This is easily done by holding the string in the blow-pipe flame and
allowing it to fuse down. Twisting the fused part has a good effect
in assisting the operation. It is desirable to use a large jet and as
powerful a flame as can be obtained during this part of the operation.

The final result should be a rod, say two or three inches long and
one-eighth of an inch thick, which will in most cases contain a large
number of air bubbles. Since the presence of drawn-out bubbles cannot
be advantageous, it is often desirable to get rid of them, and this
can conveniently be done at the present stage. The process at best
is rather tedious; it consists in drawing the quartz down very fine
before an intense flame, in order to allow the bubbles to get close
enough to the surface to burst. A considerable loss of material
invariably occurs during the process; for whenever the thin rod
separates into two bits the process of flame-drawing of threads goes
on, and entails a certain waste; moreover, the quartz in fine
filaments is probably partially volatilised.

Sooner or later, however, a sufficient length of bubble-free quartz
can be obtained. It must not be supposed that it is always necessary
to eliminate bubbles as perfectly as is contemplated in the foregoing
description of the treatment, but for special purposes it may be
essential to do so, and in any case the reader's attention is directed
to a possible source of error.

It may be mentioned in connection with this matter that crystals of
quartz may look perfectly white and clear, and yet contain impurity.
For instance, traces of sodium are generally present, and lithium was
found in large spectroscopic quantity in five out of six samples of
the purest crystals in my laboratory. The presence of lithium in rock
crystal has also been detected by Tegetmeier (Vied. Ann, xli. p.
19, 1890).

After some practice in preparing rods and freeing them of bubbles the
operator will notice a distinct difference in the fusibility of the
samples of quartz he investigates, though all may appear equally pure
to the unaided eye. It should be mentioned, however, that high
infusibility cannot always be taken as a test of purity, for the most
infusible, or rather most viscous, sample examined by the writer
contained more lithium than some less viscous samples.

Fig. 65.

During the process of freeing the quartz from bubbles the lithium and
sodium will be found to burn away, or a