ly than externally. And again, if the wind increase into a ftorm, the wa- PHYSICS. ter may appear white, especially near the fhore, because the rude agitation breaks it into foam or froth; fo much do whitenefs and blackness 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 most light of any; fo their fuperficial particles are of a fpecular nature, I fhall further endeavour to fhew, by making specular bodies white, and a white body specular.

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 fpherical looking-glafs; and a number of them lying near one another, made the glass they were fastened to, manifeftly appear a white body. And as many parts of the sky, especially 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 stars; fo, many lucid bodies, if too small to be fingly discerned 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 transparent, yet, by its power of reflecting fome incident rays of light, is, in fome meafure, a natural fpeculum, being long agitated with a whisk or spoon lofes its transparency, 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 compass is but small;. and they are not thick enough set to reflect so many little images or rays of the lucid body, as go to produce a vigorous fenfation of whiteness.. 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 itself 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 glass vial, a convenient quantity of fpirit of turpentine, which will not incorporate with water, yet is almost as clear and colourless as that; and these 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 whitish; but if by vehemently fhaking the glass for a competent time, a further comminution of the oil be made into far more numerous and fmaller globules, whilft it is alfo thereby more thoroughly confounded

PHYSICS. Confounded with the water, the mixture will appear of a much greater

whitenefs, and almost like milk; tho' if the glass be let alone a while, the colour will gradually fade as the oily globules grow fewer and larger, and at length quite vanish; 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; because 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 whitifh. And hence, perhaps, it is, that we find hot water usually 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 most part, be as well convex as fmooth; tho' it feems not easy to prove, that when diaphanous bodies are reduced into white powders, each corpufcle must 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 phyfical 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 glass, be exposed to luminous rays, you shall 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 discern a distance between the luminous lines, there is no doubt, as far as I can guess 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 neceffary 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 microscope, to confift principally of little flender icicles of feveral shapes, which afford fuch numerous lines of light as we speak 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 qualified to reflect, in a confused 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 seemingly entire, exchanged their tranfparency for whitenefs; the ignition and extinction having cracked each Jump into a multitude of minute bodies, and thereby given it a multitude of new surfaces. And even with coloured diaphanous bodies, there may, by this way, be a great degree of whiteness 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 goldsmith's fashion, is of a lovely white; but if, with a piece of smooth steel, a part of it be burnish'd, that part presently loses its whiteness, and turns to a speculum, 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 stood out on the surface of the filver, into one continued regular plain.

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 blacknes. 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 should rife up in number lefs 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. motion communicated to them by thofe corpufcles, and confequently reflect but few of them, or those but languidly, towards the eye; as when a ball, thrown against a floor, rebounds a great way upwards, but very little or not at all when thrown against mud or water; because the parts yield, and receive into themselves the motion which should reflect the ball back. But this laft manner of accounting for blackness I barely propose, without either adopting, or abfolutely rejecting it; for the hardness of touchstones, black marble, and of other bodies that are black, and solid, feems to render it fomewhat improbable that they should be of fo yielding a nature; unless we fay that fome bodies may be more difpofed to yield to the impulfes of the corpufcles of light, by reafon of a peculiar texture, than others, which, by particular trials, appear to be fofter than they. Both the solutions, however, agree in this, that black bodies reflect but little of the light which falls on them. And it is not impoffible that, int fome cafes, both the difpofition of the fuperficial particles as to figure and pofition, and the yielding of the body, or fome of its parts, may jointly contribute to render a body black. The confiderations which induced me to propose this notion of blackness, are principally these:

(1.) Whitenefs and blacknefs being generally reputed contrary qualities, whitenefs depending, as I faid, upon the difpofition of the parts of the body, to reflect light plentifully, it feem'd probable, that blackness might depend upon a contrary difpofition of furface; but upon this I fhall not infift. However, if a body, of an uniform colour, be placed, part in the fun-beams, and part in the fhade, that part, which is not illumined, will appear nearer allied to blackness than the other, from which more. light is reflected to the eye; dark colours alfo feem the blacker, the less is the light they are viewed in: and all things feem black in the dark, when they yield no rays to make impreffions upon our organs of fight; fo that fhadow and darkness are near a-kin: and fhadow, we know, is but a privation of light; blacknefs, accordingly, feems to proceed from the want of rays reflected from the black body to the eye; tho' the bodies we call black, as marble, jet, &c. are not perfectly fo, for if they were, we fhould not fee them at all. But notwithstanding the rays which fall on the fides of those erect particles we mentioned, do few of them return outwards; yet fuch as fall upon the points of those cylinders, cones, or pyramids, may be thence reflected to the eye, tho' they make but a faint impreffion there; because they are mixed with a great proportion of little fhades. Thus, having procured a large piece of black marble to be well polished, and brought to the form of a large spherical concave fpeculum, the infide thereof was a kind of dark looking-glafs, wherein I could plainly fee a little image of the fun, when it fhone thereon. But this image was very far from offending or dazling my eyes, as it would have done from another fpeculum; and tho' this were large, I could not, in a long time, fet a piece of wood on fire with it; tho' a far lefs fpeculum of the fame form, and of a more reflecting material, would prefently have made it flame. And having expofed to the fun a pretty large mortar

of white marble, polifh'd on the infide, we found that it reflected a great PHYSICS. quantity of glaring light, but fo fcatter'd, that we could not make the reflected rays meet in any fuch confpicuous focus as that we obferv'd in the black marble; tho' by holding a candle, in the night-time, at a convenient distance, we were able to procure a concourse of a few reflected rays at about two inches distance from the bottom of the mortar. But we found the heat of the fun-beams fo difperfedly reflected, to be very languid, even compared to the focus of the black marble: and the little picture of the fun that appear'd upon the white marble as a speculum, was very faint, and exceedingly ill defined.

(2.) Taking two pieces, the one of black, and the other of white marble, whofe furfaces were plain and polifh'd; and cafting on them fucceffively the rays of the fame candle, in fuch a manner that the adjacent fuperficies being shaded by an opake and perforated body, the incident rays pafs'd thro' a round hole of about half an inch in diameter, the circle of light that appear'd on the white marble was, in comparison, very bright, but very ill defined; whilst that on the black marble was far lefs luminous, but much better defined.

(3.) When we look upon a piece of linen that has small holes in it, they appear very black; fo that men are often deceiv'd in taking holes for fpots of ink and painters, to reprefent holes, make ufe of black; the reason whereof seems to be, that the rays which fall on thofe holes, penetrate fo deep, that none are reflected back to the eye. And in a narrow well, part of the mouth feems black, because the incident rays are reflected downwards, from one fide to another, till they can no longer rebound to the eye. We may confider too, that if different parts of the fame piece of black velvet be ftroak'd oppofite ways, there will appear two diftinct kinds of blackness, the one far darker than the other; probably becaufe in the less obfcure part of the velvet, the little filken piles, whereof 'tis compos'd, being inclined, there is a greater part of each of them turn'd to the eye; whilft in the other part the piles of filk being more erect, there are by far fewer rays fent outwards from the lateral parts of each pile: fo that most of thofe reflected to the eye, come from the tops of the piles, which make but a fmall part of the whole fuperficies of the velvet. This explanation I propofe, not that I think the blacknefs of the velvet proceeds from the caufe affign'd; fince each fingle pile of filk is black by reafon of its texture, in what pofition foever it be view'd; but because the greater blackness of a fingle tuft feems to proceed from the greater defect of rays reflected thence, and from the want of thofe parts of a surface that reflect rays, and the multitude of those fhaded parts that reflect none. And I have often obferv'd, that the pofition of particular bodies, far greater than piles of filk, may, notwithstanding each of them hath a colour of its own, make one part of their aggregate appear far darker than another. Thus a heap of carrots appear of a much darker colour when view'd with their points, than with their fides obverted to the eye.