omit, that we have instances also of this kind even in metalline Bodies and
animal; for those several Colours which are observed to follow each other
upon the polisht surface of hardned Steel, when it is by a sufficient
degree of heat gradually tempered or softened, are produced, from nothing
else but a certain thin _Lamina_ of a _vitrum_ or _vitrified_ part of the
Metal, which by that degree of heat, and the concurring action of the
ambient Air, is driven out and fixed on the surface of the Steel.

And this hints to me a very probable (at least, if not the true) cause of
the hardning and tempering of Steel, which has not, I think, been yet
given, nor, that I know of been so much as thought of by any. And that is
this, that the hardness of it arises from a greater proportion of a
vitrified Substance interspersed through the pores of the Steel. And that
the tempering or softning of it arises from the proportionate or smaller
parcels of it left within those pores. This will seem the more probable, if
we consider these Particulars.

First, That the pure parts of Metals are of themselves very _flexible_ and
_tuff_; that is, will indure bending and hammering, and yet retain their
continuity.

Next, That the Parts of all vitrified Substances, as all kinds of Glass,
the _Scoria_ of Metals, &c. are very hard, and also very brittle, being
neither _flexible_ nor _malleable_, but may by hammering or beating be
broken into small parts or powders.

Thirdly, That all Metals (excepting Gold and Silver, which do not so much
with the bare fire, unless assisted by other saline Bodies) do more or less
_vitrifie_ by the strength of fire, that is, are corroded by a saline
Substance, which I elsewhere shew to be the true cause of fire; and are