3 To give you the first Instance, I shall need but to remind you of what I told you a little after the beginning of this Essay, touching the Blew and Red and Yellow, that may be produc'd upon a piece of temper'd Steel, for these Colours though they be very Vivid, yet if you break the Steel they adorn, they will appear to be but Superficial; not only the innermost parts of the Metall, but those that are within a hairs breadth of the Superficies, having not any of these Colours, but retaining that of the Steel it self. Besides that, we may as well confirm this Observation, as some other particulars we elsewhere deliver concerning Colours, by the following Experiment which we purposely made.
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