not Soak, and by which it could not be Wrought, it nevertheless lost
its Colour. And these Instances (_Pyrophilus_) I am the more carefull to
mention to you, that you may not be much Surpris'd or Discourag'd, if you
should sometimes miss of performing punctually what I affirm my self to
have done in point of changing Colours; since in these Experiments the
over-sight or neglect of such little Circumstances, as in many others would
not be perhaps considerable, may occasion the mis-carrying of a Trial. And
I was willing also to take this occasion of Advertising you in the
repeating of the Experiments mention'd in this Treatise, to make use of the
Juices of Vegetables, and other things prepar'd for your Trials, as soon as
ever they are ready, lest one or other of them grow less fit, if not quite
unfit by delay; and to estimate the Event of the Trials by the Change, 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 n