PHYSICS.

(4.) I have obferv'd in a darken'd room, that if the fun-beams which came in at the hole were receiv'd upon white, or any other colour, and directed to a convenient part of the room, they would manifeftly increase the light of that part; but if we fubftituted either a piece of black cloth or black velvet, it would fo damp the incident rays, that the faid place would be less illumin'd than before, when it receiv'd its light only from the weak and oblique reflections of the floor and walls of a pretty large room; over which the beams that came in at the hole, were confufedly and in a broken manner difpers'd.

(5.) And to fhew that the rays which fall on black bodies, as they do not rebound outwards to the eye, fo they are reflected towards the body itfelf, as the nature of thofe erect particles to which we have imputed blacknefs requires; we fhall add an experiment, that will at the fame time confirm our doctrine of whitenefs. We took, then, a broad and large tile, and having whited over one half of its fuperficies, and black'd the other, we expos'd it to the fummer fun. And having let it lie there a convenient time, we found that whilft the whited part of the tile remain'd cool, the black'd part of it was grown very hot. And for further fatisfaction, we have fometimes left upon the furface of the tile a part that retain❜d its native red; and expofing all to the fun, we obferv'd the latter to have contracted a heat in comparison of the white part, but inferior to that of the black. 'Tis alfo remarkable, that rooms hung with black are not only darker than they would be otherwife, but warmer too. I have known a great lady, of a tender conftitution, complain that fhe commonly took cold upon going into the air, after having made any long vifit to perfons whofe rooms were hung with black. And this is not the only lady I have heard complain of the warmth of fuch rooms; which, tho' perhaps it may partly be imputed to the effluvia of thofe materials wherewith the hangings were dyed, yet probably the warmth in this cafe depends chiefly upon the fame caufe with darkness; for upon expofing two pieces of filk, the one white, the other black, in the fame window to the fun, I have often found the former confiderably heated, when the latter has remain'd cool.

(6.) A virtuofo of unfufpected credit acquainted me, that in a hot climate he had, by carefully blacking the fhells of eggs, and expofing them to the fun, feen them thereby well roafted, in no long time. But in England, the fun's rays feem not to be fufficiently strong to produce fuch an effect; for having expos'd eggs in the fummer feafon thereto, they acquired indeed a confiderable degree of heat, but not enough to roast them.

(7.) Laftly, our conjectures about the nature of blackness, may be fomewhat confirmed by the obfervation of the blind man, formerly mention'd, who difcerns colours with his fingers; for he fays, that he feels a greater roughness upon the furfaces of black bodies, than upon those of red, yellow, or green. And Bartholine tells us, that a blind earl of Mansfield could diftinguifh white from black only by the touch; which might fufficiently argue a great difference in the afperities, or fuperficial textures

of the bodies of thofe two colours; if the learned relator had affirm'd PHYSICS. the matter upon his own knowledge. Let us next take in the affiftance

of our experiments, purposely made to bring us farther acquainted with the nature of white and black.

1. Take any quantity of fair water, heat it, and add thereto as much The nature of good common fublimate as it will diffolve, or till fome of it lie untouched whiteness and blackness at the bottom of the liquor; then filter this folution thro' cap-caper, hewn by exand, to a fpoonful or two of the clear, add four or five drops of good periments. limpid fpirit of urine: fhake them together, and immediately the whole mixture will appear white, like milk. After this, if you prefently add a convenient proportion of rectify'd Aqua fortis, the whiteness will immediately difappear, and the whole mixture become transparent; which you may, if you please, again reduce to a confiderable degree of whitenefs, by pouring thereinto more fresh fpirit of urine. It is not neceffary to employ either Aqua fortis, or fpirit of urine, about this experiment; for we have made it with other liquors.

And

2. Make a ftrong infufion of bruis'd galls in fair water; and having. filtered it into a clean vial, add more of the fame fluid to it, till you have made it somewhat transparent, and fufficiently diluted the colour for the credit of the experiment. In this infufion, shake a convenient quantity of a clear, but very strong folution of vitriol; and you shall immediately see the mixture turn black, almost like ink and if, prefently after, you drop into this mixture a fmall quantity of good oil of vitriol, and, by fhaking. the vial, fuddenly difperfe it thro' the two other liquors; you will fee the dark colour of the whole prefently begin to diffipate, grow clear, tranfparent, and lofe its inky blacknefs; which may be again reftored by the affufion of a fmall quantity of a strong folution of falt of tartar. tho' both these atramentous liquors will feem very pale, if you write with a clean pen dipt in them; yet that is common to them, with fome forts of ink, which prove very good when dry; as I have found, that when. thefe were carefully made, what I wrote with either, efpecially with the former, would, after a while, turn fufficiently black. This experiment of deftroying and reftoring blacknefs, we have likewife try'd in common ink; tho' with this it fucceeds not fo well, and but very flowly; because the gum ufually employ'd in making it, oppofes the operations of the faline liquors. And tho' it be taken for granted, that bodies will not precipitate with alkalizate falts, which have not been first diffolved in fome acid menftruum; yet I have found, upon trial, that many vegetables, barely infufed, or but flightly boiled in common water, afford, upon the bare affufion of a strong and clear lixivium of pot-afhes, a large quantity of coagulated matter; fuch as I have had in the precipitations of vegetable substances, by means of acids; and that this matter was eafily feparable from the reft of the liquor; being left behind by it in the filtre. And, from the firft ink mentioned in this experiment, I could, by filtration, feparate a confiderable quantity of a very black pulverable fubftance. And when the ink was made clear again, by the oil of vitriol, the affufion

of

PHYSICS. of diffolved falt of tartar feem'd but to precipitate, and thereby unite, and render confpicuous, the corpufcles of the black mixture, that had been before difperfed, into very minute and fingly invifible particles, by the refolving power of the highly corrofive oil of vitriol. And that galls are not abfolutely neceffary to make atramentous liquors, appears from the following experiment. We boil'd dry'd rofe-leaves for a while in fair water, and into two or three fpoons-ful of the decoction, fhook a few drops of a strong and well filtred folution of vitriol; whereupon the mixture immediately turn'd black: and presently shaking herein a juft proportion of Aqua fortis, we changed it from a black to a deep red ink; which, by the affufion of a little spirit of urine, may be reduced immediately to an opake blackish colour.

3. In these experiments, the infufion of galls, the decoction of rofes, and the folution of vitriol, have each their own colour; but we may fuddenly produce a blacknefs, by mixing an infufion of orpiment, and a folution of minium, both whereof fhall be limpid and colourless. And with thefe liquors may be exhibited a curious and furprizing phenomenon, if made and applied in the following manner: 1. Take of the strongest unflaked lime about two parts, of yellow orpiment one part, of fair water fifteen or fixteen parts; beat the lime grofly, and powder the orpiment, with care to avoid the noxious duft: and having put these two ingredients into the water, let them remain there for two or three hours; ftirring the mixture from time to time. Thus you'll obtain a fetid liquor; the clear part whereof must be poured off from the reft, or gain'd by the filtre. 2. In the mean time burn a piece of cork, and quench it, whilft fired, for several times fucceffively in fair water; and having, by this means, reduced it to a coal, you may eafily, by grinding it with a folution of gum-arabic in water, bring it to the colour and confiftence of a good black ink. 3. Take any quantity of red lead, and two or three times its weight of vinegar, or rather the weak fpirit of it; and, putting the powder and that into a glass vial, let them infufe in fome confiderably warm place for two or three hours, till the liquor has acquired a fweet tafte. Matters being thus prepared, write what you please with a clean pen dipt in the folution of the red lead; which, if filtred, will prove fo clear, as to be invifible upon the paper. Over what is thus written, you may draw any characters or letters you please, with a pen dipt in the black ink made with cork. And, laftly, to fhew the experiment, dip a linen rag in the fetid folution of the lime and orpiment, which is alfo limpid, and draw it over the written paper; and this will at once both wipe out the ftrokes of the black ink, and render all that was wrote with the invifible ink confpicuously black.

4. If pieces of white hart's-horn be, with a moderate degree of fire, diftilled in a glafs retort, they will, after the feparation of the phlegm, fpirit, volatile falt, and the loofer and lighter parts of the oleaginous fubftance, remain one behind of a coal-black colour. And even ivory itfelf, when skilfully burnt, affords painters one of the best and deepest blacks

they have. Yet, in the inftance of diftill'd hart's horn, the operation be- PHYSICS. ing made in glafs veffels, carefully clofed, no extraneous black fubftance infinuates itfelf into the white horn; but the whiteness is deftroyed, and the blackness generated only by a change of texture made in the burnt body by the recefs of fome parts, and the tranfpofition of others. And tho' I remember not to have ever found the Caput mortuum of distilled hart's-horn to pass from a black to a true whitenefs, whilft it continued in clofe veffels; yet, having taken out the coal-black fragments, and calcined them in open veffels, I could, in few hours, quite deftroy that blackness, and, without fenfibly changing their bulk or figure, reduce them to a degree of whitenefs: fo much do these two colours depend upon the dif pofition of the little parts that the bodies wherein they are to be met with, confift of. And we find, that if white-wine tartar, or the white crystals of fuch tartar, are burnt, without being truly calcined, the Caput mortuum will be black. But if the calcination be continued till the tartar is perfectly reduced to afhes, and kept long enough in a strong fire, the remaining calx will be white. And fo we fee, that not only other vegetable fubftances, but even white woods, as the hazel, will yield a black charcoal, and afterwards whitish afhes. Thus alfo animal fubftances, naturally white, as bones and egg-fhells, grow black, upon being burnt, and white again, when perfectly calcined.

5. Yet I much question whether the rule, adufta nigra, perusta alba, will hold as univerfally as is prefumed; for I have feveral examples to alledge against it. By burning alabafter so as to make it appear to boil. almost like milk, and to reduce it to a very fine powder, it would not grow black at all, but retain its pure and native whitenefs: and tho', by keeping it longer than ufual in the fire, I produced a faint yellow in that part of the powder which lay nearest the top of the crucible; yet a curious and experienced ftone-cutter told me he had found, that if alabafter, or plaister of Paris, be very long kept in a strong fire, the whole heap of burnt powder would exchange its whitenefs for a much deeper colour than the yellow I obferved. Lead calcined with a strong fire, turns at length, to minium, whofe colour we know is a deep red; and if this minium be again urged with a strong fire, you will foon find a glaffy brittle body, darker than minium, than any white calx or glafs. 'Tis known among chymifts, that the white calx of antimony, by a more vehement operation of the fire, may be melted into a glafs; which we have obtain❜d of a red colour far deeper than that of the calx of burnt antimony and tho' common glafs of antimony, being ufually adulterated with borax, have its colour thereby diluted, often to a very pale yellow: yet not only ours, made more genuinely, was, as we faid, of a colour lefs remote from black than the calx; but, by melting it once or twice more, we found the colour heighten'd. And if you burn blue unfophifticated vitriol very flowly, and with a gentle degree of heat, you may obferve, that when 'tis burnt only fo far as to rub to powder betwixt your fingers, it will be of a white, or whitifh colour: but if you profe

cute

PHYSICS. cute the calcination, this body will pafs thro' other colours, as a grey, a yellowish, and a red: and if you continue it in a long and vehement fire, by that time it comes to be thoroughly calcined, it will be of a dark purple, nearer to black, not only than the first calx, but than the vitriol, before it at all felt the fire. I might add, that Crocus Martis, made by the lafting violence of the reverberated flames, is not fo near to white as the iron or steel that afforded it, before its calcination.

These inftances may fuffice to fhew, that minerals are to be excepted from the foremention'd rule; which, tho' it feldom fails in fubftances belonging to the animal or vegetable kingdom, may yet be suspected even in fome of these, if Belonius fay true, that charcoal, made of the wood of oxycedar is white. And I could not find, tho' hart's-horn, and other white bodies, will turn black in retorts, by heat, that camphire would at all lose its whitenefs; tho' I have purposely kept it in fuch a heat as made it melt and boil.

6. And tho' I could not, in clofed glaffes, blacken camphire by heat, but it would fublime to the fides and top of the veffel in its natural form ; yet being fet on fire in the free air, it fends out a great smoke: and having, purposely, upon fome of it, whilft flaming, clapt a large glass, almost in the form of a hive, with a hole at the top, it continued burning, fo as to line all the infide of the glafs with a foot as black as ink; and in fo great a quantity, that the clofenefs of the veffel confider'd, almost all that part of the camphire which took fire, feem'd to have been changed into that deep black substance.

7. I took rectified oil of vitriol, and, by degrees, mixed with it a convenient proportion of the effential oil of wormwood; and, warily distilling the mixture in a retort, there remain'd a fcarce credible quantity of dry matter, black as a coal. And because the oil of wormwood, tho' a chymical oil, and drawn by a virtuofo, feem'd to have fomewhat in it of the colour of the plant, I fubftituted in its room, the pure and fubtile oil of winter-favory and gradually mixing it with an equal weight of the fame oil of vitriol, and diftilling them, as before, in a retort, even these two clear liquors left me a confiderable proportion of a fubftance black as pitch; which I keep by me as a rarity.

8. Take a little yellow wax, fcraped, or thinly fliced, and putting it into a convenient glafs, pour to it a confiderable quantity of fpirit of wine; and, placing the veffel in warm fand, increase the heat by degrees, till the fpirit of wine juft begins to boil; and by continuing that degree of heat, you will quickly find the wax diffolv'd: then taking it off, you may either fuffer it to cool as haftily as with fafety to the glafs it can, or pour it, whilft yet hot, into a filtre of paper; and either in the glafs where it cools, or in the filtre, you will foon find the wax and menftruum together reduced into a white fubflance almost like butter; which, by letting the fpirit exhale, will fhrink into a much less bulk, but still retain its whitenefs. 'Tis a pretty phenomenon in working of this magiftery of wax, that the yellownefs vanishes, and neither appears in the