Page:Cyclopaedia, Chambers - Volume 2.djvu/100

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 * Kind and Virtue, by how much anyone is f mailer, by


 * fo much is its attractive Power greater, in proportion


 * to its Bulk. This Power we find Wronger in faialj

' Difference of their Weights j and the Reafun is, that
 * Magnets than in large ones, regard being had to the


 * the Particles of fmall Magnets being nearer each other,


 * more eafily unite their Forces intimately together, and


 * aft conjointly. For the fame Reafon the Rays of


 * Light, being; of all other Bodies the molt minute, it


 * may be expected that their attractive Powers fhould be

' they are, may be gathered from the following Rules :
 * of all others the ftrongeft. And how ftrong in effect


 * The Attraction of a Ray of Light, according to the


 * Quantity of its Matter, is to the Gravity which any pro-


 * jecled Body has, according likewife to the Quantity of


 * its Matter, in a Ratio compounded of the Velocity of


 * the Ray of Light, to the Velocity of that projected


 * Body, and of the Bending or Curvature of the Line,

1 the Bending or Curvature defcribed by that projected
 * which the Ray defcribes in the Place of Refraction, to

' to the refracting Surface be the fame with that of the
 * Body 5 provided, however, the Inclination of the Ray


 * projected Body to the Horizon. From which Propor-


 * tion I gather, that the Attraction of the Rays of Light is


 * above i,ooc,ooo,oco,ooo,ooo times greater than the Gra-


 * vity of Bodies on the Surface of the Earth, in pro-


 * portion to the Quantity of Matter in each, if the Light


 * pafs from the Sun to the Earth in the Space of feven


 * Minutes. But now as in Algebra, where affirmative


 * Quantities ceafe, there negative ones begin ; fo in Me-


 * chanics, where Attraction ceafes, there the repelling


 * Power mult fucceed : Therefore a Ray of Light, as


 * foon as it is caft off from the luminous Body by the vi-

4 Sphere of its Attraction, is propelled with an immenfe 4 Velocity.' See Attraction and Refulfion.
 * brating Motion of its Parts, and is got out of the

The wonderful Divifibility of the Parts of Matter is no where more apparent than in the Minutenefs of the Parti- cles of Light. Dr. Niewentiit has computed, that an Inch of Candle, when converted to Light, becomes divided into atfotJi 7040 Parts. The Expansion or Extenfion of any Por- tion of Light is inconceivable 5 Dr. Hook fhews 'tis as unlimited as the: Univerfe j proving it from the immenfe Diftance of forne of the fixed Stars, the Light whereof becomes fenfib'le to the Eye by means of a Telefcope ; nor, adds he, is it only the great Bodies of the Sun or Stars that are thus able to difperfe their Light thro' the vaftExpanfum-of the Univerfe ; but the fmalleft Spark of a lucid Body muft do the fame, even the fmalleft Globule {truck from a Steel by a Flint.

Dr. j 1 Graemefimd affirms a lucid Body to be that, which emits or gives Fire a Motion in right Lines 5 and makes the Difference between Light and Heat to confift in this, That to produce the former, the fiery Particles muft enter the Eye in a rectilinear Motion, which is not required in the latter : On the contrary, an irregular Motion feems more proper for it, as appears from the Rays coming directly from the Sun to the Top of the Mountain, which have not near that Effect with thofe in the Valley, agitated with an irregular Motion by feveral Reflections. Whe- ther or no there be always Light where there is Fire, is difputed among Authors, as alfo whether or no there be any luminous Body without Heat ; Heat being a Motion that may be infinitely diminished, and Lighta Matter that may be infinitely rare ; to which we may add, that no Heat is fenfible to us, unlefs it be more intenfe than that of our Organs of Senfe.

Sir Ifaac Newton obferves, that Bodies and Light act mutually on one another ; Bodies on Light, in emitting, reflecting, refracting, and inflecting it 5 and Light on Bo- dies, by heating them, and putting their Parts into a vi- brating Motion, wherein Heat principally confifts. For all fix'd Bodies, he obferves, when heated beyond a certain Degree, do emit Light, and fhine ; which Shining, £5?c. appears to be owing to the vibrating Motion of the Parts j and all Bodies abounding in earthy and fulphu- reous Particles, if fufficiently agitated, emit Light, which way foever that Agitation be effected. Thus Sea- Water fhines in a Storm, Quick-iilver when fhaken in Vacuo, Cats or Horfes when rubb'd in the dark ; and Wood, Fifti, and Flefh when putrefied.

The late Mr. Hatohbee has furnifh'd us with a great Variety of In dances of the artificial Production of Light on the Attrition of Bodies naturally not luminous 5 as of Amber rubb'd on Woollen Cloth in vacuo, of Glafs on Woollen, of Glafs on Glafs, of Oyfter-Shells on Woollen, and of Woollen on Woollen, all in •vacuo. On the feveral Experiments whereof, he makes the following Reflec- tions : That different forts of Bodies afford remarkably different kinds of Lizht, different both in Colour and in Force ; That the "Effects of an Attrition are various, ac- cording to the different Preparations and Managements of

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the Bodies that are to endure it j ahd that Bodies which have yielded a particular Light, may be brought by Friction to yield no more of that Light.

M. Bernoulli found by Experiment that Mercury amal- gamated with Tin, and rubb'd on Gltfs, produced a corifiderable Light'ai the Air j that Gold rubb'd on Glafs did it {till in a greater degree : but that of all others the molt exquifite Light was that produced by the Attrition of a Diamond ; being equally vivid with that of a burning Coal briskly agitated with the Bellows.

Mr. Boyle tells us of a piece of mining rotten Wood, which upon exhaufting the Air from it was extinguifh'd ; but upon its Re-admifiion, feemed to come to life again, and /hone as before : being no doubt a real Flame, and like other Flames not to be preferved without Air. See Fbofphorus.

That the Farticks of Light are attraeled by thofe of other Bodies, is evident from innumerable Experiments. This Phenomenon was firft obferved by Sir I. Newton, who found by repeated Tryals, that the Rays of Light in their paffage near the Edges of Bodies, whether opake or tranfparent, as Pieces of Metals, the Edges of Knives, broken Glaffes, &c. are diverted out of the right Lines, and always inflected or bent towards thofe Bodies. This Action of Bodies on Light is found to exert itfelf at a fen- fible Diftance, tho it always mcreafes as the Diftance is diminifh'd ■■, as appears very fenfibly in the paffage of a. Ray between the Edges of two thin Planes at different Apertures, in which there is fomething very peculiar, the Attraction of one Edge being increafed as the other is brought nearer it. The Rays of Light in their paffage out of Glafs into a Vacuum are not only inflected towards the Glafs, but if they fall too obliquely, will revert back again to the Gtaft, and be totally reflected. The Caufe of which Reflection cannot be attributed to any Refi- nance of the Vacuum, but muft be entirely owing to tome Forceor Power in the Glafs, which attracts or draws back the Rays as they were pafling into the Vacuum. And this appears farther from hence, that if you wet the pofterior Surface of the Glafs with Water, Oil, Honey, or a Solu- tion of Quickfilver, then the Rays which would other- wife have been reflected will pafs into and through that Liquor: which fhews that the Rays are not reflected till they come to that posterior Surface of the Glafs, nor even till they begin to go out of it •> for if at their going out they fall into any of the forefaid Mediums, they will not then be reflected, but perfift in their former Courfe, the Attraction of the Glafs being in this Cafe counter-balanced by that of the Liquor.

From this mutual Attraction between the Particles of" Light and other Bodies, arifes two other grand Pheno- mena, which we call the Reflexion and Rej 'ruction of Light*

We know that the Determination of a Body in Motion is chang'd by the Interpofition of another Body in its way. Thus Light impinging on the Surface of folid Bo- dies, fhould be turn 'd out of its courfe, and beaten back or reflected, fo as like other falling Bodies to make tho Angle of its Reflexion equal to that of Incidence. This 'tis found by experience Light does, and yet the Caufe of this Effect is different from that juft now affign'd : the Rays o£ Light axe not reflected by ftriking on the very Parts of the reflecting Bodies, but by fome Pewer equably diffufed throughout the whole Surface of the Body, whereby it acts on the Light, either attracting or repelling it without Contact : by which fame Power, in other Cir- cumftances the Rays are refracted 5 and by which' alfb, the Rays are firft emitted from the luminous Body; as is abundantly proved, by great variety of Arguments, by Sir I. Newton. See Reflexion.

That great Author puts it part doubt, that all thofe Rays which are reflected, tho they approach the Body infinitely near, yet never touch it ; and that rhofe which do really ftrike on the folid Parts of Bodies, adhere to 'em, and are as it were extinguifh'd and loft. If it be ask'd, how it happens, flnce we afcribe the Reflexion of the Rays to the Action of the whole Surface of the Body without Contact 5 how, I fay, it happens that all the Rays are not reflected from every Surface, but while fome are reflected, others pais through and are refracted :

The Anfwer given by Sir J. Newtan is as follows Every

Ray of Light in its paffage thro any refracting Subftance, is put into a certain tranfient Conftitution or State, which in the progrefs of the Ray returns at equal intervals, and difpofes the Ray at every Return to be eafily tranfmitted thro the next refracting Surface, and between the Returns to be eafily reflected by it: which Alternation of Re- flexion and Tranfmiffion appears to be propagated from every Surface and to all Diflanccs. What kind of Action or Difpofition this is, and whether it confiftin a circulating or vibrating Motion of the Ray or the Medium, or fomewhat elfe, he does nor inquire 5 but allows thofe who are fond of Hvpothefes to fuppofe that the Rays

of