PHYSICS. the magnitude of the former pores. And in what capacity foever these corpufcles of a liquor come to be lodged or harboured in fuch pores, the furface of the body will commonly have its afperity altered, and the incident light, that meets with a groffer liquor in the little cavities that before contained nothing but air, or fome more fubtile fluid, will have its rays either refracted, abforbed, or reflected more or lefs irregularly than if the body had been dry. Thus we fee that even fair water falling on white paper, linen, &c. will, for fome fuch reafon as those affigned, immediately alter the colour of them, and for the most part render it deeper than that of the unmoiften'd parts of the fame bodies. In like manner, when, during the fummer, the high-ways are dry and dufty, if there fall much rain, they quickly appear of a darker colour than before. Thus alfo if a drop of oil be let fall upon white paper, that part of the paper which by imbibing the liquor acquires a greater continuity, and fome tranfparency, will appear much darker than the reft; many of the incident rays of light being now tranfmitted, that would, otherwise, be reflected to the eye. Secondly, a liquor may alter the colour of a body, by freeing it from thofe things that prevent its appearing of its genuine colour; and tho' this may be faid to be rather a reftoration of a body, or a restitution of "which transmit one fort of light moft ་་ glaffes were fo thick and maffy, that << no light could get thro them, I que"ftion not but they would, like all other "opake bodies, appear of one and the "fame colour in all pofitions of the eye. "For all coloured bodies, fo far as my "obfervation reaches, may be seen thro, "if made fufficiently thin, and therefore "are in fome measure tranfparent, and dif"fer only in degree of transparency from "tinged and tranfparent liquors; thefe liquors, as well as those bodies, by a fufficient thickness becoming opake. A tranfparent body, that looks of any colour "by tranfmitted light, may alfo look of "the fame colour by reflected light, the "light of that colour being reflected by "the farther furface of the body, or by "the air beyond it. And then the re"flected colour will be diminished, and perhaps ceafe, by making the body very thick, and pitching it on the back"fide, to diminish the reflexion of its "farther furface, fo that the light re"flected from the tinging particles may "predominate. In fuch cafes the colour ་་ 46 "of the reflected light will be apt to vafrom that of the light tranfmitted." Newton. Optic. p. 159-163. ry This doctrine is farther confirmed, by mixing coloured light, fo as to compound a beam of light of the fame colour and nature with a beam of the fun's direct light, and making experiments therein. Thus Sir Ifaac Newton found, that fuch light is endued with all the properties of a beam of the fun's light; natural bodies appearing in it of the fame colours. they have in day-light. Thus cinnabar appeared of the fame red colour in it as it does by day, and if the green-making and blue-making rays were ftopped, its redness became more full and lively; but if the red-making rays were intercepted, it became yellow or green, or of fome other colour, according to the forts of rays which were not intercepted. So gold in this light appears of the fame yellow colour as in day-light; but by topping a due quantity of the yellow-making rays, it appears white like filver; which fhews, that its yellownefs arifes from the excefs of the intercepted rays tinging that whiteness with their colours when they are let pafs. See Newton. Optic. p. 163-167. its native colour, than a change; yet there here happens a change of the PHYSICS. colour which the body afforded before this operation. And fuch a change a liquor may work, either by diffolving or corroding, or by fome fuch means carrying off the matter, which either veiled or difguized the colour that afterwards appears. Thus we reftore old pieces of dirty gold to a clear fhining yellow, by putting them into the fire, and into Aqua fortis, which takes off the acquired filth from the metal; and there is alfo an eafy way to restore filver coins to their due luftre, by fetching off that which difcoloured them. I have likewife a chymical liquor, which I employ to restore pieces of cloth spotted with greafe, to their proper colour, by imbuing the fpotted part therewith; which incorporating with the greafe, and yet being of a very volatile nature, eafily carries it away with itfelf. And I have fometimes try'd, that by rubbing upon a good touchftone a certain metalline mixture fo compounded, that the impreffion it left upon the stone appeared of a very different colour from that of gold; yet a little Aqua fortis would immediately make the golden colour difclofe itfelf, by dif folving only the other metalline corpufcles that concealed thofe of the gold. Thirdly, a liquor may alter the colour of a body, by comminuting its parts; and principally after two ways: Firft, by disjoining and diffipating thofe clusters of particles which stick more loofely together, only by means of fome cement that is easily diffoluble; and this feems to be the cafe in fome of the following experiments, where the colour of many corpufcles brought to cohere, by being precipitated together, is deftroyed by the affusion of very fharp and piercing liquors. Secondly, By dividing the groffer and more folid particles into minute ones, which will be, for the part, otherwife fhaped than the entire corpufcles fo divided; as happens in a piece of wood reduced to fplinters or chips, or when a piece of cryftal heated red-hot, and quenched in cold water, cracks into a multitude of little fragments, which, tho' they do not fall afunder, alter the difpofition of the body of it, as to its manner of reflecting the light. most There is a fourth way oppofite to the third, whereby a liquor may change the colour of another body, efpecially a fluid, by procuring the coalition of feveral particles that before lay fcattered, and too remote from each other to exhibit the colour that afterwards appears. Thus, when a folution of gold has been fo diluted, that I queftioned whether the liquor really imbibed any of the metal, I have been quickly fatisfied it did, by pouring a little mercury into it; for the gold therein would foon clothe the furface of the quickfilver with a thin film of its own livery. And by this way of bringing the minute parts of bodies together, in fuch numbers as to make them a fit object for the eye, many of thofe colours feem to be generated which are produced by precipitation; efpecially if made with fair water; as when refinous gums, diffolved in spirit of wine, are let fall again, if the fpirit be greatly diluted with that weakening liquor. And thus out of the rectified and tranfparent butter of antimony, there will, by the bare mixture of fair water, be plentifully precipitated VOL. II. D that PHYSICS. that milk-white fubftance, which, by having its loofer falts well washed off, is turned into Mercurius vita. A fifth way wherein a liquor may change the colour of a body, is by dislocating its parts, and changing their order; at the fame time, perhaps, alfo altering the pofition of the fingle corpufcles, as well as their fituation in refpect of one another. What fhare particular commotions or diflocations of the parts of a body may have in changing its colour, is evident from the mutations of that quality obfervable in quickfilver, and fome other concretes, long kept by chymifts in a convenient heat, tho' in clofe veffels; and alfo in the obvious degenerations thereof in bruised cherries, and other fruit: and that fuch liquors as we have been speaking of, may greatly difcompofe the textures of many bodies, and thereby alter the difpofition of their fuperficial parts, the great commotion made in metals and other fubftances by Aqua fortis, oil of vitriol, &c. eafily perfuade us. And what fuch a varied fituation of parts may do towards diverfifying the manner wherein they reflect the light, may be gueffed, in fome measure, by the pulverization of tranfparent glafs, which thereby becomes a white powder; but ftill better, by the experiments we fhall hereafter fet down, with relation to the production and deftruction of colours by fubtile, faline liquors. And in fome chymical oils, as particularly in that of lemon-peel, the tranfpofition of the parts, confequent upon bare concuffion, will reprefent on the furfaces of the bubbles thereby occafioned, exceeding lively colours, which, when the bubbles relapfe into the rest of the oil, immediately vanish. I know not whether I should mention, as a distinct method, that power whereby a liquor may alter the colour of another body, by putting the parts of it into motion; for tho', poffibly, the motion fo produced, feldom, of a fudden, changes the colour of the body whofe parts are agitated; yet this feems to be one of the moft general, tho' not immediate caufes of the quick change of colours in bodies; for the parts being put into motion by the adventitious liquor, many of them, that were before united, may be thereby disjoined; and when that motion ceafes or decreases, others of them may stick together in a new order: by which means the motion may fometimes produce permanent changes of colours; as in the experiment we shall hereafter mention, where a fnow-white body is immediately turned into a yellow, by the bare affufion of fair water. And tho when you rub good blue vitriol upon the clean blade of a knife, it will not impart its latent colour thereto; yet if you moiften that fubftance with common water, the particles of the liquor disjoining thofe of the ftone, and thereby giving them the various agitation requifite to fluid bodies, the metalline corpufcles of the vitriol thus diffolved will lodge themfelves in throngs in the fmall pores of the iron, and give the furface of it the genuine colour of copper. Laftly, The most important way, by which a liquor may alter the colour of another body, is by affociating the faline, or other more rigid corpufcles of the liquor, with the particles of the body whereon it is employ'd to work. For these adventitious corpufcles uniting with the pro tuberant tuberant particles of the furface of a coloured fubftance, muft neceffari- PHYSICS. ly alter their bignefs, and most commonly their shape. And that the colours of bodies greatly depend upon the bulk and figure of their fuperficial particles, feems probable, fince many ancient and modern philofophers have thought, that all colours might, in the general, be accounted for from them alone. The diverfification hereof will, in our cafe, be attended with these two circumstances; the one, that the protuberant particles being increased in bulk, muft often be varied as to the clofenefs or laxity of their order; fewer of them being contained within the fame fenfible space than before; or elfe, by approaching one another, they muft ftreighten the pores; and perhaps too by the manner of their affociating themselves with the protuberant particles, intercept new pores. And thus, alfo, these adventitious corpufcles may produce a great change, as well in the little cavities, as in the protuberances of a coloured body; for they may likewife, by lodging themselves in thofe little cavities, fill them up; and it may eafily happen, that they shall not only fill the pores they infinuate themselves into, but have their upper parts extant above them: and partly by fuch new protuberances, partly by increafing the bulk of the former, thefe extraneous corpufcles may much alter the number and bignefs of the fuperficial pores, changing the old ones, and intercepting new; and very probably the order of the little extancies, and confequently that of the little depreffions, in point of fituation will be altered likewife. Thus if you diffolve quickfilver in fome kinds of Aqua fortis, the faline particles of the menftruum affociating themselves with the mercurial globules, will make a green folution, which afterwards eafily degenerates; and red lead, diffolved in fpirit of vinegar, yields a clear folution, the redness of the lead being deftroyed by the liquor. But a better inftance is afforded from copper; for I have try'd, that if upon a copper plate you let fome drops of weak Aqua fortis reft for a while, the corpufcles of the menftruum joining with thofe of the metal, will produce a very fenfible afperity upon the furface of the plate, and thence coagulate into very minute grains of a pale blue vitriol; tho', if upon another part of the fame plate you fuffer a little ftrong fpirit of urine to remain for a competent time, the rough furface will be adorned with a deeper and richer blue. And the fame Aqua fortis, that fuddenly changes the rednefs of minium into a darker colour, will, being put upon crude lead, produce a whitish fubftance,as with copper it did a bluish,and as with iron it gives a reddish, and on white quills a yellowish colour: fo much may the coalition of the parts of the fame liquor, with the differently figured particles of folid bodies feveral ways turn the differingly difpofed furfaces rough, and diverfify the colour of thofe bodies. And 'twill be easily believed, that in many changes of colour, which happen upon the diffolutions of metals, and precipitations made with oil of tartar, &c. a coalition of faline corpufcles may be made with the particles of the body diffolved or precipitated; if we confider how much the vitriol of a metal may be heavier than the pure metalline part thereof, upon account of the D 2 faline PHYSICS. faline parts coagulated therewith; and that in feveral precipitations the weight of the calx, for the fame reafon, much exceeds that of the metal when it was firft expofed to be diffolved. Whether all bo parent? But I am not aiming to establish a particular theory of colours, only defign to offer fome experiments and obfervations on the subject of them. I fhall, therefore, add but two particulars more, with relation to the forms of fuperficial afperity in coloured bodies. And firft, I fay, that there are many other means whereby true and permanent colours may be produced in bodies, befides thofe practicable by the help of liquors; for proof whereof feveral examples might be alledged; but I need only refer to what we have already obferved of the change of colours fuddenly made on tempered fteel and lead, by the operation of heat, without the interpofition of a liquor. Secondly, I obferve, what is of more importance to our prefent fubject, that tho' nature and art may, in fome cafes, fo change the afperity of the fuperficial parts of a body, as to alter its colour by any of the mentioned ways a-part, yet 'tis generally by two or three, or perhaps by more of them together, that the effect is produced; and if it be confidered how variously thofe feveral ways, with others of the like nature, may be compounded and applied, it will not appear furprizing, that fuch fruitful principles fhould be fitted to change or generate many differing colours. Hitherto we have confidered the little protuberances, or other fuperdies are tranf- ficial particles which conftitute the roughnefs of bodies, as if we took it for granted that they must be perfectly opake, and impenetrable to the rays of light, and fo contribute to the variety of colours, as they ftop more or lefs thereof, and reflect them to the eye particularly mixed with more or lefs of fhade. I have often thought it worth a ferious inquiry, whether or no particles of matter, each of them fingly infenfible, may not yet feparately confift of many minuter particles, betwixt which we may conceive little junctures, where they adhere to one another, in fome degree pervious to the prodigioufly fubtile corpufcles of the rays of light; whence confequently thofe particles would,in fuch a degree,be diaphanous, For as perfectly opake bodies can only reflect the incident rays of light,those that are diaphanous refract them too; and that refraction has a great * *Refrangibility of the rays of light is their difpofition to be turned out of their way, in paffing out of one transparent body into another; reflexibility, their difpofition to be turned back into the fame medium from the furface of any other. Sir Ifaac Newton firft fhewed the precife difference between them, in the Philofophical Tranfactions, N°. 80. A.D. 1671. and in feveral fubfequent pieces which occur in the fame Tranfactions, fo far vindicated his difcovery from feveral hand |