Although annealed glass will break with
the greatest facility by unequal pressure (or vibration of the particles
from a centre), yet it is found to withstand uniform or equable internal
pressure greater than either of the metals.
The property of welding by contact at about
a red heat is peculiar to Glass, whether with or without lead. But for
this useful fact in the ornamental department of Flint Glass, as regards
the addition of handles, feet, rings, &c., the manufacturer would be
left without those manipulatory decorations of his art. So perfect are
the weldings, that at the points of union they become as completely one
homogeneous mass as if the whole were gathered in one piece; yet, the
slightest sulphurous or carbonaceous film intervening between the parts
required to be welded, will cause the cohesion utterly to fail.
Another peculiarity, of which the
manufacturer avails himself, is the liability of Glass to fracture
instantaneously by contact with a cold metal, or by any sudden chill;
this causes instant contraction of the part affected, which cracks
through the mass by a slight blow, as perfectly as if cut with a diamond.
Welding by contact, and separation by contraction, are therefore two
principles, without which the manipulations of the Glass-blower would
be comparatively inoperative.
Having disposed of a few of the peculiarities
and eccentricities of Glass, we proceed to give some account of its
manipulatory conditions. One of these depends upon a constant rotary
motion being given by the workmen to the Glass gathered upon a hollow
blowing-iron, which will, when done with judgment, not only prevent
the Glass from falling from its position, but if rapidly rotated,
will increase its diameter on the principle of centrifugal force.
If a depressed form