by the burning glasses used in their experiments, or some other causes which they had overlooked.

However, Dr. Priestley informs us, that Mr. Michell endeavoured to ascertain the momentum of light with still greater accuracy, and that his endeavours were not altogether without success. Ilaving found that the instrument he used, acquired, from the impulse of the rays of light, a velocity of an inch in a second of time, he inferred that the quantity of matter contained in the rays falling upon the instrument in that time, amounted to no more than the 12 hundred millionth part of a grain. In the experiment, the light was collected from a surface of about three square feet; and as this surface reflected only about the half of what fell upon it, the quantity of matter contained in the solar rays, incident upon a square foot and a half of surface, in a second of time, ought to be no more than the 12 hundred milliouth part of a grain, or upon one square foot only, the 18 hundred millionth part of a grain. But as the density of the rays of light at the surface of the sun, is 45,000 times greater than at the earth, there ought to issue from a square foot of the sun's surface, in one second of time, the 40 thousandth part of a grain of matter; that is, a little more than two grains a day, or about 4,752,000 grains, which is about 670 pounds avoirdupois, in 6000 years, the time since the creation; a quantity which would have shortened the sun's semidiameter by no more than about 10 feet, if it be supposed of no greater density than water only.

The expansion or extension of any portion of light, is inconceivable. Dr. Hook shows that it is as unlimited as the universe; which he proves from the immense distance of many of the fixed stars, which only become visible to the eye by the best telescopes. Nor, adds he, are they only the great bodies of the sun or stars that are thus liable to disperse their light through the vast expanse of the universe, but the smallest spark of a lucid body must do the same, even the smallest globule struck from a steel by a flint.

The intensity of different lights, or of the same light in different circumstances, affords a curious subject of speculation. M. Bouguer, Traité d'Optique, found that when one light is from 60 to 80 times less than another, its presence or absence will not be perceived by an ordinary eye; that the moon's light, when she is 192 18 high above the horizon, is but about of her light at 66° 11' high; and when one limb just touched the horizon, her light was but the 2000th part of her light at 66° 11' high; and that hence light is diminished in the proportion of 3 to 1 by traversing 7469 toises of dense air. He found also, that the centre of the sun's disc is consider. ably more luminous than the edges of it; whereas both the primary and secondary planets are more luminous at their edges than near their centres : that, farther, the light of the sun is about 300,000 times greater than that of the moon; and therefore it is no wonder that philosophers have had so little success in their attempts to collect the light of the moon with burning-glasses; for, should one of the largest of them even increase the light 1000 times, it will still leave the light of the moon in the focus of the glass, 300 times less than the intensity of the common light of the sun.

Dr. Smith, in his Optics, vol. 1, pa. 29, thought he had proved that the light of the full moon would be only the 90,900th part of the full day-light

if no rays were lost at the moon. But Mr. Robins, in his Tracts, vol. 2, pa 225, shows that this is too great by one half. And Mr. Michell, by a more easy and accurate mode of computation, found that the density of the sun's light on the surface of the moon is but the 4,000th part of the density at the sun; and that therefore, as the moon is nearly of the same apparent magnitude as the sun, if she reflected to us all the light received on her surface, it would be only the 45,000th part of our day light, or that which we receive from the sun. Admitting therefore, with M. Bouguer, that the moon light is only the 300,000th part of the day or sun's light, Mr. Michell concludes that the moon reflects no more than between the 6th and 7th part of what she receives.

Professor Leslie, on the contrary, says, "The light of the moon has the opposite character of excessive debility. The action of her rays on the photometer is quite imperceptible: nor could I render it visible, even by collecting them in the focus of a large burning-glass. But I was enabled to form some estimate, by an indirect mode of comparison. I selected a small table of logarithms on which I could barely read the figures, by the light of the full moon: on retiring gradually backwards from a wax candle set to burn in a dark. ened room, I found the figures now become indistinct, beyond the distance of 15 feet. The force of the light received from the candle must have been only the 1350th part of a degree, for

re

"This estimate is double what has been assigned by the celebrated Bouguer; and my respect for the conclusions of that able observer has induced me, where the limit was dubious, to lean more to the side of defect than excess. If I have erred therefore, I presume it is in representing the lunar illumination rather too small than too large. But neither of these computations will agree with the current opinion, that the moon derives her light merely from the sun. In fact, if the moon flected and dispersed in every direction the whole of the light which she receives from the sun, it would, before it reached us, be spread over the concavity of a sphere equal to the lunar orbit. But the earth's orbit having its diameter about 224 times that of the moon, and the surface of a sphere being equal to four of its great circles, the secondary light which would reach the earth must be attenuated not less than two hundred thousand times, for 4 (224) 2 ≈ 200,704. Such perfect reflection, however, cannot be admitted. If we examine the face of the moon with a good telescope, we discern round spots of extraordinary brightness, and perceive large spaces which are remarkably obscure. It is evident then, that but a very small part of the incident light must be reflected, the rest being absorbed. The quantity of reflection from paper, plaster, and other white rough surfaces, according to Bouguer himself, constitutes only the 150th part of the whole incidence. If the exterior crust of the moon resem. bled, therefore, any earthy body with which we

are acquainted, her pale borrowed light would be at least one hundred times feebler than is actually observed. Hence I am disposed to think, that the rays of the moon are principally, if not entirely, discharged from her own mass, and that the lunar surface is of a nature analogous to the carbonate of barytes and other phosphorescent substances, which, after a partial calcination, are capable of being excited by the action of the solar rays to disengage their latent hight." (Leslie on Heat, p. 4: 1.)

Sir L Newton observes, that bodies and light act mutually on one another; bodies on light in emitung, reflecting, refracting, and inflecting it; and light on bodies, by heating them, and putting their parts into a vibrating motion, in which heat principally consists. For all fixed bodies, he observes, when heated beyond a certain degree, to emit light and shine; which shimng, &c. ap. pears to be owing to the vibrating motion of their parts; and all bodies, abounding in earthy and sulphureous particles, if sufficiently agitated, emit light, which way soever that agitation be effected. Thus, sea water shines in a storm; quicksilver, when shaken in vacuo; cats or horses, when rubbed in the dark; and wood, fish, and flesh, when putrefied.

Light proceeding from putrescent animal and vegetable substances, as well as from glow-worms, is mentioned by Aristotle. And Bartholin mentions four kinds of luminous insects, two of which have wings: but in hot climates it is said they are found in much greater numbers, and of different species. Columna observes, that their light is not extinguished immediately on the death of the animal. The first distinct account that occurs of light proceeding from putrescent animal flesh, is that which is given by Fabricius ab Aquapendente, in 1592, de Visione, &c. page 45. And Bartholin gives an account of a similar appearance, which happened at Montpelier in 1641, in his treatise De Lace Animalium.

Mr. Boyle speaks of a piece of shining rotten wood, which was extinguished ir vacuo; but upon re-admitting the air, it revived again, and shone as before; though he could not perceive that it was increased in condensed air. But in Birch's History of the Royal Society, vol. ii. p. 254, there is an account of the light of a shining fish, which was rendered more vivid by putting the fish into a condensing engine. The fish called whitings were those commonly used by Mr. Boyle in his experiments: though in a discourse read before 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. Birch's Hist. vol. ii. pa. 70. In 1672, Mr. Boyle accidentally observed light issuing from flesh meat; and, among other remarks on this subject, he observes that extreme cold extinguishes the light of shining wood; probably because extreme cold checks the putrefaction, which is the cause of the light. The shell-fish called Pholas is remarkable for its luminous quality. The luminousness of the sea has been also a subject of frequent observation. See Phosphorus, Putrefaction, and the latter part of this article.

Mr. Hawksbee, and many writers on the sub. ject of electricity since his time, have produced a great variety of instances of the artificial production of light, by the attrition of bodies naturally not luminous; as of amber rubbed on woollen cloth in vacuo; of glass on woollen, of glass on glass, of oyster shells on woollen, and of woollen on woollen, all in vacuo. See ELECTRICITY, &c.

VOL VII.

Of the Attraction of Light. That the particles of light are attracted by those of other bodies, is evident from numerous experiments. This pheno menon was observed by Sir Isaac Newton, who found by repeated trials, that the rays of light, in their passage near the edges of bodies, are diverted out of the right lines, and always inflected or bent towards those bodies, whether they be opaque or transparent, as species of metals, the edges of knives, broken glasses, &c. See INFLEC TION and RAYS. The curious observations that had been made on this subject by Dr. Hook and Grimaldi, led Sir Isaac Newton to repeat and diversify their experiments, and to pursue them much farther than they had done. For a particular account of his experiments and observations, see his treatise on Optics, p. 293, &c.

This action of bodies on light is found to exert itself at a sensible distance, though it always increases as the distance is diminished; as appears very sensibly in the passage of a ray between the edges of two thin planes at different apertures; which is attended with this peculiar circumstance, that the attraction of one edge is increased as the other is brought nearer it. The rays of light, in their passage out of glass into a vacuum, are not only inflected towards the glass, but if they fall too obliquely, they will revert back again to the glass, and be totally reflected. Now the cause of this reflection cannot be attributed to any resistance of the vacuum, but must be entirely owing to some force or power in the glass, which attracts or draws back the rays as they were passing into the vacuum. And this appears farther from hence, that if we wet the back surface of the glass with water, oil, honey, or a solution of quicksilver, then the rays which would otherwise have been reflected, will pervade and pass through that liquor; which shows that the rays are not reflected till they come to that back surface of the glass, nor even till they begin to go out of it; for if, at their going out, they fall into any of the aforesaid mediums, they will not then be reflected, but will persist in their former course; the attraction of the glass being in this case counterbalanced by that of the liquor.

M. Maraldi prosecuted experiments similar to those of Sir Isaac Newton on inflected light. And his observations chiefly respect the inflection of light towards other bodies, by which their shadows are partially illuminated. Acad. Paris. 1723, Mem. p. 159. See also Priestley's Hist. p. 521, &c.

M. Mairan, without attempting the discovery of new facts, endeavoured to explain the old ones by the hypothesis of an atmosphere surrounding all bodies; and consequently two reflections and refractions of light that impinges upon them, one at the surface of the atmosphere, and the other at the surface of the body itself. This atmosphere he supposed to be of a variable density and refractive power, like the air.

M. du Tour succeeded Mairan, and imagined that he could account for all the phenomena by the help of an atmosphere of a uniform density, but of a less refractive power than the air surrounding all bodies. Du Tour also varied the Newtonian experiments, and discovered more than three fringes in the colours produced by the inflection of light. He farther concludes that the refracting atmospheres, surrounding all kinds of bodies, are of the same size; for when he used a great variety of substances, and of different sizes too he always found coloured streaks of the same dimensions. He also observes that his hypo

E

thesis contradicts an

observation of Sir Isaac Newton, viz. that those rays are the most inflected which pass the nearest to any body. Mem. de Math. et de Phys. vol. v. p. 650, or Priestley's Hist. p. 531.

M. Le Cat found that objects sometimes appear magnified by means of the inflection of light. Looking at a distant steeple, when a wire, of a less diameter than the pupil of his eye, was held pretty near to it, and drawing it several times between that object and his eye, he was surprised to find that every time the wire passed before his eye, the steeple seemed to change its place, and some hills beyond the steeple seemed to have the same motion, just as if a lens had been drawn between them and his eye. This discovery led him to several others depending on the inflection of the rays of light. Thus, he magnified small objects, as the head of a pin, by viewing them through a small hole in a card, so that the rays which formed the image must necessarily pass so near the circumference of the hole as to be attracted by it. He exhibited also other app.arances of a similar nature. Traité des Sons, p. 299. Pricetley, ali supra, p. 537.

Reflection and Refraction of Lit. From the mutual attraction between the particles of light and other bodies, arise two other grand phenomena. besides the inflection of light, which are called the reflection and refraction of light. It is well known that the determination of bodies in motion, especially elastic ones, is changed by the interposition of other bodies in their way; thus also light, impinging on the surfaces of bodies, should be turned out of its course, and beaten back or reflected, so as, like other striking bodies, to make the angle of its reflection equal to the angle of incidence. This, it is found by experience, light does; and yet the cause of this effect is ditlerent from that just now assigned: for the ays of light are not reflected by striking on the very parts of the reflecting Lodies, but by some power equally diffused over the whole surface of the body, by which it acts on the light, either attracting or repelling it, without contact : by which same power, in other circumstances, the rays are refracted; and by which also the rays are first emitted from the luminous body, as Newton abundantly proves by a great variety of arguments. See REFLECTION, REFRACTION, OPTICS.

Whence comes the light afforded by ignited bodies? whether it have been previously imbibed by them? whether the commencement of ignition be distinctive of the same temperature in all bodies ? whether the great planetary sources of light be bedies in a state of combustion, or merely luminous upon principles very different from any which our experiments can point out? whether the momentum of the particles of light, or their disposition for chemical combination, be the most effectual in the changes produced by its agency? - these, and numerous other interesting questions, must be left for future research and investigation. See COMBUSTION.

The production of light by inflammation is an object of great importance to society at large, as well as to the chemist. It appears to arise immediately from the strong ignition of a body while rapidly decomposing Mest solid bodies, in combustion, are kept, putly from a want of the access of air, and partly from the vicinity of conducting bodies, at a low degree of ignition. But when vapours rapidly escape into the air, it may, and

does frequently happen, that the combustion, instead of being carried on merely at the surface of the mass, penetrates to a considerable depth within, and from this, as well as from the imperfect conducting power of the surrounding air, a white heat, or very strong ignition, is produced. The effect of lamps and candles depends upon these considerations. A combustible fluid, most commonly of the nature of fat oil, is put in a situation to be absorbed between the filaments of cotton, linen, fine wire, or asbestos. The extremity of this fibrous substance, called the wick, is then considerably heated. The oil evaporates, and its vapour takes fire. In this situation, the wick, being enveloped with flame, is kept at such a temperature, that the oil continually boils, is evaporated, burns, and by these means keeps up a constant flame. Much of the perfection of this experiment depends on the nature, quantities, and figure, of the materials employed.

LIGHT (Chemical Properties of). Of whatever this substance may consist, or however it may be produced, we see abundant reason to convince us that it is a very important agent in many of the great chemical changes that are taking place in the visible world; while even in the laboratory its effects are often demonstrable in a multiplicity of experi ments.

To ascertain in some degree (for we have still much ignorance upon this subject) the chemical properties of light, we shall first observe that the rays emitted from the sun, are not simple as was formerly conceived, but consist of three distinct kinds, each of them differently refrangible; coloritic, or those producing the effect of light and colours, calorific, or those producing heat, and deoxydizing, or those which have a tendency to reduce metallic oxyds to their metalline state. Into some classes of bodies the whole of these enter; into others, the first, second, or third kind alone. From some classes of bodies they are either severally or generally extricated without alteration; and with others, the whole or some particular kind combines, and constitutes one of their component parts.

We are

Whether any one of these three distinct kinds of rays is capable of being generated and secreted by other bodies, is perhaps doubtful: but that the light which many substances emit is derived a' extra, is evident from the experiments of father Beccaria, and several other philosophers, by which it appears that these only become luminous after having been for a long time exposed to the light; and lose their luminous property soon after they have been deprived of it. also indebted to Mr. Canton for some very interesting experiments on this subject, and for discovering a composition which possesses this proHe calcined some perty in a remarkable deg te. oyster shells in a goed cual fire for half an hour, and then pounded and sifted the purest part of them; three parts of this powder was mixed with one part of the flower of sulphur and rammed into a crucille, which was kept red hot for an beur. The brightest parts of the mixture were then scraped off and kept for use in a dry place well stopped. When this composition is exposed for a few seconds to the light, it becomes sufficiently luminous to enable a person to distinguish the hour on a watch by it. After some time it ceases to shine, Lut recovers this property on being again exposed to the light. See PиosPHORUS, and

PYROPHORUS.

But Fight net caly cuters into bedies; it also

combines with them, and constitutes one of their compound parts. Other experiments of Mr. Canton, and more recent ones of Dr. Hulme, very sufficiently prove this. It has been long known, that different kinds of meat and of fishes on the point of putrefac tion, become luminous in the dark, and of course give out light. This is the case in particular with the whiting, the herring, and the mackerel. When four drams of either of these are put into a phial, containing two ounces of sea-water, or of pure water, bolding in solution half a dram of common salt, or two drams of sulphat of magnesia, if the phial be put into a dark place, a luminous ring appears on the surface of the liquid within three days, and the whole liquid, when agitated, becomes luminous and continues in that state for some time. When these liquids are frozen, the light disappears, but is again emitted as soon as they are thawed. A moderate heat increases the luminousness, but a boiling heat extinguishes it akogether. The light is extinguished also by water, Ime-water, water impregnated with carbonic acid gas, or sulphurated hydrogen gas, fermented liquors, spirituous liquors, acids, alkalies, and water saturated with a variety of salts, as sal ammoniac, comraon salt, sulphat of magnesia; but the light appears again when these solutions are diluted with water. This light produces no sensible effect on the thermometer. Light, therefore, is a component part of such substances, and perhaps the first that makes its escape when they are beginning to be decomposed.

Not unfrequently, however, the light absorbed and afterwards emitted by bodies, is emitted not generally, but by a decomposition of its calorific rays; and often only a particular kind of ray is absorbed, while the rest are reflected. Hence the cause of the different colours of bodies: such colours depending upon the affinity of particular bodies for particular rays, and their want of affinity for others.

Hence the absorption of light by bodies, produces very sensible changes in them, and changes that operate below their surface. Plants, for instance, may be made to vegetate in the dark tolerably well: but in such case, not only is their surface white or colourless, but they have scarcely any taste, and contain but a very small proportion of combustible matter. In a very short time, however, after exposure to light, their colour becomes green, their taste is rendered much more intense, and the quantity of combustible matter is considerably increased. Lov. age, mint, carraways, tansy, and many other plants have been the subject of such experiments, by being buried long in deep mines, when it has been found that they have uniformly, in process of time, so completely lost their colour, aroma, and in some instances the very form of their leaves, as to ren der a stranger to the fact, incapable of classifying er identifying them; yet upon re-exposure to the light, they have again exhibited their peculiar cha

racters.

Light has also a considerable influence in the germination of seeds. Ingenhouse found, that seeds always germinate faster in the dark than when exposed to the light. His experiments were repeated by Sennebier with equal success; but the Abbé Bertholin has objected to this conclusion, and affirmed that the difference adverted to proceeded not from difference in the quantity of light, but of moisture; the moisture evaporating much faster from seeds in the light than from those in the shade. Yet Sennebier repeated his experiments with every possible attention to equality

of moisture, and still the same result followed. We may safely conclude, therefore, that light, though highly useful to the subsequent growth and perfection of vegetables, retards germination. Sanpear, however, has lately advanced, and upon strong grounds, that light is merely injurious to germination, in consequence of the heat it produces; for when the direct rays of the sun were intercepted, though light was admitted, the germination of seeds was not retarded sensibly.

Another very remarkable instance of the agency of light, is in the reduction of the metallic oxyds. The red oxyds of mercury and lead become much lighter when exposed to the sun; and the white salts of silver called luna cornea, in the same situation, soon become black, and the oxyd is reduced. The oxyd of gold may be reduced in the same manner. This change occurs, without requiring the least increase of temperature, or in any way being affected by heat or cold, and it occurs equally well in close as in open vessels, and only in the surface immediately exposed to the light. Light then has the property of separating oxygen from the oxyds.

This reduction of metallic oxyds was till lately sup posed to be produced by the calorific rays of light, but Dr. Wollaston, MM. Ritter and Bortmann have lately ascertained that muriat of silver is blackened most rapidly, when it is placed beyond the violet ray (the ray which was formerly conceived to be the most powerful in producing this effect) and entirely out of the prismatic spectrum. Hence it follows, that the change is produced, not merely by the calorific rays, but by rays which are incapable of rendering objects visible: rays which are more refrangible than the calorific, as they extend beyond the violet end of the spectrum. And hence, independently of the calorific and colorific, we obtain knowledge of a third set of rays; and perhaps the greater number of the chemical changes produced on bodies by solar light, is owing to this last set of rays, which, from its first noticed effect has been denominated, as we have already observed, deoxydizing. See CALORIC, COMBUSTION, and HEAT.

That the calorific and illuminating or colorific rays of the sun are not the same, was first determined by Dr. Herschell, it having been only conjectured before, in consequence of his observing that the most refrangible rays have the least heating power, and that the heating power gradually increases, as the refrangibility diminishes; whence he was led to suspect that the heating power does not stop at the end of the visible spectruni, but is continued beyond it at the red extremity. He placed, therefore, the thermometer by which he had ascertained this difference of power beyond the boundary of the red ray, but still in the line of the spectrum; and it rose still higher than it had done, when exposed to the red ray which possesses the greatest heating power of all the coloured rays. On shifting the thermometer still further, it continued to rise, and the rise did not reach its maximum till the thermometer was half an inch beyond the utmost extremity of the red ray. When shifted still further, it sunk a little; but the power of heating was sensible at the distance of an inch and a half from the red ray. These experiments have been since fully confirmed by other philosophers; and it bence follows, that there are rays emitted from the sun which produce heat, but have not the power of illuminating, and that these are the rays which produce the greatest quantity of heat. Consequently caloric is emitted from the sun in rays, and the rays of caloric are not the same with the rays of light.