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