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.
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