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the light, as to exhibit particular colours. And because thefe fubtile parts PHYSICS. were fo eafily foluble, even in cold water, I concluded, that they must abound with falts, and perhaps contain much of the effential falt of the wood. And, to try whether thefe fubtile parts were volatile enough to be diftilled, without diffolving their texture, I carefully fubmitted fome of the tinged liquor, in very low veffels, to the gentle heat of a lamp-furnace; but found all that came over, to be as limpid and colourlefs as rockwater: whilst the liquor remaining behind, was of fuch a deep blue, that it must be oppofed to a very ftrong light, to appear of any other colour. I took, likewise, a vial, fill'd with fpirit of wine, and a little falt of hart's-horn, and found, that there was a certain proportion to be met with, betwixt that liquor, and the falt, which render'd the mixture fit to exhibit fome little variety of colours, not obfervable in ordinary liquors, as it was variously fituated, with regard to the light and the eye: but this change of colour came very far fhort of that we had admired in our tin&ture. However, I fufpected that the tinging particles abounded with falts, whose texture, and the colour thence arifing, would probably be altered by piercing acids; and pouring into a small vial of the impregnated water a very little fpirit of vinegar, I found, that the blue colour immediately vanished, while the golden one remained: and which way foever I turn'd the vial, either to, or from the light, I found the liquor conftantly appear'd of a yellowish colour, and no other. Upon this, I imagin'd, that the acid falts of the vinegar, having been able to deprive the liquor of its blue colour, a fulphureous falt, which is of a contrary nature, would mortify the faline parts of the vinegar, and deftroy their effects. And, accordingly, having placed my felf betwixt the window and the vial, and let fall into the fame liquor a few drops of oil of tartar, per deliquium: I found, that immediately upon the diffufion of this liquor, the impregnated water was restored to its former blue colour. And this oil of tartar, being very ponderous, and falling directly to the bottom of the vial; it was easy to obferve, that for a little while the lower part of the liquor appeared of a deep blue, whilft all the upper part retained its former yellownefs; which it immediately loft, as foon as the oil of tartar was diffused thro' the whole: and the liquor thus reftored, being view'd, either against, or from the light, exhibited the fame phenomena with the tinged water,' before the adventitious fluids were poured into it.

Kircher fpeaks of this nephritic wood, in the following manner: ""Tis a white Mexican wood, and tho', generally, fuppofed to com"municate only a blue colour to water; yet we have found, by re"peated experiments, that 'twill turn it into all kinds of colours. The tree itself, 'tis faid, commonly grows to a large fize, with a thick even "trunk, like a pear-tree, leaves refembling thofe of rue, and fmall ob"long yellow flowers, growing in clusters. The plant is cold and moist,' "and nearly of a middle temper. The wood of this tree, made into a

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cup, turns the water put into it, first of a perfect blue colour, like the "buglofs-flower; and the longer it ftands herein, the deeper the colour

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grows. But if this colour'd water be put into a glass globe, and exposed to the light, there will not appear the leaft fign of blueness therein; but the liquor will be clear and limpid, like pure water. Yet if "this globe of glass be turn'd to a more fhady place, the whole liquor "will appear of a pleasant green; and if to a place ftill more fhaded, "of a reddish colour. And thus 'twill change its hue, according to the objects whereto 'tis expofed. But, after being placed in the dark, "'twill regain its own blue caft." In this account, I obferve the following particulars: First, the author calls it a white Mexican wood; tho', not to mention that Monardes fays, 'tis brought from New Spain, the wood that we have met with, and employ'd as Lignum Nephriticum, was not white, but, for the most part, of a dark colour, not unlike that of the fadder-colour'd wood of juniper. 'Tis true, Monardes fays, that the wood is white; and it is alfo affirmed, that what is of a fadder colour, is adulterated, by being imbued with the tincture of a vegetable. But having enquired of the most eminent of our English druggifts, he peremptorily deny'd it. And, indeed, having confidered fome of the faireft round pieces of this wood, that I could procure, I took notice, in one or two of them, that 'twas the external part only that appeared white, whilft the more internal part was of the other colour; the contrary whereto would, probably, have appeared, if the wood had been adul terated after the manner just mentioned. And I have, at present, by me, a piece of fuch wood, which, for about an inch next the bark, is white, and then fuddenly paffes to the above-faid colour: and yet this wood, by the tincture it affords in water, feems to have its colour'd part genuine; for the white part appears, upon trial, to be much less endow'd with the tinging property.

Next, our author tells us, that the infufion of this wood expos'd in a fpherical vial to the light, looks like spring-water; and adds, that there is no tincture to be feen in it: but herein our obfervation and his do not agree; for the liquor, which oppos'd to the darker part of a room exhibits a sky-colour, did conftantly with us, when held against the light, appear yellowish or reddifh, according as its tincture was more or lefs deep. So that if there be no mistake in the cafe, his white nephritic wood, and the fadder-colour'd one, which we employ'd, were not of the fame nature. What he mentions of the cup made of this wood, we have not try'd; but as for what he fays, that this wood tinges the water with all forts of colours, that is much more than any of thofe pieces we have hitherto employ'd was able to do: the change of colours difcernible in a vial full of water impregnated by any of them, and directed towards a place more light or obfcure, being far from affording any fuch variety. And as for what he tells us, that in the dark this infufion will refume a blue colour, I wish he had inform'd us how he try'd it. But having fometimes brought a round long-neck'd vial fill'd with the tincture of this wood into the darken'd room already often mention'd, and holding it fometimes near the fun-beams that enter'd at the hole, and fometimes partly in them

and partly out of them; varying alfo the pofition of the glass, and view- PHYSICS. ing it from several parts of the room, it difclos'd a much greater variety of colours, than it does in an ordinary enlighten'd room: for befides the ufual ones, it exhibited a red in fome parts, and a green in others; with intermediate colours produced by the different degrees and odd mixtures of light and fhade.

'Tis furprizing in this experiment, that the blue tincture may be deftroy'd or reftor'd, whilft the yellowish or reddish one continues as it was. That falts are of a confiderable ufe in ftriking colours, the many experiments which the dyers trade affords us, will fhew: and as far as we have hitherto try'd, thofe liquors, in general, which greatly abound in acid falts, have the power of destroying the blue colour in this infufion; as those that abound in fulphureous ones, have the virtue to reftore it. And by the way, this obfervation hints to us a new and eafy method of discovering in many liquors whether it be an acid or a fulphureous falt that is predominant. That fuch a difcovery is often very difficult, and may frequently be of great ufe, he who is no stranger to the various properties and effects of falts, and knows of how great moment it is to be able to diftinguish their tribes, will readily conceive.

Now suppose I would try whether alum, tho' it be plainly a mix'd body, To find whether an acid abounds rather with an acid than a fulphureous falt, I turn my back to or a fulphurethe light, and holding a small vial full of the tincture of Lignum Nephri- ous falt predo-ticum in my hand, which view'd in that position appears blue; then drop- minates in a ping into it a little of a strong folution of alum made in fair water, and liquor. finding upon the affusion and shaking of the whole, that the blueness formerly confpicuous in our tincture prefently vanishes, I am thereby induced to fuppofe, that the falt predominant in alum is acid. But if, on the other hand, I would examine, whether falt of urine, or of hart's-horn, is rather of a falino-fulphureous, than of an acid nature; I drop a little of the faline spirit of either, into the nephritic tincture; and finding that the. blue colour is thereby rather deepened than deftroyed,. I collect, that the falts which conftitute these fpirits, are rather fulphureous than acid. And,, to fatisfy myself yet farther in this particular, I take a fmall vial of fresh tincture, and placing both it and myfelf, with regard to the light, as before; I drop into the infufion just as much diftilled vinegar, or other acid liquor, as will ferve to deprive it of its bluenefs; then, without changing: my posture, I drop, and shake into the fame vial, a fmall proportion of spirit of hart's-horn, or urine: and finding, upon this, the tincture immediately to recover its blue colour, I am thereby confirmed in my opinion of the fulphureous nature of thefe falts.

It is much doubted to what fort of falt that which is predominant in quick-lime belongs; but we have been perfuaded to refer it rather to the lixiviate, than acid kind, by obferving, that tho' an evaporated infufion. of it will scarce yield fuch a falt as afhes, and other alkaline bodies do; yet if we deprive our nephritic tincture of its bluenefs, by juft fo much diftilled vinegar as is requifite to make that colour vanifh; the lixivium

of

PHYSICS. of quick-lime will, immediately, upon its affufion, recal the banished colour, tho' not fo powerfully as either of the fulphureous liquors, formerly mentioned. And I guess at the ftrength of the liquors, thus examined, by the quantity of them; which is fufficient to deftroy, or restore the blue colour of our tincture. But whether as to liquors, wherein neither acid nor alkaline falts are eminently predominant, our tincture will enable us to conjecture any thing more, than that fuch falts do not abound in them, I leave to further trial; for I find not, that fpirit of wine, fpirit of tartar, freed from acidity, or chymical oil of turpentine, have any remarkable power to deprive our tincture of its blue colour, or to restore it, when, upon the affufion of vinegar, it has difappeared.

Different colours obferved in the fame piece of glass.

11. We may also fee in a mineral body, fomething very near of kin to the changeable quality of the tincture of Lignum Nephriticum; for I have feveral flat pieces of glafs of the thickness of ordinary panes for windows, one of which being interpofed 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 viewed, when the beams of light pafs not fo much thro❜ it, as they are reflected from it to the eye, the yellow feems to degenerate into a pale blue, fomewhat like that of a turquoife. And, what is ftrange, if in a certain position, you hold one of these plates perpendicular to the horizon, fo that the fun may fhine upon one half of it, whilft the other half is fhaded, you may fee the part illumin'd, of a much fainter yellow than the fhaded part, which will appear more richly coloured. And if the glass be not held perpendicular, but parallel to the horizon, you may fee the fhaded part of a golden colour, whilft the other appears confiderably blue: and as you remove any part of the glafs, thus held horizontally, into the fun-beams, or the fhade, it will, in the twinkling of an eye, seem to pass from one of those colours to the other; and the rays paffing thro' it, and received by a fheet of white paper held near, colour it with a yellow, fomewhat bordering upon a red: yet the glafs may be fo opposed to the fun, as to throw a mixed colour upon paper, in fome parts more inclined to yellow, and in others to a blue.

In making these experiments with glass, you must take notice, that as one of the fides has its fuperficial parts difpofed to reflect the blue colour, that fide must be held next to the eye. I have myfelf made glaffes proper to exhibit an experiment, not unlike the laft mentioned, by laying fome filver, very finely foliated, upon glafs, and giving it, by degrees, a much stronger fire than is requifite to tinge glafs of other colours. And this experiment, tho' made without a furnace, is the more confiderable; because a skilful painter, who allow'd 'twas with filver he coloured his glaffes yellow, told me, that, when to burn them, he lays on the plates nothing but a calx of filver, calcined without corrofive liquors, and temper'd with fair water, the plates are tinged of a fine yellow, that looks of a golden colour, which part foever be turn'd to, or from the light; whilft we have found, more than once, that fome pieces

of glafs, prepared after our manner, tho', when held against the light, PHYSICS. they appeared of a transparent yellow; yet viewed with one's back turn'd to the light, exhibited an opake blue.

12. There are but few fimple, and primary colours; from the various The fimple and compofitions whereof, all the reft refult: for tho' painters imitate the hues primary colours of those numerous different colours, to be met with in the works of nabut few. ture, and of art; yet I have not found, that to exhibit this ftrange variety, they need employ any more than white, black, red, blue and yellow; these five, variously compounded, and re-compounded, being fufficient to exhibit fuch a variety, as thofe, who are altogether ftrangers to the painter's pallet, can hardly imagine. Thus black and white, differently mixed, make a vaft number of lighter and darker greys; blue and yellow make a great variety of greens; red and yellow make orange-tawny; red, with a little white, makes a carnation; red, with an eye of blue, makes a purple: and, by thefe fimple compofitions again compounded, the skilful painter can produce what kind of colour he pleases; and a great many more than we have names for. But, to render the rules about the production of colours fit to be relied on, the corpufcles, whereof the feveral pigments confift, must be fuch as do not deftroy one another's texture; for, in cafe they do, the emerging colour may be very different from what would result from the mixture of other agreeing pigments of the fame colours.

13. It may also help to discover the nature of colours, to know, that the The fun's light light of the fun, paffing thro' diaphanous bodies of different hues, may be fain'd with the colours of tinged of the fame compound colour, as if it proceeded from painters co- transparent lours of the fame denomination; tho' the latter be exhibited by reflection, bodies, in and manifeftly compounded of material pigments. Wherefore, to try the paffing thro compofition of colours, we provided feveral plates of tinged glafs, which be-them. ing laid, two at a time, upon one another; the object, view'd thro' them both, appeared of a compound colour: which agrees with what we deliver'd of looking against the light thro' paper of different colours. But we thought the experiment would be more fatisfactory, if we procured the fun-beams to be fo tinged in their paffage thro' plates of glafs, as to exhibit the compound colour upon white paper. And tho', by reafon of the thicknefs of the glaffes, the effect was but faint, even when the fun fhone ftrong; yet we eafily remedy'd that, by collecting his rays, with a convex glafs; which increafed the light, at the point where they met, fufficiently to make the compound colour very manifeft upon the paper. By this means we obferved, that the rays paffing thro' blue and yellow, compofed a green; that an intenfe and moderate red, did, with yellow, make different degrees of saffron, and orange-tawny; that green and blue made a colour partaking of both, like what fome Latin writers call Pavonaceus; and that red and blue made a purple. To which we might add other colours that we produced by the combinations of glaffes differently tinged, did I not want proper words to exprefs them in our language. And having expofed four or five forts of coloured glafs, and other tranfparent bodies to the fun, and caft the reflected light upon white paper, held near them; the light VOL. II.

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