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est dust they meet with ; and the san ap- the moon's light, when she is 19° 16' high pears to continue of his ancient dimensions, above the horizon, is about one-third of her and his attendants move in their ancient or. light, at 66° 11' high; and when one limb bits.” He therefore conjectures, that all just touched the horizon, her light was but the the phenomena of light may be more pro. 2,000th part of her light at 66° 11' high; and perly solved, by supposing all space filled that hence light is diminished in the proporwith a subtle elastic fluid, which is not visi. tion of three to one, by traversing 7,469 ble when at rest, but which, by its vibra- toises of dense air. He found also, that the tions, affects the fine sense in the eye, as centre of the sun's disc is considerably more those of the air affect the grosser organs of luminous than the edges of it; whereas the ear; and even that different degrees of both the primary and secondary planets the vibration of this medium may cause the are more luminous at their edges than near appearances of different colours. Franklin's their centres : that, further, the light of the Exper, and Observ. 1769, p. 264.
sun is about 300,000 times greater than The celebrated Euler has also maintain that of the moon; and therefore it is no ed the same hypothesis, in his “ Theoria Lii- wonder that philosophers have had so little cis et Colorum.” In the summary of his success in their attempts to collect the light arguments against the common opinion, re- of the moon with burning glasses; for, cited in Acad. Berl. 1752, p. 271, besides should one of the largest of them even inthe objections above-mentioned, he doubts crease the light 1,000 times, it will still the possibility, that particles of matter, leave the light of the moon in the focus of moving with the amazing velocity of light, the glass, 300 times less than the intensity should penetrate transparent substances of the common light of the sun. with so much ease. In whatever manner Dr. Smith, in his optics, vol. i. p. 29, they are transmitted, those bodies must thought he had proved that the light of the have pores, disposed in right lines, and in full moon would be only the 90,900th part all possible directions, to serve as canals of the fall day-light, if no rays were lost at for the passage of the rays; but such a the moon. But Mr. Robins, in his Tracts, structure must take away all solid matter vol. ii. p. 225, shows that this is too great from those bodies, and all coherence among one half. And Mr. Michell, by a more their parts, if they do contain any solid mat. easy and accurate mode of computation, ter.
found that the density of the sun's light on Among modern philosophers who have the surface of the moon, is but the 45,000th supported this doctrine, Dr. Young has part of the density at the sun; and, that shown much ability in his experimental and therefore, as the moon is nearly of the same theoretical researches, in his memoirs in the apparent magnitude as the sun, if she re“ Philosophical Transactions,” which have flected to us all the light received on her been republished in his “ Lectures," and in surface, it would be only the 45,000th part “ Nicholson's Journal."
of our day-light, or that which we receive The expansion or extension of any from the sun. Admitting, therefore, with portion of light is inconceivable. Dr. M. Bouguer, that the moon's light is only Hook shows, that it is as unlimited as the 300,000th part of the day, or sun's light; the universe, which he proves from the im- Mr. Michell concludes that the moon remense distance of many of the fixed stars, flects no more than between the 6th and 7th which only become visible to the eye by part of what she receives. the best telescopes. “Nor," adds le, Sir I. Newton long ago observed, that they only the great bodies of the sun or bodies and light act mutually on one anstars that are thus liable to disperse their other; bodies on light, in emitting, reflectlight through the vast expanse of the uni- ing, refracting, and inflecting it; and light verse, but the smallest spark of a lucid body on bodies, by heating them, and putting must do the same, even the smallest glo- their parts into a vibrating motion, in which bule struck from a steel by a flint."
heat principally consists. For all fixed The intensity of different lights, or of the bodies, he observes, when heated besame light in different circumstances, affords yond a certain degree, do emit light and a curious subject of speculation. M. Bou- shine. guer, Traité de Optique, found that when This action of bodies on light is found to one light is from sixty to eighty times less exert itself at a sensible distance, though it than another, its presence or absence will always increases as the distance is diminish. not be perceived by an ordinary eye ; that ed, as appears very sensibly in the passage
of a ray between the edges of two very thin same power, in other circumstances, the planes, at different apertures ; which is at- rays are refracted; and by which also the tended with this peculiar circumstance, that rays are first emitted from the luminous the attraction of one edge is increased as body; as Newton abundantly proves by a the other is brought nearer it.
great variety of arguments. See REFLECThe rays of light, in their passage out of Tion and REFRACTION. glass into a vacuum, are not only inflected That great author put it past doubt, that towards the glass, but if they fall too ob all those rays which are reflected do not liquely, they will revert back again to the really touch the body, though they approach glass, and be totally reflected. Now the it intinitely near; and that those which cause of this reflection cannot be attributed strike on the parts of solid bodies adhere to to any resistance of the vacuum, but must them, and are, as it were, extinguished and be entirely owing to some force or power lost. Since the reflection of the rays is as. in the glass, which attracts or draws back cribed to the action of the whole surface of the rays as they were passing into the va. the body without contact: if it be asked cuum. And this appears further from hence, how it happens that all the rays are not that if you wet the back surface of the glass reflected from every surface, but that, while with water, oil, honey, or a solution of some are reflected, others pass through and quicksilver, then the rays which would are refracted ? the answer given by Newctherwise have been reflected, will pervade ton is as follows: Every ray of light, in its and pass through that liquor ; which shows passage through any refracting surface, is that the rays are not reflected till they put into a certain transient constitution or come to that back surface of the glass, nor state, which in the progress of the ray reeven till they begin to go out of it; for if at turns at equal intervals, and disposes the ray their going out they fall into any of the at every return to be easily transmitted aforesaid mediums, they will not then be through the next refracting surface, and bereflected, but will persist in their former tween the returns to be easily reflected by it: course, the attraction of the glass being in which alteration of reflection and transmisthis case counterbalanced by that of the sion, it appears, is propagated from every surliquor.
face, and to all distances. What kind of action M. Maraldi prosecuted experiments simi. or disposition this is, and whether it consists lar to those of Sir I. Newton, on inflected in a circulating or vibrating motion of the light. And his observations chiefly respect ray, or the medium, or something else, he the inflection of light towards other bodies, does not inquire; but allows those who are by which their shadows are partially illumi. fond of hypothesis to suppose that the rays nated. Acad. Paris 1723, Mem. p. 159. of light, by impinging on any reflecting or See also Priestley's Hist. p. 521, &c. refracting surface, excite vibrations in the
From the mutual attraction between the reflecting or refracting medium, and by that particles of light and other bodies, arise two means agitate the solid parts of the body. other grand phenomena, besides the inflec- These vibrations, thus produced in the tion of light, which are called the reflection medium, move faster than the rays, so as to and refraction of light. It is well known overtake them; and when any ray is in that that the determination of bodies in motion, part of the vibration which conspires with especially elastic ones, is changed by the its motion, its velocity is increasell, and so interposition of other bodies in their way; it easily breaks through a refracting surface; thus also light, impinging on the surfaces of but when it is in a contrary part of the vi. bodies, should be turned out of its course, bration, which impedes its motion, it is and beaten back or reflected, so as, like easily reflected; and thus every ray is suc. other striking bodies, to make the angle of cessively disposed to be easily reflected or its reflection equal to the angle of inci. transmitted by every vibration which meets dence. This, it is found by experience, light it. These returns in the disposition of any does; and yet the cause of the effect is dif- ray to be reflected, he calls fits of easy ferent from that just now assigned, for the reflection; and the returns in the disposirays of light are not reflected by striking tion to be transmitted, he calls fils of easy on the very parts of the relecting bodies, transmission; also the space between the but by some power equally diffused over returns, the interval of the fits. Hence the whole surface of the body, by which it then the reason why the surfaces of all thick acts on the light, either attracting or re- transparent bodies reflect part of the light pelling it, without contact : by which incident upon therii, and refract the rest is that some rays, at their incidence, are in continues to rise till removed half an inch fits of easy reflection, and others of easy beyond the extremity of the red ray. The transmission. For the properties of reflect ball of the thermometer employed for this ed light, see MIRROR, Optics, &c. purpose should be extremely small, and
Again, a ray of light passing out of one should be blackened with Indian ink. An medium into another of different density, air thermometer is better adapted than a and in its passage making an oblique angle mercurial one, to exhibit the minute change with the surface that separates the mediums, of temperature that ensues. These invisible will be refracted, or turned out of its direc- heat-making rays may be reflected by the tion; because the rays are more strongly mirror, and refracted by the lens, exactly attracted by a denser, than by a rarer in the same manner as the rays of light. medium. That these rays are not refracted Beyond the confines of the spectrum on by striking on the solid parts of bodies, but the other side, viz. a little beyond the violet that this is effected without a real contact, and ray, the thermometer is not affected; bút by the same force by which they are emitted in this place it is remarkable that there are and reflected, only exerting itself differently also invisible rays of a different kind, which in different circumstances, is proved, in a exert all the chemical effects of the rays of great measure, by the same arguments by light, and even with greater energy. One which it is demonstrated that reflection is of the chemical properties of light is, that it performed without contact.
speedily changes from white to black the When light is refracted by a prism, or fresh precipitated muriate of silver. This other transparent body, it is divided into effect is produced most rapidly by the direct rays exciting the sensation of different light of the sun; and the rays, as separated colours; namely, red, orange, yellow, green, by the prism, have this property in various blue, indigo, and violet. This is the enu- degrees. The blue rays, for example, effect meration followed by Newton and others, a change of the muriate of silver in fifteen which supposes seven rays refrangible in the seconds, which the red require twenty above order, the red being least refrangible minutes to accomplish; and, generally speakand the violet most so, and tbat the other ing, the power diminishes as we recede from tints are produced by mixture. The image the violet extremity. But entirely out of formed by the different rays, thus separated, the spectrum, and beyond the violet rays, forms the solar spectrum. Dr. Wollaston the effect is still produced. Hence it ap. has shewn, by looking through the prism at pears that the solar beams consist of three a narrow line of light, that the primitive distinct kinds of rays; of those that excite colours are only red, green, blue, and heat, and promote oxydation ; of illuminatviolet.
ingrays; and of de-oxydizing rays. A strikHeat and light are not present in corres. ing illustration of the different power of these ponding degrees, in different parts of the various rays, is furnished by their effect on solar spectrum ; for, generally speaking, phosphorus. In the rays beyond the red those rays illuminate most that have the extremity, phosphorus is heated, smokes, least heating power. The rays in the centre and emits white fumes ; but these are preof the spectrum have the greatest illumi- sently suppressed on exposing it to the denating power, as may be ascertained by oxydizing rays which lie beyond the violet viewing, successively in each, a small body, extremity. such as the head of a common nail. It will There is an exception, however, as stated be seen most distinctly in the light green, by Dr. Wollaston, to the de-oxydizing or deep yellow rays, and less plainly to- power of the rays above-mentioned. The wards either extremity of the spectrum. substance, termed gum-guiacum, has the . The heating power of the rays follows a property, when exposed to the light, of different order. If the bulb of a sensible changing from a yellowish colour to green; thermometer be moved, in succession, and this effect he has ascertained to be conthrough the differently coloured rays, it will nected with the absorption of oxygen. Now be found to indicate the greatest heat in in the most refrangible rays, which would the red rays, next in the green, and so on, fall beyond the violet extremity, he found in a diminishing progression, to the violet. that this substance became green, and was When the thermometer is removed entirely again changed to yellow by the least refranout of the contines of the red rays, but with gible. This is precisely the reverse of what its ball still in the line of the spectrum, it happens to muriate of silver, which is rises even higher than in the red rays; and blackened, or de-oxydized, by the most
refrangible; and has its colour restored, or known to happen; but light is evolved also, is again oxygenized, in the least refrangible in other instances, where nothing like comrays.
bustion goes forwards. Thus, fresh-preCertain bodies have the property of ab- pared pure magnesia, added suddenly to sorbing the rays of light in their totality, of highly concentrated sulphuric acid, exhibits retaining them for some time, and of again a red heat. evolving them unchanged, and unaccom- Whence comes the light afforded by ig. panied by sensible heat. Thus, in an expe. nited bodies? whether it have been previriment of Du Fay, a diamond exposed to ously imbibed by them? whether the comthe sun, and immediately covered with mencement of ignition be distinctive of the black wax, shone in the dark, on removing same temperature in all bodies? whether the wax, at the expiration of several the great planetary sources of light be bomonths. Bodies possessing this property, are dies in a state of combustion, or merely,lucalled solar phosphori : such are Canton's, minous upon-principles very different from Baldwin's, Homberg's, and the Bolognian · any which our experiments can point out? phosphori. To the same class belong whether the momentum of the particles several natural bodies which retain light, of light, or their disposition for chemical and give it ont unchanged. Thus, snow is combination, be the most effectual in the a natural solar phosphorus. So also is, changes produced by its agency?—these, occasionally, the sea when agitated; putrid and numerous other interesting questions, fish have a similar property; and the glow- must be left for future research and investi. worm belongs to the same class. These gation. See COMBUSTION. phenomena are independent of every thing The production of light by inflammation like combustion ; for artificial phosphori, is an object of great importance to society after exposure to the sun's rays, shine in the at large, as well as to the chemist. It apdark when placed in the vacuum of an air- pears to arise immediately from the strong pump, or under water, &c. where no air is ignition of a body while rapidly decompresent to effect combustion.
posing. Most solid bodies in combustion From solar phosphori, the extrication of are kept, "partly from a want of the access light is facilitated by the application of an of air, and partly from the vicinity of con. elevated temperature; and, after having ducting bodies, at a low degree of ignition. ceased to shine at the ordinary tempera. But when vapours rapidly escape into the ture, they again emit light when exposed to air, it may, and does frequently happen, an increase of heat. Several bodies, which that the combustion, instead of being cardo not otherwise give out light, evolve it, ried on merely at the surface of the mass, or become phosphorescent when heated. penetrates to a considerable depth within, Thus, powdered Auate of lime becomes and from this, as well as from the imperluminous when thrown on an iron plate, fect conducting power of the surrounding raised to a temperature rather above that air, a white heat, or very strong ignition, is of boiling water. The yolk of an egg, when produced. The effect of lamps and candried, becomes luminous on being heated; dles depends upon these considerations. A and so also does tallow during liquefaction. . combustible fluid, most cominonly of the To exhibit the last mentioned fact, it is nature of fat oil, is put in a situation to be merely necessary to place a lump of tallow absorbed between the filaments of cotton, on a coal, heated below ignition, making linen, fine wire, or asbestos. The extremity the experiment in a dark room. Attrition of this fibrous substance, called the wick, is also evolves light, in many instances, by then considerably heated. The oil evapothe part rubbed becoming ignitêd. Thus, rates, and its vapour takes fire. In this sirock crystal, and other hard stones, shine tuation the wick, being enveloped with when rubbed against each other; and two flame, is kept at such a temperature, that pieces of common bonnet cane, rubbed the oil continually boils, is evaporated, strongly against each other in the dark, burns, and by these means keeps up a conemit a faint light; most probably from the stant flame. Much of the perfection of silex they contain: and two pieces of borax this experiment depends on the nature, have the same property much more remark- quantities, and figure of the materials made ably.
use of. If the wick be too large, it will supLight is disengaged in various cases of ply a greater quantity of the Anid than can chemical combination. Whenever combus. be well decomposed. Its evaporation will tion is a part of the phenomena, this is well therefore diminish the temperature, and consequently the light, and afford a fuligi. suitable contrivance into a cylindrical cavinous column, which will pass through the ty, which contains the oil; and there are centre of the flame, and fly off in the form other precautions in the construction of the of smoke. The magnitude of the wick may, apparatus, by which the oil is regularly supfrom time to time, in candles, be reduced, as plied, the access of air is duly permitted, to length, by snuffing; but this operation as well within as without the circle formed will not remedy the evils which arise from by the upper edge of this cylindrical wick, too great a diameter. If the oil be not suf- and this edge can be raised or lowered at ficiently combustible, the ignition will be pleasure. Hence the possessor has it in his but moderate, and the flame yellow; and power to regulate the surface of the wick, the same effect will be produced, if the air so that the greatest flame consistent with be not sufficiently pure and abundant. An perfect combustion may be produced ; and experiment to this effect may be made by the steadiness of the flame is secured by a including the flame of a small candle or glass shade or tube, which surrounds it, and lamp in a glass tube of about one inch in in a certain degree accelerates the current diameter, standing on the surface of a ta- of air. ble. The air which passes between the In the illumination by candles, where the glass and the table, will be sufficient to fused matter is contained in a cup or cavity maintain a very bright flame; but if a me- of the matter not yet fused, it is of some tallic covering, perforated with a hole of consequevce, whether the substance be fusi. about a quarter of an inch diameter, be laid sible at a high or low temperature. The difupon the upper orifice of the tube, the com- ference betweck wax and tallow candles bustion will be so far impeded, that the arises from this property. Wax being less flame will be perceptibly yellower. The fusible, will admit of a thinner wick, and hole may then be more or less closed at needs no snnffing; but in a tallow candle pleasure by sliding a small piece of metal, it is absolutely necessary to bave a large for example a shilling, over it. The con- wick, capable of taking up the tallow as it sequence will be, that the flame will become melts. more and more yellow, will at length emit The difference of effect in illumination smoke, and if the hole he entirely closed, between a thick and a thin wick cannot be extinction will follow.
better shown, than by remarking the ap. The smell arising from the volatile parts, pearances produced by both. When a can. which pass off not well consumed from a dle with a thick wick is first lighted, and lamp or candle, must be different according the wick snuffed short, the flame is perfect to the nature of those parts. This depends and luminons, unless its diameter be very chiefly on the oil, but in some measure upon great; in which last case, there is an opaque the wick. When a candle with a cotton part in the middle, where the combustion wick is blown out, the smell is considerably is impeded for wait of air. As the wick more offensive, than if the wick be of linen, becomes longer, the space between its upor of rush; but less offensive than if the per extremity and the apex of the fame is supply of the combustion had been oil. diminished ; and, consequently, the oil, Whenever a candle or lamp is removed, the which issues from that extremity, having a combustion is in some measure impeded by less space of ignition to pass through, is less the stream of cold air, against which it completely burned, and passes off partly in strikes. Smoke is accordingly emitted from smoke. This evil continues to increase, upits anterior side, and the peculiar smell is til at' length the upper extremity of the perceived. From this imperfection, lamps wick projects beyond the flame, and forms are much less adapted to be carried from a support for an accumulation of soot, place to place than candles,
which is afforded by the imperfect combus. From the necessity of the access of air, tion. A candle in this situation affords there will be more light produced from a scarcely one-tenth of the light, which the Jamp with a number of small wicks, than due combustion of its materials would pro. with one large one, or from a number of duce; and tallow candles, on this account, small candles, than the same quantity of require continual suufting. But, on the tallow used to make a single large one. In contrary, if we consider the wax candle, we the lamp of Argand, the wick consists of a find, that as its wick lengthens, the light in. web of cloth in the form of a pipe or tube, deed becomes less, and the cup becomes the longitudinal fibres of which are thicker filled with melted wax. The wick, howthan the circular or.es. This is passed by a ever, being thin and flexible, does not long