Experiments and Considerations Touching Colours (1664)

by Robert Boyle

their Order, fewer of them being contain'd within the ſame Senſible (though Minute) ſpace than before; or elſe by approaching to one another, they muſt Straighten the Pores, and it may be too, they will by their manner of Aſſociating themſelves with the Protuberant Particles, intercept new Pores. And this invites me to conſider farther, that the Adventitious Corpuſcles, I have been ſpeaking of, may likewiſe produce a great Change as well in the Little Cavities or Pores as in the Protuberances of a Colour'd Body; for beſides what we have juſt now taken notice of, they may by Lodging themſelves in thoſe little Cavities, fill them up, and it may well happen, that they may not only fill the Pores they Inſinuate themſelves into, but likewiſe have their Upper Parts extant above them; and partly by theſe new Protuberances, partly by Increaſing the Bulk of the former, theſe Extraneous Corpuſcles may much alter the Number and Bigneſs of the Surfaces Pores, changing the Old and Intercepting new ones. And then 'tis Odds, but the Order of the Little Extancies, and conſequently that of the Little Depreſſions in point of Situation will be alter'd likewiſe: as if you diſſolve Quick-ſilver in ſome kind of Aqua-fortis,

the Saline Particles of the Menstruum Aſſociating themſelves with the Mercurial Corpuſcles, will make a Green Solution, which afterwards eaſily enough Degenerates. And Red Lead or Minium being Diſſolv'd in Spirit of Vinegar, yields not a Red, but a Clear Solution, the Redneſs of the Lead being by the Liquor Deſtroy'd. But a better Inſtance may be taken from Copper, for I have try'd, that if upon a Copper-plate you let ſome Drops of weak Aqua-fortis reſt for a while, the Corpuſcles of the Menſtruum, joyning with thoſe of the Metall, will produce a very ſenſible Aſperity upon the Surface of the Plate, and will Concoagulate that way into very minute Grains of a Pale Blew Vitriol; whereas if upon another part of the ſame Plate you ſuffer a little ſtrong Spirit of Urine to reſt a competent time, you ſhall find the Aſperated Surface adorn'd with a Deeper and Richer Blew. And the ſame Aqua-fortis, that will quickly change the Redneſs of Red Lead into a Darker Colour, will, being put upon Crude Lead, produce a Whitiſh Subſtance, as with Copper it did a Blewiſh. And as with Iron it will produce a Reddiſh, and on White Quills a Yellowiſh, ſo much may the Coalition of the Parts of the ſame

Liquor, with the differingly Figur'd Particles of Stable Bodies, divers ways Aſperate the differingly Diſpos'd Surfaces, and to Diverſifie the Colour of thoſe Bodies. And you'l eaſily believe, that in many changes of Colour, that happen upon the Diſſolutions of Metalls, and Precipitations made with Oyl of Tartar, and the like Fix'd Salts, there may Intervene a Coalition of Saline Corpuſcles with the Particles of the Body Diſſolv'd or Precipitated, if you examine how much the Vitriol of a Metall may be Heavier than the Metalline part of it alone, upon the Score of the Saline parts Concoagulated therewith, and, that in Several Precipitations the weight of the Calx does for the ſame Reaſon much exceed that of the Metall, when it was firſt put in to be Diſſolv'd.

26. But, Pyrophilus, to conſider theſe Matters more particularly would be to forget that I declar'd againſt Adventuring, at leaſt for this time, at particular Theories of Colours, and that accordingly you may juſtly expect from me rather Experiments than Speculations, and therefore I ſhall Diſmiſs this Subject of the Forms of Superficial Aſperity in Colour'd Bodies, as ſoon as I ſhall but have nam'd to you by way of Supplement to what we have

hitherto Diſcours'd in this Section, a Couple of Particulars, (which you'l eaſily grant me) The one, That there are divers other ways for the ſpeedy Production even of True and Permanent Colours in Bodies, beſides thoſe Practicable by the help of Liquors; for proof of which Advertiſement, though ſeveral Examples might be alleged, yet I ſhall need but Re-mind you of what I mention'd to you above, touching the change of Colours ſuddenly made on Temper'd Steel, and on Lead, by the Operation of Heat, without the Intervention of a Liquor. But the other particular I am to obſerve to you is of more Importance to our preſent Subject and it is, That though Nature and Art may in ſome caſes ſo change the Aſperity of the Superficial parts of a Body, as to change its Colour by either of the ways I have propos'd Single or Unaſſiſted, yet for the moſt part 'tis by two or three, or perhaps by more of the fore-mention'd ways Aſſociated together, that the Effect is produc'd, and if you conſider how Variouſly thoſe ſeveral ways and ſome others Ally'd unto them, which I have left unmention'd, may be Compounded and Apply'd, you will not much wonder that ſuch fruitfull, whether Principles (or Manners of Diverſification)

ſhould be fitted to Change or Generate no ſmall ſtore of Differing Colours.

27. Hitherto, Pyrophilus, we have in diſcourſing of the Aſperity of Bodies conſider'd the little Protuberances of other Superficial particles which make up that Roughneſs, as if we took it for granted, that they muſt be perfectly Opacous and Impenetrable by the Beams of Light, and ſo, muſt contribute to the Variety of Colours as they terminate more or leſs Light, and reflect it to the Eye mix'd with more or leſs of thus or thus mingl'd Shades. But to deal Ingenuouſly with you, Pyrophilus, before I proceed any further, I muſt not conceal from you, that I have often thought it worth a Serious Enquiry, whether or no Particles of Matter, each of them ſing'y Inſenſible, and therefore ſmall enough to be capable of being ſuch Minute Particles as the Atomiſts both of old and of late have (not abſurdly) called Corpuſcula Coloris, may not yet conſiſt each of them of divers yet Minuter Particles, betwixt which we may conceive little Commiſſures where they Adhere to one another, and, however, may not be Porous enough to be, at leaſt in ſome degree, Pervious to the unimaginably ſubtile Corpuſcles that make up the Beams of

Light, and conſequently to be in ſuch a degree Diaphanous. For, Pyrophilus, that the propoſed Enquiry may be of moment to him that ſearches after the Nature of Colour, you'l eaſily grant, if you conſider, that whereas Perfectly Opacous bodies can but reflect the incident Beams of Light, thoſe that are Diaphanous are qualified to refract them too, and that Refraction has ſuch a ſtroak in the Production of Colours, as you cannot but have taken notice of, and perhaps admir'd in the Colours generated by the Trajection of Light through Drops of Water that exhibit a Rain-bow, through Priſmatical glaſſes, and through divers other Tranſparent bodies. But 'tis like, Pyrophilus, you'l more eaſily allow that about this matter 'tis rather Important to have a Certainty, than that 'tis Rational to entertain a Doubt; wherefore I muſt mention to you ſome of the Reaſons that make me think it may need a further Enquiry, for I find that in a Darkned Room, where the Light is permitted to enter but at One hole, the little wandering Particles of Duſt, that are commonly called Motes, and, unleſs in the Sunbeams, are not taken notice of by the unaſſiſted Sight, I have, I ſay, often obſerv'd, that theſe roving Corpuſcles being look'd on by an Eye plac'd on one ſide of the

Beams that enter'd the Little hole, and by the Darkneſs having its Pupill much Enlarg'd, I could diſcern that theſe Motes as ſoon as they came within the compaſs of the Luminous, whether Cylinder or Inverted Cone, if I may ſo call it, that was made up by the Unclouded Beams of the Sun, did in certain poſitions appear adorn'd with very vivid Colours, like thoſe of the Rain-bow, or rather like thoſe of very Minute, but Sparkling fragments of Diamonds; and as ſoon as the Continuance of their Motion had brought them to an Inconvenient poſition in reference to the Light and the Eye, they were only viſible without Darting any lively Colours as before, which ſeems to argue that theſe little Motes, or minute Fragments, of ſeveral ſorts of bodies reputed Opacous, and only crumbled as to their Exteriour and Looſer parts into Duſt, did not barely Reflect the Beams that fell upon them, but remit them to the Eye Refracted too. We may alſo obſerve, that ſeveral Bodies, (as well ſome of a Vegetable, as others of an Animal nature) which are wont to paſs for Opacous, appear in great part Tranſparent, when they are reduc'd into Thin parts, and held againſt a powerful Light. This I have not only taken notice of in pieces of Ivory reduc'd but into Thick leaves, as alſo in divers conſiderable

Thick ſhells of Fiſhes, and in ſhaving of Wood, but I have alſo found that a piece of Deal, far thicker than one would eaſily imagine, being purpoſly interpoſed betwixt my Eye plac'd in a Room, and the clear Daylight, was not only ſomewhat Tranſparent, but (perhaps by reaſon of its Gummous nature) appear'd quite through of a lovely Red. And in the Darkned Room above mention'd, Bodies held againſt the hole at which the Light enter'd, appear'd far leſs Opacous then they would elſewhere have done, inſomuch that I could eaſily and plainly ſee through the whole Thickneſs of my Hand, the Motions of a Body plac'd (at a very near diſtance indeed, but yet) beyond it. And even in Minerals, the Opacity is not always ſo great as many think, if the Body be made Thin, for White Marble though of a pretty Thickneſs, being within a Due diſtance plac'd betwixt the Eye and a Convenient Light, will Suffer the Motions of ones Finger to be well diſcern'd through it, and ſo will pieces, Thick enough, of many common Flints. But above all, that Inſtance is remarkable, that is afforded us by Muſcovie glaſs, (which ſome call Selenites, others Lapis Specularis) for though plates of this Mineral, though but of a moderate Thickneſs, do often appear Opacous, yet if