Experiments and Considerations Touching Colours (1664)

by Robert Boyle

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.