that the internal parts of the Glass have a contractive power inwards, and
the external parts are incapable of such a Contraction, and the figure of
it being spherical; it follows, that the superficial parts must bear
against each other, and keep one another from being condens'd into a less
room, in the same manner as the stones of an Arch conduce to the upholding
each other in that Figure. And this is made more probable by another
Experiment which was communicated to me by an excellent Person, whose
extraordinary Abilities in all kind of Knowledg, especially in that of
Natural things, and his generous Disposition in communicating, incouraged
me to have recourse to him on many occasions. The Experiment was this:
Small Glass-balls (about the bigness of that represented in the _Figure
&._) would, upon rubbing or scratching the inward Surface, fly all
insunder, with a pretty brisk noise; whereas neither before nor after the
inner Surface had been thus scratcht, did there appear any flaw or crack.
And putting the pieces of one of those broken ones together again, the
flaws appeared much after the manner of the black lines on the Figure, _&._
These Balls were small, but exceeding thick bubbles of Glass, which being
crack'd off from the _Puntilion_ whilst very hot, and so suffered to cool
without nealing them in the Oven over the Furnace, do thereby (being made
of white Glass, which cools much quicker then green Glass, and is thereby
made much brittler) acquire a very _porous_ and very brittle _texture_: so
that if with the point of a Needle or Bodkin, the inside of any of them be
rubbed prety hard, and then laid on a Table, it will, within a very little
while, break into many pieces with a brisk noise, and throw the parts above
a span asunder on the Table: Now though the pieces are not so small as
those of a _fulminating_ drop, yet they as plainly shew, that the outward
parts of the Glass have a great _Conatus_ to fly asunder, were they not
held together by the _tenacity_ of the parts of the inward Surface: for we
see as soon as those parts are crazed by hard rubbing, and thereby their
tenacity spoiled, the springiness of the more outward parts quickly makes a