Experiments and Considerations Touching Colours (1664) by Robert Boyle

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[6] _Album quippe & agrum, hoc quidem asperum esse dicit, hoc vero laeve.
de Sensu & Sensib. 3. 3._

[7] Epist. 2. pag. 45.

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
_Superficieculae_, that Look some one way, and some another, enough of them
Obverted to his Eye, to afford like a broken Looking-glass, a confused
Idaea, 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 (quae
12 Italica sunt) consiciunt. Nocte vero sub splendissima luna, duplatum
iter consumunt aut triplatum. Neque id incommode fit, cum nivium
reverberatione lunaris splendoris sublimes & declives campos illustret, ac
etiam montium praecipitia ac noxias feras a lorge 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.

[8] Gent. Septen. Histor. lib. 4 cap. 13.

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 _Phaenomenon_, 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.

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