fy'd, and thereby the pores of
the Iron are more open, if then by means of dipping it in cold water it be
suddenly cold, and the parts hardned, that is, stay'd in that same degree
of _Expansion_ they were in when hot, the parts become very hard and
brittle, and that upon the same account almost as small parcels of glass
quenched in water grow brittle, which we have already explicated. If after
this the piece of Steel be held in some convenient heat, till by degrees
certain colours appear upon the surface of the brightned metal, the very
hard and brittle tone of the metal, by degrees relaxes and becomes much
more tough and soft; namely, the action of the heat does by degrees loosen
the parts of the Steel that were before streached or set _atilt_ as it
were, and stayed open by each other, whereby they become relaxed and set at
liberty, whence some of the more brittle interjacent parts are thrust out
and melted into a thin skin on the surface of the Steel, which from no
colour increases to a deep Purple, and so onward by these _gradations_ or
consecutions, _White, Yellow, Orange, Minium, Scarlet, Purple, Blew,
Watchet,_ &c. and the parts within are more conveniently, and
proportionately mixt; and so they gradually subside into a texture which is
much better proportion'd and closer joyn'd, whence that rigidnesse of parts
ceases, and the parts begin to acquire their former _ductilness_.

Now, that 'tis nothing but the vitrify'd metal that sticks upon the surface
of the colour'd body, is evident from this, that if by any means it be
scraped and rubb'd off, the metal underneath it is white and clear; and if
it be kept longer in the fire, so as to increase to a considerable
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