THE
EXPERIMENTAL HISTORY
OF COLOURS.
PART. II.
Of the Nature of Whiteneſs and Blackneſs.
CHAP. I.
| 1. |
Hough after what I have acknowledged, Pyrophilus, of the Abſtruſe Nature of Colours in particular, you will eaſily believe, that I pretend not to give you a Satisfactory account of Whiteneſs and Blackneſs; Yet not wholly to fruſtrate your Expectation of my offering ſomething by way of Specimen towards the Explication of ſome Colours in particular, I ſhall make choice of Theſe as the moſt Simple Ones, (and by reaſon of their mutual Oppoſition the Leaſt hardly explicable) about which to preſent you my Thoughts, upon condition you will take them at moſt to be my Conjectures, not my Opinions.
2. When I apply'd my Self to conſider, how the cauſe of Whiteneſs might be explan'd by Intelligible and Mechanical Principles, I remembred not to have met with any thing among the Antient Corpuſcularian Philoſophers, touching the Quality we call Whiteneſs, ſave that Democritus is by Ariſtotle ſaid to have aſcrib'd the Whiteneſs of Bodies to their Smoothneſs, and on the contrary their Blackneſs to their Aſperity.6 But though about the Latter of thoſe Qualities his Opinion be allowable, as we ſhall ſee anon, yet that he heeds a Favourable Interpretation in what is Deliver'd concerning the Firſt, (at leaſt if his Doctrine be not Mis-repreſented in this point, as it has been in many others) we ſhall quickly have Occaſion to manifeſt. But amongſt the Moderns, the moſt Learned Gaſſendus in his Ingenious Epiſtle publiſh'd in the Year 1642. De apparente Magnitudine ſolis humilis & ſublimis, reviving the Atomical Philoſophy, has, though but Incidentally, deliver'd ſomething towards the Explication of Whiteneſs upon Mechanical Principles: And becauſe no Man that I know of, has done ſo before him, I ſhall, to be ſure to do him Right, give you his Senſe in his own Words:7 Cogites velim (ſays he) lucem quidem in Diaphano nullius coloris videri, ſed in Opaco tamen terminante Candicare, ac tantò magis, quantò denſior ſeu collectior fuerit. Deinde aquam non eſſe quidem coloris ex ſe candidi & radium tamen ex eâ reflexum verſus oculum candicare. Rurſus cum plana aquæ Superficies non niſi ex una parte eam reflexionem faciat: ſi contigerit tamen illam in aliquot bullas intumeſcere, bullam unamquamque reflectionem facere, & candoris ſpeciem creare certa Superficiei parte. Ad hæc Spumam ex aqua pura non alia ratione videri candeſcere & albeſcerere quam quod ſit congeries confertiſſima minutiſſimarum bullarum, quarum unaquæque ſuum radium reflectit, unde continens candor alborve apparet. Denique Nivem nihil aliud videri quam ſpeciem puriſſimæ ſpumæ ex bullulis quam minutiſſimis & confertiſſimis cohærentis. Sed ridiculam me exhibeam, ſi tales meas nugas uberius proponem.
3. But though in this paſſage, that very Ingenous Perſon has Anticipated part of what I ſhould ſay; Yet I preſume you will for all that expect, that I ſhould give you a fuller Account of that Notion of Whiteneſs, which I have the leaſt Exceptions to, and of the Particulars whence I deduce it, which to do, I muſt mention to you the following Experiments and Obſervations.
Whiteneſs then conſider'd as a Quality in the Object, ſeems chiefly to depend upon this, That the Superficies of the Body that is call'd White, is Aſperated by almoſt innumerable Small Surfaces, which being of an almoſt Specular Nature, are alſo ſo Plac'd, that ſome Looking this way, and ſome that way, they yet Reflect the Rays of Light that fall on them, not towards one another, but outwards towards the Spectators Eye. In this Rude and General account of Whiteneſs, it ſeems that beſides thoſe Qualities, which are common to Bodies of other Colours, as for inſtance the Minuteneſs and Number of the Superficial parts, the two chief things attributed to Bodies as White are made to be, Firſt, that its Little Protuberances and Superficial parts be of ſomewhat a Specular Nature, that they may as little Looking-glaſſes each of them Reflect the Beams it receives, (or the little Picture of the Sun made on it) without otherwiſe conſiderably Altering them; whereas in moſt other Colours, they are wont to be much Chang'd, by being alſo Refracted, or by being Return'd to the Eye, mixt with Shades or otherwiſe. And next, that its Superficial parts be ſo Situated, that they Retain not the Incident Rays of Light by Reflecting them Inwards, but Send them almoſt all Back, ſo that the Outermoſt Corpuſcles of a White Body, having their Various Little Surfaces of a Specular Nature, a Man can from no place Behold the Body, but that there will be among thoſe Innumerable Superficieculæ, that Look ſome one way, and ſome another, enough of them Obverted to his Eye, to afford like a broken Looking-glaſs, a confuſed Idæa, or Repreſentation of Light, and make ſuch an Impreſſion on the Organ, as that for which Men are wont to call a Body White. But this Notion will perhaps be beſt Explan'd by the ſame Experiments and Obſervations, on which it is Built, And therefore I ſhall now advance to Them.
4. And in the firſt place I conſider, that the Sun and other Powerfully Lucid Bodies, are not only wont to Offend, which we call to Dazle our Eyes, but that if any Colour be to be Aſcrib'd to them as they are Lucid, it ſeems it ſhould be Whiteneſs: For the Sun at Noon-day, and in Clear weather, and when his Face is leſs Troubled, and as it were Stained by the Steams of Sublunary Bodies, and when his Beams have much leſs of the Atmoſphere to Traject in their Paſſage to our Eyes, appears of a Colour more approaching to White, than when nearer the Horizon, the Interpoſition of certain Sorts of Fumes and Vapours make him oftentimes appear either Red, or at leaſt more Yellow. And when the Sun Shines upon that Natural Looking-glaſs, a Smooth water, that part of it, which appears to this or that particular Beholder, the moſt Shin'd on, does to his Eye ſeem far Whiter than the reſt. And here I ſhall add, that I have ſometimes had the Opportunity to obſerve a thing, that may make to my preſent purpoſe, namely, that when the Sun was Veil'd over as it were, with a Thin White Cloud, and yet was too Bright to be Look'd upon Directly without Dazling, by caſting my Eyes upon a Smooth water, as we ſometimes do to obſerve Eclipſes without prejudice to our Eyes, the Sun then not far from the Meridian, appear'd to me not Red, but ſo White, that 'twas not without ſome Wonder, that I made the Obſervation. Beſides, though we in Engliſh are wont to ſay, a thing is Red hot, as an Expreſſion of its being Superlatively Ignitum, (if I may ſo Speak for want of a proper Engliſh word) yet in the Forges of Smiths, and the Furnaces of other Artificers, by that which they call a White heat, they mean a further Degree of Ignition, than by that which both they and we call a Red heat.
5. Secondly, I conſider, that common Experience informs us, that as much Light Over-powers the Eye, ſo when the Ground is covered with Snow, (a Body extremely White) thoſe that have Weak Eyes are wont to complain of too much Light: And even thoſe that have not, are generally Senſible of an Extraordinary meaſure of Light in the Air; and if they are fain to Look very long upon the Snow, find their Sight Offended by it. On which occaſion we may call to mind what Xenophon relates, that his Cyrus marching his Army for divers days through Mountains covered with Snow, the Dazling ſplendor of its Whiteneſs prejudic'd the Sight of very many of his Souldiers, and Blinded ſome of them; and other Stories of that Nature be met with in Writers of good Note. And the like has been affirm'd to me by credible Perſons of my own Acquaintance, and eſpecially by one who though Skill'd in Phyſick and not Ancient confeſs'd to me when I purpoſely ask'd him, that not only during his ſtay in Muſcovy, he found his Eyes much Impair'd, by being reduc'd frequently to Travel in the Snow, but that the Weakneſs of his Eyes did not Leave him when he left that Country, but has follow'd him into theſe Parts, and yet continues to Trouble him. And to this doth agree what I as well as others have obſerv'd, namely, that when I Travell'd by Night, when the Ground was all cover'd with Snow, though the Night otherwiſe would not have been Lightſome, yet I could very well ſee to Chooſe my way. But much more Remarkable to my preſent purpoſe is that, which I have met with in Olaus Magnus,8 concerning the way of Travelling in Winter in the Northern Regions, where the Days of that Seaſon are ſo very Short; for after other things not needfull to be here Tranſcribed: Iter, ſays he, Diurnum duo ſcilicet montana milliaria (quæ 12 Italica ſunt) conſiciunt. Nocte verò ſub ſplendiſſima luna, duplatum iter conſumunt aut triplatum. Neque id incommodè fit, cum nivium reverberatione lunaris ſplendoris ſublimes & declives campos illuſtret, ac etiam montium præcipitia ac noxias feras à lorgè proſpiciant evitandas. Which Teſtimony I the leſs Scruple to allege, becauſe that it agrees very well with what has been Affirm'd to me by a Phyſician of Moſco, whom the Notion I have been Treating of concerning Whiteneſs invited me to ask whether he could not See much farther when he Travell'd by Night in Ruſſia than he could do in England, or elſewhere, when there was no Snow upon the Ground; For this Ingenious Perſon inform'd me, that he could See Things at a farr greater Diſtance, and with more Clearneſs, when he Travell'd by Night on the Ruſſian Snow, though without the Aſſiſtance of Moon-ſhine, than we in theſe Parts would eaſily be perſwaded. Though it ſeems not unlikely to me, that the Intenſeneſs of the Cold may contribute ſomething to the conſiderableneſs of the Effect, by much Clearing the Air of Darkiſh Steams, which in theſe more Temperate Climates are wont to Thicken it in Snowy weather: For having purpoſely inquir'd of this Doctor, and conſulted that Ingenious Navigator Captain James's Voyage hereafter to be further mention'd, I find both their Relations agree in this, that in Dark Froſty Nights they could Diſcover more Stars, and See the reſt Clearer than we in England are wont to do.
6. I know indeed that divers Learned Men think, that Snow ſo ſtrongly Affects our Eye, not by a Borrow'd, but a Native Light; But I venture to give it as a Proof, that White Bodies reflect more Light than Others, becauſe having once purpoſely plac'd a parcel of Snow in a Room carefully Darkned, that no Celeſtial Light might come to fall upon it; neither I, nor an ingenous Perſon, (Skill'd in Opticks) whom I deſir'd for a Witneſs, could find, that it had any other Light than what it receiv'd. And however, 'tis uſual among thoſe that Travel in Dark Nights, that the Guides wear ſomething of White to be Diſcern'd by, there being ſcarce any Night ſo Dark, but that in the Free Air there remains ſome Light, though Broken and Debilitated perhaps by a thouſand Reflections from the Opacous Corpuſcles that Swim in the Air, and lend it to one another before it comes to arrive at the Eye.
7. Thirdly, And the better to ſhew that White Bodies reflect ſtore of Light, in comparſon of thoſe that are otherwiſe Colour'd, I did in the Darkn'd Room, formerly mention'd, hold not far from the Hole, at which the Light was admitted, a Sheet only of White Paper, from whence caſting the Sun-beams upon a White Wall, whereunto it was Obverted, it manifeſtly appear'd both to Me, and to the Perſon I took for a Witneſs of the Experiment, that it Reflected a far greater Light, than any of the other Colours formerly mention'd, the Light ſo thrown upon one Wall notably Enlightning it, and by it a good part of the Room. And yet further to ſhow you, that White Bodies Reflect the Beams From them, and not Towards themſelves, Let me add, that Ordinary Burning-glaſſes, ſuch as are wont to be employ'd to light Tobacco, will not in a great while Burn, or ſo much as Diſcolour a Sheet of White Paper. Inſomuch that even when I was a Boy, and Lov'd to make Tryals with Burning-glaſſes, I could not but wonder at this Odd Phænomenon, which ſet me very Early upon Gueſſing at the Nature of Whiteneſs, eſpecially becauſe I took notice, that the Image of the Sun upon a White Paper was not ſo well Defin'd (the Light ſeeming too Diffus'd) as upon Black, and becauſe I try'd, that Blacking over the Paper with Ink, not only the Ink would be quickly Dry'd up, but the Paper that I could not Burn before, would be quickly ſet on Fire. I have alſo try'd, that by expoſing my Hand with a Thin Black Glove over it to the Warm Sun, it was thereby very quickly and conſiderably more Heated, than if I took off the Glove, and held my Hand Naked, or put on it another Glove of Thin but White Leather. And having thus ſhewn you, Pyrophilus, that White Bodies reflect the moſt Light of any, let us now proceed, to conſider what is further to be taken notice of in them, in order to our preſent Enquiry.
8. And Fourthly, whereas among the Diſpoſitions we attributed to White Bodies, we alſo intimated this, That ſuch Bodies are apt, like Speculums, though but Imperfect ones, to Reflect the Light that falls on them Untroubled or Unſtain'd, we ſhall beſides other particulars to be met with in theſe Papers, offer you this in favour of the Conjecture; That in the Darkned Room ſeveral times mention'd in this Treatſe, we try'd that the Sun-beams being caſt from a Coloured Body upon a neighbouring White Wall, the Determinate Colour of the Body was from the Wall reflected to the Eye; whereas we could in divers caſes manifeſtly Alter the Colour arriving at the Eye, by Subſtituting at a convenient Diſtance, a (conveniently) Colour'd (and Gloſſy) Body inſtead of the White Wall. As by throwing the Beams from a Yellow Body upon a Blew, there would be Exhibited a kind of Green, as in the Experiments about Colours is more fully Declar'd.
9. I know not whether I ſhould on this Occaſion take notice, that when, as when looking upon the Calm and Smooth Surface of a River betwixt my Eye and the Sun, it appear'd to be a natural Speculum, wherein that Part which Reflected to my Eye the Entire and defin'd Image of the Sun, and the Beams leſs remote from thoſe which exhibited That Image, appear'd indeed of a great and Whitiſh Brightneſs, but the reſt Comparatively Dark enough: if afterwards the Superficies chanc'd to be a little, but not much troubled, by a gentle Breath of Wind, and thereby reduc'd into a Multitude of Small and Smooth Speculums, the Surface of the River would ſuitably to the Doctrine lately deliver'd, at a Diſtance appear very much of Kin to White, though it would loſe that Brightneſs or Whiteneſs upon the Return of the Surface to Calmneſs and an Uniform Level. And I have ſometimes for Tryals ſake brought in by a Lenticular Glaſs, the Image of a River, Shin'd upon by the Sun, into an Upper Room Darkn'd, and Diſtant about a Quarter of a Mile from the River, by which means the Numerous Declining Surfaces of the Water appear'd ſo Contracted, that upon the Body that receiv'd the Images, the whole River appear'd a very White Object at two or three paces diſtance. But if we drew Near it, this Whiteneſs appear'd to proceed from an Innumerable company of Lucid Reflections, from the ſeveral Gently wav'd Superficies of the Water, which look'd Near at hand like a Multitude of very Little, but Shining Scales of Fiſh, of which many did every moment Diſappear, and as many were by the Sun, Wind and River generated anew. But though this Obſervation ſeem'd Sufficiently to diſcover, how the Appearing Whiteneſs in that caſe was Produc'd, yet in ſome other caſes Water may have the Same, though not ſo Vivid a Colour upon other Accounts; for oftentimes it happens that the Smooth Surface of the Water does appear Bright or Whitiſh, by reaſon of the Reflection not immediatly of the Images of the Sun, but of the Brightneſs of the Sky; and in ſuch caſes a Convenient Wind may where it paſſes along make the Surface look Black, by cauſing many ſuch Furrows and Cavities, as may make the Inflected Superficies of the Water reflect the Brightneſs of the Sky rather Inward than Outward. And again if the Wind increaſe into a Storm, the Water may appear White, eſpecially near the Shore and the Ship, namely becauſe the Rude Agitation Breaks it into Fome or Froth. So much do Whiteneſs and Blackneſs depend upon the Diſpoſition of the Superficial parts of a Body to Reflect the Beams of Light Inward or Outward. But that as White Bodies reflect the moſt Light of any, ſo there Superficial Particles are, in the Senſe newly Deliver'd, of a Specular Nature, I ſhall now further endeavour to ſhew both by the making of Specular bodies White, and the making of a White body Specular.
10. In the Fifth place then, I will inform You, that (not to repeat what Gaſſendus obſerves concerning Water) I have for Curioſity ſake Diſtill'd Quickſilver in a Cucurbit, fitted with a Capacious Glaſs-head, and obſerv'd that when the Operation was perform'd by the Degrees of Fire requiſite for my purpoſe, there would ſtick to the Inſide of the Alembick a multitude of Little round drops of Mercury. And as you know that Mercury is a Specular Body, ſo each of theſe Little drops was a ſmall round Looking-glaſs, and a Multitude of them lying Thick and Near one another, they did both in my Judgment, and that of thoſe I Invited to ſee it, make the Glaſs they were faſtened to, appear manifeſtly a White Body. And yet as I ſaid, this Whiteneſs depended upon the Minuteneſs and Nearneſs of the Little Mercurial Globuli, the Convexity of whoſe Surfaces fitted them to repreſent in a Narrow compaſs a Multitude of Little Lucid Images to differingly ſituated Beholders. And here let me obſerve a thing that ſeems much to countenance the Notion I have been recommending: namely, that whereas divers parts of the Sky, and eſpecially the Milky-way, do to the naked Eye appear White, (as the name it ſelf imports) yet the Galaxie look'd upon through the Teleſcope, does not ſhew White, but appears to be made up of a Vaſt multitude of Little Starrs; ſo that a Multitude of Lucid Bodies, if they be ſo Small that they cannot Singly or apart be diſcern'd by the Eye, and if they be ſufficiently Thick ſet by one another, may by their confus'd beams appear to the Eye One White Body. And why it is not poſſible, that the like may be done, when a Multitude of Bright and Little Corpuſcles being crowded together, are made to ſend together Vivid beams to the Eye, though they Shine but as the Planets by a Borrow'd Light?
11. But to return to our Experiments. We may take notice, That the White of an Egg, though in part Tranſparent, yet by its power of Reflecting ſome Incident Rays of Light, is in ſome meaſure a Natural Speculum, being long agitated with a Whisk or Spoon, loſes its Tranſparency, and becomes very White, by being turn'd into Froth, that is into an Aggregate of Numerous ſmall Bubbles, whoſe Convex Superficies fits them to Reflect the Light every way Outwards. And 'tis worth Noting, that when Water, for inſtance, is Agitated into Froth, if the Bubbles be Great and Few, the Whiteneſs will be but Faint, becauſe the number of Specula within a Narrow compaſs is but Small, and they are not Thick ſet enough to Reflect ſo Many Little Images or Beams of the Lucid Body, as are requiſite to produce a Vigorous ſenſation of Whiteneſs: And partly leaſt it ſhould be ſaid, that the Whiteneſs of ſuch Globulous Particles proceeds from the Air Included in the Froth; (which to make good, it ſhould be prov'd that the Air it ſelf is White) and partly to illuſtrate the better the Notion we have propos'd of Whiteneſs, I ſhall add, that I purpoſely made this Experiment, I took a quantity Fair water, & put to it in a clear Glaſs phial, a convenient quantity of Oyl or Spirit of Turpentine, becauſe that Liquor will not incorporate with Water, and yet is almoſt as Clear and Colourleſs as it; theſe being Gently Shaken together, the Agitation breaks the Oyl (which as I ſaid, is Indiſpos'd to Mix like Wine or Milk per minima with the Water) into a Multitude of Little Globes, which each of them Reflecting Outwards a Lucid Image, make the Imperfect Mixture of the two Liquors appear Whitiſh; but if by Vehemently Shaking the Glaſs for a competent time you make a further Comminution of the Oyl into far more Numerous and Smaller Globuli, and thereby confound it alſo better with the Water, the Mixture will appear of a Much greater Whiteneſs, and almoſt like Milk; whereas if the Glaſs be a while let alone, the Colour will by degrees Impair, as the Oyly globes grow Fewer and Bigger, and at length will quite Vaniſh, leaving both the Liquors Diſtinct and Diaphanous as before. And ſuch a Tryal hath not ill ſucceeded, when inſteed of the Colourleſs Oyl of Turpentine I took a Yellow Mixture made of a good Proportion of Crude Turpentine diſſolv'd in that Liquor; and (if I mis-remember not) it alſo Succeeded better than one would expect, when I employ'd an Oyl brought by Filings of Copper infuſed in it, to a deep Green. And this (by the way) may be the Reaſon, why often times when the Oyls of ſome Spices and of Anniſeeds &c. are Diſtilled in a Limbec with Water, the Water (as I have ſeveral times obſerv'd) comes over Whitiſh, and will perhaps continue ſo for a good while, becauſe if the Fire be made too Strong, the ſubtile Chymical Oyl is thereby much Agitated and Broken, and Blended with the Water in ſuch Numerous and Minute Globules, as cannot eaſily in a ſhort time Emerge to the Top of the Water, and whilſt they Remain in it, make it, for the Reaſon newly intimated, look Whitiſh; and perhaps upon the ſame Ground a cauſe may be rendred, why Hot water is obſerv'd to be uſually more Opacous and Whitiſh, than the ſame Water Cold, the Agitation turning the more Spirituous or otherwiſe Conveniently Diſpos'd Particles of the Water into Vapours, thereby Producing in the Body of the Liquor a Multitude of Small Bubbles, which interrupt the Free paſſage, that the Beams of Light would elſe have Every way, and from the Innermoſt parts of the Water Reflect many of them Outwards. Theſe and the like Examples, Pyrophilus, have induc'd me to Suſpect, that the Superficial Particles of White bodies, may for the Moſt part be as well Convex as Smooth; I content my ſelf to ſay Suſpect and for the moſt part, becauſe it ſeems not Eaſie to prove, that when Diaphanous bodies, as we ſhall ſee by and by, are reduc'd into White Powders, each Corpuſcle muſt needs be of a Convex Superficies, ſince perhaps it may Suffice that Specular Surfaces look ſeverally ways. For (as we have ſeen) when a Diaphanous Body comes to be reduc'd to very Minute parts, it thereby requires a Multitude of Little Surfaces within a Narrow compaſs. And though each of theſe ſhould not be of a Figure Convenient to Reflect a Round Image of the Sun, yet even from ſuch an Inconveniently Figur'd body, there may be Reflected ſome (either Streight or Crooked) Phyſical Line of Light, which Line I call Phyſical, becauſe it has ſome Breadth in it, and in which Line in many caſes ſome Refraction of the Light falling upon the Body it depends on, may contribute to the Brightneſs, as if a Slender Wire, or Solid Cylinder of Glaſs be expos'd to the Light, you ſhall ſee in ſome part of it a vivid Line of Light, and if we were able to draw out and lay together a Multitude of theſe Little Wires or Thrids of Glaſs, ſo Slender, that the Eye could not diſcern a Diſtance betwixt the Luminous Lines, there is little doubt (as far as I can gueſs by a Tryal purpoſely made with very Slender, but far leſs Slender Thrids of Glaſs, whoſe Aggregate was Look'd upon one way White) but the whole Phyſical Superficies compos'd of them, would to the Eye appear White, and if ſo, it will not be always neceſſary that the Figure of thoſe Corpuſcles, that make a Body appear White, ſhould be Globulous. And as for Snow it ſelf, though the Learned Gaſſendus (as we have ſeen above) makes it to ſeem nothing elſe but a pure Frozen Froth, conſiſting of exceedingly Minute and Thickſet Bubbles; yet I ſee no neceſſity of Admitting that, ſince not only by the Variouſly and Curiouſly Figur'd Snow, that I have divers times had the Opportunity with Pleaſure to obſerve, but alſo by the Common Snow, it rather doth appear both to the Naked Eye, and in a Microſcope, often, if not moſt commonly, to conſiſt principally of Little Slender Icicles of ſeveral Shapes, which afford ſuch Numerous Lines of Light, as we have been newly Speaking of.
12. Sixthly, If you take a Diaphanous Body, as for inſtance a Piece of Glaſs, and reduce it to Powder, the ſame Body, which when it was Entire, freely Tranſmitted the Beams of Light, acquiring by Contuſion a multitude of Minute Surfaces, each of which is as it were a Little, but Imperfect Speculum, is qualify'd to Reflect in a Confus'd manner, ſo many either Beams, or Little and Singly Unobſervable Images of the Lucid Body, that from a Diaphanous it Degenerates into a White Body. And I remember, I have for Trials ſake taken Lumps of Rock Cryſtal, and Heating them Red hot in a Crucible, I found according to my Expectation, that being Quench'd in Fair water, even thoſe that remain'd in ſeemingly entire Lumps exchang'd their Tranſlucency for Whiteneſs, the Ignition and Extinction having as it were Crack'd each Lump into a multitude of Minute Bodies, and thereby given it a great multitude of new Surfaces. And ev'n with Diaphanous Bodies, that are Colour'd, there may be this way a Greater Degree of Whiteneſs produced, than one would lightly think; as I remember, I have by Contuſion obtain'd Whitiſh Powders of Granates, Glaſs of Antimony, and Emeralds finely Beaten, and you may more eaſily make the Experiment, by taking Good Venereal Vitriol of a Deep Blew, and comparing with ſome of the Entire Cryſtalls purpoſely reſerv'd, ſome of the Subtile Powder of the ſame Salt, which will Comparatively exhibit a very conſiderable degree of Whitiſhneſs.
13. Seventhly, And as by a Change of Poſition in the Parts, a Body that is not White, may be made White, ſo by a Slight change of the Texture of its Surface, a White Body may be Depriv'd of its Whiteneſs. For if, (as I have try'd in Gold-ſmiths Shops) you take a piece of Silver that has been freſhly Boyl'd, as the Artificers call it, (which is done by, firſt Bruſhing, and then Decocting it with Salt and Tartar, and perhaps ſome other Ingredients) you ſhall find it to be of a Lovely White. But if you take a piece of Smooth Steel, and therewith Burniſh a part of it, which may be preſently done, you ſhall find that Part will Loſe its Whiteneſs, and turn a Speculum, looking almoſt every where Dark, as other Looking-glaſſes do, which may not a little confirm our Doctrine. For by this we may gueſs, what it is chiefly that made the Body White before, by conſidering that all that was done to deprive it of that Whiteneſs, was only to Depreſs the Little Protuberances that were before on the Surface of the Silver into one Continu'd Superficies, and thereby effect this, that now the Image of the Lucid Body, and conſequently a Kind of Whiteneſs ſhall appear to your Eye, but in ſome place of the greater Silver Looking-glaſs (whence the Beams reflected at an Angle Equal to that wherewith they fall on it, may reach your Eye) whilſt the Aſperity remain'd Undeſtroy'd, the Light falling on innumerable Little Specula Obverted ſome one way, and ſome another, did from all Senſibly Diſtinguiſhable parts of the Superficies reflect confus'd Beams or Repreſentations of Light to the Beholders Eye, from whence ſoever he chance to Look upon it. And among the Experiments annex'd to this Diſcourſe, you will find One, wherein by the Change of Texture in Bodies, Whiteneſs is in a Trice both Generated and Deſtroy'd.
CHAP. II.
1. What we have Diſcours'd of Whiteneſs, may ſomewhat Aſſiſt us to form a Notion of Blackneſs, thoſe two Qualities being Contrary enough to Illuſtrate each other. Yet among the Antient Philoſophers I find leſs Aſſiſtance to form a Notion of Blackneſs than of Whiteneſs, only Democritus in the paſſage above Recited out of Aristotle has given a General Hint of the Cauſe of this Colour, by referring the Blackneſs of Bodies to their Aſperity. But this I call but a General Hint, becauſe thoſe Bodies that are Green, and Purple, and Blew, ſeem to be ſo as well as Black ones, upon the Account of their Superficial Aſperity. But among the Moderns, the formerly mention'd Gaſſendus, perhaps invited by this Hint of Democritus, has Incidentally in another Epiſtle given us, though a very Short, yet a ſomewhat Clearer account of the Nature of Blackneſs in theſe words: Existimare par est corpora ſuâpte Naturâ nigra constare ex particulis, quarum Superficieculæ ſcabræ ſint, nec facilè lucem extrorſum reflectant. I wiſh this Ingenious Man had enlarg'd himſelf upon this Subject; For indeed it ſeems, that as that which makes a Body White, is chiefly ſuch a Diſpoſition of its Parts, that it Reflects (I mean without much Interruption) more of the Light that falls on it, than Bodies of any other Colour do, ſo that which makes a Body Black is principally a Peculiar kind of Texture, chiefly of its Superficial Particle, whereby it does as it were Dead the Light that falls on it, ſo that very little is Reflected Outwards to the Eye.
2. And this Texture may be Explicated two, and perhaps more than two ſeveral ways, whereof the firſt is by Suppoſing in the Superficies of the Black Body a Particular kind of Aſperity, whereby the Superficial Particles reflect but Few of the Incident Beams Outwards, and the reſt Inwards towards the Body it ſelf. As if for Inſtance, we ſhould conceive the Surface of a Black Body to be Aſperated by an almoſt Numberleſs throng of Little Cylinders, Pyramids, Cones, and other ſuch Corpuſcles, which by their being Thick Set and Erected, reflect the Beams of Light from one to another Inwards, and ſend them too and fro ſo often, that at length they are Loſt before they can come to Rebound out again to the Eye. And this is the firſt of the two mention'd ways of Explicating Blackneſs. The other way is by Suppoſing the Texture of Black Bodies to be ſuch, that either by their Yielding to the Beams of Light, or upon ſome other Account, they do as it were Dead the Beams of Light, and keep them from being Reflected in any Plenty, or with any Conſiderable Vigour of Motion, Outwards. According to this Notion it may be ſaid, that the Corpuſcles that make up the Beams of Light, whether they be Solary Effluviums, or Minute Particles of ſome Ætherial Subſtance, Thruſting on one another from the Lucid Body, do, falling on Black Bodies, meet with ſuch a Texture, that ſuch Bodies receive Into themſelves, and Retain almoſt all the Motion communicated to them by the Corpuſcles that make up the Beams of Light, and conſequently Reflect but Few of them, or thoſe but Languidly, towards the Eye, it happening here almoſt in like manner as to a ball, which thrown againſt a Stone or Floor, would Rebound a great way Upwards, but Rebounds very Little or not at all, when it is thrown againſt Water, or Mud, or a Looſe Net, becauſe the Parts yield, and receive into themſelves the Motion, on whoſe Account the Ball ſhould be Reflected Outwards. But this Laſt way of Explicating Blackneſs, I ſhall content my Self to have Propos'd, without either Adopting it, or abſolutely Rejecting it. For the Hardneſs of Touchſtones, Black Marble and other Bodies, that being Black are Solid, ſeem to make it ſomewhat Improbable, that ſuch Bodies ſhould be of ſo Yielding a Texture, unleſs we ſhould ſay, that ſome Bodies may be more Diſpos'd to Yield to the Impulſes of the Corpuſcles of Light by reaſon of a Peculiar Texture, than other Bodies, that in other Tryals appear to be Softer than they. But though the Former of theſe two Explications of Blackneſs be that, by which we ſhall Endeavour to give an Account of it, yet as we ſaid, we ſhall not Abſolutely Reject this Latter, partly becauſe they both Agree in this, that Black Bodies Reflect but Little of the Light that falls on them, and partly becauſe it is not Impoſſible, that in ſome Caſes both the Diſpoſition of the Superficial particles, as to Figure and Poſition, and the Yielding of the Body, or ſome of its Parts, may joyntly, though not in an Equal meaſure concurr to the rendring of a Body Black. The Conſiderations that induc'd me to propoſe this Notion of Blackneſs, as I Explan'd it, are principally theſe:
3. Firſt, That as I lately ſaid, Whiteneſs and Blackneſs being generally reputed to be Contrary Qualities, Whiteneſs depending as I ſaid upon the Diſpoſition of the Parts of a Body to Reflect much Light, it ſeems likely, that Blackneſs may depend upon a Contrary Diſpoſition of the Black Bodies Surface; But upon this I ſhall not Inſiſt.
4. Next then we ſee, that if a Body of One and the ſame Colour be plac'd, part in the Sun-beams, and part in the Shade, that part which is not Shin'd on will appear more of Kin to Blackneſs than the other, from which more Light Rebounds to the Eye; And Dark Colours ſeem the Blacker, the leſs Light they are Look'd upon in, and we think all Things Black in the Dark, when they ſend no Beams to make Impreſſions on our Organs of Sight, ſo that Shadows and Darkneſs are near of Kin, and Shaddow we know is but a Privation of Light; and accordingly Blackneſs ſeems to proceed from the Paucity of Beams Reflected from the Black Body to the Eye, I ſay the Paucity of Beams, becauſe thoſe Bodies that we call Black, as Marble, Jeat, &c. are Short of being perfectly ſo, elſe we ſhould not See them at all. But though the Beams that fall on the Sides of thoſe Erected Particles that we have been mentioning, do Few of them return Outwards, yet thoſe that fall upon the Points of thoſe Cylinders, Cones, or Pyramids, may thence Rebound to the Eye, though they make there but a Faint Impreſſion, becauſe they Arrive not there, but Mingl'd with a great Proportion of Little Shades. This may be Confirm'd by my having procur'd a Large piece of Black Marble well Poliſh'd, and brought to the Form of a Large Sphærical and Concave Speculum; For on the Inſide this Marble being well Poliſh'd, was a kind of Dark Looking-glaſs, wherein I could plainly ſee a Little Image of the Sun, when that Shin'd upon it. But this Image was very far from Offending and Dazling my Eyes, as it would have done from another Speculum; Nor, though the Speculum were Large, could I in a Long time, or in a Hot Sun ſet a piece of Wood on Fire, though a far leſs Speculum of the ſame Form, and of a more Reflecting Matter, would have made it Flame in a Trice.
5. And on this Occaſion we may as well in Reference to ſomething formerly deliver'd concerning Whiteneſs, as in Reference to what has been newly ſaid, Subjoyn what we further obſerv'd touching the Differing Reflections of Light from White and Black Marble, namely, that having taking a pretty Large Mortar of White Marble, New and Poliſh'd in the Inſide, and Expos'd it to the Sun, we found that it Reflected a great deal of Glaring Light, but ſo Diſpers'd, that we could not make the Reflected Beams concurr in any ſuch Conſpicuous Focus, as that newly taken notice of in the Black Marble, though perhaps there may enough of them be made to meet near the Bottom, to make ſome Kind of Focus, eſpecially ſince by holding in the Night-time a Candle at a convenient Diſtance, we were able to procure a Concourſe of ſome, though not many of the Reflected Beams, at about two Inches diſtant from the Bottom of the Mortar: But we found the Heat even of the Sunbeams ſo Diſperſedly Reflected to be very Languid, even in Compariſon of the Black Marbles Focus. And the Little Picture of the Sun, that appear'd upon the White Marble as a Speculum, was but very Faint and exceeding ill Defin'd. Secondly, That taking two pieces of Plain and Poliſh'd Surfaces, and caſting on them Succeſſively the Beams of the Same Candle, In ſuch manner, as that the Neighbouring Superficies being Shaded by an Opacous and Perforated Body, the Incident Beams were permitted to paſs but through a Round Hole of about Half an Inch Diameter, the Circle of Light that appear'd on the White Marble was in Compariſon very Bright, but very ill Defin'd; whereas that on the Black Marble was far leſs Luminous, but much more preciſely Defin'd.
6. Thirdly, When you Look upon a piece of Linnen that has Small Holes in it, thoſe Holes appear very Black, and Men are often deceiv'd in taking Holes for Spots of Ink; And Painters to repreſent Holes, make uſe of Black, the Reaſon of which ſeems to be, that the Beams that fall on thoſe Holes, fall into them So Deep, that none of them is Reflected back to the Eye. And in narrow Wells part of the Mouth ſeems Black, becauſe the Incident Beams are Reflected Downwards from one ſide to another, till they can no more Rebound to the Eye.
We may conſider too, that if Differing parts of the ſame piece of Black Velvet be ſtroak'd Oppoſite ways, the piece of Velvet will appear of two Diſtinct kinds of Blackneſs, the one far Darker than the other, of which Diſparity the Reaſon Seems to be, that in the Leſs obſcure part of the Velvet, the Little Silken Piles whereof 'tis made up, being Inclin'd, there is a Greater part of each of them Obverted to the Eye, whereas in the other part the Piles of Silk being more Erected, there are far Fewer Beams Reflected Outwards from the Lateral parts of each Pile, So that moſt of thoſe that Rebound to the Eye, come from the Tops of the Piles, which make but a ſmall part of the whole Superficies, that may be cover'd by the piece of Velvet. Which Explication I propoſe, not that I think the Blackneſs of the Velvet proceeds from the Cauſe aſſign'd, ſince each Single Pile of Silk is Black by reaſon of its Texture, in what Poſition ſoever you Look upon it; But that the Greater Blackneſs of one of theſe Tuffts ſeems to proceed from the Greater Paucity of Beams Reflected from it, and that from the Fewneſs of thoſe Parts of a Surface that Reflect Beams, and the Multitude of thoſe Shaded Parts that Reflect none. And I remember, that I have oftentimes obſerv'd, that the Poſition of Particular Bodies far greater than Piles of Silk in reference to the Eye, may notwithſtanding their having each of them a Colour of its own, make one part of their Aggregate appear far Darker than the other; For I have near Great Towns often taken notice, that a Cart-load of Carrots pack'd up, appear'd of a much Darker Colour when Look'd upon, where the Points of the Carrots were Obverted to the Eye, than where the Sides of them were ſo.
7. Fourthly, In a Darkned Room, I purpoſely obſerv'd, that if the Sun-beams, which came in at the Hole were receiv'd upon White or any other Colour, and directed to a Convenient place of the Room, they would Manifeſtly, though not all Equally, Encreaſe the Light of that Part; whereas if we Subſtituted, either a piece of Black Cloth or Black Velvet, it would ſo Dead the Incident Beams, that the place (newly mention'd) whereto I Obverted the Black Body, would be Leſs Enlightned than it was before, when it received its Light but from the Weak and Oblique Reflections of the Floor and Walls of a pretty Large Room, through which the Beams that came in at the Hole were Confuſedly and Brokenly Diſpers'd.
8. Fifthly, And to ſhew that the Beams that fall on Black Bodies, as they do not Rebound Outwards to the Eye, ſo they are Reflected towards the Body it ſelf, as the Nature of thoſe Erected Particles to which we have imputed Blackneſs, requires, we will add an Experiment that will alſo confirm our Doctrine touching Whiteneſs; Namely, that we took a Broad and Large Tile, and having Whitened over one half of the Superficies of it, and Black'd the other, we expos'd it to the Summer Sun; And having let it lye there a convenient time (for the Difference is more Apparent, if it have not lain there too long) we found, as we expected, that whilſt the Whited part of the Tile remained Cool enough, the Black'd part of the ſame Tile was grown not only Senſible, but very Hot, (ſometimes to a ſtrong Degree.) And to ſatisfie ſome of our Friends the more, we have ſometimes left upon the Surface of the Tile, beſides the White and Black parts thereof, a part that Retain'd the native Red of the Tile it ſelf, and Expoſing them to the Sun, we obſerv'd this Laſt mention'd to have Contracted a Heat in compariſon of the White, but a Heat Inferiour to that of the Black, of which the Reaſon ſeems to be, that the Superficial Particles of Black Bodies, being, as we ſaid, more Erected, than thoſe of White or Red ones, the Corpuſcles of Light falling on their ſides, being for the moſt part Reflected Inwards from one Particle to another, and thereby engag'd as it were and kept from Rebounding Upwards, they communicate their brisk Motion, wherewith they were impell'd againſt the Black Body, (upon whoſe account had they fallen upon a White Body, they would have been Reflected Outwards) to the Small parts of the Black Body, and thereby Produce in thoſe Small parts ſuch an Agitation, as (when we feel it) we are wont to call Heat. I have been lately inform'd, that an Obſervation near of Kin to Ours, has been made by ſome Learned Men in France and Italy, by long Expoſing to a very Hot Sun, two pieces of Marble, the one White, the other Black; But though the Obſervation be worthy of them, and may confirm the ſame Truth with Our Experiment, yet beſides that our Tryal needs not the Summer, nor any Great Heat to ſucceed, It ſeems to have this Advantage above the other, that whereas Bodies more Solid, and of a Cloſer Texture, though they uſe to be more Slowly Heated, are wont to receive a Greater Degree of Heat from the Sun or Fire, than (Cæteris paribus) Bodies of a Slightest Texture; I have found by the Information of Stone-cutters, and by other ways of Enquiry, that Black Marble is much Solider and Harder than White, ſo that poſſibly the Difference betwixt the Degrees of Heat they receive from the Sunbeams will by many be aſcrib'd to the Difference of their Texture, rather than to that of their Colour, though I think our Experiment will make it Probable enough that the greater part of that Difference may well be aſcrib'd to that Diſpoſition of Parts, which makes the one Reflect the Sunbeams Inward; and the other Outwards. And with this Doctrine accords very well, that Rooms hung with Black, are not only Darker than elſe they would be, but are wont to be Warmer too; Inſomuch that I have known a great Lady, whoſe Conſtitution was ſomewhat Tender, complain that ſhe was wont to catch Cold, when ſhe went out into the Air, after having made any long Viſits to Perſons, whoſe Rooms were hung with Black. And this is not the only Lady I have heard complain of the Warmth of ſuch Rooms, which though perhaps it may be partly imputed to the Effluvia of thoſe Materials wherewith the hangings were Dy'd, yet probably the Warmth of ſuch Rooms depends chiefly upon the ſame Cauſe that the Darkneſs does; As (not to repeat what I formerly Noted touching my Gloves,) to ſatisfie ſome Curious Perſons of that Sex, I have convinc'd them, by Tryall, that of two Pieces of Silken Stuff given me by themſelves, and expos'd in their Preſence, to the ſame Window, Shin'd on by that Sun, the White was conſiderably Heated, when the Black was not ſo much as Senſibly ſo.
9. Sixthly, I remember, that Acquainting one Day a Virtuoſo of Unſuſpected Credit, that had Viſited Hot Countries, with part of what I have here Deliver'd concerning Blackneſs, he Related to me by way of Confirmation of it, a very notable Experiment, which he had both others make, and Made himſelf in a Warm Climate, namely, that having carefully Black'd over Eggs, and Expos'd them to the Hot Sun, they were thereby in no very Long time well Roaſted, to which Effect I conceive the Heat of the Climate muſt have Concurr'd with the Diſpoſition of the Black Surface to Reflect the Sunbeams Inward, for I remember, that having made that among other Tryals in England, though in Summer-time, the Eggs I Expos'd, acquir'd indeed a conſiderable Degree of Heat, but yet not ſo Intenſe a One, as prov'd Sufficient to Roaſt them.
10. Seventhly, and Laſtly, Our Conjectures at the Nature of Blackneſs may be ſomewhat Confirm'd by the (formerly mention'd) Obſervation of the Blind Dutch-man, that Diſcerns Colours with his Fingers; for he Says, that he Feels a greater Roughneſs upon the Surfaces of Black Bodies, than upon thoſe of Red, or Yellow, or Green. And I remember, that the Diligent Bartholinus ſays,9 that a Blind Earl of Mansfield could Diſtinguiſh White from Black only by the Touch, which would Sufficiently Argue a great Diſparity in the Aſperities, or other Superficial Textures of Bodies of thoſe two Colours, if the Learn'd Relator had Affirm'd the Matter upon his own Knowledge.
II. Theſe, Pyrophilus, are the chief things that Occurr to me at preſent, about the Nature of Whiteneſs and Blackneſs, which it they have Rendred it ſo much as Probable, that in Moſt; or at leaſt Many Caſes, the Cauſes of theſe Qualities may be ſuch as I have Adventur'd to Deliver, it is as much as I Pretend to; for till I have Opportunity to Examine the Matter by ſome further Tryals, I am not ſure, but that in ſome White and Black Bodies, there may Concurr to the Colour ſome peculiar Texture or Diſpoſition of the Body, whereby the Motion of the Small Corpuſcles that make up the Incident Beams of Light, may be Differingly Modify'd, before they reach the Eye, eſpecially in this, that White Bodies do not only Copiouſly Reflect thoſe Incident Corpuſcles Outwards, but Reflect them Briskly, and do not otherwiſe Alter them in the manner of their Motion. Nor ſhall I now ſtay to Enquire, whether ſome of thoſe other ways, (as a Diſpoſition to Alter the Velocity, the Rotation, or the Order and Manner of Appulſe ſo the Eye of the Reflected Corpuſcles that Compos'd the Incident Beams of Light) which we mention'd when we conſider'd the Production of Colours in General, may not in ſome Caſes be Applicable to thoſe of White and Black Bodies: For I am yet ſo much a Seeker in this Matter, and ſo little Wedded to the Opinions I have propos'd, that what I am to add ſhall be but the Beginning of a Collection of Experiments and Obſervation towards the Hiſtory of Whiteneſs and Blackneſs, without at preſent interpoſing my Explications of them, that ſo, I may aſſiſt your Enquires without much Fore-ſtalling or Biaſſing your Judgment.