are more obvious in divers sorts of fruits, as
Cherries, Plums, &c. wherein, according as the Vegetable sap is sweetned,
or otherwise ripened, by passing from one degree to another of Maturation,
the external part of the fruit passes likewise from one to another Colour.
But one of the noblest Instances I have met with of this kind, is not so
obvious; and that is the way of tempering Steel to make Gravers, Drills,
Springs, and other Mechanical Instruments, which we have divers times both
made Artificers practise in our presence, and tryed our selves, after the
following manner, First, the slender Steel to be tempered is to be hardened
by heating as much of it as is requisite among glowing Coals, till it be
glowing hot, but it must not be quenched assoon as it is taken from the
fire (for that would make it too brittle, and spoil it) but must be held
over a bason of water, till it descend from a White heat to a Red one,
which assoon as ever you perceive, you must immediately quench as much as
you desire to harden in the cold water. The Steel thus hardened, will, if
it be good, look somewhat White and must be made bright at the end, that
its change of Colours may be there conspicuous; and then holding it so in
the flame of a Candle, that the bright end may be, for about half an inch,
or more, out of the flame, that the smoak do not stain or sully the
brightness of it, you shall after a while see that clean end, which is
almost contiguous to the flame, pass very nimbly from one Colour to
another, as from a brighter Yellow, to a deeper and reddish Yellow, which
Artificers call a _sanguine_, and from that to a fainter first, and then a
a deeper Blew. And to bring home this Experiment to our present purpose, it
is found by daily Experience, that each of these succeeding Colours argue
such a change made in the texture of the Steel, that if it be taken from
the flame, and immediately quenched in the tallow (whereby it is setled in
whatever temper it had before) when it is Yellow, it is of such a hardness
as makes it fit for Gravers Drills, and such like tools; but if it be kept
a few minutes longer in the flame till it grow Blew, it becomes much
softer, and unfit to make Gravers for Metalls, but fit to make Springs for
Watches, and such like Instruments, which are therefore commonly of that
Colour; and if the Steel be kept in the flame, after that this deep Blew
hath disclosed it self, it will grow so soft, as to need to be new hard