ly than externally. And again, if the wind increase into a ftorm, the wa- PHYSICS. ter may appear white, efpecially near the fhore, because the rude agitation breaks it into foam or froth; fo much do whitenefs and blacknefs depend upon the difpofition of the fuperficial parts of a body, to reflect the rays of light inward or outward. But that as white bodies reflect the moft light of any; fo their fuperficial particles are of a fpecular nature, I fhall further endeavour to fhew, by making fpecular bodies white, and a white body fpecular.

5. Upon diftilling quickfilver in a cucurbit, fitted with a capacious glafs-head, I have obferved, that when the operation was performed by proper degrees of fire, there would ftick to the infide of the alembic a multitude of little round drops of mercury. And as mercury is a fpecular body, each of thefe little drops was a small spherical looking-glafs ; and a number of them lying near one another, made the glafs they were faftened to, manifeftly appear a white body. And as many parts of the sky, efpecially the milky way, appear white to the naked eye; yet the galaxy, viewed thro' a telescope, does not appear white, but to be made up of a vaft multitude of little ftars; fo, many lucid bodies, if too fmall to be fingly difcerned by the eye, and fet fufficiently thick by one another, may, by their united rays, appear to the eye as one white body: and why may not the like happen, when a multitude of bright, little corpufcles, crowded together, are made jointly to reflect vivid rays to the eye, tho' they shine by a borrowed light?

But to return to our experiments; we may take notice, that the white of an egg, which tho' in part tranfparent, yet, by its power of reflecting fome incident rays of light, is, in fome measure, a natural fpeculum, being long agitated with a whisk or fpoon lofes its tranfparency, and becomes a very white froth; that is, an aggregate of numerous fmall bubbles, whofe convex fuperficies fit them to reflect the light every way outwards. And 'tis worth obferving that when water, for inftance, is agitated into froth, if the bubbles be great and few, the whitenefs will be but faint, because the number of fpecula within a narrow compafs is but fmall; and they are not thick enough fet to reflect fo many little images or rays of the lucid body, as go to produce a vigorous fenfation of whitenefs. And left it fhould be faid, that the whitenefs of fuch globular particles proceeds from the air included in the froth, (tho' who can prove that the air itfelf is white?) and at the fame time to illuftrate our doctrine of whiteness, I fhall add this experiment. I put to fome fair water, contained in a glafs vial, a convenient quantity of fpirit of turpentine, which will not incorporate with water, yet is almost as clear and colourlefs as that; and thefe being well fhaken together, I found the agitation broke the oil into a multitude of little globes, which each of them reflecting outwards a lucid image, made the imperfect mixture of the two liquors, appear whitifh; but if by vehemently fhaking the glass for a competent time, a further comminution of the oil be made into far more numerous and finaller globules, whilft it is alfo thereby more thoroughly

confounded

PHYSICS. confounded with the water, the mixture will appear of a much greater. whitenefs, and almoft like milk; tho' if the glafs be let alone awhile, the colour will gradually fade as the oily globules grow fewer and larger, and at length quite vanifh; leaving both the liquors diftinct and diaphanous as before. And fuch a trial hath fucceeded, when inftead of oil, or fpirit of turpentine, I took a yellow mixture, made with a large proportion of crude turpentine, diffolved in that liquor; and it alfo fucceeded better than one would expect, when I employ'd an oil brought to a deep green, by infufing copper filings therein. Thus aromatics, diftilled with water, often yield a white liquor, which may long continue of that colour; becaufe if the fire be made too ftrong, the fubtile oil is thereby much agitated, broken, and blended with the water, in fuch numerous and minute globules, as cannot eafily, in a fhort time, emerge to the top of the water, and, whilft they remain therein, make it look whitish. And hence, perhaps, it is, that we find hot water ufually more opake and whitish than the fame when cold; the agitation turning the more volatile particles of the water into vapours, and thereby producing, in the body of the liquor, a multitude of fmall bubbles, which interrupt the free paffage of the rays of light, and from the innermoft parts of the water, reflect many of them outwards. Thefe, and the like examples, have induced me to fufpect, that the fuperficial particles of white bodies, may, for the mcft part, be as well convex as fmooth; tho' it feems not eafy to prove, that when diaphanous bodies are reduced into white powders, each corpufcle muft needs be of a convex fuperficies; and, perhaps, it may here fuffice that the fpecular furfaces look feveral ways. We have feen that when a diaphanous body is reduced to very minute parts, it thereby acquires a multitude of little furfaces within a narrow compafs; and tho' each of thofe fhould not be of a convenient figure to reflect a round image of the fun, they may reflect fome physical line of light, wherein fome refraction of that which falls upon the body, whereon it depends, may often contribute to its whiteness. Thus if a flender wire, or folid cylinder of glafs, be expofed to luminous rays, you fhall fee, in fome part thereof, a vivid line of light; and if we were able to draw out and lay together a multitude of thefe little wires or threads of glass, fo flender, that the eye could not difcern a diftance between the luminous lines, there is no doubt, as far as I can guefs by a trial of this kind purpofely made, that the whole phyfical fuperficies, compofed of them, would appear white to the eye; and if fo, it is not always neceflary that the figure of thofe corpufcles, that make a body appear white, fhould be fpherical: and fnow itfelf commonly appears both to the naked eye, and when viewed thro' a microfcope, to confift principally of little flender icicles of feveral fhapes, which afford fuch numerous lines of light as we fpeak of.

6. If you take a diaphanous body, as, for inftance, a piece of glafs, and reduce it to powder, the fame body which, when entire, freely tranfmitted the rays of light, acquiring, by contufion, a multitude of minute' furfaces, each of which is, as it were, a little fpeculum, becomes there

by

by qualified to reflect, in a confufed manner, fo many rays, or little and PHYSICS. fingly unobfervable images of the lucid body, that from a diaphanous, it degenerates into a white fubftance. And heating lumps of crystal redhot, in a crucible, I have found, that, upon quenching 'em in fair water, even those which remained feemingly entire, exchanged their tranfparency for whitenefs; the ignition and extinction having cracked each lump into a multitude of minute bodies, and thereby given it a multitude of new furfaces. And even with coloured diaphanous bodies, there may, by this way, be a great degree of whitenefs produced. I have, by contufion, obtained whitish powders from granats, glafs of antimony, and emeralds; but the experiment is more eafily made, by comparing deep blue pulverized vitriol of copper, with fome of the entire crystals of the fame, for this will, comparatively, exhibit a confiderable degree of whitenefs.

7. And as by a change of pofition in the parts of differently coloured bodies, they may be rendered white; fo by a flight change, in the texture of its furface, a white body may be deprived of that property. A piece of filver newly boiled, with falt and tartar, after the goldfmith's fashion, is of a lovely white; but if, with a piece of fmooth steel, a part of it be burnifh'd, that part prefently lofes its whitenefs, and turns to a fpeculum, almost every where dark, like other mirrors; which adds a great confirmation to our doctrine. For hence we fee what it chiefly is that made the body white before; fince all that was done to deprive it of that whitenefs, was only to deprefs the little irregular protuberancies, that ftood out on the furface of the filver, into one continued regular plain.

blackness.

8. What we have faid of whitenefs may affift us to form a notion of The nature of blackness; those two qualities being fufficiently oppofite to illuftrate each other. And as that which makes a body white, is chiefly fuch a difpofition of its parts, as difpofes it to reflect more of the light that falls on it, than bodies of different colours; fo that which renders a body black, is principally a peculiar kind of texture of its fuperficial particles; whereby it damps the light that falls on it, fo that very little is reflected to the eye.

9. This texture is explicable two feveral ways; and firft, by fuppofing, in the fuperficies of the black body, a particular kind of afperity; whence the fuperficial particles reflect few of the incident rays outwards, and the reft inwards, upon the body itfelf: as if, for inftance, the furface of a black body fhould rife up in numberlefs little cylinders, pyramids, cones, &c. which, by being thick fet and erect, throw the rays of light from one to another inwards, fo often, that, at length, they are loft before they can come out again to the eye. The other way fuppofes the textures of black bodies either to yield to the rays of light, or, upon fome other account, to ftifle and keep them from being reflected in any number, or with any confiderable vigour outwards. According to this notion it may be faid, that the corpufcles, which compofe the rays of light, thrufting one another from the lucid body, and falling on black fubftances, meet with fuch a texture, that they receive into themfelves, and retain almost all the VOL. II.

F

motion

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 lefs 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, so they are reflected towards the body itself, 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 facisfaction, 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 comparifon 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 she 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 darknefs; 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 ftrong 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 blacknefs, 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 roughnefs 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 diftinguish white from black only by the touch; which might fufficiently argue a great difference in the afperities, or fuperficial textures

of

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, purpofely made to bring us farther acquainted with the nature of white and black.

whiteness and

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 blackness fhewn at the bottom of the liquor; then filter this folution thro' cap-caper, by experiments. and, to a fpoonful or two of the clear, add four or five drops of good 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 tranfparent which you may, if you pleafe, 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.

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 fomewhat tranfparent, and fufficiently diluted the colour for the credit of the experiment. In this infufion, shake a convenient quantity of a clear, but very ftrong folution of vitriol; and you fhall immediately fee 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 ftrong folution of falt of tartar. And 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; becaufe 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 ftrong and clear lixivium of pot-afhes, a largo quantity of coagulated matter; fuch as I have had in the precipitations of vegetable fubftances, by means of acids; and that this matter was easily feparable from the reft of the liquor; being left behind by it in the filtre. And, from the first 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 affusion

of