excessively rarified by the heat of the glass-Furnace.

But that there may be such an expansion of the aerial substance contained
in those little _blebbs_ or bubbles in the body of the drop, this following
Experiment will make more evident.

* * * * *

Take a small Glass-Cane about a foot long, seal up one end of it
_hermetically_, then put in a very small bubble of Glass, almost of the
shape of an Essence-viol with the open mouth towards the sealed end, then
draw out the other end of the Pipe very small, and fill the whole Cylinder
with water, then set this Tube by the Fire till the Water begin to boyl,
and the Air in the bubble be in good part rarified and driven out, then by
sucking at the smalling Pipe, more of the Air or vapours in the bubble may
be suck'd out, so that it may sink to the bottom; when it is sunk to the
bottom, in the flame of a Candle, or Lamp, nip up the slender Pipe and let
it cool: whereupon it is obvious to observe, first, that the Water by
degrees will subside and shrink into much less room: Next, that the Air or
vapours in the Glass will expand themselves so, as to buoy up the little
Glass: Thirdly, that all about the inside of the Glass-pipe there will
appear an infinite number of small bubbles, which as the Water grows colder
and colder will swell bigger and bigger, and many of them buoy themselves
up and break at the top.

From this _Disceding_ of the heat in Glass drops, that is, by the quenching
or cooling Irradiations propagated from the Surface upwards and inwards, by
the lines CT, CT, DT, DE, &c. the bubbles in the drop have room to expand
themselves a little, and the parts of the Glass contract themselves; but
this operation being too quick for the sluggish parts of the Glass, the