the most common acceptation of the word, signifies that invisible etherial matter which makes objects perceptible to our senses of seeing. Figuratively, it is also used for whatever conveys instruction to our minds, and likewise for that instruction itself.

For an account of the chemical properties of light, see Chemistry Index; and for its physical properties, see Optics.

Light independent of Heat. In general, a very considerable degree of heat is requisite to the emission of light from any body; but there are several exceptions to this, especially in light proceeding from putrefied substances and phosphorus, together with that of luminous animals, and other similar appearances. Light proceeding from putrefied animal and vegetable substances, as well as from glow-worms, is mentioned by Aristotle. Thomas Bartholin mentions four kinds of luminous insects, two with wings, and two without; but in hot climates travellers say they are found in much greater numbers, and of different species. Columna, an industrious naturalist, observes, that their light is not extinguished immediately upon the death of the animal.

The first distinct account that we meet with of light proceeding from putrefied animal flesh is that which De Visonne, given by Fabricius ab Aquapendente; who says, that when three Roman youths, residing at Padua, had bought a lamb, and had eaten part of it on Easter day 1562, several pieces of the remainder, which they kept till the day following, shone like so many candles when they were casually viewed in the dark. Part of this luminous flesh was immediately sent to Aquapendente, who was professor of anatomy in that city. He observed, that both the lean and the fat of this meat shone with a whitish kind of light; and also took notice, that some pieces of kid's flesh, which had happened to have lain in contact with it, were luminous, as well as the fingers and other parts of the bodies of those persons who touched it. Those parts, he observed, shone the most which were soft to the touch, and seemed to be transparent in candlelight; but where the flesh was thick and solid, or where a bone was near the outside, it did not shine.

After this appearance, we find no account of any other similar to it, before that which was observed by Bartholin, and of which he gives a very pompous description in his ingenious treatise already quoted. This happened at Montpelier in 1641, when a poor old woman had bought a piece of flesh in the market, intending to make use of it the day following. But happening not to be able to sleep well that night, and her bed and pantry being in the same room, she observed so much light come from the flesh, as to illuminate all the place where it hung. A part of this luminous flesh was carried as a curiosity to Henry Bourbon, duke of Condé, the governor of the place, who viewed it for several hours with the greatest astonishment.

This light was observed to be whitish; and not to cover the whole surface of the flesh, but certain parts only, as if gems of unequal splendour had been scattered over it. This flesh was kept till it began to putrefy, when the light vanished; which, as some religious people fancied, it did in the form of a cross.

It is natural to expect, that the almost universal experimental philosopher Mr Boyle should try the effect of his air-pump upon these luminous substances. Accordingly, we find that he did not fail to do it; when he presently found that the light of rotten wood was extinguished in vacuo, and revived again on the admission of the air, even after a long continuance in vacuo; but the extinguishing of this light was not so complete immediately upon exhausting the receiver, as some little time afterwards. He could not perceive, however, that the light of rotten wood was increased in condensed air; but this, he imagined, might arise from his not being able to judge very well of the degree of light, through so thick and cloudy a glass vessel as he then made use of; but we find that the light of a shining fish, which was put into a condensing engine before the Royal Society, in 1668, was rendered more vivid by that means. The principal of Mr Boyle's experiments were made in October 1667.

This philosopher attended to a great variety of circumstances relating to this curious phenomenon. Among other things he observed, that change of air was not necessary to the maintenance of this light; for it continued a long time when a piece of the wood was put into a very small glass hermetically sealed, and it made no difference when this tube which contained the wood was put into an exhausted receiver. This he also observed with respect to a luminous fish, which he put into water, and placed in the same circumstances. He also found, that the light of shining fishes had other properties in common with that of shining wood; but the latter, he says, was presently quenched with water, spirit of wine, a greater variety of saline mixtures, and other fluids. Water, however, did not quench all the light of some shining veal on which he tried it, though spirit of wine destroyed its virtue presently.

Mr Boyle's observation of light proceeding from flesh meat was quite casual. On the 13th of February 1662, one of his servants was greatly alarmed with the shining of some veal, which had been kept a few days, but had no bad smell, and was in a state very proper for use. The servant immediately made his master acquainted with this extraordinary appearance; and though he was then in bed, he ordered it to be immediately brought to him, and he examined it with the greatest attention. Suspecting that the state of the atmosphere had some share in the production of this phenomenon, he takes notice, after describing the appearance, that the wind was south-west and blustering, the air hot for the season, the moon was past its last quarter, and the mercury in the barometer was at 29½ inches.

Mr Boyle was often disappointed in his experiments on shining fishes; finding that they did not always shine, shine in the very same circumstances, as far as he could judge, with others which had shined before. At one time that they failed to shine, according to his expectations, he observed that the weather was variable, and not without some days of frost and snow. In general he made use of whitings, finding them the fittest for his purpose. In a discourse, however, upon this subject at the Royal Society in 1681, it was asserted, that, of all fishy substances, the eggs of lobsters, after they had been boiled, shone the brightest. Olig. Jacobus observes, that, upon opening a sea-polypus, it was so luminous as to startle several persons who saw it; and he says, that the more putrid the fish was, the more luminous it grew. The nails also, and the fingers of the persons who touched it, became luminous; and the black liquor which issued from the animal, and which is its bile, shone also, but with a very faint light.

Mr Boyle draws a minute comparison between the light of burning coals and that of shining wood or fish, showing in what particulars they agree, and in what they differ. Among other things he observes, that extreme cold extinguishes the light of shining wood, as appeared when a piece of it was put into a glass tube, and held in a frigorific mixture. He also found that rotten wood did not waste itself by shining, and that the application of a thermometer to it did not discover the least degree of heat.

There is a remarkable shell-fish called Pholas, which forms for itself holes in various kinds of stone, &c., las, a remarkably That this fish is luminous, was noticed by Pliny; who luminous observes, &c. Light. observes, that it shines in the mouth of the person who eats it, and, if it touch his hands or clothes, makes them luminous. He also says that the light depends upon its moisture. The light of this fish has furnished matter for various observations and experiments to M. Reaumur, and the Bolognian academicians, especially Beccarius, who took so much pains with the subject of phosphorescent light.

M. Reaumur observes, that, whereas other fishes give light when they tend to putrefaction, this is more luminous in proportion to its being fresh; and that when they are dried, their light will revive if they be moistened either with fresh or salt water, but that brandy immediately extinguishes it. He endeavoured to make this light permanent, but none of his schemes succeeded.

The attention of the Bolognian academicians was engaged to this subject by M. F. Marfilius, in 1724, who brought a number of these fishes, and the stones in which they were enclosed, to Bologna, on purpose for their examination.

Beccarius observed, that though this fish ceased to shine when it became putrid; yet that in its most putrid state, it would shine, and make the water in which it was immersed luminous, when it was agitated. Galeatus and Montius found, that wine or vinegar extinguished this light; that in common oil it continued some days; but in rectified spirit of wine or urine, hardly a minute.

In order to observe in what manner this light was affected by different degrees of heat, they made use of a Reaumur's thermometer, and found that water rendered luminous by these fishes increased in light till the heat arrived to 45 degrees; but that it then became suddenly extinct, and could not be revived.

In the experiments of Beccarius, a solution of sea salt increased the light of the luminous water; a solution of nitre did not increase it quite so much. Sal ammoniac diminished it a little, oil of tartar per deliquium nearly extinguished it, and the acids entirely. This water poured upon fresh calcined gypsum, rock crystal, cerule, or sugar, became more luminous. He also tried the effects of it when poured upon various other substances, but there was nothing very remarkable in them. Afterwards, using luminous milk, he found that oil of vitriol extinguished the light, but that oil of tartar increased it.

This gentleman had the curiosity to try how differently coloured substances were affected by this kind of light; and having, for this purpose, dipped several ribbons in it, the white came out the brightest, next to this was the yellow, and then the green; the other colours could hardly be perceived. It was not, however, any particular colour, but only light that was perceived in this case. He then dipped boards painted with the different colours, and also glass tubes, filled with substances of different colours, in water rendered luminous by the fishes. In both these cases the red was hardly visible, the yellow was the brightest, and the violet the dullest. But on the boards the blue was nearly equal to the yellow, and the green more languid; whereas in the glasses, the blue was inferior to the green.

Of all the liquors into which he put the pholas, milk was rendered the most luminous. A single pholas made seven ounces of milk so luminous, that the faces of persons might be distinguished by it, and it looked as if it was transparent.

Air appeared to be necessary to this light; for when Beccarius put the luminous milk into glass tubes, no agitation would make it shine, unless bubbles of air were mixed with it. Also Montius and Galeatus found, that, in an exhausted receiver, the pholas lost its light, but the water was sometimes made more luminous; which they ascribed to the rising of bubbles of air through it.

Beccarius, as well as Reaumur, had many schemes to render the light of these pholases permanent. For this purpose he kneaded the juice into a kind of paste, with flour, and found that it would give light when it was immersed in warm water; but it answered best to preserve the fish in honey. In any other method of preservation, the property of becoming luminous would not continue longer than six months, but in honey it had lasted above a year; and then it would, when plunged in warm water, give as much light as ever it had done.

Similar, in some respects, to those observations on Asla Cerfa the light of the pholas, was that which was observed renfa, to proceed from wood which was moist, but not in a p. 485 putrid state, which was very conspicuous in the dark.

That the sea is sometimes luminous, especially when light from it is put in motion by the dashing of oars or the sea water-beating of it against a ship, has been observed with admiration by a great number of persons. Mr Boyle, after reciting all the circumstances of this appearance, as far as he could collect them from the accounts of navigators; as its being extended as far as the eye could reach, and at other times being visible only when the water was dashed against some other body; that, in some seas, this phenomenon is accompanied by some particular winds, but not in others; and that sometimes one part of the sea will be luminous, when another part, not far from it, will not be so; concludes with saying, that he could not help suspecting that these odd phenomena, belonging to great masses of water, were in some measure owing to some cosmical law or custom of the terrestrial globe, or at least of the planetary vortex.

Some curious observations on the shining of some Dr Beale's fishes, and the pickle in which they were immersed, were made by Dr Beale, in May 1665; and had they fishes been properly attended to and purified, might have led to the discovery of the cause of this appearance. Having put some boiled mackerel into water, together with salt and sweet herbs; when the cook was, some time after, stirring it, in order to take out some of the fillies, the observer, that, at the first motion, the water was very luminous; and that the fish shining through the water added much to the light which the water yielded. The water was of itself thick and blackish, rather than of any other colour; and yet it shone on being stirred, and at the same time the fishes appeared more luminous than the water. Wherever the drops of this water, after it had been stirred, fell to the ground, they shined; and the children in the family diverted themselves with taking the drops, which were as broad as a penny, and running with them about the house. The cook observed, that, when she turned up that side of the fish that was lowest, no light came from it; Light; and that, when the water had settled for some time, it did not shine at all. The day following, the water gave but little light, and only after a brisk agitation, though the fishes continued to shine as well from the inside as the outside, and especially about the throat, and such places as seemed to have been a little broken in the boiling.

When in the light of the sun, he examined, with a microscope, a small piece of a fish which had shined very much the night before, he found nothing remarkable on its surface, except that he thought he perceived what he calls a steam, rather dark than luminous, arising like a very small and almost imperceptible sparkle. Of the sparkles he had no doubt; but he thought it possible that the steam might be a deception of the sight, or some dust in the air.

Finding the fish to be quite dry, he moistened it with his spittle; and then observed that it gave a little light, though but for a short time. The fish at that time was not fetid, nor yet insipid to the best discerning palate. Two of the fishes he kept two or three days longer for farther trial; but, the weather being very hot, they became fetid; and, contrary to his expectations, there was no more light produced either by the agitation of the water or in the fish.

Father Bourzes, in his voyage to the Indies in 1704, took particular notice of the luminous appearance of the sea. The light was sometimes so great, that he could easily read the title of a book by it, though he was nine or ten feet from the surface of the water. Sometimes he could easily distinguish, in the wake of a ship, the particles that were luminous from those that were not; and they appeared not to be all of the same figure. Some of them were like points of light, and others such as stars appear to the naked eye. Some of them were like globes, of a line or two in diameter; and others as big as one's head. Sometimes they formed themselves into squares of three or four inches long, and one or two broad. Sometimes all these different figures were visible at the same time; and sometimes they were what he calls varices of light, which at one particular time appeared and disappeared immediately like flashes of lightning.

Nor did only the wake of the ship produce this light, but fishes also, in swimming, left so luminous a track behind them, that both their size and species might be distinguished by it. When he took some of the water out of the sea, and stirred it ever so little with his hand, in the dark, he always saw in it an infinite number of bright particles; and he had the same appearance whenever he dipped a piece of linen in the sea, and wrung it in a dark place, even though it was half dry; and he observed, that when the sparkles fell upon any thing that was solid, it would continue shining for some hours together.

After mentioning several circumstances which did not contribute to this appearance, this father observes, that it depends very much upon the quality of the water; and he was pretty sure that this light is the greatest when the water is fattest, and fullest of foam. For in the main sea, he says, the water is not everywhere equally pure; and that sometimes, if linen be dipped in the sea, it is clammy when it is drawn up again; and he often observed, that when the wake of the ship was the brightest, the water was the most fat and glutinous, and that linen moistened with it produced a great deal of light, if it was stirred or moved briskly. Besides, in some parts of the sea, he saw a substance like saw dust, sometimes red and sometimes yellow; and when he drew up the water in those places, it was always viscous and glutinous. The sailors told him, that it was the spawn of whales; that there are great quantities of it in the north; and that sometimes, in the night, they appeared all over of a bright light, without being put in motion by any vessel or fish passing by them.

As a confirmation of this conjecture, that the more glutinous the sea water is, the more it is disposed to become luminous, he observes, that one day they took a fish which was called a bonite, the inside of the mouth of which was so luminous, that, without any other light, he could read the same characters which he had before read by the light in the wake of the ship; and the mouth of this fish was full of a viscous matter, which, when it was rubbed upon a piece of wood, made it immediately all over luminous; though, when the moisture was dried up, the light was extinguished.

The abbé Nollet was much struck with the luminousness of the sea when he was at Venice in 1749; let's theory, and, after taking a great deal of pains to ascertain the circumstances of it, concluded that it was occasioned by a shining insect; and having examined the water very often, he at length did find a small insect, which he particularly describes, and to which he attributes the light. The same hypothesis had also occurred to M. Vianelli, professor of medicine in Chioggia near Venice; and both he and M. Grizzellini, a physician in Venice, have given drawings of the insects from which they imagined this light to proceed.

The abbé was the more confirmed in his hypothesis, by observing, some time after, the motion of some luminous particles in the sea. For, going into the water, and keeping his head just above the surface, he saw them dart from the bottom, which was covered with weeds, to the top, in a manner which he thought very much resembled the motions of insects; though, when he endeavoured to catch them, he only found some luminous spots upon his handkerchief, which were enlarged when he pressed them with his finger.

M. le Roi, making a voyage on the Mediterranean, Observa- presently after the abbé Nollet made his observations of M. at Venice, took notice, that in the daytime, the prov le Roi, of the ship in motion threw up many small particles, which, falling upon the water, rolled upon the surface Memoires of the sea for a few seconds before they mixed with Prefectes, it; and in the night the same particles, as he con vol iii. 144, included, had the appearance of fire. Taking a quantity of the water, the same small sparks appeared whenever it was agitated; but, as was observed with respect to Dr Beale's experiments, every successive agitation produced a less effect than the preceding, except after being suffered to rest a while; for then a fresh agitation would make it almost as luminous as the first. This water, he observed, would retain its property of shining by agitation a day or two; but it disappeared immediately on being set on the fire, though it was not made to boil.

This gentleman, after giving much attention to this phenomenon, concludes, that it is not occasioned by any any thinning insects, as the abbé Nollet imagined; especially as, after carefully examining some of the luminous points, which he caught upon an handkerchief, he found them to be round like large pins heads, but with nothing of the appearance of any animal, though he viewed them with a microscope. He also found, that the mixture of a little spirit of wine with water just drawn from the sea, would give the appearance of a great number of little sparks, which would continue visible longer than those in the ocean. All the acids, and various other liquors, produced the same effect, though not quite so conspicuously; but no fresh agitation would make them luminous again. M. le Roy is far from asserting that there are no luminous insects in the sea. He even supposes that the abbé Nollet and M. Vianelli had found them. But he was satisfied that the sea is luminous chiefly on some other account, though he does not so much as advance a conjecture about what it is.

M. Ant. Martin made many experiments on the light of fishes, with a view to discover the cause of the light of the sea. He thought that he had reason to conclude, from a great variety of experiments, that all sea fishes have this property; but that it is not to be found in any that are produced in fresh water. Nothing depended upon the colour of the fishes, except that he thought that the white ones, and especially those that had white scales, were a little more luminous than others. This light, he found, was increased by a small quantity of salt; and also by a small degree of warmth, though a greater degree extinguished it. This agrees with another observation of his, that it depends entirely upon a kind of moisture which they had about them, and which a small degree of heat would expel, when an oiliness remained which did not give this light, but would burn in the fire. Light from the shells of birds or beasts is not so bright, he says, as that which proceeds from fish. Human bodies, he says, have sometimes emitted light about the time that they began to putrefy, and the walls and roof of a place in which dead bodies had often been exposed, had a kind of dew or clamminess upon it, which was sometimes luminous; and he imagined that the lights which are said to be seen in burying-grounds may be owing to this cause.

From some experiments made by Mr Canton, he concludes, that the luminousness of sea water is owing to the slimy and other putrefactive substances it contains. On the evening of the 14th of June 1768, he put a small fresh whiting into a gallon of sea water, in a pan which was about 14 inches in diameter, and took notice that neither the whiting nor the water, when agitated, gave any light. A Fahrenheit's thermometer, in the cellar where the pan was placed, stood at 54°. The 15th, at night, that part of the fish which was even with the surface of the water was luminous, but the water itself was dark. He drew the end of a stick through it, from one side of the pan to the other; and the water appeared luminous behind the stick all the way, but gave light only where it was disturbed. When all the water was stirred, the whole became luminous, and appeared like milk, giving a considerable degree of light to the sides of the pan; and it continued to do so for some time after it was at rest. The water was most luminous when the fish had been in it about 28 hours; but would not give any light by being stirred, after it had been in it three days.

He then put a gallon of fresh water into one pan, and an equal quantity of sea water into another; and into each pan he put a fresh herring of about three ounces. The next night the whole surface of the sea water was luminous without being stirred; but it was much more so when it was put in motion; and the upper part of the herring, which was considerably below the surface of the water, was also very bright; while at the same time the fresh water, and the fish that was in it, were quite dark. There were several very bright luminous spots on different parts of the surface of the sea water; and the whole, when viewed by the light of a candle, seemed covered with a greasy scum. The third night, the light of the sea water while at rest, was very little, if at all less than before; but when stirred, its light was so great as to discover the time by a watch, and the fish in it appeared as a dark substance. After this, its light was evidently decreasing, but was not quite gone before the 7th night. The fresh water and the fish in it were perfectly dark during the whole time. The thermometer was generally above 60°.

The preceding experiments were made with sea water; but he now made use of other water, into which he put common or sea salt, till he found, by an hydrometer, that it was of the same specific gravity with the sea water; and, at the same time, in another gallon of water, he dissolved two pounds of salt; and into each of these waters he put a small fresh herring. The next evening the whole surface of the artificial sea water was luminous without being stirred; but gave much more light when it was disturbed. It appeared exactly like the real sea water in the preceding experiment; its light lasted about the same time, and went off in the same manner; while the other water, which was almost as salt as it could be made, never gave any light. The herring which was taken out of it the seventh night, and washed from its salt, was found firm and sweet; but the other herring was very soft and putrid, much more so than that which had been kept as long in fresh water. If a herring, in warm weather, be put into 10 gallons of artificial sea water, instead of one, the water, he says, will still become luminous, but its light will not be so strong.

It appeared by some of the first observations on this subject, that heat extinguishes the light of putrefactive substances. Mr Canton also attended to this circumstance; and observes, that though the greatest summer heat is well known to promote putrefaction, yet 20 degrees more than that of the human blood seems to hinder it. For putting a small piece of a luminous fish into a thin glass ball, he found, that water of the heat of 118 degrees would extinguish its light in less than half a minute; but that, on taking it out of the water, it would begin to recover its light in about 10 seconds; but it was never afterwards so bright as before.

Mr Canton made the same observation that Mr Ant. Martin had done, viz. that several kinds of river fish could not be made to give light, in the same circumstances in which any sea fish became luminous. He says, however, that a piece of carp made the water very luminous, LIG

Luminous, though the outside, or scaly part of it, did not shine at all.

For the sake of those persons who may choose to repeat his experiments, he observes, that artificial sea water may be made without the use of an hydrometer, by the proportion of four ounces avoidupois of salt to seven pints of water, wine measure.

A very elaborate paper on the subject by Dr Hulme appeared in the Philosophical Transactions for 1800, to which we refer our readers, and to Chemistry, p. 451.

From undoubted observations, however, it appears, that in many places of the ocean it is covered with luminous insects to a very considerable extent. M. Dagelet, a French astronomer who returned from the Terra Australis in the year 1774, brought with him several kinds of worms which shine in water when it is set in motion; and M. Rigaud, in a paper inserted (if we are not mistaken) in the Journal des Scavans for the month of March 1770, affirms, that the luminous surface of the sea, from the port of Brest to the Antilles, contains an immense quantity of little, round, shining polypuses of about a quarter of a line in diameter. Other learned men, who acknowledge the existence of these luminous animals, cannot, however, be persuaded to consider them as the cause of all that light and scintillation that appear on the surface of the ocean; they think that some substance of the phosphorus kind, arising from putrefaction, must be admitted as one of the causes of this phenomenon. M. Godehouse has published curious observations on a kind of fish called in French bonite, already mentioned; and though he has observed, and accurately described, several of the luminous insects that are found in sea water, he is, nevertheless, of opinion, that the scintillation and flaming light of the sea proceed from the oily and greasy substances with which it is impregnated.

The abbe Nollet was long of opinion, that the light of the sea proceeded from electricity (A); though he afterwards seemed inclined to think, that this phenomenon was caused by small animals, either by their luminous aspect, or at least by some liquor or effluvia which they emitted. He did not, however, exclude other causes; among these, the spawn or fry of fish deserves to be noticed. M. Dagelet, falling into the bay of Antongil, in the island of Madagascar, observed a prodigious quantity of fry which covered the surface of the sea above a mile in length, and which he at first took for banks of sand on account of their colour; they exhaled a disagreeable odour, and the sea had appeared with uncommon splendor some days before. The same accurate observer, perceiving the sea remarkably luminous in the road of the Cape of Good Hope during a perfect calm, remarked, that the oars of the canoes produced a whitish and pearly kind of lustre; when he took in his hand the water which contained this phosphorus, he discerned in it, for some minutes, globules of light as large as the heads of pins. When he pressed these globules, they appeared to his touch like a soft and thin pulp; and some days after the sea was covered near the coasts with whole banks of these little fish in innumerable multitudes.

To putrefaction, also, some are willing to attribute that luminous appearance which goes by the name of ignis fatuus, to which the credulous vulgar ascribe very extraordinary and especially mischievous powers. It is most frequently observed in boggy places and near rivers, though sometimes also in dry places. By its appearance benighted travellers are said to have been sometimes misled into marshy places, taking the light which they saw before them for a candle at a distance; from which seemingly mischievous property it has been thought by the vulgar to be a spirit of a malignant nature, and been named accordingly Will with a wolf, or Jack with a lantern; for the same reason also it probably had its Latin name ignis fatuus.

This kind of light is said to be frequent about burying places and dunghills. Some countries are also remarkable for it, as about Bologna in Italy, and some parts of Spain and Ethiopia. Its forms are so uncertain and variable that they can scarce be described, especially as few philosophical observers ever had the good fortune to meet with it. Dr Derham, however, happened one night to perceive one of them, and got so near that he could have a very advantageous view of it. This is by no means easy to be obtained; for, among other singularities of the ignis fatuus, it is observed to avoid the approach of any person, and fly from place to place as if it was animated. That which Dr Derham observed was in some boggy ground betwixt two rocky hills; and the night was dark and calm; by which means, probably, he was enabled to advance within two or three yards of it. It appeared like a complete body of light without any division, so that he was sure it could not be occasioned by insects as some have supposed; the separate lights of which he could not have failed to distinguish, had it been occasioned by them. The light kept dancing about a dead thistle, till a very slight motion of the air, occasioned, as he supposed, by his near approach to it, made it jump to another place; after which it kept flying before him as he advanced. M. Beccaria endeavoured to procure all the intelligence he could concerning this phenomenon, by inquiring of all his acquaintance who might have had an opportunity of observing it. Thus he obtained information that two of these lights appeared in the plains about Bologna, the one to the north, and the other to the south, of that city, and were to be seen almost every dark night, especially that to the eastward, giving a light equal to an ordinary faggot. The latter appeared to a gentleman of his acquaintance as he was travelling; moved constantly before him for about a mile; and gave a better light than a torch which was carried before him. Both these appearances gave a very strong light, and were constantly in motion, though this various and uncertain. Sometimes they would rise, sometimes sink; but commonly they would hover about six feet from the ground; they would also frequently disappear on a sudden,

(A) This hypothesis was also maintained in a treatise published at Venice in 1746, by an officer in the Austrian service, under the title, Dell' Elettricismo. den, and appear again in some other place. They differed also in size and figure, sometimes spreading prettily wide, and then contracting themselves; sometimes breaking into two, and then joining again. Sometimes they would appear like waves, at others they would seem to drop sparks of fire: they were but little affected by the wind; and in wet and rainy weather were frequently observed to cast a stronger light than in dry weather: they were also observed more frequently when snow lay upon the ground, than in the hottest summer; but he was assured that there was not a dark night throughout the whole year in which they were not to be seen. The ground to the eastward of Bologna, where the largest of these appearances was observed, is a hard chalky soil mixed with clay, which will retain the moisture for a long time, but breaks and cracks in hot weather. On the mountains, where the soil is of a looser texture, and less capable of retaining moisture, the ignis fatuus were less.

From the best information which M. Beccaria was able to procure, he found that these lights were very frequent about rivers and brooks. He concludes his narrative with the following singular account:

"An intelligent gentleman travelling in the evening, between eight and nine, in a mountainous road about ten miles south of Bologna, perceived a light which shone very strangely upon some stones which lay on the banks of the river Riverde. It seemed to be about two feet above the stones, and not far from the water. In size and figure it had the appearance of a parallelepiped, somewhat more than a foot in length, and half a foot high, the longest side being parallel to the horizon. Its light was so strong, that he could plainly discern by it part of a neighbouring hedge and the water of the river; only in the east corner of it the light was rather faint, and the square figure less perfect, as if it was cut off or darkened by the segment of a circle. On examining it a little nearer, he was surprised to find that it changed gradually from a bright red, first to a yellowish, and then to a pale colour, in proportion as he drew nearer; and when he came to the place itself, it quite vanished. Upon this he stepped back, and not only saw it again, but found that the farther he went from it, the stronger and brighter it grew. When he examined the place of this luminous appearance, he could perceive no smell nor any other mark of fire."

This account was confirmed by another gentleman, who informed M. Beccaria, that he had seen the same light five or six different times in spring and in autumn; and that it always appeared of the same shape, and in the very same place. One night in particular, he observed it come out of a neighbouring field to settle in the usual place.

A very remarkable account of an ignis fatuus is given by Dr Shaw in his Travels to the Holy Land. It appeared in the valleys of Mount Ephraim, and attended him and his company for more than an hour. Sometimes it would appear globular, or in the shape of the flame of a candle; at others it would spread to such a degree as to involve the whole company in a pale inoffensive light, then contract itself, and suddenly disappear; but in less than a minute would appear again; sometimes running swiftly along, it would expand itself at certain intervals over more than two or three acres of the adjacent mountains. The atmosphere from the beginning of the evening had been remarkably thick and hazy; and the dew, as they felt it on the bridles of their horses, was very clammy and unctuous.

Lights resembling the ignis fatuus are sometimes observed at sea, skipping about the masts and rigging of ships; and Dr Shaw informs us, that he has seen them in such weather as that just mentioned when he saw the ignis fatuus in Palestine. Similar appearances have been observed in various other situations; and we are told of one which appeared about the bed of a woman in Milan, surrounding it as well as her body entirely. This light fled from the hand which approached it; but was at length entirely dispersed by the motion of the air. Of the same kind also, most probably, are those small luminous appearances which sometimes appear in houses or near them, called in Scotland Elf candles, and which are supposed to portend the death of some person about the house. In general these lights are harmless, though not always; for we have accounts of some luminous vapours which would encompass stacks of hay and corn, and set them on fire; so that they became objects of great terror and concern to the country people. Of these it was observed, that they would avoid a drawn sword, or sharp-pointed iron instrument, and that they would be driven away by a great noise; both which methods were made use of to dissipate them: and it was likewise observed, that they came from some distance, as it were on purpose to do mischief.

Several philosophers have endeavoured to account for these appearances, but hitherto with no great success; nor indeed does there seem to be sufficient data for solving all their phenomena. Willoughby, Ray, and others, have imagined that the light was occasioned by a number of shining insects; but this opinion was never supported in such a manner as to gain much ground. The ignis fatuus seen by Dr Derham above mentioned, as well as all the other instances we have related seem to demonstrate the contrary. Sir Isaac Newton calls it a vapour shining without heat; and supposes that there is the same difference between the vapour of ignis fatuus and flame, that there is between the shining or rotten wood and burning coals. But though this seems generally to be the case, there are still some exceptions, as has been instance in the vapours which set fire to the stacks of corn. Dr Priestley supposes that the light is of the same nature with that produced by putrefactive substances; and others are of opinion, that the electrical fluid is principally concerned; but none have attempted to give any particular solution of the phenomena.

From the frequent appearance of the ignis fatuus in marshes, moist ground, burying places, and dung-hills, we are naturally led to conclude, that putrefaction is concerned in the production of it. This process, we know, is attended with the emission of an aqueous steam, together with a quantity of fixed, inflammable, and azotic airs, all blended together in the form of vapour. It is likewise attended with some degree of heat; and we know that there are some vapours, that of sulphur particularly, which become luminous, with a degree of heat much less than that sufficient to set fire to combustible bodies. There is no inconsistency, therefore, in supposing that the putrid vapour... vapour may be capable of shining with a still smaller degree of heat than that of sulphur, and consequently become luminous by that which putrefaction alone affords. This would account for the ignis fatuus, were it only a steady luminous vapour arising from places where putrid matters are contained; but its extreme mobility, and flying from one place to another on the approach of any person, cannot be accounted for on this principle. If one quantity of the putrid vapour becomes luminous by means of heat, all the rest ought to do so likewise; so that, though we may allow heat and putrefaction to be concerned, yet of necessity we must have recourse to some other agent, which cannot be any other than electricity. Without this, it is impossible to conceive how any body of moveable vapour should not be carried away by the wind; but so far is this from being the case, that the ignis fatui, described by M. Beccaria, were but little affected by the wind. It is besides proved by undoubted experiment, that electricity always is attended with some degree of heat; and this, however small, may be sufficient to give a luminous property to any vapour on which it acts strongly; not to mention, that the electric fluid itself is no other than that of light, and may therefore by its action easily produce a luminous appearance independent of any vapour.

We have a strong proof that electricity is concerned, or indeed the principal agent, in producing the ignis fatuus, from an experiment related by Dr Priestley of a flame of this kind being artificially produced. A gentleman, who had been making many electrical experiments for a whole afternoon in a small room, on going out of it, observed a flame following him at some little distance. This, we have no reason to doubt, was a true ignis fatuus, and the circumstances necessary to produce it were then present, viz. an atmosphere impregnated with animal vapour, and likewise strongly electrified. Both these circumstances undoubtedly must have taken place in the present case; for the quantity of perspiration emitted by a human body is by no means inconsiderable; and it, as well as the electricity, would be collected by reason of the smallness of the room. In this case, however, there seems to have been a considerable difference between the artificial ignis fatuus and those commonly met with; for this flame followed the gentleman as he went out of the room; but the natural ones commonly fly from those who approach them. This may be accounted for, from a difference between the electricity of the atmosphere in the one room and the other; in which case the flame would naturally be attracted towards that place where the electricity was either different in quality or in quantity; but in the natural way, where all bodies may be supposed equally electrified for a great way round, a repulsion will naturally take place. Still, however, this does not seem to be always the case. In those instances where travellers have been attended by an ignis fatuus, we cannot suppose it to have been influenced by any other power than what we call attraction, and which electricity is very capable of producing. Its keeping at some distance is likewise easily accounted for; as we know that bodies possessed of different quantities of electricity may be made to attract one another for a certain space, and then repel without having ever come into contact.

On this principle we may account for the light which surrounded the woman at Milan, but fled from the hand of any other person. On the same principle may we account for these mischievous vapours which set fire to the hay and corn stacks, but were driven away by presenting to them a pointed iron instrument, or by making a noise. Both these are known to have a great effect upon the electric matter; and by means of either, even lightning may occasionally be made to fall upon or to avoid particular places, according to the circumstances by which the general mass happens to be affected at that time.

On the whole, therefore, it seems most probable, that the ignis fatuus is a collection of vapour of the putrefied kind, very much affected by electricity; according to the degree of which, it will either give a weak or strong light, or even set fire to certain substances disposed to receive its operation. This opinion seems greatly to be confirmed from some luminous appearances observed in privies, where the putrid vapours have even collected themselves into balls, and exploded violently on the approach of a candle. This last effect, however, we cannot so well ascribe to the electricity, as to the accension of the inflammable air which frequently abounds in such places.

In the appendix to Dr Priestley's third volume of Experiments and Observations on Air, Mr Warltire gives an account of some very remarkable ignis fatuus, which he observed on the road to Bromsgrove, about five miles from Birmingham. The time of observation was the 12th of December 1776, before day-light. A great many of these lights were playing in an adjacent field, in different directions; from some of which there suddenly sprung up bright branches of light, something resembling the explosion of a rocket that contained many brilliant stars, if the discharge was upwards, instead of the usual direction; and the hedge and trees on each side of the hedge were illuminated. This appearance continued but a few seconds, and then the jack-a-lanterns played as before. Mr Warltire was not near enough to observe if the apparent explosions were attended with any report.

Cronstedt gives it as his opinion, that ignis fatuus, as well as the meteors called falling stars, are owing to collections of inflammable air raised to a great height in the atmosphere. But, with regard to the latter, the vast height at which they move evidently shows that they cannot be the effect of any gravitating vapour whatever; for the lightest inflammable air is one-twelfth of that of the common atmosphere; and we have no reason to believe, that at the distance of 40 or 50 miles from the earth, the latter has near one-twelfth of its weight at the surface. From the account given by Mr Warltire, we should be apt to conclude, that there is a strong affinity betwixt the ignis fatuus and fire balls, inasmuch that the one might be very easily converted into the other. From this then we must ascribe an electrical origin to the one as well as the other. Electricity, we know, can assume both these appearances, as is evident in the case of points: or even when the atmosphere is violently electrified, as around the string of an electrical kite, which always will appear to be surrounded with a blue flame in the night, if the electricity be very strong.

On the whole, it appears, that electricity acting up- on a small quantity of atmospheric air, with a certain degree of vigour, will produce an appearance resembling an ignis fatuus; with a superior force it will produce a fire ball; and a sudden increase of electrical power might produce those sparks and apparent explosions observed by Mr Warlire. The only difficulty therefore is, Why does electricity exert its power upon one portion of the atmosphere rather than another, seeing it has an opportunity of diffusing itself equally through the whole? To this it seems impossible to give any other reason than that we see the fact is so; and that in all cases where there is a quantity of electrified air or vapour, there will be an accumulation in one part rather than another. Thus, in the experiment already related, where the gentleman perceived a blue flame following him, the whole air of the room was electrified, but the greatest power of the fluid was exerted on that which gave the luminous appearance.

With regard to the uses of the ignis fatuus in the system of nature, we can only say, that they seem to be accidental appearances resulting from the motion of the electric fluid, and, no doubt, like other meteors subservient to the preservation of its equilibrium, and thus are useful in preventing those dreadful commotions which ensue when a proper medium for so doing is deficient.

A light in some respects similar to those above mentioned has been found to proceed from that celebrated chemical production called phosphorus, which always tends to decompose itself, so as to take fire by the access of air only. Phosphorus, therefore, when it emits light, is properly a body ignited; though when a very small quantity of it is used, as what is left after drawing it over paper, or what may be dissolved in essential oil, the heat is not sensible. But perhaps the matter which emits the light in what we call putrefactive substances may be similar to it, though it be generated by a different process, and burn with a less degree of heat. Putrefaction does not seem to be necessary to the light of glow-worms or of the phalopes; and yet their light is sufficiently similar to that of shining wood or flesh. Electric light is unquestionably similar to that of phosphorus, though the source of it is apparently very different.

Kunckel formed his phosphorus into a kind of pills about the size of peas, which being moistened a little, and scraped in the dark, yielded a very considerable light, but not without smoke. The light was much more pleasing when eight or ten of these pills were put into a glass of water; for being shaken in the dark, the whole glass seemed to be filled with light. Kunckel also reduced his phosphorus into the form of larger stones; which being warmed by a person's hand, and rubbed upon paper, would describe letters that were very legible in the dark.

The greatest variety of experiments with the light of phosphorus was made by Dr Slare; who says, that the liquid phosphorus (which is nothing more than the solid phosphorus dissolved in any of the essential oils) would not hurt even a lady's hand; or that, when the hands or face were washed with it, it would not only make them visible to other persons in the dark, but that the light was so considerable as to make other neighbouring objects visible.

When the solid phosphorus is quite immersed in water, he observes that it ceases to thine; but that if any part of it chance to emerge, or get into the air, it will thine though the glass be hermetically sealed. In a large glass he kept it without water for several days; and yet it continued shining, with very little diminution of its light or weight. If the letters that were written with this phosphorus were warmed by the fire, they presently became dark lines, which continued upon the paper, like ink. To try how much light was given by a small quantity of this phosphorus, he observed that it continued to flame in the open air for seven or eight days; the light being visible whenever he shut his window.

As air was generally thought to contain the pabulum of flame, Dr Slare was determined to try this with respect to phosphorus; and for this purpose he placed a large piece of it in a receiver; but upon exhausting it, he perceived that it became more luminous, and that, upon admitting the air, it returned to its former state. This property of the light of phosphorus, which is the very reverse of that of shining wood and fishes, was also ascertained by several very accurate experiments of Mr Haukbee's.

Endeavouring to blow the phosphorus into a flame with a pair of bellows, Dr Slare found that it was presently blown out, and that it was a considerable time before the light revived again. All liquors would extinguish this light when the phosphorus was put into them; nor would it thine or burn, though it was even boiled in the most inflammable liquors, as oil of olives, spirit of turpentine, or even spirit of wine.

In order to keep his phosphorus from conflagration, he used to put it in a glass of water; and sometimes he has seen it, when thus immersed in water, make such bright and vigorous coruscations in the air, as, he says, would surprise and frighten those who are not used to the phenomenon. This fiery meteor, he says, is contracted in its passage through the water, but expands as soon as it gets above it. If any person would make this experiment to advantage, he informs them that the glass must be deep and cylindrical, and not above three quarters filled with water. This effect he perceived in warm weather only, and never in cold.

The phosphorus of which we have been treating is prepared from urine; but in some cases the sweat, which is similar to urine, has been observed to be phosphoraceous, without any preparation. This once happened to a person who used to eat great quantities of salt, and who was a little subject to the gout, after sweating with violent exercise. Stripping himself in the dark, his shirt seemed to be all on fire, which surprised him very much. Upon examination, red spots were found upon his shirt; and the physician who was present perceived an urinous smell, though it had nothing in it of volatile alkali, but of the muriatic acid; the same, he says, that issues from cabbage much salted, and strongly fermented.

The easiest method of accounting for all these kinds of lights, perhaps, is from electricity. If light consists counted for in a certain vibration of the electric fluid *, then it follows, that in whatever substances such a vibration takes place, there light must appear, whether in putrefactive animal substances, tea water, phosphorus, or anything else. We know that the electric matter pervades all terrestrial terrestrial substances, and is very liable to be set in motion from causes of which we are ignorant. The action of the air by which putrefaction is produced may be one of these causes; and it can by no means appear surprising that the electric matter should act in the bodies of living animals in such a manner as to produce a permanent light, when we certainly know it acts in some of them so powerfully as to produce a shock similar to that of a charged phial.—On this subject we shall only observe farther, that when this vibration becomes so powerful as to penetrate the solid substance of the body itself, the luminous body then becomes transparent, as in the milk mentioned in the former part of this article; but when it is only superficial, the body, though it emits light, is itself opaque.

Light from Diamonds. Among luminous bodies the diamond is to be reckoned; as some diamonds are known to shine in the dark. But on account of the feebleness of their splendour, it is necessary for the person who is to observe them, previously to stay in the dark at least a quarter of an hour; that the pupil of the eye may be dilated and enlarged, and to rendered capable of receiving a large quantity of the rays of light. M. du Fay has also observed, that the eyes ought to be shut for this time, or at least one of them; and that, in that case, the light of the diamond is afterwards only seen by that eye which has been shut. Before the diamond is brought into the dark room, it must be exposed to the sunshine, or at least to the open daylight, to imbibe a sufficient quantity of rays; and this is done in one minute, or even less; eight or ten seconds having been found to furnish as much light as a stone is capable of receiving: and when brought into the dark, its light continues about twelve or thirteen minutes, weakening all the while, by insensible degrees. It is very remarkable, that in bodies so extremely similar to each other as diamonds are, some should have this property of imbuing the sun's rays, and shining in the dark, and that others should not; yet if it is found to be by experiment, and the most nearly resembling stones shall be found one to have this property, and another to be destitute of it; while many of the most dissimilar have the property in common. There seems to be no rule, nor even the least traces of any imperfect rule of judging, which diamonds have, and which have not this property; their natural brightnesses, their purity, their size, or their shape, contribute nothing to it; and all that has been yet discovered of the least regularity among them, is, that all yellow diamonds have this property. This is supposed to arise from their having more sulphur in their composition, and therefore illuminating more readily, or emitting a more visible flame.

The burning of diamonds is a term used among the jewellers, for putting them into a fierce fire, as they frequently do, when they are fouled with brown, or yellow, or the like; this always divests them of their colour, without doing them the least sensible injury. M. du Fay, having been informed of this common practice, formed a conjecture, that the difference of diamonds in their shining or not shining in the dark, was owing to it; and that either all those which had been burnt, or all those which had not, were those which alone shone in the dark. But this was found an erroneous conjecture; for two diamonds, one lucid in the dark, the other not, were both burnt, and afterwards both were found to retain the same properties they had before. It is not only the open sunshine, or open daylight, which gives to these diamonds the power of shining in the dark; they receive it in the same manner even if laid under a glass, or plunged in water or in milk.

M. du Fay tried whether it was possible to make the diamond retain, for any longer time, the light it naturally parts with so soon; and found, that if the diamond, after being exposed to the light, be covered with black wax, it will shine in the dark, as well fix hours afterwards as at the time it was first impregnated with the light.

The imbibing light, in this manner, being so nice a property as not to be found in several diamonds, it was not to be supposed that it would be found in any other stones; accordingly, on trial, the ruby, the sapphire, and the topaz, were found wholly destitute of it; and among a large number of rough emeralds, one only was found to possess it. Such is the strange uncertainty of these accidents.

All the other less precious stones were tried, and found not to possess this property of imbibing light from the daylight or sunshine, but they all became luminous by the different means of heating or friction; with this difference, that some acquired it by one of these methods, and others by the other; each being unaffected by that which gave the property to the other. The diamond becomes luminous by all these ways.

Beccarius also discovered, that diamonds have the property of the Bolognian phosphorus, about the same time that it occurred to M. du Fay. Com. Bonon. vol. ii. p. 276. M. du Fay likewise observed, that the common topaz, when calcined, had all the properties of this phosphorus; and pursuing the discovery, he found the same property, in a great degree, in the beryllites, gypsum, limestone, and marble: though he was obliged to dissolve some very hard substances of this kind in acids, before calcination could produce this change in them; and with some substances he could not succeed even thus; especially with flint stones, river sand, jaspers, agates, and rock crystal.

Light from Plants. In Sweden a very curious phenomenon has been observed on certain flowers by M. Haggert, lecturer in natural history. One evening he perceived a faint flash of light repeatedly dart from a marigold. Surprised at such an uncommon appearance, he resolved to examine it with attention; and, to be assured it was no deception of the eye, he placed a man near him, with orders to make a signal at the moment when he observed the light. They both saw it constantly at the same moment.

The light was most brilliant on marigolds of an orange or flame colour; but scarcely visible on pale ones.

The flash was frequently seen on the same flower two or three times in quick succession, but more commonly at intervals of several minutes; and when several flowers in the same place emitted their light together, it could be observed at a considerable distance.

This phenomenon was remarked in the months of July and August at sunset, and for half an hour, when the atmosphere was clear; but after a rainy day, or when the air was loaded with vapours, nothing of it was seen.

The following flowers emitted flashes, more or less vivid, in this order: 1. The marigold, *calendula officinalis*. 2. Monk's-hood, *tropaeolum majus*. 3. The orange lily, *lilium bulbiferum*. 4. The Indian pink, *tagetes patula et erecta*.

To discover whether some little insects or phosphoric worms might not be the cause of it, the flowers were carefully examined, even with a microscope, without any such thing being found.

From the rapidity of the flash, and other circumstances, it may be conjectured that there is something of electricity in this phenomenon. It is well known, that when the pitil of a flower is impregnated, the pollen bursts away by its elasticity, with which electricity may be combined. But M. Haggern, after having observed the flash from the orange lily, the antherae of which are a considerable space distant from the petals, found that the light proceeded from the petals only; whence he concludes, that this electric light is caused by the pollen, which, in flying off, is scattered on the petals. Whatever be the cause, the effect is singular and highly curious.

**Lights**, in Painting, are those parts of a piece which are illuminated, or that lie open to the luminary by which the piece is supposed to be enlightened; and which, for this reason, are painted in bright vivid colours.

In this sense, light is opposed to shadow.

Different lights have very different effects on a picture, and occasion a difference in the management of every part. A great deal therefore depends on the painter's choosing a proper light for his piece to be illuminated by; and a great deal more, in the conduct of the lights and shadows, when the luminary is pitched upon.

The strength and relievo of a figure, as well as its gracefulness, depend entirely on the management of the lights, and the joining of those to the shadows.

The light a figure receives is either direct or reflected; to each of which special regard must be had. The doctrine of lights and shadows makes that part of painting called **clair-obscur**.

**Light-Horse**, an ancient term in our English customs, signifying an ordinary cavalier or horseman lightly armed, and so as to enter a corps or regiment; in opposition to the men at arms, who were heavily accoutred, and armed at all points. See Light-Horse.

**Light-House**, a building erected upon a cape or promontory on the sea coast, or upon some rock in the sea, and having on its top in the night-time a great fire, or light formed by candles, which is constantly attended by some careful person, so as to be seen at a great distance from the land. It is used to direct the shipping on the coast, that might otherwise run ashore, or steer an improper course, when the darkness of the night and the uncertainty of currents, &c., might render their situation with regard to the shore extremely doubtful. Lamp-lights are, on many accounts, preferable to coal fires or candles; and the effect of these may be increased by placing them either behind glass hemispheres, or before properly disposed glass or metal reflectors, which last method is now very generally adopted. See Beacons.

**Light-Room**, a small apartment, enclosed with glass windows, near the magazine of a ship of war. It is used to contain the lights by which the gunner and his assistants are enabled to fill cartridges with powder to be ready for action.

**Lighter**, a large, open, flat-bottomed vessel, generally managed with oars, and employed to carry goods to or from a ship when she is to be laden or delivered.—There are also some lighters furnished with a deck throughout their whole length, in order to contain those merchandises which would be damaged by rainy weather: these are usually called close lighters.