Experiments and considerations touching colours first occasionally written, among some other essays to a friend, and now suffer'd to come abroad as the beginning of an experimental history of colours / by the Honourable Robert Boyle ...

EXPERIMENT XL.

The Experiment I am now to mention to you, Pyrophilus, is that which both you, and all the other Virtuosi that have seen it, have been pleas'd to think very strange; and indeed of all the Experiments of Co∣lours, I have yet met with, it seems to be the fittest to recommend the Doctrine pro∣pos'd in this Treatise, and to shew that we need not suppose, that all Colours must necessarily be Inherent Qualities, flowing from the Substantial Forms of the Bodies they are said to belong to, since by a bare Mechanical change of Texture in the Mi∣nute parts of Bodies, two Colours may in

a moment be Generated quite De novo, and utterly Destroy'd. For there is this difference betwixt the following Experi∣ment, and most of the others deliver'd in these Papers, that in this, the Colour that a Body already had, is not chang'd into ano∣ther, but betwixt two Bodies, each of them apart devoid of Colour, there is in a mo∣ment generated a very deep Colour, and which if it were let alone, would be per∣manent; and yet by a very small Parcel of a third Body, that has no Colour of its own, (lest some may pretend I know not what Antipathy betwixt Colours) this otherwise permanent Colour will be in another trice so quite Destroy'd, that there will remain no foot-stepts either of it or of any other Colour in the whole Mix∣ture.

The Experiment is very easie, and it is thus perform'd: Take good common Sub∣limate, and fully satiate with it what quan∣tity of Water you please, Filtre the Solu∣tion carefully through clean and close Paper, that it may drop down as Clear and Colourless as Fountain water. Then when you'l shew the Experiment, put of it about a Spoonfull into a small Wine-glass, or any other convenient Vessel made of clear Glass, and droping in three or four

drops of good Oyl of Tartar, per Deli∣quium, well Filtred that it may likewise be without Colour, these two Limpid Li∣quors will in the twinkling of an Eye turn into an Opacous mixture of a deep Orange Colour, which by keeping the Glass conti∣nually shaking in your hand, you must pre∣serve from setling too soon to the Bottom; And when the Spectators have a little be∣held this first Change, then you must pre∣sently drop in about four or five drops of Oyl of Vitriol, and continuing to shake the Glass pretty strongly, that it may the Nimbler diffuse it self, the whole Colour, if you have gone Skilfully to work, will immediately disappear, and all the Liquor in the Glass will be Clear and Colourless as before, without so much as a Sediment at the Bottom. But for the more gracefull Trial of this Experiment, 'twill not be amiss to observe, First, That there should not be taken too much of the Solution of Subli∣mate, nor too much of the Oyl of Tartar drop'd in, to avoid the necessity of putting in so much Oyl of Vitriol as may make an Ebullition, and perhaps run over the Glass. Secondly, That 'tis convenient to keep the Glass always a little shaking, both for the better mixing of the Liquors, and to keep the Yellow Substance from Subsiding, which

else it would in a short time do, though when 'tis subsided it will retain its Colour, and also be capable of being depriv'd of it by the Oyl newly mention'd. Thirdly, That if any Yellow matter stick at the sides of the Glass, 'tis but inclining the Glass, till the clarify'd Liquor can wash alongst it, and the Liquor will presently imbibe it, and deprive it of its Colour.

Many have somewhat wondred, how I came to light upon this Experiment, but the Notions or Conjectures I have about the differing Natures of the several Tribes of Salts, having led me to devise the Expe∣riment, it will not be difficult for me to give you the Chymical Reason, if I may so speak, of the Phaenomenon. Having then observ'd, that Mercury being dissolv'd in some Menstruums, would yield a dark Yel∣low Precipitate, and supposing that, as to this, common Water, and the Salts that stick to the Mercury would be equivalent to those Acid Menstruums, which work upon the Quick-silver, upon the account of their Saline particles, I substituted a So∣lution of Sublimate in fair Water, instead of a Solution of Mercury in Aqua-fortis, or Spirit of Nitre, that simple Solution being both clearer and free from that very offensive smell, which accompanies the So∣lutions

of Mercury made with those other corrosive Liquors; then I consider'd, that That, which makes the Yellow Colour, is indeed but a Precipitate made by the means of the Oyl of Tartar, which we drop in, and which, as Chymists know, does gene∣rally precipitate Metalline Bodies corroded by Acid Salts; so that the Colour in our case results from the Coalition of the Mer∣curial particles with the Saline ones, where∣with they were formerly associated, and with the Alcalizate particles of the Salt of Tartar that swim up and down in the Oyl. Wherefore considering also, that very many of the effects of Lixiviate Liquors, upon the Solutions of other Bodies, may be de∣stroy'd by Acid Menstruums, as I elsewhere more particularly declare, I concluded, that if I chose a very potently Acid Li∣quor, which by its Incisive power might undo the work of the Oyl of Tartar, and disperse again those Particles, which the other had by Precipitation associated, into such minute Corpuseles as were before singly Inconspicuous, they would become Inconspicuous again, and consequently leave the Liquor as Colourless as before the Precipitation was made.

This, as I said, Pyrophilus seems to be the Chymical reason of this Experiment, that

is such a reason, as, supposing the truth of those Chymical Notions I have elsewhere I hope evinc'd, may give such an account of the Phaenomena as Chymical Notions can supply us with; but I both here and else∣where make use of this way of speaking, to intimate that I am sufficiently aware of the difference betwixt a Chymical Expli∣cation of a Phaenomenon, and one that is truly Philosophical or Mechanical; as in our present case, I tell you something, when I tell you that the Yellowness of the Mer∣curial Solution, and the Oyl of Tartar is produc'd by the Precipitation occasion'd by the affusion of the latter of those Li∣quors, and that the destruction of the Co∣lour proceeds from the Dissipation of that Curdl'd matter, whose Texture is destroy'd, and which is dissolv'd into Minute and In∣visible particles by the potently Acid Men∣struum, which is the reason, why there remains no Sediment in the Bottom, be∣cause the infused Oyl takes it up, and re∣solves it into hidden or invisible Parts, as Water does Salt or Sugar. But when I have told you all this, I am far from think∣ing I have told all that such an Inquisitive Person as your self would know, for I presume you would desire as well as I to learn (at least) why the Particles of the

Mercury, of the Tartar, and of the Acid Salts convening together, should make ra∣ther an Orange Colour than a Red, or a Blew, or a Green, for 'tis not enough to say what I related a little before, that divers Mercurial Solutions, though otherwise made, would yield a Yellow precipitate, because the Question will recurr concern∣ing them; and to give it a satisfactory an∣swer, is, I freely acknowledge, more than I dare as yet pretend to.

But to confirm my conjecture about the Chymical reason of our Experiment, I may add, that as I have (viz. pag. 34^th. of this Treatise) elsewhere (on another oc∣casion) told you, with Saline Liquors of another kind and nature than Salt of Tar∣tar, (namely, with Spirit of Urine, and Liquors of kin to that) I can make the Mercury precipitate out of the first simple Solution quite of another Colour than that hitherto mention'd; Nay, if instead of altering the Precipitating Liquor, I alter'd the Texture of the Sublimate in such a way as my Notions about Salt requir'd, I could produce the same Phaenomenon. For having purposely Sublim'd together Equal parts (or thereabout) of Sal-Armoniack and Sublimate, first diligently Mix'd, the ascen∣ding Flowers being dissolv'd in fair Water,

and Filtred, gave a Solution Limpid and Colourless, like that of the other Subli∣mates, and yet an Alcaly drop'd into this Liquor did not turn it Yellow but White. And upon the same Grounds we may with Quick-silver, without the help of common Sublimate, prepare another sort of Flow∣ers dissoluble in Water without Disco∣louring it, with which I could likewise do what I newly mention'd; to which I shall add, (what possibly you'l somewhat won∣der at) That so much does the Colour depend upon the Texture resulting from the Convention of the several sorts of Cor∣puseles, that though in our Experiment, Oyl of Vitriol destroys the Yellow Co∣lour, yet with Quick-silver and fair Wa∣ter, by the help of Oyl of Vitriol alone, we may easily make a kind of Precipitate of a fair and permanent Yellow, as you will e're long (in the forty second Expe∣ment of this third Part) be taught. And I may further add, that I chose Oyl of Vitriol, not so much for any other or pecu∣liar Quality, as for its being, when 'tis well rectify'd, (which 'tis somewhat hazardous to bring it to be) not only devoid of Co∣lour and in Smells, but extremely Strong and Incisive; For though common and un∣dephlegmated Aqua-fortis will not perform

the same thing well, yet that which is made exceeding Strong by being carefully De∣phlegm'd, will do it pretty well, though not so well as Oyl of Vitriol which is so Strong, that even without Rectification it may for a need be made use of. I will not here tell you what I have try'd, that I may be able to deprive at pleasure the Preci∣pitate that one of the Sulphureous Liquors had made, by the copious Affusion of the other: Because I found, though this Ex∣periment is too ticklish to let me give a full account of it in few words, I shall therefore tell you, that it is not only for once, that the other above-mention'd Ex∣periment may be made, the same Nume∣rical parcels of Liquor being still imploy'd in it; for after I have Clarify'd the Orange Colour'd Liquor, by the addition of as lit∣tle of the Oyl of Viriol as will suffice to perform the effect, I can again at pleasure re-produce the Opacous Colour, by the dropping in of fresh Oyl of Tartar, and destroy it again by the Re-affusion of more of the Acid Menstruum; and yet oftner if I please, can I with these two contrariant Liquors recall and disperse the Colour, though by reason of the addition of so much new Liquor, in reference to the Mercurial particles, the Colour will at length appear more dilute and faint.

An improvement of the fortieth Ex∣periment.

And, Pyrophilus, to confirm yet further the Notions that led me to think on the propos'd Experiment, I shall acquaint you with another, which when I had conve∣niency I have sometimes added to it, and which has to the Spectators appear'd little less Odd than the first; And though be∣cause the Liquor, requisite to make the Trial succeed well, must be on purpose pre∣par'd anew a while before, because it will not long retain its fitness for this work, I do but seldome annex this Experiment to the other, yet I shall tell you how I devis'd it, and how I make it. If you boyl Crude Antimony in a strong and clear Lixivium, you shall separate a Substance from it, which some Modern Chymists are pleas'd to call its Sulphur, but how deservedly I shall not here examine, having elsewhere done it in an Opportune place; wherefore I shall now but need to take notice, that when this suppos'd Sulphur (not now to call it ra∣ther a kind of Crocus) is let fall by the Liquor upon its Refrigeration, it often set∣tles in Flakes, or such like parcels of a Yel∣low Substance, (which being by the pre∣cedent

dissolution reduc'd into Minute parts, may peradventure be made to take Fire much more easily than the Grosser Powder of unprepar'd Antimony would have done.) Considering therefore, that common Sulphur boyl'd in a Lixivium may be Precipitated out of it by Rhenish∣wine or White-wine, which are Sowrish Liquors, and have in them, as I elsewhere shew, an Acid Salt; and having found also by Trial, that with other Acid Liquors I could Precipitate out of Lixiviate Sol∣vents some other Mineral concretions a∣bounding with Sulphureons parts, of which sort is crude Antimony, I concluded it to be easie to Precipitate the Antimony dis∣solv'd, as was lately mention'd, with the Acid Oyl of Vitriol; and though common Sulphur yields a White Precipitate, which the Chymists call Lac Sulphuris, yet I sup∣pos'd the Precipitated Antimony would be of a deep Yellow Colour, as well, if made with Oyl of Vitriol, as if made only by Refrigeration and length of Time. From this 'twas easie to deduce this Experiment, that if you put into one Glass some of the freshly Impregnated and Filtrated Solution of Antimony, and into another some of the Orange-Colour'd Mixture, (which I formerly shew'd you how to make with a

Mercurial Solution and Oyl of Tartar) a few drops of Oyl of Vitriol dropp'd into the last mention'd Glass, would, as I told you before, turn the Deep Yellow mixture into a Cleer Liquor; whereas a little of the same Oyl dropp'd out of the same Viol into the other Glass would presently (but not without some ill sent) turn the mode∣rately cleer Solution into a Deep Yellow Substance. But this, as I said, succeeds not well, unless you employ a Lixivium that has but newly dissolv'd Antimony, and has not yet let it fall. But yet in Summer time, if your Lixivium have been duly Impreg∣nated and well Filtred after it is quite cold, it will for some dayes (perhaps much longer than I had occasion to try) retain Antimony enough to exhibit, upon the Affusion of the Corrosive Oyl, as much of a good Yellow Substance as is necessary to satisfie the Beholders of the Possibility of the Experiment.

Reflections upon the X L. Experiment Compared with the X. and XX.

The Knowledge of the Distinction of Salts which we have propos'd, whereby they are discriminated into Acid, Volatile,

or Salsuginous (if I may for Distinction sake so call the Fugitive Salts of Animal Substances) and fix d or Alcalizate, may possibly (by that little part which we have already deliver'd, of what we could say of its Applicableness) appear of so much Use in Natural Philosophy (especially in the Practick part of it) that I doubt not but it will be no Unwelcome Corollary of the Preceding Experiment, if by the help of it I teach you to distinguish, which of those Salts is Predominant in Chymical Liquors, as well as whether any of them be so or not. For though in our Notes upon the X. and XX. Experiments I have shown you a way by means of the Tincture of Lignum Ne∣phriticum, or of Syrrup of Violets, to dis∣cover whether a propounded Salt be Acid or not, yet you can thereby only find in ge∣neral that such and such Salts belong not to the Tribe of Acids, but cannot determine whether they belong to the Tribe of Uri∣nous Salts (under which for distinction sake I comprehend all those Volatile Salts of Animal or other Substances that are con∣trary to Acids) or to that of Alcalies. For as well the one as the other of these Salino-Sulphurous Salts will restore the Caeruleous Colour to the Tincture of Lignum Nephri∣ticum, and turn that of Syrrup of Violets

into Green. Wherefore this XL. Experi∣ment does opportunely supply the defici∣ency of those. For being sollicitous to find out some ready wayes of discriminating the Tribes of Chymical Salts, I found that all those I thought fit to make Tryal of, would, if they were of a Lixiviate Nature, make with Sublimate dissolv'd in Fair Water an Orange Tawny Precipitate; whereas if they were of an Urinous Nature the Pre∣cipitate would be White and Milky. So that having alwayes by me some Syrrup of Violets and some Solution of Sublimate, I can by the help of the first of those Liquors discover in a trice, whether the propounded Salt or Saline Body be of an Acid Nature or no, if it be I need (you know) inquire no further; but if it be not, I can very ea∣sily, and as readily distinguish between the other two kinds of Salts, by the White or Orange-Colour that is immediately pro∣duc'd, by letting fall a few Drops or Grains of the Salt to be examin'd, into a spoonfull of the cleer Solution of Sublimate. For Example, it has been suppos'd by some eminently Learned, That when Sal Armo∣niack being mingled with an Alcaly is forc'd from it by the Fire in close Vessels, the Volatile Salt that will thereby be obtain'd (if the Operation be skilfully perform'd,)

is but a more fine and subtile sort of Sal Armoniack, which, 'tis presum'd, this Ope∣ration do's but more exquisitely purifie, than common Solutions, Filtrations, and Coa∣gulations. But this Opinion may be easily thown to be Erroneous, as by other Argu∣ments, so particularly by the lately deliver'd Method of distinguishing the Tribes of Salts. For the Saline Spirit of Sal Armo∣niack, as it is in many other manifest Qua∣lities very like the Spirit of Urine, so like, that it will in a trice make Syrrup of Vio∣lets of a Lovely Green, turn a Solution of good Verdigrease into an Excellent Azure, and make the Solution of a Sublimate yield a White Precipitate, insomuch that in most (for I say not all of the Experiments) where I Aim onely at producing a sudden change of Colour, I scruple not to use Spi∣rit of Sal Armoniack when it is at hand, in∣stead of Spirit of Urine, as indeed it seems chiefly to consist (besides the flegm that helps to make it fluid) of the Volatile Uri∣nous Salt (yet not excluding that of Soot) that abounds in the Sal Armoniack and is set at liberty from the Sea Salt wherewith it was formerly associated, and clogg'd, by the Operation of the Alcaly, that divides the Ingredients of Sal Armoniack, and retains that Sea Salt with it self. What use may be

made of the like way of exploration in that inquiry which puzzles so many Modern Naturalists, whether the Rich Pigment (which we have often had occasion to men∣tion) belongs to the Vegetable or Animal Kingdome, you may find in another place where I give you some account of what I try'd about Cocheneel. But I think it need∣less to exemplifie here our Method by any other Instances, many such being to be met with in divers parts of this Treatise; but I will rather advertise you, that, by this way of examining Chymical Liquors, you may not onely in most Cases conclude Affirma∣tively, but in some Cases Negatively. As since Spirit of Wine, and as far as I have try'd, those Chymical Oyles which Artists call Essential, did not (when I us'd them as I had us'd the several Families of Salts up∣on that Syrrup) turn Syrrup of Violets. Red or Green, nor the Solution of Subli∣mate White or Yellow, I inferr'd it may thence be probably argued, that either they are destitute of Salt, or have such as belongs not to either of the three Grand families already often mention'd. When I went to examine the Spirit of Oak or of such like Concretes forced over through a Retort, I found by this means amongst others, that (as I elsewhere show) those Chymists are

much mistaken in it, that account it a simple Liquor, and one of their Hypostatical Prin∣ciples: for not to mention what flegm it may have, I found that with a few drops of one of this sort of Spirits mix'd with a good proportion of Syrrup of Violets, I could change the Colour and make it Purplish, by the affinity of which Colour to Redness, I conjectur'd that this Spirit had some Acid Corpuseles in it, and accordingly I found that as it would destroy the Blewness of a Tincture of Lignum Nephriticum, so being put upon Corals it would Corrode them, as common Spirit of Vinegar, and other Acid Liquors are wont to do. And farther to examine whether there were not a great part of the Liquor that was not of an Acid nature, having separated the Sour or Vine∣gar-like part from the rest, which (if I mi∣stake not) is far the more Copious, we con∣cluded as we had conjectured, the other or remaining part, though it had a strong taste as well as smell, to be of a nature differing from that of either of the three sorts of Salts above mention'd, since it did as little as Spi∣rit of Wine, and Chymical Oyls, alter the Colour either of Syrrup of Violets or So∣lution of Sublimate, whence we also in∣ferr'd that the change that had been made of that Syrrup into a Purple Colour, was

effected by the Vinegar, that was one of the two Ingredients of the Liquor, which was wont to pass for a Simple or Uncompoun∣ded Spirit. And, upon this account, 'twas of the Spirit of Oak (and the like Concretes) freed from it's Vinegar that I elsewhere told you, that I had not then observ'd it, (and I have repeated the Tryal but very lately) to destroy the Caeruleous Tincture of Lignum Nephriticum. But this onely, en passant; for the Chief thing I had to add was this, That by the same way may be ex∣amin'd and discover'd, divers changes that are produc'd in Bodies either by Nature only, or by Art; either of them being able by changing the Texture of some Concretes I could name, to qualifie them to Operate after a New manner upon the above men∣tion'd Syrrup, or Solution, or both. And by this means, to tell you that upon the by, I have been able to discover, that there may be made Bodies, which though they run per Deliquium, as readily as Salt of Tartar, be∣long in other respects, not to the family of Alcaliz, much less to that of Salsuginous, or that of Acid Salts. Perhaps too, I may know a way of making a highly operative Saline Body that shall neither change the Colour of Syrrup of Violets, nor Precipi∣tate the Solution of Sublimate; And, I can

likewise if I please conceal by what Liquors I perform such changes of Colour, as I have been mentioning to you, by quite al∣tering the Texture of some ordinary Chy∣mical productions, the Exploration of which is the main use of the fortieth Expe∣riment, which I think teaches not a little, if it teach us to discover the nature of those things (in reference to Salt) that are ob∣tain'd by the ordinary Chymical Analysis of mix'd Bodyes, though perhaps there may be other Bodyes prepar'd by Chymistry which may have the same Effects in the change of Colours; and yet be produc'd not from what Chymists call the Resolu∣tion of Bodies, but from their Composi∣tion. But the discoursing of things of this nature is more proper for another place. I shall now onely add, what might perhaps have been more seasonably told you before; That the Reason why the way of Explora∣tion of Salts hitherto deliver'd, succeeds in the Solution of Sublimate, depends upon the particular Texture of that Solution, as well as upon the differing Natures of the Saline Liquors imploy'd to Precipitate it. For Gold dissolv'd in Aqua Regia, whether you Precipitate it with Oyl of Tartar which is an Alcaly, or with Spirit of Urne, or Sal Armoniack which belongs to the family of

Volatile Salts, will either way afford a Yellow substance: though with such an Acid Liquor, as, I say not Spirit of Salt, the Bo∣dy that yields it, being upon the matter an Ingredient of Aqua Regis, but Oyl of Vitriol it self, I did not find that I could Precipitate the Metall out of the Solution; or destroy the Colour of it, though the same Oyl of Vitriol would readily Precipitate Silver dissolv'd in Aqua-fortis. And if you dissolve pure Silver in Aqua-fortis, and suffer it to shoot into Crystals, the cleer Solution of these made in fair Water, will afford a very White Precipitate, whether it be made with an Alcaly, or an Acid Spi∣rit, as that of Salt, whereas, which may seem somewhat strange, with Spirit of Sal Ar∣moniack (that I us'd was made of Quick-lime) I could obtain no such White Pre∣cipitate; that Volatile Spirit, nor (as I remember) that of Urine, scarce doing a∣ny more than striking down a very small quantity of Matter, which was neither White nor Whitish, so that the remain∣ing Liquor being suffer'd to evaporate till the superfluous Moisture was gone, the greatest part of the Metalline Corpuscles with the Saline ones that had imbib'd them, concoagulated into Salt, as is usual in such Solutions, wherein the Metall has not been Precipitated.