s kept fairly hot--it need
not be soft; interrupt any other work often enough to ensure this.
Then, directing the flame chiefly on the thin rod, begin to melt and
pull the glass over the edges of the gap. When the gap is closed get
the lump very hot, so that all the glass is well melted together, and
then, if necessary, pull the excess of glass off, as before described.

It must be remembered that this and the method of the previous section
are emergency methods, and never give such nice joints as a
manipulation which avoids them, i.e. when the ends of the tubes are
perfectly straight and true to begin with. Also note that, as the
tubes cannot be kept in rotation while being patched, it is as well to
work at as low a temperature as possible, consistently with the other
conditions, or the glass will tend to run down and form a drop,
leaving a correspondingly thin place behind.

Fig. 20.

Sec. 28. A very common fault in cutting a tube of about an inch in
diameter is to leave it with a projecting point, as shown. This can
be slowly chipped off by the pliers, using the jaws to crush and grind
away the edge of the projection; it is fatal to attempt to break off
large pieces of glass all at once.

Sec. 29. It will be convenient here to mention some methods of cutting
large tubes. With tubes up to an inch and a half in diameter, and
even over this--provided that the glass is not very thick--we may
proceed as follows: Make a good scratch about half an inch long, and
pretty deep, i.e. pass the knife backwards and forwards two or three
times. Press a point of melted glass exactly on one end of the
scratch; the glass point even when pressed out of shape should not be
as large as a button one-twelfth of an inch in diameter. If this
fails at first, repeat the operation two or three times.

Fig. 21.

If a crack does not form, touch the hot place with the cold end of the
nail. If no success is obtained, try the other end of the scratch.
If failure still pursues the operator, let him make another cut on the
opposite side of the tube and try again. In general, the tube will
yield the first or second time the hot drop of glass is applied.
Never apply the drop at the centre of the scratch, or a ragged crack,
which may run in any direction, will result. Very often, with a large
tube, the crack formed by a successful operation will only extend a
short distance. In this case it is desirable to entice the crack
ro