Experiments and considerations touching colours first occasionally written, among some other essays to a friend, and now suffer'd to come abroad as the beginning of an experimental history of colours / by the Honourable Robert Boyle ...

EXPERIMENT XLVI.

We have often taken notice, as of a re∣markable thing, that Metalls as they appear to the Eye, before they come to be farther alter'd by other Bodyes, do exhibit Co∣lours very different from those which the Fire and the Menstruum, either apart, or both together, do produce in them; especially considering that these Metalline Bodyes are after all these disguises reducible not only to their former Metalline Consistence and o∣ther more radical properties, but to their Colour too, as if Nature had given divers Metalls to each of them a double Colour, an External, and an Internal; But though upon a more attentive Consideration of this difference of Colours, it seem'd propable to me, that divers (for I say not all) of those Colours which we have just now call'd In∣ternal, are rather produc'd by the Coalition of Metalline Particles with those of the Salts, or other Bodyes employ'd to work on them, than by the bare alteration of the parts of the Metalls themselves: and though therefore we may call the obvious Colours, Natural or Common, & the others Adven∣titious, yet because such changes of Colours, from whatsoever cause they be resolv'd to

proceed may be properly enough taken in to illustrate our present Subject, we shall not scruple to take notice of some of them, especially because there are among them such as are produc'd without the interven∣vention of Saline Menstruums. Of the Adven∣titious Colours of Metalline Bodies the Chief sorts seem to be these three. The first, such Colours as are produc'd without other Additaments by the Action of the fire upon Metalls. The next such as emerge from the Coalition of Metalline Particles with those of some Menstruum imploy'd to Corrode a Metall or Precipitate it; And the last, The Colours afforded by Metalline Bodyes ei∣ther Colliquated with, or otherwise Pene∣trating into, other Bodies, especially fusible ones. But these (Pyrophilus,) are only as I told you, the Chief sorts of the adventi∣tious Colours of Metalls, for there may o∣thers belong to them, of which I shall here∣after have occasion to take notice of some, and of which also there possibly may be others that I never took notice of.

And to begin with the first sort of Colours, 'tis well enough known to Chymists, that Tin being Calcin'd by fire alone is wont to afford a White Calx, and Lead Calcin'd by fire alone affords that most Common Red-Powder we call Minium: Copper al∣so

Calcin'd per se, by a long or violent fire, is wont to yield (as far as I have had occa∣sion to take notice of it) a very Dark or Blackish Powder; That Iron likewise may by the Action of Reverberated flames be turn'd into a Colour almost like that of Saffron, may be easily deduc'd from the Preparation of that Powder, which by rea∣son of its Colour and of the Metall 'tis made of is by Chymists call'd, Crocus Martis per se. And that Mercury made by the stress of Fire, may be turn'd into a Red Powder, which Chymists call Precipitate per se, I elsewhere more particularly declare.

Annotation I.

It is not unworthy the Admonishing you, (Pyrophilus,) and it agrees very well with our Conjectures about the dependence of the change of a Body's Colour upon that of its Texture, that the same Metall may be the successive operation of the fire receive divers Adventitious Colours, as is evident in Lead, which before it come to so deep a Colour as that of Minium, may pass through divers others.

Annotation II.

Not only the Calces, but the Glasses of Metalls, Vitrify'd per se, may be of Colours differing from the Natural or Obvious Co∣lour of the Metall; as I have observ'd in the Glass of Lead, made by long exposing Crude Lead to a violent fire, and what I have observ'd about the Glass or Slagg of Copper, (of which I can show you some of an odd kind of Texture,) may be else∣where more conveniently related. I have likewise seen a piece of very Dark Glass, which an Ingenious Artificer that show'd it me profess'd himself to have made of Silver alone by an extreme Violence (which seems to be no more than is needfull) of the fire.

Annotation III.

Minerals also by the Action of the Fire may be brought to afford Colours very differing from their own, as I not long since noted to you about the variously Colour'd Flowers of Antimony, to which we may add the Whitish Grey-Colour of its Calx, and the Yellow or Reddish Colour of the Glass, where into that Calx may be flux'd.

And I remember, that I elsewhere told

you, that Vitriol Calcin'd with a very gentle heat, and afterwards with higher and high∣er degrees of it, may be made to pass through several Colours before it descends to a Dark Purplish Colour, whereto a strong fire is wont at length to reduce it. But to insist on the Colours produc'd by the Operation of fire upon several Minerals would take up farr more time than I have now to spare.