from the air to the hydrogen or carbonic acid. But whaever it may be due to, it doubtless subserves many useful purposes.

The light of the little organic lamp illuminates the insect's path, and probably discloses to its minute and sensitive eyes that of which it is in quest, although at times it may be a source of danger, as when it serves as a mark for some voracious bird, which, like Cowper's nightingale, is in want of a supper.

Numerous species of insects belonging to the genus Elater are phosphorescent, and they are generally known as fireflies, and are referred to by Southey in "Madoc," and by Longfellow, who in the "Song of Hiawatha" gives us the red man's idea of their character.

As we have said, the different kinds of fireflies are very numerous. Kirby and Spence state that from Chili to the south of the United States, there are seventy distinct species. The Elater noctilucus of Latreille has perhaps been most studied. It is of a dark-brown color, attains to a length of about one inch and a half, and has two yellow spots on its back, which shine very brightly at night. Hidden under the wing-cases there exist two other luminous spots, so that when the insect is flying it shows four lights of great brilliancy as such lights go. The light it emits is more vivid than that given out by the glow-worms, and it is said that the light emitted by the two spots on its back is sufficient alone to read small print by,

Phosphorescence is often an accompaniment of the cessation of animal and vegetable life. The bodies of most marine animals shine after death, and phosphorescence has been observed in the dead flesh of man, lamb, and calf. Dead fish, n.ore especially the herring and mackerel, are noted for their shining appearance when they have been exposed to the air for some time; but beyond having noticed what favors and what disfavors the phenomenon, we are very little better off in our knowledge of the subject than the ancients were.

It appears to have been made out that the phenomenon is not due to the presence of animalculæ, and from other observations it is believed that the phosphorescence is the result of some state which precedes putrefaction. From this it would seem to follow that the phosphorescence in these cases may be a physical one, allied to that presented by minerals such as we will presently describe.

The luminous appearance of decayed wood in the dark, which will probably be a sight the reader is quite familiar with, has been known from the earliest times, and is mentioned by Pliny, who refers to the light emitted by "the trunk of the oak when it has become rotten with old age." The luminosity displayed here has been attributed to a cobweb-like fungus; and respecting its physical cause it has been found that moisture increases it, and that an atmosphere of pure nitrogen is as favorable to its manifestation as one of pure oxygen. Decaying potatoes likewise emit a faint light in the dark.

But perhaps the most remarkable example of undoubted vegetable phosphorescence is that furnished by a red mushroom, the Agaricus olearius. During the night it emits a bluish light which is complementary to its color; in other words it is an organic instance unique in its wav, of the reciprocity of radiation and absorption, for during the day the fungus absorbs certain rays of the sun, and gives ont during the night somewhat similar though much less intense rays.

The behavior of the fungus has been studied by MM. Delille and Fabre, and from their separate observations it would appear that the young mushroor is phosphorescent for many successive nights, even when uprooted from the olive-tree at the foot of which it grows; that dampness or dryness of the air does not appear to influence its light, and that no elevation of temperature can be observed in the parts which shine. It would seem, therefore, not improbable that we have here a case of phosphorescenco similar to that of sulphide of calcium, and many other substances which require first to bo exposed to the rays of the sun before they will shine on their own account in the dark.

The cases of mineral phosphorescence are of surpassing interest, because of the readiness with which in most instances the phenomena may be produced, the softness and beauty of the light emitted, and the possibility there now seems of some of these phosphorescent substances being utilized for making luminous paints.

In November, 1877, a patent was applied for by Profes sor Balmain, under the title "Improvements in painting, varnishing, and whitewashing," and the patent covers tho mixing of phosphorescent substances with any vehicle that will form what is commonly called a paint, wash, or varnish. Small cards coated with Balmain's luminous paint are offered for sale, labeled "A trap to catch a sumbeam," and with such a trap the reader may try some very interesting experiments.

It will be found that if the card be exposed to the direct rays of the sun it shines with a somewhat violent light when it is removed at once into a dark room. This placing of a phosphorescent substance in sunshine is termed inso lation, from the Latin in, into; and sol, the sun.

There are a great many substances which are phosphorescent after insolation besides the sulphide of calcium, or Canton's phosphorus, as it is commonly called, which forms the basis of Balmain's paint. When the card is no longer luminous in the dark, take it into the sunshine again for a few minutes, and have resting on the paint some object-as, e.g., a penny-piece.

Upon taking the card into the dark cellar once more, it will be found that there is this time a dark, immutable and immovable shadow surrounded by a luminous surface. The paint may likewise be excited by holding it close to a gaslight, but it will be found that after a few experiments the paint has lost its phosphorescent property, owing to the absorption of the heat rays. This antagonism of the heat radiations to the manifestation of phosphorescence after insolation was known more than a century ago, Wilson having in 1775 pointed out that the rays of the violet end of the spectrum, where there is least heat, cause a vivid phosphorescence in the sulphide of calcium, while the rays at the red end, where there is most heat, canse the phosphorescence produced by the other rays to cease. The phosphorescent sulphide of calcium was prepared by Canton by heating intensely for one hour a mixture of three parts of sifted calcined oyster-shells with one part of sulphur, which is materially the same as the plan adopted by Balmain, who heats together lime and sulphur, and the pro luct is then for painting purposes mixed with mastic varnish and a little turpentine. The nature of the light emitted varies with the method employed to prepare the sulphide, an orange-colored phosphorescence being obtained from sulphide of calcium which has been prepared from oyster-shells, while the light is much more refrangible, bluish, when the sulphide is made from carbonate of lime which has been precipitated.

Among the other substances which become phosphores

cent by insolation we may mention the diamond, and the following salts of lime-the nitrate, carbonate, phosphate, and arseniate. The sulphide of barium is likewise a remarkable phosphorescent body, and is said to have been the first substance that was known to become phosphorescent after insolation.

It is variously known as solar phosphorus, Bologna stone, or Bologna phosphorus, and may be made in the following manner: Mix the finely-powdered heavy spar, or sulphate, with gum, and calcine the paste thus obtained. The product of calcination is the sulphide of barium.

The length of time during which a substance continues to phosphoresce after insolation varies with its nature, as one would expect, and while some give out light for hours, others do not exhibit it even after the lapse of a second. Becquerel found, for example, that fluor-spar is seen to be phosphorescent only when not more than onethousandth of a second has passed since it was insolated. For such delicate determinations of duration he employed the phosphoroscope. The body to be experimented upon is placed in a cell within the instrument, and between two disks which are made to revolve. Each disk may have one or more sectorial apertures which are not opposite to another, so that upon turning the handle when there is nothing in the phosphoroscope the observer sees no light coming from the aperture next to, and passing his eye. When, however, a phosphorescent substance is in the cell, it receives a charge of light, if one may so speak, as the aperture in the disk furthest from the observer passes it, and if the light it emits after this sudden and short insolation last for a small fraction of a second, the observer on the other side sees it when the aperture in the disk nearest to him passes his eye.

Heat alone will produce phosphorescence in some bodies, and one of the most remarkable in this respect is the fluor-spar, or fluoride of calcium, which we have just seen is so weakly phosphorescent after insolation. If powdered fluor-spar be put on a plate of heated iron, not hot enough to be red, the powder will shine with a vivid phosphoric light. And a variety of the fluor-spar, called chlorophane, emits light at a temperature so low as 20° to 25° C. In illustration of this portion of our subject, the reader may try the following interesting and simple experiment: Heat one of the fire-irons, say the poker, to redness. Take it now into the cellar, and when it has cooled just sufficiently to emit no further light, rub the heated end over the whitewashed wall. The end of the poker is now illuminated by a white phosphoric light, and upon bringing it into daylight it will be seen that some of the whitewash has adhered to the iron. It is remarkable that whitewash is also said to be very faintly phosphorescent after insolation.

Many substances emit a phosphoric light when they are struck in the dark, and among these are chlorate of potash, felspar and sugar. Take two pieces of lump sugar into a dark room and strike them together. Every now and again faint flashes of light will be observed, thus furnishing us with a simple example of phosphorescence produced by percussion.

We have now to describe the remarkable cases of phosphorescence to be seen in empty space, under circumstances that have been discovered by Mr. Crookes.

A fairly good vacuum transmits an electric spark, while in a still better vacuum, well-balanced and very light bodies begin to move when the sun's rays fall on them. It has recently been shown that when the vacuum has been made so perfect that there is within the vacuous vessel a pressure of only about one millionth of an atmo

sphere, extraordinary phosphorescent phenomena are observed if the vessel be connected with an induction coil. At such a pressure the inner surface of the glass glow with a rich light, whose color, Crookes has shown, depends upon the nature of the glass vessel used; uranium glass giving a dark-green phosphorescence, English glass a blue, and soft German glass a bright apple-green phosphorescence. Crookes regards this phosphorescence as produced by the bombardment of the remaining molecules of gs against the sides of the glass, and his experiments would appear to show that these molecules are shot off the negative pole in straight lines, like rays of light. Many minerals placed in the path of the flying molecules exhibit a brilliant phosphorescence. A diamond, for example, that was mounted in the centre of an exhausted bulb, shone with as much light as a candle, phosphorescing with a bright-green light when the negative discharge was directed on to it. A collection of diamonds, lent to Crookes by Professor Maskelyne, exhibited, when treated in the same manner, the following colors of phosphorescence: apricot, red, orange, yellowish-green, pale-green and blue. Under similar circumstances rubies shine with a brilliant rich red color, as if they are glowing hot, and they emit this color of phosphorescent light whatever may be their natural color.

Many other curious facts were discovered in this investigation, and not the least important of these was that a sort of shadow is produced by an obstructing body placed in the path of the rushing molecules. Within a pearshaped bulb subsequently exhausted to the proper degree, there was placed a cross (b) so that it would be in the way of air particles rushing from the negative pole (a), when the apparatus was joined up to the induction coil. Under the influence of the air particles flying from a, all parts of the bulb, save a cross-shaped space at the broad end, soon exhibited a phosphorescent light presenting the appear. ance given at c d. Upon now shaking the aluminium cross, b, off its hinge, the perfectly fresh dark space at c d became luminous under the bombardment of the air particles, and so luminous in comparison with the wearied bacaground that had been phosphorescing from the commencement, that now the observer beheld a luminous cross on a comparatively black ground, ef. We see, therefore, that the negatively electrified molecules of air remaining in the bulb dash against anything that is in front, and cast shadows, as it were, of obstacles which stand in their way; that, where they are stopped by the glass, light is produced by the sudden arrest of velocity, and we may further add that it is accompanied by a rise of temperature.

We have now one more example of phosphorescence to consider, and then we have done, and it is that of the phosphorus with which we started. Perhaps, in none of the other cases we have mentioned can it be positively said that combustion is going on, but in this there is no doubt. Phosphorus very readily combines with the oxygen of the air, i.e., in ordinary language, it readily burns, and when it burns it gives out light. If it be burning fiercely it will give out a light that may dazzle the eyes, and the higher oxide of phosphorus will be formed. If, on the other hand, it be burning very slowly, the lower oxide is formed, and only a very faint light is emitted. It will therefore be seen that the phosphorescence of the match-track is due to the combustion of the trace of phosphorus left on it, the friction of the operation rising its temperature sufficiently to make it burn in the air.

We have seen, then, that there are many substancesanimal, vegetable, and mineral-which, under certain circumstances, are self-luminous, emitting a faint light,

trembling, of the phosphorescent body's molecules may be produced by the beating of air particles against it, as in the phenomena Crookes has so successfully studied, or by the wash of ether waves, as in the case of insolation.

Turning from matters theoretical to those practical, we cannot say as yet that phosphorescence has been utilized' in the affairs of life. It has, however, been proposed to use Balmain's luminous paint for painting the interiors of railway carriages, among other pur

poses, to the end that the phosphorescent mixture, after drinking in the sunbeams falling athwart the rushing train, might give them out again in dark tunnels for the benefit of passengers. The white man is, in short, treading in the steps of his red brother, who has for long been known to utilize the light of the wah-wah-tysee-attached to his hands and feet for night-traveling, and within his home for the benefit of his industrious squaw performing her evening wigwam duties.

A PHOSPHORESCENT DIAMOND.