Experiments and Considerations Touching Colours (1664) by Robert Boyle

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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 suapte Natura nigra constare ex
particulis, quarum Superficieculae scabrae sint, nec facile 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
AEtherial 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 Sphaerical 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 (_Caeteris 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.

[9] Hist. Anatom. Cent. 3. Hist. 44.

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.

* * * * *

EXPERIMENT
IN
CONSORT,
Touching
Whiteness & Blackness.

* * * * *

EXPERIMENT I.

Having promis'd in the 114, and 115. Pages of the foregoing Discourse of
Whiteness and Blackness, to shew, that those two Colours may by a change of
Texture in bodies, each of them apart Diaphanous and Colourless, be at
pleasure and in a trice as well Generated as Destroy'd, We shall begin with
Experiments that may acquit us of that promise.

Take then what Quantity you please of Fair Water, and having Heated it, put
into it as much good Common Sublimate, as it is able to Dissolve, and (to
be sure of having it well glutted:) continue putting in the Sublimate, till
some of it lye Untouch'd in the bottom of the Liquor, Filter this Solution
through Cap-paper, to have it cleer and limpid, and into a spoonfull or two
thereof, (put into a clean glass vessel,) shake about four or five drops
(according as you took more or less of this Solution) of good limpid
Spirits of Urine, and immediately the whole mixture will appear White like
Milk, to which mixture if you presently add a convenient proportion of
Rectifi'd _Aqua Fortis_ (for the number of drops is hard to determine,
because of the Differing Strength of the liquor, but easily found by tryal)
the Whiteness will presently disappear, and the whole mixture become
Transparent, which you may, if you please, again reduce to a good degree of
Whiteness (though inferiour to the first) onely by a more copious affusion
of fresh Spirit of Urine. _N_. First, That it is not so necessary to employ
either _Aqua Fortis_ or Spirit of Urine about this Experiment, but that we
have made it with other liquors instead of these, of which perhaps more
elsewhere. Secondly, That this Experiment, though not made with the same
_Menstruums_, nor producing the same Colour is yet much of Kin to that
other to be mentioned in this Tract among our other Experiments of Colours,
about turning a Solution of Praecipitate into an Orange-colour, and the
Chymical Reason being much alike in both, the annexing it to one of them
may suffice FOR both.

_EXPERIMENT II._

Make a strong Infusion of broken Galls in Fair Water, and having Filtred it
into a clean Vial, add more of the same liquor to it, till you have made it
somewhat Transparent, and sufficiently diluted the Colour, for the credit
of the Experiment, lest otherwise the Darkness of the liquor might make it
be objected, that 'twas already almost Ink; Into this Infusion shake a
convenient quantity of a Cleer, but very strong Solution of Vitriol, and
you shall immediately see the mixture turn Black almost like Ink, and such
a way of producing Blackness is vulgar enough; but if presently after you
doe upon this mixture drop a small quantity of good oyl of Vitriol, and, by
shaking the Vial disperse it nimbly through the two other liquors, you
shall (if you perform your part well, and have employ'd oyl of Vitriol
Cleer and Strong enough) see the Darkness of the liquor presently begin to
be discuss'd, and grow pretty Cleer and Transparent, losing its Inky
Blackness, which you may again restore to it by the affusion of a small
quantity of a very strong Solution of Salt of Tartar. And though neither of
these Atramentous liquors will seem other than very Pale Ink, if you write
with a clean Pen dipt in them, yet that is common to them with some sorts
of Ink that prove very good when Dry, as I have also found, that when I
made these carefully, what I wrote with either of them, especially with the
Former, would when throughly Dry grow Black enough not to appear bad Ink.
This Experiment of taking away and restoring Blackness from and to the
liquors, we have likewise tryed in Common Ink; but there it succeeds not so
well, and but very slowly, by reason that the Gum wont to be employed in
the making it, does by its Tenacity oppose the operations of the above
mention'd Saline liquors. But to consider Gum no more, what some kind of
Praecipitation may have to do in the producing and destroying of Inks
without it, I have elsewhere given you some occasion and assistance to
enquire; But I must not now stay to do so my self, only I shall take notice
to you, that though it be taken for granted that bodies will not be
Praecipitated by Alcalizat Salts, that have not first been dissolved in some
Acid _Menstruums_, yet I have found upon tryals, which my conjectures lead
me to make on purpose, That divers Vegetables _barely infus'd_, or, _but
slightly decocted in common water_, would, upon the affusion of a Strong
and Cleer _Lixivium_ of Potashes, and much more of some other Praecipitating
liquors that I sometimes employ, afford good store of a Crudled matter,
such as I have had in the Praecipitations of Vegetable substances, by the
intervention of Acid things, and that this matter was easily separable from
the rest of the liquor, being left behind by it in the Filtre; and in
making the first Ink mention'd in this Experiment, I found that I could by
Filtration separate pretty store of a very Black pulverable substance, that
remain'd in the Filtre, and when the Ink was made Cleer again by the Oyl of
Vitriol, the affusion of dissolv'd _Sal Tartari_ seem'd but to Praecipitate,
and thereby to Unite and render Conspicuous the particles of the Black
mixture that had before been dispers'd into very Minute and singly
Invisible particles by the Incisive and resolving power of the highly
Corrosive Oyl of Vitriol.

And to manifest, _Pyrophilus_, that Galls are not so requisite as many
suppose to the making Atramentous Liquors, we have sometimes made the
following Experiment, We took dryed Rose leaves and Decocted them for a
while in Fair Water, into two or three spoonfulls of this Decoction we
shook a few drops of a strong and well filtrated Solution of Vitriol (which
perhaps had it been Green would have done as well) and immediately the
mixture did turn Black, and when into this mixture presently after it was
made, we shook a just Proportion of _Aqua Fortis_, we turn'd it from a
Black Ink to a deep Red one, which by the affusion of a little Spirit of
Urine may be reduc'd immediately to an Opacous and Blackish Colour. And in
regard, _Pyrophilus_, that in the former Experiments, both the Infusion of
Galls, and the Decoction of Roses, and the Solution of Copperis employ'd
about them, are endow'd each of them with its own Colour, there may be a
more noble Experiment of the sudden production of Blackness made by the
way mention'd in the Second Section of the Second Part of our Essays, for
though upon the Confusion of the two Liquors there mention'd, there do
immediately emerge a very Black mixture, yet both the Infusion of
_Orpiment_ and the Solution of _Minium_ were before their being joyn'd
together, Limpid and Colourless.

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