PHYSICS.

ores.

And as by feveral metals, and other minerals, we can give various co lours to glafs; fo, on the other hand, from the different colours that miAn eafy method neral ores, or other mineral powders, by being melted with glass, disclose of examining therein, a probable conjecture may be often made of the metal, or known mineral, that a propos'd ore either contains, or is nearest allied to. And this eafy method of examining ores, may, in fome cafes, prove very ferviceable; but great circumfpection is requifite to keep it from proving fallacious, upon account of the variations of colour, producible by the different proportions that may happen, betwixt the ore and the glass; by the richness and poornefs of the ore itself; by the degree of fire; and efpecially by the length of time, during which the matter is kept in fusion. There is another way, different from thofe already mentioned, whereby metals may be brought to exhibit adventitious colours. And, in this cafe, the metal does not fo much impart a colour to another body, as receive one from it; or rather, both bodies, by the new texture resulting from their mixture, produce a new colour. I will not here infift upon the examples afforded us by yellow orpiment, and common fea-falt; from which, fublimed together, chymifts unanimously affirm their white or cry-ftalline arfenic to be made: but 'tis worth noting, that tho' yellow orpiment be acknowledged by far the most predominant of the two ingredients of arfenic; yet arfenic, duly added to the highest coloured metal, copper, when in fufion, gives it a whitenefs both within and without.. Thus, alfo, Lapis Calaminaris changes and improves the colour of copper, by turning it into brafs. And I have sometimes, by the help of zink, duly. mixed after a certain manner, given to copper as rich a golden colour as ever I faw in the best true gold..

The way of making counterfeit gems.

Upon knowing the different methods of producing the adventitious colours of metals and minerals in bodies capable of vitrification, depends. the pretty art of making counterfeit gems: for whilft pure fand, or cal-cined crystal, gives the body in their preparation; 'tis, for the moft part, fome metalline or mineral calx, mixed in a fmall proportion with it, that gives the colour. Calcined lead, fufed with fine white fand, or cry-ftal, reduced by ignitions and fubfequent extinctions in water, to a subtile powder, will, of itself, be brought, by a due decoction, to give a clear mafs, coloured like a German amethyft. But this colour may be eafily over-powered by thofe of several other mineral pigments, fo that with a glafs of lead you may emulate the fresh and lovely green of an eme-rald; tho', in many cafes, the colour which the lead itself, upon vitrification, tends to, may vitiate that of the pigment defign'd to appear in the mafs. Thefe colours alfo depend fo much upon the texture of the materials, that we have made the glafs of lead itself, compofed of about three parts of litharge, or Minium, fufed with one of cryftal, or fand, very finely powdered, pafs thro' different colours, according as we kept it more or lefs in fufion. But the degrees of coction, and other circumstances, may so vary the colour produced, that, in a small crucible, I have had frag

ments of the fame mafs; in fome of which, perhaps, not fo big as a ha- PHYSICS. zel-nut, four diftinct colours might be difcerned.

Befides the three mentioned forts of adventitious colours in metals, there may be others reducible under the fame head; of which I fhall in

stance only in two.

The first is afforded us from the practice of fcarlet-dyers. A most famous master in this art affured me, that neither he, nor others, can strike the lovely colour, called the bow-dye, unless their materials be boiled in veffels of a particular metal. Secondly, metals will afford uncommon colours, by imbuing feveral bodies with folutions of them, made in proper menftrua. Thus, tho' copper, plentifully diffolved in Aqua fortis, will communicate to feveral bodies the colour of that folution; yet fome other metals will not; as I have often try'd. Gold, diffolved in Aqua regia, dyes the nails and skin, the hafts of knives, and other things made of ivory, not with a yellow, but a purple colour; which, tho' it manifeft itself but flowly, is very durable, and can scarce ever be washed out. Thus we formerly faid, that the purer cryftals of fine filver made with Aqua fortis, tho' they appear white, will presently dye the skin and nails of a black, not to be washed off like ordinary ink. And many other bodies may, in the fame manner, be dy'd, fome of a black, and others of a blackish colour.

rent from their

Mineral folutions, alfo, may produce colours different from thofe of Mineral foluthe liquors themfelves. I fhall not fetch an example of this from what tions may give happens in the falting of beef, which often appears green, and fometimes colours diffeof a reddish colour, when boil'd; nor fhall Î infift on the practice of giv-own. ing, by falt-peter, and a certain proportion of common falt, a fine rednefs to fuch flesh as would otherwife appear purely white: I rather chufe to fay, that I have feveral times found a folution of the fulphur of vitriol, or even of common fulphur, tho' the liquor appeared clear, immediately to tinge a piece of new coin, or other clean filver, fometimes with a golden, fometimes with a deeper, and more reddish colour; according to the ftrength of the folution, and the quantity of it, that chanced to adhere to the metal. This renders it the lefs furprizing, that the water of the hot fpring at Bath, abounding with diffolved fubftances of a very fulphureous nature, fhould, for a while, tinge fuch pieces of coin yellow, as are for a due time immersed in it. And even vegetable liquors, whether by degeneration, or by altering the texture of the body that imbibes them, may ftain other fubftances with colours very different from their own. 'Tis affirmed, that the green juice of Alcanna dyes the skin and nails of a lafting red.

50. Meeting, in an Italian author, with a way of preparing what he The method of calls a Lacca of vegetables, by which the Italians mean a kind of extract preparing a yellow vegetar fit for painting, like that rich Lacca, in English commonly call'd lac, em- ble lac. ploy'd by painters as a glorious red; and finding the experiment confiderable, but very imperfectly related, we improved upon it in the following

manner:

PHYSICS. manner: tho' the thing, as there delivered, is, by the Italians, esteem'd a great fecret.

Put what quantity of powdered turmeric you please into fair water; adding to every pound of that liquor a large fpoonful of a very strong lixivium of pot-afhes, clarify'd by filtration; let all this fimmer over a foft fire in a clean glazed earthen veffel, till you find, by the immersion of a fheet of white paper, that the liquor is fufficiently impregnated with the golden tincture of the turmeric: then take the decoction off the fire, and filtre or ftrain it; and afterwards, leifurely dropping into it a strong folution of roch-alum, the decoction will, as it were, be curdled; and the tinged part of it either emerge, fubfide, or fwim up and down in little yellow flakes: when, if you pour this mixture into a funnel, lined with cap paper, the liquor that formerly filtred yellow, will now pafs clear, and leave its tinged parts behind in the filtre; into which fair water must be fo often poured, till the matter therein contain'd be dulcify'd; that is, till the water paffes thro' it, as taftelefs as when it was poured on. But if, without filtration, you would obtain the flakes of this vegetable lac, pour a large quantity of fair water upon the decoction, after the affufion of the aluminous folution; and you fhall find the liquor to grow clearer, and the lac to fettle together at the bottom, or emerge to the top of the water: tho', fometimes, having not employ'd a fufficient quantity of fair water, the lac has partly fubfided, and partly emerged, leaving all the liquor clear in the middle. But, to make this lac fit for use, it must, by repeated affufions of fresh water, be dulcify'd from the adhering falts, as well as that feparated by the filtre; and be fpread to dry leifurely upon pieces of cloth, with brown paper, chalk, or bricks under them, to imbibe the moisture.

'Tis fuppofed, that the magiftery of vegetables, obtain❜d by this means, confifts only of the more foluble and coloured parts of the respective plant that affords it. But I must take the liberty to queftion the fuppofition: for, according to my notion of falts, alum, tho' to fenfe a homogeneous body, ought not to be reckoned among true falts, but is to be look'd upon as a kind of magiftery; fince, as native vitriol contains both a faline fubftance, and a metal corroded and affociated with it; fo alum, (which is fo near allied to vitriol, that, in fome places of England, the fame ftone will fometimes afford both) feems manifeftly to contain a peculiar kind of acid spirit generated in the bowels of the earth, and a kind of stony matter diffolved by it. In making our ordinary alum, 'tis true, the workmen use the ashes of a fea-weed, vulgarly called kelp, and urine; yet I am inform'd, that here in England, there is, befides the factitious alum, another fort made by nature, without the help of thofe additions. Now, confidering this compofition of alum, and that alkaline falts precipitate what acid ones have diffolved; I could not but fufpect that the curdled matter, call'd the magiftery of vegetables, may have in it a confiderable proportion of a ftony fubftance, precipitated out of the alum by the lixi

vium wherein the vegetable had been boiled. And, to fhew there is no PHYSICS. neceffity thatall the curdled fubftance muft belong to the vegetable; I took a strong solution of alum, and having filtred it; by pouring in a convenient quantity of a strong solution of pot-afhes, I prefently turn'd the mixture into a white curdled matter; which, committed to the filtre, left a great quantity of a very white ftony calx, that seem'd to be of a mineral nature, as well from other figns, as that little bits of it being put upon a live coal, they did neither melt, nor fly away, whilft that was blown upon; and a quantity of this white fubftance, being for a long time kept in a red-hot crucible, was neither diminished nor fpoiled: hot water too, wherein I kept another parcel of fuch calx, feem'd only to wash away the loofer falts from the ftony fubftance. And I have, by gentle exhalation, recovered from the liquor that paffed thro' the filtre, and left this calx behind, a body, in appearance like falt; for 'twas very white, and confifted of innumerable exceeding flender fhining particles, which would, in part, eafily melt at the flame of a candle, and, in part, fly away with fome little noife. I have likewife, with urinous falts, fuch as the spirit of falarmoniac, as well as with the spirit of urine, and even with ftale urine undiftilled, eafily precipitated fuch a white calx out of a limpid folution of alum. Upon the whole, circumfpection is required in judging of the nature of aluminous liquors by precipitation; otherwise, we may fometimes imagine that to be precipitated out of a liquor by alum, which is rather precipitated out of alum by the liquor.

This method of making lacs we have also practifed with madder, which yielded us a red lac; and with rue, which afforded an extract nearly of the fame colour with that of its leaves. But, because 'tis here, principally, the alkaline falt of the pot-afhes, which enables the water fo powerfully to extract the tincture of the vegetables; I fear the decoction will not always be of the very fame colour with the vegetable it is made of. For lixivious falts, tho', by penetrating and opening the bodies of vegetables, they prepare and difpofe them to part with their tincture readily; yet they not only draw out fome tinctures, but likewife alter them: as will eafily appear from feveral of the experiments already delivered. And tho alum be of an acid nature, and may, in fone cafes, deftroy the adventitious colours produced by the alkali, and restore the former; yet we have produced examples, that, in many cafes, an acid will not reftore a vegetable fubftance to the colour destroy'd by an alkaline falt; but make it affume a third, very different from both. Befides, I have, after this manner, made magifteries of brazil, of cochineal, and of other things, red, yellow, or green; that appeared, fome of them, of a rich colour, and others of no bad one: in fome, however, the colour of the lac feem'd rather inferior to that of the plant; and in others again, both very diffe rent, and much worse.

But 'tis time to conclude this sketch of a history of colours.

I fhall

PHYSICS.

I fhall attempt to build no theory* upon the experiments and obfervations here delivered; but leave the fubject to the profecution of others. I only make it my request, that the reader would not presently conclude me mistaken in giving the matters of fact concerning the changes of colours fet down, tho' he fhould not immediately find them exactly agreeable to his own experiments. For, befides the contingencies to which this kind of trials is obnoxious, the omiffion or variation of a feemingly inconfiderable circumftance, may hinder the fuccefs of an experiment,