4 We took a good quantity of clean Lead, and melted it with a ſtrong Fire, and then immediately pouring it out into a clean Veſſel of a convenient ſhape and matter, (we us'd one of Iron, that the great and ſudden Heat might not injure it) and then carefully and nimbly taking off the Scum that floated on the top, we perceiv'd, as we expected, the ſmooth and
gloſſie Surface of the melted matter, to be adorn'd with a very glorious Colour, which being as Tranſitory as Delightfull, did almoſt immediately give place to another vivid Colour, and that was as quickly ſucceeded by a third, and this as it were chas'd away by a fourth, and ſo theſe wonderfully vivid Colours ſucceſſively appear'd and vaniſh'd, (yet the ſame now and then appearing the ſecond time) till the Metall ceaſing to be hot enough to afford any longer this pleaſing Spectacle, the Colours that chanc'd to adorn the Surface, when the Lead thus began to cool, remain'd upon it; but were ſo Superficial, that how little ſoever we ſcrap'd off the Surface of the Lead, we did in ſuch places ſcrape off all the Colour, and diſcover only that which is natural to the Metall it ſelf, which receiving its adventitious Colours, only when the heat was very Intenſe, and in that part which was expos'd to the comparatively very cold Air, (which by other Experiments ſeems to abound with ſubtil Saline parts, perhaps not uncapable of working upon Lead ſo diſpos'd:) Theſe things I ſay, together with my obſerving that whatever parts of the ſo ſtrongly melted Lead were expos'd a while to the Air, turn'd into a kind of Scum or Litharge,
how bright and clean ſoever they appear'd before, ſuggeſted to me ſome Thoughts or Ravings, which I have not now time to acquaint You with. One that did not know me, Pyrophilus, would perchance think I endeavour'd to impoſe upon You by relating this Experiment, which I have ſeveral times try'd, but the Reaſon why the Phænomena mention'd have not been taken notice of, may be, that unleſs Lead be brought to a much higher degree of Fuſion or Fluidity than is uſual, or than is indeed requiſite to make it melt, the Phænomena I mention'd will ſcarce at all diſcloſe themſelves; And we have alſo obſerv'd that this ſucceſſive appearing and vaniſhing of vivid Colours, was wont to be impair'd or determin'd whilſt the Metal expos'd to the Air remain'd yet hotter than one would readily ſuſpect. And one thing I muſt further Note, of which I leave You to ſearch after the Reaſon, namely, that the ſame Colours did not always and regularly ſucceed one another, as is uſually in Steel, but in the diverſify'd Order mention'd in this following Note, which I was ſcarce able to write down, the ſucceſſion of the Colours was ſo very quick, whether that proceeded from the differing degrees of Heat in the Lead expos'd to the cool Air, or from ſome
other Reaſon, I leave you to examine.
[Blew, Yellow, Purple, Blew; Green, Purple, Blew, Yellow, Red; Purple, Blew, Yellow and Blew, Yellow, Blew, Purple, Green mixt, Yellow, Red, Blew, Green, Yellow, Red, Purple, Green.]
5. The Atomiſts of Old, and ſome Learned men of late, have attempted to explicate the variety of Colours in Opacous bodies from the various Figures of their Superficial parts; the attempt is Ingenious, and the Doctrine ſeems partly True, but I confeſs I think there are divers other things that muſt be taken in as concurrent to produce thoſe differing forms of Aſperity, whereon the Colours of Opacous bodies ſeem to depend. To declare this a little, we muſt aſſume, that the Surfaces of all ſuch Bodies how Smooth or polite ſoever they may appear to our Dull Sight and Touch, are exactly ſmooth only in a popular, or at moſt in a Phyſical ſenſe, but not in a ſtrict and rigid ſenſe.
6. This, excellent Microſcopes ſhew us in many Bodies, that ſeem Smooth to our naked Eyes; and this not only as to the little Hillocks or Protuberancies that ſwell
above that which may be conceiv'd to be the Plain or Level of the conſider'd Surface, for it is obvious enough to thoſe that are any thing converſant with ſuch Glaſſes, but as to numerous Depreſſions beneath that Level, of which ſort of Cavities by the help of a Microſcope, which the greateſt Artificer that makes them, judges to be the greateſt Magnifying Glaſs in Europe, except one that equals it, we have on the Surface of a thin piece of Cork that appear'd ſmooth to the Eye, obſerv'd about ſixty in a Row, within the length of leſs then an 31 and 32 part of an Inch, (for the Glaſs takes in no longer a ſpace at one view) and theſe Cavities (which made that little piece of Cork look almoſt like an empty Honey-comb) were not only very diſtinct, and figur'd like one another, but of a conſiderable bigneſs, and a ſcarce credible depth; inſomuch that their diſtinct ſhadows as well as ſides were plainly diſcern'd and eaſiy to be reckon'd, and might have been well diſtinguiſh'd, though they had been ten times leſſer than they were; which I thought it not amiſs to mention to you Pyrophilus upon the by, that you may thence make ſome Eſtimate, what a ſtrange Inequality, and what a multitude of little Shades, there may really be, in a
ſcarce ſenſible part of the Phyſical ſuperficies, though the naked Eye ſees no ſuch matter. And as Excellent Microſcopes ſhew us this Ruggedneſs in many Bodies that paſs for Smooth, ſo there are divers Experiments, though we muſt not now ſtay to urge them, which ſeem to perſwade us of the ſame thing as to the reſt of ſuch Bodies as we are now treating off; So, that there is no ſenſible part of an Opacous body, that may not be conceiv'd to be made up of a multitude of ſingly inſenſible Corpuſcles, but in the giving theſe ſurfaces that diſpoſition, which makes them alter the Light that reflects thence to the Eye after the manner requiſite to make the Object appear Green, Blew, &c. the Figures of theſe Particles have a great, but not the only ſtroak. 'Tis true indeed that the protuberant Particles may be of very great variety of Figures, Sphærical, Elliptical, Conical, Cylindrical, Polyedrical, and ſome very irregular, and that according to the Nature of theſe, and the ſituation of the Lucid body, the Light muſt be variouſly affected, after one manner from Surfaces (I now ſpeak of Phyſical Surfaces) conſiſting of Sphaerical, and in another from thoſe that are made up of Conical or Cylindrical Corpuſcles; ſome
being fitted to reflect more of the incident Beams of Light, others leſs, and ſome towards one part, others towards another. But beſides this difference of Shape, there may be divers other things that may eminently concurr to vary the forms of Aſperity that Colours ſo much depend on. For, willingly allowing the Figure of the Particles in the firſt place, I conſider ſecondly, that the ſuperficial Corpuſcles, if I may ſo call them, may be bigger in one Body, and leſs in another, and conſequently fitted to allay the Light falling on them with greater ſhades. Next, the protuberant Particles may be ſet more or leſs cloſe together, that is, there may be a greater or a ſmaller number of them within the compaſs of one, than within the compaſs of another ſmall part of the Surface of the ſame Extent, and how much theſe Qualities may ſerve to produce Colour may be ſomewhat gueſs'd at, by that which happens in the Agitation of Water; for if the Bubbles that are thereby made be Great, and but Few, the Water will ſcarce acquire a ſenſible Colour, but if it be reduc'd to a Froth, conſiſting of Bubbles, which being very Minute and Contiguous to each other, are a multitude of them crowded into a narrow Room, the Water (turned to