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 IX.

VVhen we Dissolve in Aqua Fortis a mixture of Gold and Silver melted into one Lump, it usually happens that the Powder of Gold that falls to the bottom, as not be∣ing Dissoluble by that Menstruum, will not have its own Yellow, but appear of a Black Colour, though neither the Gold, nor the Silver, nor the Aqua Fortis did before ma∣nifest any Blackness. And divers Alchy∣mists, when they make Solutions of Mine∣rals they would Examine, are very Glad, if they see a Black Powder Praecipirated to the Bottom, taking it for a Hopefull Sign, that those Particles are of a Golden Na∣ture,

which appear in a Colour so ordinary to Gold parted from other Metalls by Aqua Fortis, that it is a trouble to the Refiner to Reduce the Praecipitated Calx to its Na∣tive Colour. For though, (as we have try'd,) that may be Quickly enough done by Fire, which will make this Gold look very Gloriously (as indeed 'tis at least one of the Best wayes that is Practis'd for the Refining of Gold,) yet it requires both Watchfulness and Skill, to give it such a Degree of Fire as will serve to Restore it to its Lustre, without giving it such a One, as may bring it to Fusion, to which the Minuteness of the Corpuscles it consists of makes the Powder very apt. And this brings into my Mind, that having taken a Flat and Bright piece of Gold, that was Re∣fin'd by a Curious and Skilfull Person on purpose to Trye to what height of Purity Gold could be brought by Art, I found that this very piece, as Glorious as it look'd, be∣ing rubb'd a little upon a piece of fine clean Linnen, did sully it with a kind of Black; and the like I have observ'd in Refin'd Sil∣ver, which I therefore mention, because I formerly suspected that the Impurity of the Metall might have been the only Cause of what I have divers times observ'd in wea∣ring Silver-hilted Swords, Namely, that

where they rubb'd upon my Clothes, if they were of a Light-Colour'd Cloath, the Affriction would quickly Black them; and Congruously hereunto I have found Pens Blackt almost all over, when I had a while carri'd them about me in a Silver Ink-case. To which I shall only add, that whereas in these several Instances of Denigration, the Metalls are worn off, or otherwise Reduc'd into very Minute Parts, that Circumstance may prove not Unworthy your Notice.