escribed
later on, may be used. However, it is not essential to use a diamond
wheel, though it saves time.

When the cut is made to a sufficient depth proceed thus: Obtain two
strips of bibulous paper or bits of tape and twist them round the tube
on each side of the scratch, allowing not more than one-eighth of an
inch between them. Then add a few drops of water to each, till it is
thoroughly soaked, but not allowing water to run away. Dry out the
scratch by a shred of blotting paper.

Turn down the oxygas flame to the smallest dimensions, and then boldly
apply it with its hottest part playing right into the nick and at a
single point. Probably in about two seconds, or less, the tube will
break. If it does not, rotate the tube, but still so that the flame
plays in the nick. After making the tube very hot all round--if it
has not broken--apply the flame again steadily at one point for a few
seconds and then apply a bit of cold iron. If the tube does not break
at once during these processes, let it cool, and cut the groove
deeper; then try again. [Footnote: This method is continually being
reinvented and published in the various journals. It is of unknown
antiquity.]

Fig. 22.

If the tube breaks after great heating and long efforts, it will
probably leave incipient cracks running away from the break, or may
even break irregularly. A good break is nearly always one that was
easily made. If a number of rings have to be cut, or a number of cuts
made on glass tubes of about the same size, it will be found
economical in the end to mount a glazier's diamond for the purpose. A
simple but suitable apparatus is figured (Fig. 23).

Fig. 23.

The only difficulty is to regulate the position of the diamond so that
it cuts. In order to do this, carefully note its cutting angle by
preliminary trials on sheet glass, and then adjust the diamond by
clamps, or by wriggling it in a fork, as shown. Weight the board very
slightly, so as to give the small necessary pressure, and produce the
cut by rotating the tube by hand. When a cut is nearly completed take
great care that the two ends join, or irregularity will result. This
is not always easy to do unless the tube happens to be straight.
Having got a cut, start a crack by means of a fine light watchmaker's
hammer, or even a bit of fused glass, and entice the crack round the
cut by tapping with the hammer or by means of the flame pencil.

If the cut is a true "cut