t all events to disappear.

A rod of quartz, say three inches long, one-sixteenth of an inch in
diameter, and free from bubbles for half an inch of its length, even
when examined by a strong lens, is suitable for drawing into threads.
The rod is manipulated exactly in the manner described under
glass-blowing, and is finally drawn down at the bubble free part into
a needle, say 0.02 inch in diameter (No. 25 on the Birmingham wire
gauge), and 2 inches long.

Fig. 66.

There is one peculiarity about fused quartz which renders its
manipulation easier than that of glass--it is impossible to break
fused quartz, however suddenly it be thrust into the blow-pipe flame.
A rod having a diameter of three-sixteenths of an inch--and perhaps
much more--may be brought right up to the tip of the inner cone of
the oxy-gas flame and held there-till one side fuses, the other being
comparatively cool, without the slightest fear of precipitating a
smash. In seven years' experience I have never seen a bit of once
fused quartz broken by sudden heating; whether it might be done if
sufficient precautions were taken I do not know.

The reason of the fortunate peculiarity of quartz in this respect is,
I presume, to be found in the fact that quartz once it has been fused
is really a very strong material indeed, and is also probably the
least expansible substance known. From some experiments of the writer
upon the subject, it may be concluded that at the most quartz which
has been fused expands only about one-fifth as fast as flint-glass, at
all events between 20 deg. and 70 deg. C.

Sec. 84. Drawing Quartz Threads.

The thick end of the rod of quartz is held in the fingers or
occasionally in a clip. The end of the fine point is attached to a
straw arrow by means of a little sealing-wax. The arrow is laid on
the stock of a crossbow in the proper position for firing. See
Figs. 67 and 68, which practically explain themselves.

The needle is heated by the blow-pipe till a minute length is in a
state of uniform fusion; the arrow is then let fly, when it draws a
thread out with it. The arrow is preferably allowed to strike a
wooden target placed, say, 30 feet away from the bow, and a width of
black glazed calico is laid under the line of fire to catch the thread
or arrow if it falls short. The general arrangements will be obvious
from the figure.

The bow is of pine in the case where very long thin threads are
required, though f