Light in their pores; admitting that light moving with its utmost ve locity hath no momentum. But fince the light, which is moved where it illuminates, fo that the illumination proceeds above 10,000,000 of miles in a minute, or above 8c0,000 times fwifter than founds, is not found to impel the most opaque poifed or light bodies, in the line of illumination; the theory of illumination by emitted light is not reconcileable to the phænomena of light, by any argument from the fmallness of its parts, and from the fmallnefs of their momentum; which fubjects we shall speak of in due time. "The quantity of tallow which burns from a candle of the ordinary fize, in a fecond of time, is not equal in bulk to the fixth part of a drop of melted tallow. And the illumination caused by the combuftion of this finall portion of tallow, may be feen even through air, in every part which the pupil measures of a space of two or three thousand quare miles. Reafon revolts at the fuppofition that fo much light could be crowded in fo fmall a compafs in cold tallow t; we know no powers by which it can be projected, except thofe of the mutual re pulfions of its parts; we cannot from any analogy infer, that fuch a minute portion of grofs matter can concentrate and combine with fo great a quantity of the repellent matter of light; and unless we admit hypothefis as a proof of this hypothefis of the motion of light, the fuppofitions of the ingenious Mr. Canton, Mr. Melville, and others, concerning the wide intervals between the parts of light, relatively to the bulk of them, does not give any colour of truth to the notion of this emiffion of light from burning bodies. "If we fuppofe the moved light, which illuminates in all spaces between the fun and vifible bodies celeftial and terreftrial, to be projected from the fubftance of the fun; and that it moves from the fun to us in about eight minutes; all these spaces must be replenished in the first eight minutes of fun-fhine, with as much light as is fufficient for ftrong illumination, which is all that is at any time found equally diffused in these spaces; and every minute thereafter, light will be concentrated or accumulated in them. But as light can be concentrated by convex lenticular glaffes and concave mirrors, befides other means to be noticed hereafter; and as we are well affured that a concentration of light to the thoufandth part of the space in which it is ufually diffufed, is capable of exciting great heat and combustion in bodies expofed to it, wherever the fun illuminates freely; the concentration of light, refulting from the prefumed continual emiffion of it, must make the fun-fhine of every hour much hotter than that of the It is peculiarly remarkable in this treatife, that every thing loft, or not in being, which is not found. what we have o ce poffeffed, it is much the fame thing, not to be found, this is not the cafe, with things not covered. Rev. the author fuppofes Now, though, with whether it be loft or yet poffeffed or dif + Especially as the tallow obfcures the wick, blackens the cover of the candlestick or the cieling, and proves, that our light is manufactured of the materials of darkness. Rev. former hour; and in a certain time, from the first action of the fun, which is to eight minutes, as the area of a burning glafs is to the area of its real focus; this our world must be red hot and flaming: and the fun must be wafting daily, and in time incapable of emitting light, or of illuminating our earth, by reafon of the difpendium of light from every face of it, as well as from that prefented to us at any inftant. Thefe confequences do not follow the action of the fun, but are totally repugnant to our experience during thoufands of years; and the premifes from which they flow are erroneous; however ingenioufly the Rev. Dr. Horfley and others endeavour to extenuate the wafte of the fun, which must be the confequence of the fuppofed emiflion of light; unless the fun hath a power of drawing back light, as fast as he is fuppofed to emit light for the falling of comets into the fun, is a mere conjecture. And, concerning that power, it were fufficient to remark, that the existence of it is not demonftrated. But we may further obferve, that it is altogether improbable. For the cause of illumination is not fenfibly weaker in any one face of the fun than in another; and the fun acts incefiantly in illuminating the spaces and bodies around it; and it is impoffible, by any known property of matter, or law of nature, for the fame body to exert, at the fame inftant and in the fame coaft, a power of propelling any matter, and a con trary power of drawing back the fame matter again as quickly as it was propelled, and from the greatest known diftances: and it is impoffible that two fuch powets fhould not counteract each other, and rather poife the matter on which they act, in a state of a relative reit, than caufe a rapid motion of it, progreffive in fome parts, and retrograde in others, interceding thefe. It is moreover to be confidered, that if the light fuppofed to be emitted from the fun, were drawn back to the fun as fast as it is emitted, there could be no darkness in the night; because the current of light returning to the fun, muft illuminate, as well as that proceeding from it, if illumination confist in the progreffive motion of light. "Thofe who imagine that the fun emits the light by which it il Juminates, muft allow that thousands of ftars, and of fires on this our earth, are continually emitting light in addition to that of the fun . And thus the error of the hypothefis is aggravated; fince a multitude of bodies emitting light incefiantly into the fame fpaces, are not found Here our author falls into the common errour, of confounding light with fire; to which it relates merely by the fame affociation of ideas, whichmakes an elephant dance at the beat of a drum. The dumb beaft is conftantly burnt in the toes to the found of the ta-too. Rev. + By no means. Dr. H. though right in the main of this argument, is plainly no aftronomer. The planets, it is now demonftrable, tend con itantly to an obliquity, by which they will in time become comets, and from their excentricity either will fall into the fun, or be totally burnt up and diffipated by their approach to it. Dr. H. talks idly of the experience of thousands of years on a fubject, which that of millions of years cannot deter mine: Rev. This is fomething like the widow's weeping and the drunkard's water.... ing into the fea. Rev. to increase the quantity of light therein, nor to fuffer any waste of their fubitance. "The philofophers who affirm that illumination is caused by a progreffive motion of Light; when they confider how many funs, if the xed ftars be funs, and how many millions of fires, are continually emitting Light, by their hypothefis; and when they recollect that all mafles of grols ma ter are pervious to Light; must acknowledge that all known places are replenished with Light, and no place can be void of Light in any inftant of time; and our atmosphere efpecially, which is open to the fun and ftars and fires, can never be deficient of Light. And when they fur her obferve how Light, condenfed in bodies placed in the locus of a burning glafs, expands from the bodies, when the instrument of condenfation is removed, nearly in the fame manner as other elastic fluids expand when the condening power ceases; they will perceive, that by reafon of this expanfibility of Light, all places of which we have any knowledge muft contain Light in quantities which are nearly as the spaces, except in the vicinity of bodies which attract or concentrate Light; as we shall fhew hereafter. Now it the parts of Light by which we fee the fun, were emitted from the fun, they muft dart fwiftly 800,000,000 of miles through a space already fupplied abundantly with Light, without impinging on the parts of it; and they muft likewife move rapidly many leagues through the denfer parts of our atmosphere, without clafhing with the groffer atoms of air to as to be ftopped by them; and when an ethereal fluid diffused in all known fpace is affumed, they must moreover move through this ether without impediment, and with the foregoing velocity. But we have no experimental evidence of fuch motion; it was originally fuppofed, only for the fake of certain phænomena, which may, in the prefent riper ftate of natural knowledge. be explained without it, as we fhall fhew; and fince there is no fimilitude of it in nature, and it hath no conformity with the established laws of motion, it is to common understanding incredible. "If the air were equally denfe at every altitude of the atmosphere; and if the parts of air and Light, and of the ether if you will, were all difpofed fo as to be equally diftant from each other; it were poffible that Light might move rapidly and progreffively between these parts, in certain directions without touching them. But this cannot happen in all the directions in which we have fenfe of rectilinear illumination: neither can it happen in air whofe parts are unequally diftant in any two different altitudes, and are perpetually changing their places and arrangement relatively to each other, when they are agitated by winds and other cautes. And as we fee against the wind, or athwart the wind, as well as with it, and through still air in any one direction as well as in another, allowance being made for the turbid state and greater den. fity of the air near the earth; and through water and glass of any thicknefs, in which the motion of Light is not ftifled by means to be confi dered bereafter, as well in one direction as in another; although the pores of thefe fluids, and efpecially of incompreffible folid bodies cannot be continued, in all poffible directions, uninterrupted by the folid One would think this reprefentation of light, as a body, would have cured our author of this hypothefis likewife. Rev. matter matter of the fluids and bodies, fo as to give paffage to the Light in a fraight course, however it may fall on them; Light is not moved from the fun and burning bodies with a continued progreffive rapid motion." Our author brings fome collateral arguments to difprove the projection and progreffive motion of light from the luminous body, which are apt and pertinent. "If," fays he, "it were true that the parts of Light, which illuminate when we see a number of stars or candles, are emitted with great velocity from thefe bodies; thefe parts being moved in different directions towards each other, ought, either by impinging or by repelling each other when they approach, to be turned out of the rectilinear way between each luminary and the eye, and ought to take a course very different from that of the experienced illumination. According to the hypothefis, the torrent of Light from the fun is much denfer than that from a ftar; and when the fun fhines obliquely on our hori. zon, the weaker torrent of Light flowing from the ftars, ought to fall in with, or be turned out of its way by, the denfer and stronger torrent from the fun; so that stars near the zenith should be invifible to a man looking towards them from the bottom of a deep pit. But these conclufions refulting from the hypothefis are false; and therefore the hy◄ pothefis is erroneous. Again, "The phænomena of reflections of Light further fhew, that illumination doth not confift in a continued progreffive motion of its parts. For a mirror placed in the fun-beam admitted into a dark chamber, fo as to reflect the illumination, in the line of incident illumination as nearly as is poflible, does not weaken or scatter the illumination in the beam of Light; which it ought to do, if the incident and reflected Light were moved progreffively in contrary directions: and the illumination obliquely incident on a broad mirror, is not found to disturb the reflected illumination, as it ought, according to the laws of motion, if the Light of the incident and reflected beams which cross each other, were moved rapidly and progreffively; and as it ought, whether the parts of this Light impinge on each other, or fhun each other by virtue of a repulfive power; and for the like reasons the phenomena of illumination reflected regularly to a focus, by concave mirJors, in a direction oblique and contrary to that in which the Light is fuppofed to be moved from the fun to the concave mirrors; plainly fhew that Light is not moved in continued progreffion." And again, if we confider the Light contained in illuminated fpaces, we cannot from any analogy prefume that illumination is continued by any progreffive motion of Light. When a stone is thrown into stagnant water, the motion of the parts of water contiguous to the ftone, is communicated to the circumambient water; and the extreme circular waves which reach the banks, do not confift of the parts first impelled, but of the water which was contiguous to the banks; and if the ftone be fmall relatively to the water, the motion of the water is manifeftly undulatory, where it is not fenfibly progreffive. A perfon immerfed in water, can hear the collifion of two ftones in the water, at a great diftance, as the learned Dr. Franklin obferves. In this cafe the whole water hath no progreffive motion, although motion is propa gated gated in it to the greatest diflances at which the experiment has been made. "When we find motion thus excited in immenfe quantities of a grofs and heavy fluid, by forces incapable of giving fenfible progreffive motion to very small portions of it; and when we confider that the matter expanding from burning bodies doth not appear to have any confiderable momentum; we find fome reason from analogy to prefume that the motion of Light is a propagated motion, but none to believe that Light is propelled freely through Light with progreffive motion. When a bell is rung, motion is fenfibly propagated in air to the distance of many miles, without any continued progreffive motion of the air; and the force necellary to generate this motion is incomparably lefs than the force which would be neceffary to propel fuch a body of air through air with the velocity of found. Between our sense of the vibrations of air, and our fenfe of the motion of Light; between ftillness or quiefcence of air, and darkness or quiefcence of Light; between found and illumination; between the fwiftnefs of found in a grofs and heavy Auid confiderably denfe, and the greater swiftness of illumination in a rare fluid, whofe parts do not gravitate fenfibly; between the force neceffary to caufe found, and the force neceflary to caufe illumination; we perceive fome analogy. But we find none in elaftic fluids to countenance the hypothefis of the projection and rapid progreffive motion of Light through Light, or the continued propul fion of one body of Light by another. "Air propelled from the bellows, or by the explofion of gunpowder; and all other blasts of air against air, caufe eddies and devious motions of the air; and the projected air lofes its motion in the ambient air in a finall time and pace. So Light, if it be an elastic fluid as we shall prefently endeavour to fhew, being projected from the luciferous parts of the fun, or from the fire in a light-house; would in a fhort time and space bend out of the right courfe of illumination and fpend its motion in the circumjacent light." Having now come to the end, of what is publifhed, of this Effay, we fhall take leave of the prefent publication with obferving, that the author, having greatly excited our curiofity about the phænomenon of light, hath left us much in the dark about the manner in which he intends to account for it. He feems neither to fide with Sir Ifaac Newton, in allowing it to confift of minute particles neceffarily projected and progreffively propelled from the luminous body; nor with those who confider it merely as motion, propogated through an elaftic medium. To the latter, indeed, he appears, from his allufion to the propagation of found, the moft to incline; but he night as well call the common atmosphere found, or the matter of found, as call the medium, through which light is propagated, light, or the matter of light.-We would recommend to Dr. Higgins, in the profecution of this treatife to confider, that more depends on logical precifion of terins, even in natural philofophy, than he perhaps is aware of, and that it is to little purpose |