Now though quick Lime be the powder that this direction makes choice of,
yet I doubt not, but that there may be much more convenient ones found out,
one of which I have made tryal of, and found very effectual; and were it
not for discovering, by the mentioning of it, another Secret, which I am
not free to impart, I should have here inserted it.

* * * * *


Observ. IX. _Of the Colours observable in Muscovy Glass, and other thin
Bodies_.

Moscovy-glass, or _Lapis specularis_, is a Body that seems to have as many
Curiosities in its Fabrick as any common Mineral I have met with: for
first, It is transparent to a great thickness: Next, it is compounded of an
infinite number of thin flakes joyned or generated one upon another so
close & smooth, as with many hundreds of them to make one smooth and thin
Plate of a transparent flexible substance, which with care and diligence
may be flit into pieces so exceedingly thin as to be hardly perceivable by
the eye, and yet even those, which I have thought the thinnest, I have with
a good _Microscope_ found to be made up of many other Plates, yet thinner;
and it is probable, that, were our _Microscopes_ much better, we might much
further discover its divisibility. Nor are these flakes only regular as to
the smoothness of their Surfaces, but thirdly, In many Plates they may be
perceived to be terminated naturally with edges of the figure of a
_Rhomboeid_. This Figure is much more conspicuous in our English talk, much
whereof is found in the Lead Mines, and is commonly called _Spar_, and
_Kauck_, which is of the same kind of substance with the _Selenitis_, but
is seldom found in so large flakes as that is, nor is it altogether so