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 hot round Iron-Ring, which just
incompasses the place that is to be cut, or else by a _Sulphur'd_ Threed,
which is often wound about the place where the separation is to be made,
and then fired. Or Secondly, A Glass may be cracked by cooling it suddenly
in any place with Water, or the like, after it has been all leisurely and
gradually heated very hot. Both which _Phænomena_ seem manifestly to
proceed from the _expansion_ and contraction of the parts of the Glass,
which is also made more probable by this circumstance which I have
observed, that a piece of common window-glass being heated in the middle
very suddenly with a live Coal or hot Iron, does usually at the first crack
fall into pieces, whereas if the Plate has been gradually heated very hot,
and a drop of cold Water and the like be put on the middle of it, it only
flaws it, but does not break it asunder immediately.

A Fourth Argument may be drawn from this Experiment; Take a Glass-pipe, and
fit into a solid stick of Glass, so as it will but just be moved in it.
Then by degrees heat them whilst they are one within another, and they will
grow stiffer, but when they are again cold, they will be as easie to be
turned as before. This Expansion of Glass is more manifest in this
Experiment.

Take a stick of Glass of a considerable length, and fit it so between the
two ends or screws of a Lath, that it may but just easily turn, and that
the very ends of it may be just toucht and susteined thereby; then applying
the flame of the Candle to the middle of it, and heating it hot, you will
presently find the Glass to stick very fast on those points, and not
without much difficulty to be convertible on them, before that by removing