thickness, it may, by blows, be beaten off in flakes. This is further
confirm'd by this observable, that that Iron or Steel will keep longer from
rusting which is covered with this vitrify'd case: Thus also Lead will, by
degrees, be all turn'd into a litharge; for that colour which covers the
top being scum'd or shov'd aside, appears to be nothing else but a litharge
or vitrify'd Lead.

This is observable also in some sort, on Brass, Copper, Silver, Gold, Tin,
but is most conspicuous in Lead: all those Colours that cover the surface
of the Metal being nothing else, but a very thin vitrifi'd part of the
heated Metal.

The other Instance we have, is in Animal bodies, as in Pearls, Mother of
Pearl-shels, Oyster-shels, and almost all other kinds of stony shels
whatsoever. This have I also sometimes with pleasure observ'd even in
Muscles and Tendons. Further, if you take any glutinous substance and run
it exceedingly thin upon the surface of a smooth glass or a polisht
metaline body, you shall find the like effects produced: and in general,
wheresoever you meet with a transparent body thin enough, that is
terminated by reflecting bodies of differing refractions from it, there
will be a production of these pleasing and lovely colours.

Nor is it necessary, that the two _terminating_ Bodies should be both of
the same kind, as may appear by the _vitrified Laminæ_ on _Steel_, _Lead_,
and other Metals, one surface of which _Laminæ_ is contiguous to the
surface of the Metal, the other to that of the Air.

Nor is it necessary, that these colour'd _Laminæ_ should be of an even
thickness, that is, should have their edges and middles of equal thickness,
as in a Looking-glass-plate, which circumstance is only requisite to make