that is produc’d presently upon the due and sufficient Application of Actives to Passives, (as they speak) because in many cases the effects of such Mixtures may not be lasting, and the newly produc’d Colour may in a little time degenerate.  But, (Pyrophilus) I forgot to add to the two former Observations lately made about Vegetables, a third of the same Import, made in Mineral substances, by telling you, That the better to satisfie a Friend or two in this particular, I sometimes made, according to some Conjectures of mine, this Experiment; That having dissolv’d good Silver in Aqua-fortis, and Precipitated it with Spirit of Salt, upon the first Decanting of the Liquor, the remaining Matter would be purely White; but after it had lain a while uncover’d, that part of it, that was Contiguous to the Air, would not only lose its Whiteness, but appear of a very Dark and almost Blackish Colour, I say that part that was Contiguous to the Air, because if that were gently taken off, the Subjacent part of the same Mass would appear very White, till that also, having continu’d a while expos’d to the Air, would likewise Degenerate.  Now whether the Air perform these things by the means of a Subtile Salt, which we elsewhere show it not to be destitute of, or by a peircing Moisture, that is apt easily to insinuate it self into the Pores of some Bodies, and thereby change their Texture, and so their Colour; Or by solliciting the Avolation of certain parts of the Bodies, to which ’tis Contiguous; or by some other way, (which possibly I may elsewhere propose and consider) I have not now the leisure to discourse.  And for the same reason, though I could add many other Instances, of what I formerly noted touching the emergency of Redness upon the Digestion of many Bodies, insomuch that I have often seen upon the Borders of France (and probably we may have the like in England) a sort of Pears, which digested for some time with a little Wine, in a Vessel exactly clos’d, will in not many hours appear throughout of a deep Red Colour, (as also that of the Juice, wherein they are Stew’d, becomes) but ev’n on pure and white Salt of Tartar, pure Spirit of Wine, as clear as Rock-water, will (as we elsewhere declare) by long Digestion acquire a Redness; Though I say such Instances might be Multiply’d, and though there be some other Obvious changes of Colours, which happen so frequently, that they cannot but be as well Considerable as Notorious; such as is the Blackness of almost all Bodies burn’d in the open Air:  yet our haste invites us to resign you the Exercise of enquiring into the Causes of these Changes.  And certainly, the reason both why the Soots of such differing Bodies are almost all of them all Black, why so much the greater part of Vegetables should be rather Green than of any other Colour, and particularly (which more directly concerns this place) why gentle Heats do so frequently in Chymical Operations produce rather a Redness than another Colour in digested Menstruums, not only Sulphureous, as Spirit of Wine, but Saline, as Spirit of Vinegar, may be very well worth a serious Inquiry; which I shall therefore recommend to Pyrophilus and his Ingenious Friends.