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 philosophers even doubted whether objects became visible by means of anything proceeding from them, or from 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 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 Cartesian 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 thrown 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 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-Index.

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 us 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 Bofcovich and others, that each particle is endowed with an imperceptible 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 consists of particles emitted from the luminous body, or only in its vibrations of a subtile fluid, it has 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 Hartfocker 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 these 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 slight 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... sphere 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 harpsichord-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 sized shot-corn, as a counterpoise to the copperplate. 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 magnetical. 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 above mentioned, 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 above mentioned 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 copperplate 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 fifteen 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-hundredth-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 wants 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, against the Cartesians, 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 shown, that all motion propagated through a fluid diverges from a rectilinear progress into the unmoved 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 inflected 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, through crooked tubes and through straight lines; but light was never known to move in any curve, nor to inflect itself ad undas um."

To this Mr Rowning adds another proof. "The by Mr Cartesian notion of light (says he), was not that it Rowning, is propagated from luminous bodies by the emission of small..." 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 sustains 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 discous 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 final 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 has 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.—For the properties of light acting as the medium of our perceptions by the sense of sight, see the article Optics.

In the Philosophical Transactions for 1776, Dr Fordyce gives an account of some experiments upon the light produced by Inflammation. They were made to determine, whether there was any light produced by the inflammation itself, independent of ignition. Substances, he observes, begin to be luminous in the dark when heated to between 6 and 700 degrees of Fahrenheit's thermometer. If the substances be colourless, they first emit a red light; then a red mixed with yellow; and lastly, with a great degree of heat, a pure white. This whiteness, however, seems to depend greatly upon the density of the body; for the vapour at the end of the flame urged by a blow-pipe is not visibly luminous, though its heat be sufficiently great to give a white heat to glass. The colour of the ignited matter, according to our author, has an effect upon the colour of the light emitted. Thus, during the calcination of zinc, the calx of which is white, a light is produced farce inferior in beauty to that of the sun matter itself. A beautiful green is communicated by the green calx of copper to the flame of a fire into which it is thrown; and the yellow empyreumatic oil into the colour which tallow or any common oil is converted in burning, communicates a part of its own colour to the flame, which very much alters the appearance of bodies seen by candle-light from what it is by day-light. It does not, however, appear that this always holds good; for the flame of burning iron is intensely white; and yet neither the metal itself nor any of its calces are of that colour.

Light produced by the decomposition of bodies by inflammation without ignition is always blue, and produces very little heat. Thus phosphorus of urine is decomposed by mere exposure to the air, and gives but very little heat, though a considerable light is emitted. The following proof is adduced by our author that this emission of light is a true inflammation. "Take a receiver of white glass, capable of holding six or eight gallons; put into it a drachm of phosphorus finely powdered, and half an ounce of water; cork the mouth of the receiver, and tie it over with a bladder, so as to exclude the external air: incline the receiver to all sides gently, and afterwards set it to rest; the powder will adhere to the sides, and the water will drain from it. As soon as the water is sufficiently drained off, the particles of the phosphorus will become luminous, and emit a thick smoke: this will continue for some days; but at last no more light or vapour will appear. Open the receiver, and you will find that the air will have contracted, as it does from the inflammation of a candle in Van Helmont's experiment; that is, about a twentieth part. It is become unfit for inflammation; for if a lighted candle be immersed in it, it will be extinguished as well as the phosphorus, and an animal will be suffocated by it. The air then has suffered the same change as that which has served for the inflammation of other bodies; and the phosphorus is partly decomposed, the water in the receiver being impregnated with its acid, and the air saturated with its phlogiston. Blow fresh air into the receiver, and the light and smoke will immediately re-appear. In like manner it is known that sulphur will burn and give light without heat sufficient for ignition. Take a piece of iron heated nearly red hot, and throw a little gun-powder upon it. If the heat be of a proper degree, the sulphur will burn off with a blue flame, without heat sufficient for ignition; for if such heat had been produced, the gun-powder would certainly have taken fire. It is the inflammation and decomposition of the sulphur, and not its evaporation, which produces the light; for if we sublime sulphur in vessels of the most transparent glass, no light will be visible except at the very beginning, when a small portion of it burns till the air in the vessel be saturated, and rendered unfit for inflammation."

Our author is of opinion, that the light produced by inflammation is of a blue colour, from whatever body it is derived. This he endeavours to prove from an observation on the flame of a candle, the lower part of which, where the inflammation is, always appears of blue. a blue colour. "Or (says he) take a candle which has burned for some time; extinguish it by applying tallow to the wick, and let it stand to cool; afterwards set it on fire by the flame of another candle; at first no more vapour will arise than can be acted upon by the air at once; inflammation, therefore, will go on in the whole small flame, and it will be blue. When a candle burns, the following process takes place. The tallow boils in the wick; and is converted into empyreumatic oil, rising from it in the form of vapour. As it rises from every part of the wick, the volume is increased till it comes to the top, and gives to the lower part of the flame the form of the frustum of an inverted cone. The air is applied to the outer surface of the column of vapour; and there decomposing the empyreumatic oil, produces heat and blue light: the stratum of vapour, within the outer burning surface, is heated white-hot; the heat diminishes towards the centre, which, if the flame be large, is scarcely red hot; as the column rises, decomposition taking place constantly on its surface, it necessarily diminishes, and the upper part of the flame is conical. That the tallow boils in the wick, can be seen: that it is converted into empyreumatic oil, is proved by drawing the vapour, rising in the middle of the flame, where it does not burn, into a glass tube: the empyreumatic oil condenses; this also shows that the flame does not burn in the middle. That the heat is produced on the outer surface, appears, if we take a small rod of glass, and put the end of it in the blue flame on the surface; it will be heated white hot, and melt. Immerse the rod into the flame, so that the point shall be in the centre: it will melt and bend where it is in the blue flame on the surface; whereas, if the flame be large, the point which is in the centre will hardly be heated red-hot. That the empyreumatic oil is decomposed, is proved by burning a candle with a very small wick in distilling vessels; no condensation of empyreumatic oil takes place."

In the 75th volume of the Transactions, Mr Morgan treats the subject of light at some length. As a foundation for his reasoning, he assumes the following data:

1. That light is a body, and, like all others, subject to the laws of attraction. 2. That light is an heterogeneous body; and that the same attractive power operates with different degrees of force on its different parts. 3. That the light which escapes from combustibles when decomposed by heat, or by any other means, was, previous to its escape, a component part of these substances.

Hence he concludes, that when the attractive force by which the several rays of light are attached to a body is weakened, some of those rays will escape sooner than others; it being evident that those which are detained by the smallest power will soonest go off when the general attractive force is weakened. This he illustrates by the example of a mixture of spirit of wine, water, and other more fixed substances. The application of a gentle heat will carry off the spirit of wine only; a heat not much greater will evaporate the spirits and water mixed together; and a still greater degree will carry off a mixture of all the particles together.

"In like manner (says he), when the surface of a combustible is in a state of decomposition, those parts of it which are the least fixed, or which are united with the least force, will be separated first. Amongst these the indigo rays of light will make the earliest appearance.

Vol. X, Part I.

By increasing the heat, we shall mix the violet with the indigo; by increasing it still more, we shall add the blue and the green to the mixture, till at length we reach that intensity of heat which will cause all the rays to escape at the same instant, and make the flame of a combustible perfectly white. By examining the flame of a common candle, we may observe, that its lowest extremities, or the part in which the black colour of the wick terminates, discharges the least heat; and that, as the vertex of the flame is approached, a successive order of parts is passed through, in which the lowest is continually adding to the heat of that which is just above it, till we come to the top of the flame, near which all the heat is collected into a focus. At the lowest extremity, however, where the heat is inconsiderable, a blue colour may always be observed; and from this appearance, amongst others, I think it may be concluded, that the blue rays are some of those which escape from combustibles in an early period of their decomposition; and that if the decomposition could be examined in a period still more early, the colour of the flame would be violet. By an *a priori* deduction of this kind, I was led to observe, that to the external boundary of the flame of a common candle is annexed a filament of light; which if proper care be taken to prevent the escape of too much smoke, will appear most beautifully coloured with the violet and indigo rays. If sulphur or ether be burned, or any other combustible whose vapour is kindled in a small degree of heat, a blue flame will appear; which, if examined by the prism, will be found to consist of the violet, the indigo, the blue, and sometimes a small quantity of the green rays. The best mode, however, of showing the escape of some rays by that degree of heat which will not separate others till increased, is the following. Give a piece of brown paper a spherical form, by pressing it upon any hard globular substance. Gradually bring the paper thus formed to that distance from the candle at which it will begin to take fire. In this case a beautiful blue flame per may be seen hanging, as it were, by the paper till a hole is made in it; when the flame, owing to the increased action of the air upon all parts of it, becomes white, though the edges still continue of a blue or violet colour.

As a confirmation of this, it may be observed, that the very flame, which when exposed to a certain degree of heat emits only the most refrangible rays, will, if exposed to one considerably greater, emit also those which are less so. The flames of sulphur and spirit of wine, if suddenly exposed to the heat of a reverberatory, will change their blue colour for one that is perfectly white."

To obtain a more perfect knowledge of this matter, our author examined the light proceeding from combustible bodies by Mr Melville's method. Having darkened the room, he interposed between the eye and combustible a sheet of pateboard, in which was a very small hole for transmitting the light. Viewing the light which passed through this hole with a prism, he observed, that the blue and violet rays were in greater abundance than any of the rest, though all the different kinds passed through it when spirit of wine only was made use of. When the combustion of the spirit of wine was checked by throwing in sal ammoniac, the red rays disappeared, but made their appearance again as soon as the salt became heated to such a degree as to increase... increase rather than diminish the combustion of the spirits. On examining the different parts of the flame separately, it was always found that the colours varied according to the degree of heat. At the base of the flame, or where the heat was least, the indigo, violet, and blue always appeared in greatest quantity; but as the vertex was approached, the other rays appeared, and at the very top they were all visible through a prism.

Conclusions from these experiments.

From these facts Mr Morgan concludes, 1. That light, as an heterogeneous body, is gradually decomposed during combustion; thus the indigo rays escape with the least heat, and the red with the greatest; and from this again he explains the reason why flames assume different colours. "If a piece of paper (says he), impregnated with a solution of copper in nitrous acid, be set on fire, the bottom and sides of the flame are always tinged green. Now this flame is evidently in that weak state of decomposition in which the most refrangible rays escape in the greatest abundance; but of these the green rays escape most plentifully through the unignited vapour and that portion of the atmosphere which is interposed betwixt the eye and the flame. This peculiarity may be observed in greatest perfection in brass founders. Here the heat, though very strong, is scarcely sufficient to decompose the metallic vapour which escapes from the melted brass; whence the flame has a very singular appearance, the edges being green, and the body of such an appearance, as to give substances viewed by it a pallid and ghastly appearance, owing to the want of a sufficient quantity of red rays to make a perfect white."

2. Mr Morgan explains the red appearance of bodies in their last state of ignition, from the previous escape of the more refrangible rays, so that only the red ones remain. "Again, (says he), we may consider the external surface of the combustible body as annexed to an inner surface, which may be partly, but not so perfectly decomposed as itself; for the violence of the heat will be found to lessen its effects the nearer it approaches to the centre of the substance which is exposed to it. Hence we are to consider the parts which are just covered by the external surface as having lost less of their component light than the external surface itself; or the former may retain the green rays when the latter has lost both indigo, violet, blue, and green.

3. "Those parts which are nearer the centre of the body than any of the preceding, must, as they are farther from the greatest violence of the heat, have lost proportionably fewer of their rays; or while the external parts may have lost all but the red, these may have lost only the indigo and violet.

4. "The most central parts may be unaffected by the heat; and whenever the fire does reach these parts, they will immediately discharge their indigo rays, and be decomposed in the gradual manner already mentioned. A piece of rotten wood, while burning, will exemplify and confirm the preceding illustration. When influenced by the external air only, if examined through a prism, no rays will be found to escape but the orange and the red. By blowing upon the burning wood with a pair of bellows, the combustion being increased, will affect those internal parts of the body which were not acted upon before. These parts therefore will begin to lose their light, and a prism will show the green, blue, violet, and indigo, all appearing in succession. Appearances similar to the preceding may be observed in a common kitchen fire. When it is faintest, its colour is most red, the other rays having been emitted, and the combustion at a stand; but by blowing upon it in this state, its brightness will be encreased, and more and more of the rays which are yielded by the internal parts of the body will come to the eye, till at length, by continuing to blow, the combustion will be made so complete as to yield all the rays, or to make it appear perfectly white."

Our author concludes the subject with a criticism Sir Isaac Newton's definition of flame, viz., definition that it is a vapour heated red hot. In his opinion, flame of flame is an instance of combustion whose colour will be criticised by determined by the degree of decomposition which takes place. When very imperfect, only the most refrangible rays will appear. If it be very perfect, all the rays will appear, and its flame will be brilliant in proportion. But there are flames which consist of burning particles, the rays of which have partly escaped before they ascended into form of vapour. "Such (says he) would be the flame of a red hot coal, if exposed to such a heat as would gradually convert it into vapour. When the fire is very low under the furnace of an iron foundry, at the upper orifice of the chimney a red flame of this kind may be seen, which is different from the flame that appears immediately after fresh coals have been thrown upon the fire; for in consequence of adding such a supply to the burning fuel, a vast column of smoke ascends, and forms a medium so thick as to absorb most of the rays excepting the red."

Thus we have a most elaborate theory for the solving of phenomena which seem not easily to admit any solution. It is obvious, however, that the data not upon which he builds his system are altogether unfounded and hypothetical. That light is subject to the laws of attraction, cannot be proved unless we could examine it independent of any other substance whatever; that is to say, in a perfect vacuum; and even in the most perfect vacuum that can be formed, we are far from being certain that no other matter is present. Light is inflected and turned out of its course in many different ways when acting in the common atmosphere, but we have no reason to suppose that it would be the same in a perfect vacuum; at least we have not a right to lay it down as a principle to argue from, unless it were verified by experience. Even the heterogeneous nature of light seems far from being absolutely established. The refraction into different colours by the prism seems insufficient to do so; for though, by a quick revolution of these colours when painted upon any substance, we may produce a kind of white colour, it is by no means perfect, but looks as if some black had got amongst it. The opinion of those who maintain that the prismatic colours are no other than different mixtures of light and shade, seems therefore equally probable with the other. His third position, that the light emitted by combustible bodies formed part of their substance before combustion, seems still worse founded; for instead of being fixed in solid substances, all the light and heat proceeding from combustion seem entirely to come from the air. By means of heat originally applied, the substance, or part of it, is rarefied into vapour; and this vapour, we have every reason to suppose, consists of elementary fire united with the solid substance. It is this fire, heat, or light, which is afterwards thrown out from the vapour in combustion; and new supplies of it perpetually come from the atmosphere, as is abundantly shown under the articles Combustion, Fire, Flame, and many others throughout this work. We cannot therefore think it either inconsistent or very improbable, that in the beginning of combustion, when the white light is clouded with a great quantity of vapour, it should appear of a blue or violet colour; and that in proportion as this vapour is dissipated, it should appear green, yellow, red, or perfectly white: for it is observable, that in dephlegmatised air, even those flames which in the common atmosphere always appear blue, such as sulphur and spirit of wine, are then changed to a dazzling white. The pure light of the sun also seen through smoke, or even through a great quantity of aqueous vapour, appears red; and there is not the least doubt, that if we were to view the sun while he thus appears red through any blue medium, he would appear purple; and in like manner, were we to view a blue flame through a yellow medium, it would appear of a green colour.

In the same paper Mr Morgan has some curious observations upon the electric light. There is neither fluid nor solid, he says, through which the electric fluid in its passage will not appear luminous, if we do not make the quantity, through which it has to pass, too great. In his experiments on fluids, he puts them into a tube about three quarters of an inch diameter and four inches long. The orifices are then stopped up with two corks, through which two pointed wires are thrust, so that the points may approach within one eighth part of an inch of each other; and in this case the electric matter which passes through the fluid is always luminous, provided a sufficient force be used. The experiment, however, is dangerous, unless great care be taken; and the tube, unless it be very strong, will be broken by a very slight discharge. With acids the experiment succeeds more difficulty; they must be put into capillary tubes, and the wires placed very near to each other. A stripe of gold leaf one eighth of an inch diameter, and a yard long, becomes quite luminous by discharging a battery over it; and our author cannot ascertain the length to which it might be made luminous. The experiment will also succeed with Dutch metal or silver leaf. If the gold or silver leaf be put upon a glass, and that laid in water, the whole will appear most beautifully luminous on discharging a battery through it.

The better a conductor that any substance is, the greater is the difficulty of making the electric spark visible in it. Hence it requires a much greater power of electricity to make a spark visible in boiling than in cold water; the former being a much better conductor than the latter. In like manner, the mineral acids are much better conductors than common water; and, of consequence, the spark is made to appear in them with much more difficulty than in water. This appears from what has been already mentioned; and our author likewise observes, that if a few drops of acid be poured into the tube containing the water employed for this purpose, it will scarcely be possible to make the spark luminous in it by any force.

The rarity of any body greatly increases the case with which the electric spark is made visible in it; as appears from discharging a vial through rarefied air, the vapour of ether, spirit of wine, or water.

In the prosecution of his experiments upon this subject, our author cemented a ball of iron into the orifice of a tube 48 inches long, and two thirds of an inch diameter, so that it could bear the weight of the quicksilver with which the tube was filled all to a small space at the open end, which contained a few drops of water. Having inverted the tube, and plunged the open end of it into a basin of mercury, that in the tube stood nearly half an inch lower than in a barometer with which it was compared at the same time, owing to the vapour which was formed by the water; but the spark passed as brilliant through the rarefied water as it does through rarefied air. If spirit of wine be employed instead of water in this experiment, the spark will not be so luminous. In the vapour of ether a great force is requisite to make the spark luminous, but good ether will press the mercury down as far as 16 or 17 inches. By rarefying the vapour, however, the spark will pass through it with more ease.

On examining the mineral acids in vacuo, Mr Morgan could not find that any vapour escaped from them. To give them the requisite degree of tenacity, therefore, he traced a line upon glass about an eighth part of an inch broad, with a camel's hair pencil dipped in the acids: the line extended sometimes to the length of 27 inches; in which case, the electric spark would pass over the whole with great brilliancy. If by widening the line, however, or putting on a drop of acid in any particular part, the quantity was increased, the spark never appeared in that part.

The brightness of the electric light is always in proportion to its condensation. Thus, if a spark taken from a powerful electrical machine divides itself into electric brushes, or throws out sparks from the sides, by which the light is diffused over a larger surface, it thus becomes less brilliant; and in all cases in which any diffusion of light, whether electric or not, takes place, the case will be the same.

In some cases, Mr Morgan is of opinion, that, even with the electric fluid, only the more refrangible rays escape from the electric fluid, or throw out sparks from the sides, by which the light is diffused over a larger surface, it thus becomes less brilliant; and in all cases in which any diffusion of light, whether electric or not, takes place, the case will be the same.

Sometimes with the electric fluid, only the more refrangible rays escape from the electric fluid, or throw out sparks from the sides, by which the light is diffused over a larger surface, it thus becomes less brilliant; and in all cases in which any diffusion of light, whether electric or not, takes place, the case will be the same.

Our author next proceeds to examine the influence of media upon electric light; which, he says, is similar media upon their influence upon solar light, and serves to explain several phenomena.

"Let a pointed wire (says he), having a metallic hall fixed to one of its extremities, be forced obliquely into a piece of wood, so as to make a small angle with its surface; and to make the point lie about one eighth of an inch below it. Let another pointed wire, which communicates with the ground, be forced in the same manner into the same wood, so that its point may in like manner be about one eighth of an inch below the surface, and about two inches distant from the point." of the first wire. Let the wood be insulated, and a strong spark, which strikes on the metallic ball, will force its way through the interval of wood which lies between the points, and appear as red as blood. To prove that this appearance depends on the wood's absorption of all the rays but the red, I would observe, that the greater the depth of the points is below the surface, the less mixed are the rays. When they are deepest below the surface, only the red come to the eye through a prism; when raised a little nearer the surface, the red and orange appeared; when nearer still, the yellow; and so on, till, by making the spark pass through the wood very near its surface, all the rays were at length able to reach the eye. If the points be only one eighth of an inch below the surface of soft deal wood, the red, the orange, and the yellow rays will appear as the spark passes through it. But when the points are at an equal depth in a piece of harder wood, box for instance, the yellow, and perhaps the orange, will disappear. As a farther proof of this absorption of the rays, it may be observed, when the spark strikes very bright upon one side of the piece of deal, it will appear quite red on the other. In like manner, a red appearance may be given to a spark which strikes bright over the inside of a tube, merely by spreading some pitch very thinly over the outside of the same tube."

Mr Morgan now proceeds to mention some experiments which seem to militate against the doctrine he has been endeavouring to establish, rather than to support it; viz. 1. If into a Torricellian vacuum of any length a few drops of ether are conveyed, and both ends of the vacuum stopped up with metallic conductors, so that a spark may pass through it, the spark in its passage will make the following appearances. When the eye is placed close to the tube, the spark will appear perfectly white; if the eye is removed to the distance of two yards, it will appear green; but at the distance of six or seven yards, it will appear reddish.

"These changes evidently depend (says our author) on the quantity of medium through which the light passes; and the red light more particularly, which we see at the greatest distance from the tube, is accounted for on the same principle as the red light of the clouded sun, or a lighted candle."

2. Dr Priestley long ago observed the red appearance of the electric spark, when passing through inflammable air. But this appearance is very much diversified according to the quantity of medium through which the spark is beheld. At a very considerable distance the red comes unmixed to the eye; but if the eye be placed close to the tube, the spark appears white and brilliant. By increasing, however, the quantity of fluid conveyed through any portion of inflammable air, or by condensing that air, the spark may be made perfectly white. It may further be observed, that all weak explosions and sparks, when viewed at a distance, make a reddish appearance. The reason of these appearances seems to be, that the weaker the spark or explosion is, the more it is disposed to assume a red colour, when viewed at a distance. This seems to confirm what has already been mentioned as a probable hypothesis, that the different colours of light are entirely owing to the medium through which they are viewed.

On phosphoric light Mr Morgan makes some curious observations; but still argues on the same principles we have already mentioned. "Some shells (says he), prepared according to Mr Wilton's directions*, after being exposed to the sun, or to the flash of a battery, phosphoric emit a purple, others a green, and others a reddish light, light. If, with Mr Wilton, we suppose that these shells are in a state of flow combustion, may we not conclude that some are just beginning to burn, and therefore emitting the most refrangible rays; while others are in a more advanced state of combustion, and therefore emitting the least refrangible? If this conclusion be right, the shells which are emitting the purple or the green, must still retain the yellow, the orange, and the red, which will also make their appearance as soon as the combustion is sufficiently increased." In confirmation of this, Mr Morgan adduces the following experiment, viz. that if a shell, while emitting its green rays, be placed upon a warm shovel, the colour will soon be changed into a yellow mixed with red.—To the theory of flow combustion Mr Morgan makes the following objections.

1. If phosphoric shells owe their light to this cause, we must consider the word combustion, when applied to them, as implying all those circumstances which usually attend a body when on fire. On this supposition there ought to be an increase of the heat as well as of the decomposition of the combustible. But neither of these are found to take place in fact; for a phosphoric body never fails to lose its light entirely in a certain degree of heat, without losing the power of becoming phosphoric again when it has been sufficiently cooled. While very hot, the charge of the strongest battery conveyed over it has no effect.

2. When bodies are wafted by combustion, they can never be made to reassume the appearances which they previously displayed. "No power (says our author) can give to ashes the phenomena of a burning coal. But phosphoric bodies are very different in this respect; for a phosphoric shell may be made to lose all its light by exposure to heat, and again may be made as luminous as ever by exposure to the sun."

3. It is remarkable that some bodies which are most beautifully phosphoric, are at the same time the most obstinate in resisting fire. "Let us now see (says Mr Morgan) the consequence of admitting the common hypothesis, that the detention of those rays which fall upon phosphoric is owing to some force which prevents their immediate reflection, but is not adequate to their entire absorption. This force, whatever it be, cannot well be supposed to operate with equal power on all these rays. If this be not the case, we cannot well avoid concluding, that phosphoric shells will assume different colours, owing to the earlier and later escape of the different rays of light. This conclusion is justified by an experiment already mentioned; viz. that when the force is such as to admit the escape of the purple, blue, and green, we have only to lessen that force, by warming the body, and the yellow, the orange, and red escape. Beccaria has proved, that there is scarcely any body which is not phosphoric, or may not become so by heat. But as the phosphoric force is most powerful when the purple rays only escape, so we are to conclude, that it is weakest when it is able to retain the red rays only. This is agreeable to several facts. Chalk, oyster-shells, together with those phosphoric bodies whose goodness has been very much impaired by long keeping, when finely powdered, and placed within the circuit of an electrical battery, will exhibit, by their scattered particles, a shower of light; but these particles will appear reddish, or their phosphoric power will be sufficient only to detain the yellow, orange, and red rays. When spirit of wine is in a similar manner brought within the circuit of a battery, a similar effect may be discovered: its particles diverge in several directions, displaying a most beautiful golden appearance. The metallic calces are rendered phosphoric with the greatest difficulty; but even these may be scattered into a shower of red luminous particles by the electric stroke.

In a postscript to this paper, by Dr Price, it is observed, that by phosphoric force, Mr Morgan seems to mean, not the force with which a phosphoric body emits, but that with which it absorbs and retains, the light. This last force is proportioned to the degree of attraction between the phosphoric body and light; and therefore must, according to Mr Morgan's theory, be weakest when it so freely emits the light it has imbibed as not to retain those rays which adhere to it most strongly. According to Mr Morgan's theory, these are the rays which are the least refrangible. "It is, however (says Dr Price), an objection to that, the less refrangibility of rays seems to imply a less force of attraction between them and the substances which refract them; but it should be considered, that, possibly, the force of cohesion, which unites the rays of light to bodies, may be a different power from that which refracts them."

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 left to the touch, and seemed to be transparent in candle light; 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 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 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, 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. ance; 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 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. Jacobæus 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. 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, and the Bolognian academicians, especially Beccarius, who took so much pains with the subject of phosphoric 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. Marlianus, 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 faeces 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, ceruils, or ingar, 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 pholas 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 renfa, the light of the pholas, was that which was observed vol. v. 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 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 conical 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, she 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 she 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 steam, 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 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 discriminating 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, account of that he could easily read the title of a book by it, luminous though he was nine or ten feet from the surface of the water. Sometimes he could easily distinguish, in the wake of the 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 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 vortices 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 over 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 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 nousness 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 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, presently after the abbé Nollet made his observations at Venice, took notice, that in the daytime, 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, though 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 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 Roi 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. No thing 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 left. 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 greyish 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, 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 Savans 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 poly-

pufes 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. Godchoue 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 abbé 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 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 ignis fatua, that luminous appearance which goes by the name of ignis fatua, 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 fatua.

This kind of light is said to be frequent about burying places and dung-hills. 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,

(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. 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. Beccari 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 was 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, and appear again in some other place. They differed also in size and figure, sometimes spreading pretty 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. Beccari 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 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 parallelopiped, 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 perfectly, 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 stranger 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. Beccari, 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 insensible 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 these 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 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 of rotten wood and burning coals. But though this seems generally to be the case, there are still some exceptions, as has been instanced 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 dungs-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, phlogisticated, and alkaline airs, all blended together in one common 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 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 this is from being the case, that the ignis fatuus described by M. Beccari 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 as 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 putrefactive 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 accretion of the inflammable air which frequently abounds in such places.

In the appendix to Dr Priestley's third volume of experiments experiments and observations on air, Mr Warltire gives an account of some very remarkable ignes fatui, 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 ignes fatui, 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 ⅓ 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 ignes fatui 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 upon a small quantity of atmospherical 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 Warltire. 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 ignes fatui 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 are, 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 phosphoric acid 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 putrefied 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 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 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 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 flame, 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 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, sea-water, phosphorus, or any thing 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 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 splendor, 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 so rendered capable of receiving a larger 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 sun-shine, or at least to the open day-light, 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 imbibing the sun's rays, and shining in the dark, and that others should not; yet so 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 brightness, 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 the yellow diamonds have this property. This may probably arise from their having more sulphur in their composition, and therefore illuminating more readily, or emitting a more vivid 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 day-light, 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 six 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 day-light or sun-shine, but they all became luminous by the different means of heating or friction; with this difference, that some acquired it by one of these... 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 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 heliometers, gypsum, lime-stone, 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. Haggern, 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 sun-set, 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, *glandula officinalis*. 2. Monk's-hood, *tropaeolum majus*. 3. The orange-lily, *lilium bulbiferum*. 4. The Indian pink, *tagetes patula* &c.

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 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 pistil 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 chiaroscuro.

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-Horses.

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, 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.