Experiments and Considerations Touching Colours (1664) by Robert Boyle

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Having made, _Pyrophilus_, divers Tryals upon this Nephritick Wood, we
found mention made of it by the Industrious Jesuit _Kircherus_, who having
received a Cup Turned of it from the _Mexican_ Procurator of his Society,
has probably receiv'd also from him the Information he gives us concerning
that _Exotick_ Plant, and therefore partly for that Reason, and partly
because what he Writes concerning it, does not perfectly agree with what we
have deliver'd, we shall not Scruple to acquaint you in his own Words, with
as much of what he writes concerning our Wood, as is requisite to our
present purpose. _Hoc loco_ (says he)[17] _neutiquam omittendum duximus
quoddam ligni candidi Mexicani genus, quod Indigenae Coalle & Tlapazatli
vocant, quod etsi experientia hucusque non nisi Caeruleo aquam colore
tingere docuerit, nos tamen continua experientia invenimus id aquam in omne
Colorum genus transformare, quod merito cuipiam Paradoxum videri posset;
Ligni frutex grandis, ut aiunt, non raro in molem arboris excrescit,
truncus illius eft crassus, enodis, instar piri arboris, folia ciceris
foliis, aut rutae haud absimilia, flores exigui, oblongi, lutei & spicatim
digesti; est frigida & humida planta, licet parum recedat a medio
temperamento. Hujus itaque descriptae arboris lignum in poculum efformatum,
aquam eidem infusam primo in aquam intense Caeruleam, colore floris
Buglossae; tingit, & quo diutius in eo steterit, tanto intensiorem colorem
acquirit. Hanc igitur aquam si Vitreae Sphaerae infuderis, lucique exposueris,
ne ullum quidem Caerulei coloris vestigium apparebit, sed instar aquae purae
putae fontanae limpidam claramque aspicientibus se praebebit. Porro si hanc
phialam vitream versus locum magis umbrosum direxeris, totus humor
gratissimum virorem referet; si adhuc umbrosioribus locis, subrubrum, & sic
pro rerum objectarum conditione, mirum dictu, colorem mutabit; in tenebris
vero vel in vase opaco posita, Caeruleum colorem suum resumet._

[17] Kircher. Art. Mag. lucis & umbrae, _lib. 1. part. 3._

In this passage we may take notice of the following Particulars. And first,
he calls it a White _Mexican_ Wood, whereas (not to mention that
_Mornardes_ informs us that it is brought out of _Nova Hispania_) the Wood
that we have met with in several places, and employ'd as _Lignum
Nephriticum_, was not White, but for the most part of a much Darker Colour,
not unlike that of the Sadder Colour'd Wood of Juniper. 'Tis true, that
_Monardes_ himself also says, that the Wood is White; and it is affirm'd,
that the Wood which is of a Sadder Colour is Adulterated by being Imbu'd
with the Tincture of a Vegetable, in whose Decoction it is steep'd. But
having purposely enquir'd of the Eminentest of our _English_ Druggists, he
peremptorily deny'd it. And indeed, having consider'd some of the fairest
Round pieces of this Wood that I could meet with in these Parts, I had
Opportunity to take notice that in one or two of them it was the External
part of the Wood that was White, and the more Inward part that was of the
other Colour, the contrary of which would probably have appear'd, if the
Wood had been Adulterated after the afore-mention'd manner. And I have at
present by me a piece of such Wood, which for about an Inch next the Bark
is White, and then as it were abruptly passes to the above-mention'd
Colour, and yet this Wood by the Tincture, it afforded us in Water, appears
to have its Colour'd part Genuine enough; for as for the White part, it
appears upon tryal of both at once, much less enrich'd with the tingent
Property.

Next, whereas our Author tells us, that the Infusion of this Wood expos'd
in a Vial to the Light, looks like Spring-water, in which he afterwards
adds, that there is no Tincture to be seen in it, our Observation and his
agree not, for the Liquor, which opposed to the Darker part of a Room
exhibits a Sky-colour, did constantly, when held against the Light, appear
Yellowish or Reddish, according as its Tincture was more Dilute or Deep;
and then, whereas it has been already said, that the Caeruleous Colour was
by Acid Salts abolished, this Yellowish one surviv'd without any
considerable Alteration, so that unless our Author's Words be taken in a
very Limited Sense, we must conclude, that either his Memory mis-inform'd
him, or that his White _Nephritick_ Wood, and the Sadder Colour'd one which
we employ'd, were not altogether of the same Nature: What he mentions of
the Cup made of _Lignum Nephriticum_, we have not had Opportunity to try,
not having been able to procure pieces of that Wood great enough, and
otherwise fit to be turned into Cups; but as for what he says in the Title
of his Experiment, that this Wood tinges the Water with all Sorts of
Colours, that is much more than any of those pieces of Nephritick Wood that
we have hitherto employ'd, was able to make good; The change of Colours
discernable in a Vial full of Water, Impregnated by any of them, as it is
directed towards a place more Lightsome or Obscure, being far from
affording a Variety answerable to so promising a Title. And as for what he
tells us, that in the Dark the Infusion of our Wood will resume a
Caeruleous Colour, I wish he had Inform'd us how he Try'd it.

But this brings into my mind, that having sometimes for Curiosity sake,
brought a round Vial with a long Neck fill'd with the Tincture of _Lignum
Nephriticum_ into the Darken'd Room already often mention'd, and holding it
sometimes in, sometimes near the Sun-beams that enter'd at the hole, and
sometimes partly in them, and partly out of them, the Glass being held in
several postures, and look'd upon from several Neighbouring parts of the
Room, disclos'd a much greater Variety of Colours than in ordinary
inlightn'd Rooms it is wont to do; exhibiting, besides the usual Colours, a
Red in some parts, and a Green in others, besides Intermediate Colours
produc'd by the differing Degrees, and odd mixtures of Light and Shade.

By all this You may see, _Pyrophilus_, the reasonableness of what we
elsewhere had occasion to mention, when we have divers times told you, that
it is usefull to have New Experiments try'd over again, though they were,
at first, made by Knowing and Candid Men, such Reiterations of Experiments
commonly exhibiting some New Phaenomena, detecting some Mistake or hinting
some Truth, in reference to them, that was not formerly taken notice of.
And some of our friends have been pleas'd to think, that we have made no
unusefull addition to this Experiment, by shewing a way, how in a moment
our Liquor may be depriv'd of its Blewness, and restor'd to it again by the
affusion of a very few drops of Liquors, which have neither of them any
Colour at all of their own. And that which deserves some particular wonder,
is, that the Caeruleous Tincture of our Wood is subject by the former
Method to be Destroy'd or Restor'd, the Yellowish or Reddish Tincture
continuing what it was. And that you may see, that Salts are of a
considerable use in the striking of Colours, let me add to the many
Experiments which may be afforded us to this purpose by the Dyers Trade,
this Observation; That as far as we have hitherto try'd, those Liquors in
general that are strong of Acid Salts have the Power of Destroying the
Blewness of the Infusion of our Wood, and those Liquors indiscriminatly
that abound with Sulphureous Salts, (under which I comprehend the Urinous
and Volatile Salts of Animal Substances, and the Alcalisate or fixed Salts
that are made by Incineration) have the vertue of Restoring it.

_A Corollary of the Tenth Experiment._

That this Experiment, _Pyrophilus_, may be as well Usefull as Delightfull
to You, I must mind You, _Pyrophilus_, that in the newly mention'd
Observation, I have hinted to You a New and Easie way of Discovering in
many Liquors (for I dare not say in all) whether it be an Acid or
Sulphureous Salt, that is Predominant; and that such a Discovery is
oftentimes of great Difficulty, and may frequently be of great Use, he that
is not a Stranger to the various Properties and Effects of Salts, and of
how great moment it is to be able to distinguish their Tribes, may readily
conceive. But to proceed to the way of trying other Liquors by an Infusion
of our Wood, take it briefly thus. Suppose I have a mind to try whether I
conjecture aright, when I imagine that Allom, though it be plainly a Mixt
Body, does abound rather with Acid than Sulphureous Salt. To satisfie my
self herein, I turn my back to the Light, and holding a small Vial full of
the Tincture of _Lignum Nephriticum_, which look'd upon in that Position,
appears Caeruleous, I drop into it a little of a strong Solution of Allom
made in Fair Water, and finding upon the Affusion and shaking of this New
liquor, that the Blewness formerly conspicuous in our Tincture does
presently vanish, I am thereby incited to suppose, that the Salt
Praedominant in Allom belongs to the Family of Sour Salts; but if on the
other side I have a mind to examine whether or no I rightly conceive that
Salt of Urine, or of Harts-horn is rather of a Saline Sulphureous (if I may
so speak) than of an Acid Nature, I drop a little of the Saline Spirit of
either into the Nephritick Tincture, and finding that the Caeruleous Colour
is rather thereby Deepned than Destroy'd, I collect that the Salts, which
constitute these Spirits, are rather Sulphureous than Acid. And to satisfie
my self yet farther in this particular, I take a small Vial of fresh
Tincture, and placing both it and my self in reference to the Light as
formerly, I drop into the Infusion just as much Distill'd Vinegar, or other
Acid liquor as will serve to Deprive it of its Blewness (which a few drops,
if the Sour Liquor be strong, and the Vial small will suffice to do) then
without changing my Posture, I drop and shake into the same Vial a small
proportion of Spirit of Hartshorn or Urine, and finding that upon this
affusion, the Tincture immediately recovers its Caeruleous Colour, I am
thereby confirm'd firm'd in my former Opinion, of the Sulphureous Nature of
these Salts. And so, whereas it is much doubted by Some Modern Chymists to
what sort of Salt, that which is Praedominant in Quick-lime belongs, we have
been perswaded to referr it rather to Lixiviate than Acid Salts, by having
observ'd, that though an Evaporated Infusion of it will scarce yield such a
Salt, as Ashes and other Alcalizate Bodyes are wont to do, yet if we
deprive our Nephritick Tincture of its Blewness by just so much Distill'd
Vinegar as is requisite to make that Colour Vanish, the _Lixivium_ of
Quick-lime will immediately upon its Affusion recall the Banished Colour;
but not so Powerfully as either of the Sulphureous Liquors formerly
mention'd. And therefore I allow my self to guess at the _Strength_ of the
Liquors examin'd by this Experiment, by the _Quantity_ of them which is
sufficient to Destroy or Restore the Caeruleous Colour of our Tincture. But
whether concerning Liquors, wherein neither Acid nor Alcalisate Salts are
Eminently Praedominant, our Tincture will enable us to conjecture any thing
more than that such Salts are not Praedominant in them, I take not upon me
to determine here, but leave to further Tryal; For I find not that Spirit
of Wine, Spirit of Tartar freed from Acidity, or Chymical Oyl of
Turpentine, (although Liquors which must be conceiv'd very Saline, if
Chymists have, which is here no place to Dispute, rightly ascrib'd tasts to
the Saline Principle of Bodyes,) have any Remarkable Power either to
deprive our Tincture of its Caeruleous Colour, or restore it, when upon the
Affusion of Spirit of Vinegar it has disappear'd.

_EXPERIMENT XI._

And here I must not omit, _Pyrophilus_, to inform You, that we can shew You
even in a Mineral Body something that may seem very near of Kin to the
Changeable Quality of the Tincture of _Lignum Nephriticum_, for we have
several flat pieces of Glass, of the thickness of ordinary Panes for
Windows one of which being interposed betwixt the Eye and a clear Light,
appears of a Golden Colour, not much unlike that of the moderate Tincture
of our Wood, but being so look'd upon as that the Beams of light are not so
much Trajected thorough it as Reflected from it to the Eye, that Yellow
seems to degenerate into a pale Blew, somewhat like that of a Turquoise.
And what which may also appear strange, is this, that if in a certain
posture you hold one of these Plates Perpendicular to the Horizon, so that
the Sun-beams shine upon half of it, the other half being Shaded, You may
see that the part Shin'd upon will be of a much Diluter Yellow than the
Shaded part which will appear much more Richly Colour'd; and if You alter
the Posture of the Glass, so that it be not held Perpendicular, but
Parallel in reference to the Horizon, You may see, (which perhaps you will
admire) the Shaded part look of a Golden Colour, but the other that the Sun
shines freely on, will appear considerably Blew, and as you remove any part
of the Glass thus held Horizontally into the Sun-beams or Shade, it will in
the twinkling of an Eye seem to pass from one of the above mention'd
Colours to the other, the Sun-beams Trajected through it upon a sheet of
White Paper held near it, do colour it with a Yellow, somewhat bordering
upon a Red, but yet the Glass may be so oppos'd to the Sun, that it may
upon Paper project a mix'd Colour here and there more inclin'd to Yellow,
and here and there more to Blew. The other Phaenomena of this odd Glass, I
fear it would be scarce worth while to Record, and therefore I shall rather
advertise You, _First_ that in the trying of these Experiments with it, you
must take notice that one of the sides has either alone, or at least
principally its Superficial parts dispos'd to the Reflection of the Blew
Colour above nam'd, and that therefore you must have a care to keep that
side nearest to the Eye. And next, that we have our selves made Glasses not
unfit to exhibit an Experiment not unlike that I have been speaking of, by
laying upon pieces of Glass some very finely foliated Silver, and giving it
by degrees a much stronger Fire than is requisite or usual for the Tinging
of Glasses of other Colours. And this Experiment, not to mention that it
was made without a Furnace in which Artificers that Paint Glass are wont to
be very Curious, is the more considerable, because, that though a Skilfull
Painter could not deny to me that 'twas with Silver he Colour'd his Glasses
Yellow; yet he told me, that when to Burn them (as they speak) he layes on
the plates of Glass nothing but a _Calx_ of Silver Calcin'd without
Corrosive Liquors, and Temper'd with Fair Water, the Plates are Ting'd of a
fine Yellow that looks of a Golden Colour, which part soever of it you turn
to or from the Light; whereas (whether it be what an Artificer would call
Over-doing, or Burning, or else the imploying the Silver Crude that makes
the Difference,) we have found more than once, that some Pieces of Glass
prepar'd as we have related, though held against the Light they appear'd of
a Transparent Yellow, yet look'd on with ones back turn'd to the Light they
exhibited an Untransparent Blew.

_EXPERIMENT XII._

If you will allow me, _Pyrophilus_, for the avoiding of Ambiguity, to
imploy the Word Pigments, to signifie such prepared materials (as
Cochinele, Vermilion, Orpiment,) as Painters, Dyers and other Artificers
make use of to impart or imitate particular Colours, I shall be the better
understood in divers passages of the following papers, and particularly
when I tell you, That the mixing of Pigments being no inconsiderable part
of the Painters Art, it may seem an Incroachment in me to meddle with it.
But I think I may easily be excus'd (though I do not altogether pass it by)
if I restrain my self to the making of a Transient mention of some few of
their Practices about this matter; and that only so far forth, as may
warrant me to observe to you, that there are but few Simple and Primary
Colours (if I may so call them) from whose Various Compositions all the
rest do as it were Result. For though Painters can imitate the Hues (though
not always the Splendor) of those almost Numberless differing Colours that
are to be met with in the Works of Nature, and of Art, I have not yet
found, that to exhibit this strange Variety they need imploy any more than
_White_, and _Black_, and _Red_, and _Blew_, and _Yellow_; these _five_,
Variously _Compounded_, and (if I may so speak) _Decompounded_, being
sufficient to exhibit a Variety and Number of Colours, such, as those that
are altogether Strangers to the Painters Pallets, can hardly imagine.

Thus (for Instance) Black and White differingly mix'd, make a Vast company
of Lighter and Darker Grays.

Blew and Yellow make a huge Variety of Greens.

Red and Yellow make Orange Tawny.

Red with a little White makes a Carnation.

Red with an Eye of Blew, makes a Purple; and by these simple Compositions
again Compounded among themselves, the Skilfull Painter can produce what
kind of Colour he pleases, and a great many more than we have yet Names
for. But, as I intimated above, 'tis not my Design to prosecute this
Subject, though I thought it not unfit to take some Notice of it, because
we may hereafter have occasion to make use of what has been now deliver'd,
to illustrate the Generation of Intermediate Colours; concerning which we
must yet subjoyn this Caution, that to make the Rules about the Emergency
of Colours, fit to be Relied upon, the Corpuscles whereof the Pigments
consist must be such as do not Destroy one anothers Texture, for in case
they do, the produced Colour may be very Different from that which would
Result from the Mixture of other harmless Pigments of the same Colours, as
I shall have Occasion to shew ere long.

_EXPERIMENT XIII._

It may also give much light to an Enquirer into the Nature of Colours, to
know that not only in Green, but in many (if not all) other Colours, the
Light of the Sun passing through Diaphanous Bodies of differing Hues may be
tinged of the same Compound Colour, as if it came from some Painters
Colours of the same Denomination, though this later be exhibited by
Reflection, and be (as the former Experiment declares) manifestly
Compounded of material Pigments. Wherefore to try the Composition of
Colours by Trajection, we provided several Plates of Tinged Glass, which
being laid two at a time one on the top of another, the Object look'd upon
through them both, appear'd of a Compounded Colour, which agrees well with
what we have observ'd in the second Experiment, of Looking against the
Light through differingly Colour'd Papers. But we thought the Experiment
would be more Satisfactory, if we procur'd the Sun-beams to be so Ting'd in
their passage through Plates of Glass, as to exhibit the Compounded Colour
upon a Sheet of White Paper. And though by reason of the Thickness of the
Glasses, the Effect was but Faint, even when the Sun was High and Shin'd
forth clear, yet, we easily remedied that by Contracting the Beams we cast
on them by means of a Convex Burning-glass, which where it made the Beams
much converge Increas'd the Light enough to make the Compounded Colour very
manifest upon the Paper. By this means we observ'd, that the Beams
trajected through Blew and Yellow compos'd a Green, that an intense and
moderate Red did with Yellow make differing degrees of Saffron, and Orange
Tawny Colours, that Green and Blew made a Colour partaking of both, such as
that which some Latin Writers call _Pavonaceus_, that Red and Blew made a
Purple, to which we might add other Colours, that we produc'd by the
Combinations of Glasses differingly Ting'd, but that I want proper Words to
express them in our Language, and had not when we made the Tryals, the
Opportunity of consulting with a Painter, who perchance might have Suppli'd
me with some of the terms I wanted.

I know not whether it will be requisite to subjoyn on this Occasion, what I
tried concerning Reflections from Colour'd Glasses, and other Transparent
Bodies, namely, that having expos'd four or five sorts of them to the Sun,
and cast the Reflected Beams upon White Paper held near at hand, the Light
appear'd not manifestly Ting'd, but as if it had been Reflected from the
Impervious parts of a Colourless Glass, only that Reflected from the Yellow
was here and there stain'd with the same Colour, as if those Beams were not
all Reflected from the Superficial, but some from the Internal parts of the
Glass; upon which Occasion you may take notice, that a Skilfull Tradesman,
who makes such Colour'd Glass told me, that where as the Red Pigment was
but Superficial, the Yellow penetrated to the very midst of the Plate. But
for further Satisfaction, not having the Opportunity to Foliate those
Plates, and so turn them into Looking-glasses, we Foliated a Plate of
_Muscovy_ Glass, and then laying on it a little Transparent Varnish of a
Gold Colour, we expos'd it to the Sun-beams, so as to cast them upon a Body
fit to receive them, on which the Reflected Light, appearing, as we
expected, Yellow, manifested that Rebounding from the Specular part of the
_Selenitis_, it was Ting'd in its return with the Colour of the Transparent
Varnish through which it pass'd.

_EXPERIMENT XIV._

After what we have said of the Composition of Colours, it will now be
seasonable to annex some Experiments that we made in favour of those
Colours, that are taught in the Schools not to be Real, but only Apparent
and Phantastical; For we found by Tryals, that these Colours might be
Compounded, both with True and Stable Colours, and with one another, as
well as unquestionably Genuine and Lasting Colours, and that the Colours
resulting from such Compositions, would respectively deserve the same
Denominations.

For first, having by the Trajection of the Sun-beams through a Glass-prism
thrown an Iris on the Floor, I found that by placing a Blew Glass at a
convenient distance betwixt the Prism and the Iris, that part of the Iris
that was before Yellow, might be made to appear Green, though not of a
Grass Green, but of one more Dilute and Yellowish. And it seems not
improbable, that the narrow Greenish List (if I may so call it) that is
wont to be seen between the Yellow and Blew parts of the Iris, is made by
the Confusion of those two Bordering Colours.

Next, I found, that though the want of a sufficient Liveliness in either of
the Compounding Colours, or a light Error in the manner of making the
following Tryals, was enough to render some of them Unsuccessfull, yet when
all necessary Circumstances were duely observ'd, the Event was answerable
to our Expectation and Desire.

And (as I formerly Noted) that Red and Blew compound a Purple, so I could
produce this last nam'd Colour, by casting at some Distance from the Glass
the Blew part of the Prismatical Iris (as I think it may be call'd for
Distinction sake) upon a Lively Red, (for else the Experiment succeeds not
so well.) And I remember, that sometimes when I try'd this upon a piece of
Red Cloath, _that_ part of the Iris which would have been Blew, (as I try'd
by covering that part of the Cloath with a piece of White Paper) and
Compounded with the Red, wherewith the Cloath was Imbued before, appear'd
of a fair Purple, did, when I came to View it near at hand, look very Odly,
as if there were some strange Reflection or Refraction or both made in the
Hairs of which that Cloath was composed.

Calling likewise the Prismatical Iris upon a very Vivid Blew, I found that
part of it, which would else have been the Yellow, appear Green. (Another
somewhat differing Tryal, and yet fit to confirm this, you will find in the
fifteenth Experiment.)

But it may seem somewhat more strange, that though the Prismatical Iris
being made by the Refraction of Light through a Body that has no Colour at
all, must according to the Doctrine of the Schools consist of as purely
Emphatical Colours, as may be, yet even these may be Compounded with one
another, as well as Real Colours in the Grossest Pigments. For I took at
once two Triangular Glasses, and one of them being kept fixt in the same
Posture, that the Iris it projected on the Floor might not Waver, I cast on
the same Floor another Iris with the other Prism, and Moving it too and fro
to bring what part of the second Iris I pleas'd, to fall upon what part of
the first I thought fit, we did sometimes (for a small Errour suffices to
hinder the Success) obtain by this means a Green Colour in that part of the
more Stable Iris, that before was Yellow, or Blew, and frequently by
casting those Beams that in one of the Iris's made the Blew upon the Red
parts of the other Iris, we were able to produce a lovely Purple, which we
can Destroy or Recompose at pleasure, by Severing and Reapproaching the
Edges of the two Iris's.

_EXPERIMENT XV._

On this occasion, _Pyrophilus_, I shall add, that finding the Glass-prism
to be the usefullest Instrument Men have yet imploy'd about the
Contemplation of Colours, and considering that Prisms hitherto in use are
made of Glass, Transparent and Colourless, I thought it would not be amiss
to try, what change the Superinduction of a Colour, without the Destruction
of the Diaphaneity, would produce in the Colours exhibited by the Prism.
But being unable to procure one to be made of Colour'd Glass, and fearing
also that if it were not carefully made, the Thickness of it would render
it too Opacous, I endeavoured to substitute one made of Clarify'd Rosin, or
of Turpentine brought (as I elsewhere teach) to the consistence of a
Transparent Gum. But though these Endeavours were not wholly lost, yet we
found it so difficult to give these Materials their true Shape, that we
chose rather to Varnish over an ordinary Prism with some of these few
Pigments that are to be had Transparent; as accordingly we did first with
Yellow, and then with Red, or rather Crimson, made with Lake temper'd with
a convenient Oyl, and the Event was, That for want of good Transparent
Colours, (of which you know there are but very few) both the Yellow and the
Red made the Glass so Opacous, (though the Pigment were laid on but upon
two Sides of the Glass, no more being absolutely necessary) that unless I
look'd upon an Inlightned Window, or the Flame of a Candle, or some other
Luminous or very Vivid object, I could scarce discern any Colours at all,
especially when the Glass was cover'd with Red. But when I did look on such
Objects, it appear'd (as I expected) that the Colour of the Pigment had
Vitiated or Drown'd some of those which the Prism would according to its
wont have exhibited, and mingling with others, Alter'd them: as I remember,
that both to my Eyes, and others to whom I show'd it, when the Prism was
cover'd with Yellow, it made those Parts of bright Objects, where the Blew
would else have been Conspicuous, appear of a light Green. But,
_Pyrophilus_, both the Nature of the Colours, and the Degree of
Transparency, or of Darkness in the Pigment, besides divers other
Circumstances, did so vary the _Phaenomena_ of these Tryals, that till I can
procure small Colour'd Prisms, or Hollow ones that may be filled with
Tincted Liquor, or obtain Some better Pigments than those I was reduc'd to
imploy, I shall forbear to Build any thing upon what has been delivered,
and shall make no other use of it, than to invite you to prosecute the
Inquiry further.

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