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

The nature of light hath been a subject of speculation from the earliest ages of philosophy. Some of the first distinguished by the appellation of philosophers even doubted whether objects became visible by means of anything proceeding from them, or from light, the eye of the spectator. The fallacy of this notion must very soon have been apparent, because, in that case, we ought to have seen as well in the night as in the day. The opinion was therefore qualified by Empedocles and Plato; who maintained, that vision was occasioned by particles continually flying off from the surfaces of bodies which met with others proceeding from the eye; but Pythagoras ascribed it solely to the particles proceeding from the external objects and entering the pupil of the eye.

Among the modern philosophers there have been two celebrated opinions, viz. the Cartelian and Newtonian. According to the former, light is an invisible fluid present at all times and in all places, but which requires to be set in motion by an ignited or otherwise properly qualified body in order to make objects visible to us.—The Newtonians maintain, that light is not a fluid per se, but consists of a vast number of exceedingly small particles shaken off in all directions from the luminous body with inconceivable velocity by a repulsive power; and which most probably never return again to the body from which they were emitted. These particles are also said to be emitted in right lines by the body from whence they proceed; and this rectilinear direction they preserve until they are turned out of their original path by the attraction of some other body near which they pass, and which is called inflection; by passing through a medium of different density, which is called refraction; or by being thrown obliquely or directly forward by some body which opposes their passage, and which is called reflection; or, lastly, till they are totally stopped by the substance of any body into which they penetrate, and which is called their extinction. A succession of these particles following one another in an exactly straight line is called a ray of light; and this ray, in whatever manner it hath its direction changed, whether by refraction, reflection, or inflection, always preserves its recti- rectilinear course, neither is it possible by any art whatever to make it pass on in the segment of a circle, ellipse, or other curve.—From some observations on the eclipses of Jupiter's satellites, and also on the aberration of the fixed stars, it appears that the particles of light move at the rate of little less than 200,000 miles in a second of time. See Astronomy, no. 126, 127, 284.

To this doctrine concerning the nature of light several objections have been made; the most considerable of which is, That in this case, as rays of light are continually passing in different directions from every visible point, they must necessarily interfere with and destroy each other in such a manner as entirely to confound all distinct perception of objects, if not to destroy the sense of seeing altogether; not to mention the continual waste of substance which a constant emission of particles must occasion in the luminous body, and which since the creation ought to have greatly diminished the sun and stars, as well as increased the bulk of the earth and planets by the vast quantity of particles of light absorbed by them in such a long period of time.

In answer to this objection, Mr Melville gives some ingenious illustrations concerning the extreme subtlety of light, or the smallness of the particles of which it consists, and of which few persons, even of those who admit the hypothesis, have any idea. He observes, that there is probably no physical point in the visible horizon that does not send rays to every other point, unless where opaque bodies interpose. Light, in its passage from one system to another, often passes thro' torrents of light issuing from other suns and systems, without ever interfering, or being diverted in its course, either by it, or by the particles of that elastic medium which some phenomena give reason to suppose are diffused through all the mundane space. To account for this fact and others similar to it, he concludes, that the particles of which light consists must be incomparably rare, even when they are the most dense; that is, that the semidiameters of the two nearest particles, in the same or in different beams, soon after their emission, are incomparably less than their distance from one another. This difficulty concerning the non-interference of the particles of light is not solved, as he observes, by supposing with Mr Boscovich and others, that each particle is endowed with an insuperable impulsive force; because, in that case, their spheres of impulsion would even be more liable to interfere, and they would on that account be more likely to disturb one another.

The difficulty, according to Mr Canton, will nearly vanish, if a very small portion of time be allowed between the emission of every particle and the next that follows in the same direction. Suppose, for instance, that one lucid point of the sun's surface emits 150 particles in a second, which are more than sufficient to give continual light to the eye without the least appearance of intermission; yet still the particles of which it consists, will on account of their great velocity be more than 1000 miles behind each other, and thereby leave room enough for others to pass in all directions.

In order to determine whether light really consisted of particles emitted from the luminous body, or only in the vibrations of a subtile fluid, it hath been attempted to find out its momentum, or the force with which it moves. The first who set about this matter with any tolerable pretensions to accuracy was M. Mairan. Others indeed, particularly Hartsoeker and Homberg, had pretended, that in certain cases this momentum was very perceptible; but M. Mairan proved, that the effects mentioned by them were owing to currents of heated air produced by the burning-glasses used in their experiments, or to some other causes overlooked by the philosophers. To decide the matter therefore, if possible, he began with trying the effects of rays collected by lenses of four and six inches diameter, and thrown upon the needle of a compass; but the result was nothing more than some tremulous motion from whence he could draw no conclusion. After this, he and Mr du Fay constructed a kind of mill of copper, which moved with an exceeding flight impulse; but though they threw upon it the focus of a lens of seven or eight inches diameter, they were still unable to draw any conclusions from the result.

M. Mairan afterwards procured a horizontal wheel of iron three inches in diameter, having six radii, at the extremity of each of which was a small wing fixed obliquely. The axis of the wheel, which was also of iron, was suspended by a magnet. The wheel and the axis together did not weigh more than 30 grains; but though a motion was given to this wheel when the focus of the burning-glass was thrown upon the extremities of the radii, yet it was so irregular that he could not but conclude that it was occasioned by the motion of the heated air. He then intended to have made his experiment in vacuo, but he concluded that it was unnecessary. For, besides the difficulty of making a vacuum, he was persuaded that there was in our atmosphere a thinner medium which freely penetrates even glass itself, the existence of which he imagined that he had sufficiently proved in his treatise on the aurora borealis. See Aurora Borealis, no. 5.

Mr Michell some years ago endeavoured to ascertain the momentum of light in a manner still more accurate. The instrument he made use of for this purpose consisted of a very thin plate of copper, a little more than an inch square, which was fastened to one end of a slender harpichord-wire about ten inches long. To the middle of this was fixed an agate cap, such as is commonly used for small mariner's compasses, after the manner of which it was intended to turn; and at the other end of the wire was a middling fixed shot-corn, as a counterpoise to the copper-plate. The instrument had also fixed to it in the middle, at right angles to the length of the wire, and in an horizontal direction, a small bit of a very slender sewing needle, about one third, or perhaps half an inch long, which was made magnetic. In this state the whole instrument might weigh about 10 grains. It was placed on a very sharp pointed needle, on which the agate cap turned extremely freely; and to prevent its being disturbed by any motion of the air, it was included in a box, the lid and front of which were of glass. This box was about 12 inches long, six or seven inches deep, and about as much in width; the needle standing upright in the middle. At the time of making the experiment, the box was placed in such a manner that a line drawn from the sun passed at right angles to the length of it; and the instrument was brought to range in the same direction with the box, by means of the magnetical bit of needle abovementioned, and a magnet properly placed on the outside, which would retain it, though with extremely little force, in any situation. The rays of the sun were now thrown upon the copperplate abovementioned from a concave mirror of about two feet diameter, which, passing through the front-glass of the box, were collected into the focus of the mirror upon the copperplate. In consequence of this the plate began to move, with a slow motion of about an inch in a second of time, till it had moved through a space of about two inches and a half, when it struck against the back of the box. The mirror being removed, the instrument returned to its former situation by means of the little needle and magnet; and the rays of the sun being then again thrown upon it, it again began to move, and struck against the back of the box as before; and this was repeated three or four times with the same success.—The instrument was then placed the contrary way in the box to that in which it had been placed before, so that the end to which the copper-plate was affixed, and which had lain, in the former experiment, towards the right hand, now lay towards the left; and the rays of the sun being again thrown upon it, it began to move with a slow motion, and struck against the back of the box as before; and this was repeated once or twice with the same success. But by this time the copperplate began to be so much altered in its form, by the extreme heat which it underwent in each experiment, and which brought it nearly into a state of fusion, that it became very much bent, and the more so as it had been unwarily supported by the middle, half of it lying above and half below the wire to which it was fastened. By this means it now varied so much from the vertical position, that it began to act in the same manner as the sail of a windmill, being impelled by the stream of heated air which moved upwards, with a force sufficient to drive it in opposition to the impulse of the rays of light.

"If we impute, (says Dr Priestley), the motion produced in the above experiment to the impulse of the rays of light, and suppose that the instrument weighed ten grains, and acquired a velocity of one inch in a second, we shall find that the quantity of matter contained in the rays falling upon the instrument in that time amounted to no more than one twelve hundred millionth part of a grain, the velocity of light exceeding the velocity of one inch in a second in the proportion of about $12,000,000,000$ to $1$. Now the light in the above experiment was collected from a surface of about three square feet, which reflecting only about half what falls upon it, the quantity of matter contained in the rays of the sun incident upon a square foot and an half of surface in one second of time, ought to be no more than the twelve hundred millionth part of a grain, or, upon one square foot only, the eighteen hundred millionth part of a grain. But the density of the rays of light at the surface of the sun is greater than at the earth in the proportion of $45,000$ to $1$: there ought, therefore, to issue from one square foot of the sun's surface in one second of time, in order to supply the waste by light, one forty thousandth part of a grain of matter; that is, a little more than two grains in a day, or about $4,752,000$ grains, or $670$ pounds avoirdupois nearly, in $6000$ years; a quantity which would have shortened the sun's semidiameter no more than about ten feet, if it was formed of the density of water only."

The Newtonians, besides the answer just now given to the most formidable objections of their opponents, have endeavoured to prove the impossibility of light being a vibration in any fluid. Sir Isaac, in his Principia, demonstrates, that no rectilinear motion can be propagated among the particles of any fluid unless these particles lie in right lines; and he hath also shewn, that all motion propagated through a fluid diverges from a rectilinear progress into the unmoveable spaces. Hence he concludes, "a pressure on a fluid medium (i.e. a motion propagated by such a medium beyond any obstacle, which impedes any part of its motion,) cannot be propagated in right lines, but will be always inflecting and diffusing itself every way, to the quiescent medium beyond that obstacle. The power of gravity tends downwards; but the pressure of water rising from it, tends every way with an equable force, and is propagated with equal ease, and equal strength, in curves, as in straight lines. Waves, on the surface of the water, gliding by the extremes of any very large obstacle, inflect and dilate themselves, still diffusing gradually, into the quiescent water beyond that obstacle. The waves, pulses, or vibrations of the air, wherein sound consists, are manifestly inflected, though not so considerably as the waves of water; and sounds are propagated with equal ease, thro' crooked tubes, and through straight lines; but light was never known to move in any curve, nor to inflect itself ad umbrum."

To this Mr Rowning adds another proof. "The Mr Carteian notion of light, (says he,) was not that it Rowning is propagated from luminous bodies by the emission of small particles, but that it was communicated to the organ of sight by their pressure upon the materia subtilis, with which they supposed the universe to be full. But, according to this hypothesis, it could never be dark; because, when a fluid fulfils any pressure, if that fluid fills all the space it takes up, absolutely, without leaving any pores, which is the case of the supposed materia subtilis, then that pressure must necessarily be communicated equally and instantaneously to every part. And therefore, whether the sun were above or below the horizon, the pressure communicated, and consequently the light, would be the same. And farther, as the pressure would be instantaneous, so would the light, which is contrary to what is collected from the eclipses of Jupiter's satellites."

It is obvious, however, that, whatever side we take concerning the nature of light, many, indeed almost all the circumstances concerning it, are incomprehensible, and beyond the reach of human understanding.

Most of the discours flowers, by some power unknown to us, follow the sun in his course. They attend him to his evening retreat, and meet his rising lustre in the morning with the same unerring law. If a plant also is shut up in a dark room, and a small hole is afterwards opened by which the light of the sun may enter, the plant will turn towards that hole, and even alter its own shape in order to get near it; so that though it was straight before, it will in time become crooked, that it may get near the light. It is not the heat, but the light of the sun, which it thus covets; for, though a fire be kept in the room, capable of giving a much stronger heat than the sun, the plant will turn away from the fire in order to enjoy the sun's light. The green colour of plants also depends on the sun's light being allowed to shine upon them; for without this they are always white.—From this last circumstance, and likewise the property which the solar light has of blackening precipitates of silver from the nitrous acid*, it hath been thought that light either contains the phlogiston in very considerable quantity, or is itself a modification of that unknown substance. But that this cannot be the case, we have now a proof little short of demonstration, from the last experiments of Dr Priestley concerning the production of pure dephlogisticated air from pump-water, by means of the solar light. If light either were the phlogiston itself, or contained it in very considerable quantity, it is impossible the air produced by its means could be pure and dephlogisticated. See the articles Gas and Air, in the APPENDIX.—For the properties of light acting as the medium of our perceptions by the sense of sight, see the article 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 putrefactive substances and phosphorus, together with that of luminous animals, and other similar appearances. Light proceeding from putrefactive 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 putrefactive animal-flesh is that which is 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 1592, 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, was 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 splendor 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 p. 156, 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 great variety of saline mixtures, and other fluids. Water, however, did not quench all the light of some shining veal, on which he tried it, tho' 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. Mr Boyle was often disappointed in his experiments on shining-fishes; finding, that they did not always 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-polypos, 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 wane 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. That this fish is luminous, was noticed by Pliny; who observes, that it shines in the mouth of the person who eats it, and, if it touch his hands or cloaths, 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, 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; 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. Marilius, in 1724, who brought a number of these fishes, and the stones in which they were inclosed, 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 they were 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 saltpetre 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 delicatum nearly extinguished it, and the acids entirely. This water poured upon fresh calcined gypsum, rock crystal, ceruss, 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 to 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 Galactius 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 the light of the pholas, was that which was observed to proceed from wood which was moist, but not in a putrid state, which was very conspicuous in the dark.

That the sea is sometimes luminous, especially when it is put in motion by the dashing of oars or the beating... heating 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 fishes, and the pickle in which they were immersed, were made by Dr. Beal, in May 1665; and had they been properly attended to and pursued, 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 fishes, he observed, 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 shined 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 he turned up that side of the fish that was lowest, no light came from it; 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 fleam, rather dark than luminous, arising like a very small dust from the fish, and here and there a very small and almost imperceptible sparkle. Of the sparkles he had no doubt; but he thought it possible that the fleam might be a deception of the light, 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 Bourzes's wake of the ship, the particles that were luminous from account of those that were not; and they appeared not to be all luminous of the same figure. Some of them were like points of light, and others such as flares 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 there were what he calls vertices 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 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 anything 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 luminous fleams of the sea when he was at Venice in 1749; and, after taking a great deal of pains to ascertain the circumstances of it, concluded that it was occasioned by 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 abbe 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, presently after the abbe Nollet made his observations at Venice, took notice, that, in the day-time, the prow of the ship in motion threw up many small particles, which, falling upon the water, rolled upon the surface of the sea for a few seconds, before they mixed with it; and in the night the same particles, as he concluded, 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 Beal'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, tho' it was not made to boil.

This gentleman, after giving much attention to this phenomenon, concludes, that it is not occasioned by any shining insects, as the abbe 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 Roi is far from asserting that there are no luminous insects in the sea. He even supposes that the abbe 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 flesh 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 putrescent 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; 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 7th 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 two 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, 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 avoirdupois of salt, to seven pints of water, wine-measure.

From undoubted observations, however, it appears, that in many places of the ocean it is covered with luminous insects to a very considerable extent. Mr 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 1776, affirms, that the luminous surface of the sea, from the port of Bret 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. Godehoue has published curious observations on a kind of fish, called in French bonites, 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 splendour 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 luster: when he took in his hand the water which contained phosphorus, he discovered 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 ignis fatuus. That luminous appearance which goes by the name of ignis fatuus, or in common English Will-with-a-wisp, to which the credulous vulgar ascribe very extraordinary and especially mischievous powers. This phenomenon is chiefly visible in damp places; and is also said to be very often seen in burying-grounds, and near dunghills. Travellers say, that it is very frequent near Bologna in Italy, and in several parts of Spain and Ethiopia. The form and size of it are very various, and often variable.

It was the opinion of many philosophers, and especially Willoughby and Ray, that the ignis fatuus is made by shining insects; but this opinion was never well supported. Sir Isaac Newton calls it a vapour shining without heat; and supposes that there is the same difference between this vapour and flame, that there is between wood shining without heat, and burning coals of fire. That this opinion is just, and, moreover, that the light of this vapour shining without heat is of the same nature with light from putrefactive substances, according to the supporters of the putrefactive hypothesis, may be concluded from the following circumstances relating to them, as described by Dr Derham and G. B. Beccari.

The former of these gentlemen, having observed an ignis fatuus in some boggy ground, between two rocky hills, in a dark and calm night, got by degrees within two or three yards of it, and thereby had an opportunity

(a) This hypothesis was also maintained in a treatise published at Venice in 1746, by an officer in the Austrian service, under the following title, Dell' Elettrificazione. tunity of viewing it to the greatest advantage. It kept skipping about a dead thistle, till a slight motion of the air, occasioned, as he supposed, by his near approach to it, made it jump to another place; and as he advanced, it kept flying before him. He was so near to it, that, had it been the shining of glow-worms, he was satisfied that he could not but have distinguished the separate lights of which it must have consisted; whereas it was one uniform body of light. He therefore thought that it must be an ignited vapour (a).

M. Beccari made it his business to inquire concerning this phenomenon of all his acquaintance who had opportunities of observing it, either on the mountains, or on the plains. He found, that two which appeared on the plains, one to the north and the other to the east of Bologna, were to be seen almost every dark night, especially the latter; and the light they gave was equal to that of an ordinary faggot. That to the east of Bologna once appeared to a gentleman of his acquaintance as he was travelling, and kept him company above a mile, constantly moving before him, and casting a stronger light upon the road than the torch which was carried along with him. All these luminous appearances, he says, gave light enough to make all neighbouring objects visible; and they are always observed to be in motion, but this motion was various and uncertain. Sometimes they would rise up, and at other times sink; but they commonly kept hovering about six feet from the ground. They would also disappear of a sudden, and instantly appear again in some other place. They differed both in size and figure; sometimes spreading pretty wide, and then again contracting themselves; sometimes breaking into two, and then joining again; sometimes floating like waves, and dropping, as it were, sparks of fire. He was assured that there was not a dark night all the year round in which they did not appear; and that they were observed more frequently when the ground was covered with snow, than in the hottest summer: nor did rain or snow in the least hinder their appearance; but, on the contrary, they were observed more frequently, and cast a stronger light, in rainy and wet weather; nor were they much affected by the wind.

The grounds to the east of Bologna, where the largest of these appearances was seen, is, he says, a hard chalky and clayey soil, which will retain the water a long time, and afterwards, in hot weather, would break into large cracks; but on the mountains, where the ignis fatuus were smaller, the soil was of a loose sandy texture, which would not keep the water very long. According to the best information he could procure, these lights very much frequent brooks and rivers, being often observed on the banks of them; particularly, says he, because the current of air carries them thither more readily than to any other place.

This gentleman concludes his account of these appearances with the following curious narrative. An intelligent gentleman travelling in March, between eight and nine in the evening, in a mountainous road about ten miles south of Bologna, perceived a light which shone very strongly upon some stones that lay on the banks of the river Rioverde. 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 parallelo-piped, 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 distinguish 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.

His curiosity tempting him to examine this appearance a little nearer, he advanced gently towards the place; but 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 not perceive the least smell, or any other mark of fire.

This extraordinary account was confirmed to M. Beccari by another gentleman who frequently travelled the same road, and who assured him that he had seen the very same light five or six different times, in spring and autumn, and that he had always observed it to be of the very same shape, and in the same place; and he once took particular notice of its coming out of a neighbouring place, and settling itself in the figure above described.

M. Beccari owns himself to be greatly at a loss to account not only for this very remarkable appearance, but also for the ignis fatuus in general. He only says, that all persons who ever saw any of these appearances, agree, that they cast a light quite different from that of shining fires.

Dr Shaw describes an ignis fatuus which he saw in the Holy Land; the circumstances of which are so remarkable, that they well deserve to be recited, especially as some of them seem to point at the cause of the phenomenon. As he and his company were travelling by night through the valleys of Mount Ephraim, they were attended more than an hour by an ignis fatuus, which was sometimes globular, or in the form of the flame of a candle; and which would immediately afterwards spread itself so much as to involve the whole company in a pale inoffensive light, and then contract itself again, and suddenly disappear. But in less than a minute it would become visible as before; or, running along from one place to another, with a swift progressive motion, 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 upon their bridles, was unusually clammy and unctuous. In the same kind of weather, he says, he has observed those luminous appearances, which at sea ship about the masts and yards of ships, and which the sailors call corpulentis, which is a corruption of the Spanish Cuerpo santo.

A light in some respects similar to those above mentioned has been found to proceed from that celebrated light chemical... 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 pholades; 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 shine; but that if any part of it chance to emerge, or get into the air, it will shine 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. Hauksbee'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 shine 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 consuming, 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 coffee, and who was a little subject to the gout, after perspiring 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 fermenting.

The safest method of accounting for all these kinds of lights, perhaps, is from electricity. If light consists in a certain vibration of the electric fluid, then it follows, that in whatever substance such a vibration takes place, there light must appear, whether in putrefactive animal-substances, sea-water, phosphorus, or anything else. We know that the electric matter pervades all 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 vial.—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 abovementioned; but, when it is only superficial, the body, though it emits light, is itself opaque.

Light-House, in maritime affairs, a building on the sea-shore, a promontory, a rock, &c. wherein is kept a light during the night in order to direct vessels sailing near the place.

Light-Room, a small apartment, inclosed 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.