est dust they meet with; and the sun appears to continue of his ancient dimensions, and his attendants move in their ancient orbits." He therefore conjectures, that all the phenomena of light may be more properly solved, by supposing all space filled with a subtle elastic fluid, which is not visible when at rest, but which, by its vibrations, affects the fine sense in the eye, as those of the air affect the grosser organs of the ear; and even that different degrees of the vibration of this medium may cause the appearances of different colours. Franklin's Exper. and Observ. 1769, p. 264. The celebrated Euler has also maintain ed the same hypothesis, in his "Theoria Lucis et Colorum." In the summary of his arguments against the common opinion, recited in Acad. Berl. 1752, p. 271, besides the objections above-mentioned, he doubts the possibility, that particles of matter, moving with the amazing velocity of light, should penetrate transparent substances In whatever manner with so much ease. they are transmitted, those bodies must have pores, disposed in right lines, and in all possible directions, to serve as canals for the passage of the rays; but such a structure must take away all solid matter from those bodies, and all coherence among their parts, if they do contain any solid mat ter. Among modern philosophers who have supported this doctrine, Dr. Young has shown much ability in his experimental and theoretical researches, in his memoirs in the "Philosophical Transactions," which have been republished in his "Lectures," and in "Nicholson's Journal." 66 The expansion or extension of any Dr. portion of light is inconceivable. 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, 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." are The intensity of different lights, or of the same light in different circumstances, affords a curious subject of speculation. M. Bouguer, Traité de Optique, found that when one light is from sixty to eighty times less than another, its presence or absence will not be perceived by an ordinary eye; that the moon's light, when she is 19° 16' high above the horizon, is about one-third of her light, at 66° 11′ high; and when one limb just touched the horizon, her light was but the 2,000th part of her light at 66° 11' high; and that hence light is diminished in the proportion of three to one, by traversing 7,469 toises of dense air. He found also, that the centre of the sun's disc is considerably 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, further, 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 1,000 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. i. p. 29, thought he had proved that the light of the full moon would be only the 90,900th part of the fall day-light, if no rays were lost at the moon. But Mr. Robins, in his Tracts, vol. ii. p. 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 45,000th 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's 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. Sir I. Newton long ago observed, that bodies and light act mutually on one another; bodies on light, in emitting, reflecting, refracting, and inflecting it; and light on bodies, by heating them, and putting their parts into a vibrating motion, in which For all fixed heat principally consists. bodies, he observes, when heated beyond a certain degree, do emit light and shine. This action of bodies on light is found to exert itself at a sensible distance, though it always increases as the distance is diminish. ed, as appears very sensibly in the passage of a ray between the edges of two very 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 va cuum. And this appears further from hence, that if you wet the back surface of the glass with water, oil, honey, or a solution of quicksilver, then the rays which would ctherwise 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 I. 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. 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 inci dence. This, it is found by experience, light does; and yet the cause of the effect is dif ferent from that just now assigned, for the rays of light are not reflected by striking on the very parts of the reflecting bodies, 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 REFLEC TION and REFRACTION. That great author put it past doubt, that all those rays which are reflected do not really touch the body, though they approach it intinitely near; and that those which strike on the parts of solid bodies adhere to them, and are, as it were, extinguished and lost. Since the reflection of the rays is ascribed to the action of the whole surface of the body without contact: if it be asked how it happens that all the rays are not reflected from every surface, but that, while some are reflected, others pass through and are refracted? the answer given by Newton is as follows: Every ray of light, in its passage through any refracting surface, is put into a certain transient constitution or state, which in the progress of the ray returns at equal intervals, and disposes the ray at every return to be easily transmitted through the next refracting surface, and between the returns to be easily reflected by it: which alteration of reflection and transmission, it appears, is propagated from every surface, and to all distances. What kind of action or disposition this is, and whether it consists in a circulating or vibrating motion of the ray, or the medium, or something else, he does not inquire; but allows those who are fond of hypothesis to suppose that the rays of light, by impinging on any reflecting or refracting surface, excite vibrations in the reflecting or refracting medium, and by that means agitate the solid parts of the body. These vibrations, thus produced in the medium, move faster than the rays, so as to overtake them; and when any ray is in that part of the vibration which conspires with its motion, its velocity is increased, and so it easily breaks through a refi acting surface; but when it is in a contrary part of the vibration, which impedes its motion, it is easily reflected; and thus every ray is successively disposed to be easily reflected or transmitted by every vibration which meets it. These returns in the disposition of any ray to be reflected, he calls fits of easy reflection; and the returns in the disposition to be transmitted, he calls fits of easy transmission; also the space between the returns, the interval of the fits. Hence then the reason why the surfaces of all thick transparent bodies reflect part of the light incident upon them, and refract the rest is that some rays, at their incidence, are in fits of easy reflection, and others of easy transmission. For the properties of reflected light, see MIRROR, OPTICS, &c. Again, a ray of light passing out of one medium into another of different density, and in its passage making an oblique angle with the surface that separates the mediums, will be refracted, or turned out of its direction; because the rays are more strongly attracted by a denser, than by a rarer medium. That these rays are not refracted by striking on the solid parts of bodies, but that this is effected without a real contact, and by the same force by which they are emitted and reflected, only exerting itself differently in different circumstances, is proved, in a great measure, by the same arguments by which it is demonstrated that reflection is performed without contact. When light is refracted by a prism, or other transparent body, it is divided into rays exciting the sensation of different colours; namely, red, orange, yellow, green, blue, indigo, and violet. This is the enumeration followed by Newton and others, which supposes seven rays refrangible in the above order, the red being least refrangible and the violet most so, and that the other tints are produced by mixture. The image formed by the different rays, thus separated, forms the solar spectrum. Dr. Wollaston has shewn, by looking through the prism at a narrow line of light, that the primitive colours are only red, green, blue, and violet. Heat and light are not present in corresponding degrees, in different parts of the solar spectrum; for, generally speaking, those rays illuminate most that have the least heating power. The rays in the centre of the spectrum have the greatest illuminating power, as may be ascertained by viewing, successively in each, a small body, such as the head of a common nail. It will be seen most distinctly in the light green, or deep yellow rays, and less plainly towards either extremity of the spectrum. The heating power of the rays follows a different order. If the bulb of a sensible thermometer be moved, in succession, through the differently coloured rays, it will be found to indicate the greatest heat in the red rays, next in the green, and so on, in a diminishing progression, to the violet. When the thermometer is removed entirely out of the confines of the red rays, but with its ball still in the line of the spectrum, it rises even higher than in the red rays; and continues to rise till removed half an inch beyond the extremity of the red ray. The ball of the thermometer employed for this purpose should be extremely small, and should be blackened with Indian ink. An air thermometer is better adapted than a mercurial one, to exhibit the minute change of temperature that ensues. These invisible heat-making rays may be reflected by the mirror, and refracted by the lens, exactly in the same manner as the rays of light. Beyond the confines of the spectrum on the other side, viz. a little beyond the violet ray, the thermometer is not affected; bút in this place it is remarkable that there are also invisible rays of a different kind, which exert all the chemical effects of the rays of light, and even with greater energy. One of the chemical properties of light is, that it speedily changes from white to black the fresh precipitated muriate of silver. This effect is produced most rapidly by the direct light of the sun; and the rays, as separated by the prism, have this property in various degrees. The blue rays, for example, effect a change of the muriate of silver in fifteen seconds, which the red require twenty minutes to accomplish; and, generally speaking, the power diminishes as we recede from the violet extremity. But entirely out of the spectrum, and beyond the violet rays, the effect is still produced. Hence it appears that the solar beams consist of three distinct kinds of rays; of those that excite heat, and promote oxydation; of illuminatingrays; and of de-oxydizing rays. A striking illustration of the different power of these various rays, is furnished by their effect on phosphorus. In the rays beyond the red extremity, phosphorus is heated, smokes, and emits white fumes; but these are presently suppressed on exposing it to the deoxydizing rays which lie beyond the violet extremity. There is an exception, however, as stated by Dr. Wollaston, to the de-oxydizing power of the rays above-mentioned. The substance, termed gum-guiacum, has the property, when exposed to the light, of changing from a yellowish colour to green; and this effect he has ascertained to be connected with the absorption of oxygen. Now in the most refrangible rays, which would fall beyond the violet extremity, he found that this substance became green, and was again changed to yellow by the least refrangible. This is precisely the reverse of what happens to muriate of silver, which is blackened, or de-oxydized, by the most refrangible; and has its colour restored, or is again oxygenized, in the least refrangible rays. Certain bodies have the property of absorbing the rays of light in their totality, of retaining them for some time, and of again evolving them unchanged, and unaccompanied by sensible heat. Thus, in an experiment of Du Fay, a diamond exposed to the sun, and immediately covered with black wax, shone in the dark, on removing the wax, at the expiration of several months. Bodies possessing this property, are called solar phosphori: such are Canton's, Baldwin's, Homberg's, and the Bolognian phosphori. To the same class belong several natural bodies which retain light, and give it ont unchanged. Thus, snow is a natural solar phosphorus. So also is, occasionally, the sea when agitated; putrid fish have a similar property; and the glowworm belongs to the same class. These phenomena are independent of every thing like combustion; for artificial phosphori, after exposure to the sun's rays, shine in the dark when placed in the vacuum of an airpump, or under water, &c. where no air is present to effect combustion. From solar phosphori, the extrication of light is facilitated by the application of an elevated temperature; and, after having ceased to shine at the ordinary temperature, they again emit light when exposed to an increase of heat. Several bodies, which do not otherwise give out light, evolve it, or become phosphorescent when heated. Thus, powdered fluate of lime becomes luminous when thrown on an iron plate, raised to a temperature rather above that of boiling water. The yolk of an egg, when dried, becomes luminous on being heated; and so also does tallow during liquefaction. To exhibit the last mentioned fact, it is merely necessary to place a lump of tallow on a coal, heated below ignition, making the experiment in a dark room. Attrition also evolves light, in many instances, by the part rubbed becoming ignited. Thus, rock crystal, and other hard stones, shine when rubbed against each other; and two pieces of common bonnet cane, rubbed strongly against each other in the dark, emit a faint light; most probably from the silex they contain: and two pieces of borax have the same property much more remarkably. Light is disengaged in various cases of chemical combination. Whenever combus. tion is a part of the phenomena, this is well known to happen; but light is evolved also, in other instances, where nothing like combustion goes forwards. Thus, fresh-prepared pure magnesia, added suddenly to highly concentrated sulphuric acid, exhibits a red heat. 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 bodies 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 investi gation. 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. Most solid bodies in combustion are kept, partly 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 made use of. If the wick be too large, it will supply a greater quantity of the fluid than can be well decomposed. Its evaporation will therefore diminish the temperature, and consequently the light, and afford a fuliginous column, which will pass through the centre of the flame, and fly off in the form of smoke. The magnitude of the wick may, from time to time, in candles, be reduced, as to length, by snuffing; but this operation will not remedy the evils which arise from too great a diameter. If the oil be not sufficiently combustible, the ignition will be but moderate, and the flame yellow; and the same effect will be produced, if the air be not sufficiently pure and abundant. An experiment to this effect may be made by including the flame of a small candle or lamp in a glass tube of about one inch in diameter, standing on the surface of a table. The air which passes between the glass and the table, will be sufficient to maintain a very bright flame; but if a metallic covering, perforated with a hole of about a quarter of an inch diameter, be laid upon the upper orifice of the tube, the combustion will be so far impeded, that the flame will be perceptibly yellower. The hole may then be more or less closed at pleasure by sliding a small piece of metal, for example a shilling, over it. The consequence will be, that the flame will become more and more yellow, will at length emit smoke, and if the hole be entirely closed, extinction will follow. The smell arising from the volatile parts, which pass off not well consumed from a lamp or candle, must be different according to the nature of those parts. This depends chiefly on the oil, but in some measure upon the wick. When a candle with a cotton wick is blown out, the smell is considerably more offensive, than if the wick be of linen, or of rush; but less offensive than if the supply of the combustion had been oil. Whenever a candle or lamp is removed, the combustion is in some measure impeded by the stream of cold air, against which it strikes. Smoke is accordingly emitted from its anterior side, and the peculiar smell is perceived. From this imperfection, lamps are much less adapted to be carried from place to place than candles. From the necessity of the access of air, there will be more light produced from a lamp with a number of small wicks, than with one large one, or from a number of small candles, than the same quantity of tallow used to make a single large one. In the lamp of Argand, the wick consists of a web of cloth in the form of a pipe or tube, the longitudinal fibres of which are thicker than the circular ones. This is passed by a suitable contrivance into a cylindrical cavity, which contains the oil; and there are other precautions in the construction of the apparatus, by which the oil is regularly supplied, the access of air is duly permitted, as well within as without the circle formed by the upper edge of this cylindrical wick, and this edge can be raised or lowered at pleasure. Hence the possessor has it in his power to regulate the surface of the wick, so that the greatest flame consistent with perfect combustion may be produced; and the steadiness of the flame is secured by a glass shade or tube, which surrounds it, and in a certain degree accelerates the current of air. In the illumination by candles, where the fused matter is contained in a cup or cavity of the matter not yet fused, it is of some consequence, whether the substance be fusisible at a high or low temperature. The difference betwece wax and tallow candles arises from this property. Wax being less fusible, will admit of a thinner wick, and needs no snuffing; but in a tallow candle it is absolutely necessary to bave a large wick, capable of taking up the tallow as it melts. The difference of effect in illumination between a thick and a thin wick cannot be better shown, than by remarking the ap pearances produced by both. When a candle with a thick wick is first lighted, and the wick snuffed short, the flame is perfect and luminous, unless its diameter be very great; in which last case, there is an opaque part in the middle, where the combustion is impeded for want of air. As the wick becomes longer, the space between its upper extremity and the apex of the flame is diminished; and, consequently, the oil, which issues from that extremity, having a less space of ignition to pass through, is less completely burned, and passes off partly in smoke. This evil continues to increase, until at length the upper extremity of the wick projects beyond the flame, and forms a support for an accumulation of soot, which is afforded by the imperfect combus. tion. A candle in this situation affords scarcely one-tenth of the light, which the due combustion of its materials would produce; and tallow candles, on this account, require continual snuffing. But, on the contrary, if we consider the wax candle, we find, that as its wick lengthens, the light indeed becomes less, and the cup becomes filled with melted wax. The wick, how. ever, being thin and flexible, does not long |