e Barrel; but if
it be suffered to cool again, as soon as it is cold it will be as movable,
and as easie to be turned as before.

This Quality is also very observable in _Lead_, _Tin_, _Silver_,
_Antimony_, _Pitch_, _Rosin_, _Bees-wax_, _Butter_, and the like; all
which, if after they be melted you suffer gently to cool, you shall find
the parts of the upper Surface to subside and fall inwards, losing that
plumpness and smoothness it had whilst in fusion. The like I have also
observed in the cooling of _Glass of Antimony_, which does very neer
approach the nature of Glass,

But because these are all Examples taken from other materials then Glass,
and argue only, that possibly there may be the like property also in Glass,
not that really there is; we shall by three or four Experiments indeavour
to manifest that also.

And the First is an Observation that is very obvious even in these very
drops, to wit, that they are all of them terminated with an unequal or
irregular Surface, especially about the smaller part of the drop, and the
whole length of the stem; as about D, and from thence to A, the whole
Surface, which would have been round if the drop had cool'd leisurely, is,
by being quenched hastily, very irregularly flatted and pitted; which I
suppose proceeds partly from the Waters unequally cooling and pressing the
parts of the drop, and partly from the self-contracting or subsiding
quality of the substance of the Glass: For the vehemency of the heat of the
drop causes such hidden motions and bubbles in the cold Water, that some
parts of the Water bear more forcibly against one part then against
another, and consequently do more suddenly cool those parts to which they
are contiguous.

A Second Argument may be drawn from the Experiment of cutting Glasses with
a hot Iron. For in that Experiment the top of the Iron heats, and thereby
rarifies the parts of the Glass that lie just before the crack, whence each
of those agitated parts indeavouring to expand its self and get elbow-room,
thrusts off all the rest of the contiguous parts, and consequently promotes
the crack that was before begun.

A Third Argument may be drawn from the way of producing a crack in a sound
piece or plate of Glass, which is done two wayes, either First, by suddenly
heating a piece of Glass in one place more then in another. And by this
means _chymists_ usually cut off the necks of Glass-bodies, by two kinds of
Instruments, either by a glowing