e should form the concave side of the
bent tube.

[Illustration: FIG. 7.]

The exact condition in which the glass is most suitable for bending can
only be learned by making a few trials. If it is too soft in consequence
of being overheated, the sides will collapse. If, in the endeavour to
heat the side _A_ of Fig. 7 a little more than _B_, _B_ is
insufficiently heated, the tube will be likely to break on the convex
side _B_. If the bent tube be likely to become flattened, and this
cannot always be prevented in bending very thin tubes, the fault may be
avoided by blowing gently into one end of the tube whilst bending it,
for which purpose the other end should be closed beforehand. A tube
already flattened may, to some extent, be blown into shape after
closing one end and re-heating the bent portion, but it is not easy to
give it a really good shape.

When making a bend like that in Fig. 7, to secure that the arms of the
tube _C_ and _D_, and the curve at _B_, shall be in one plane, the tube
should be held in a position perpendicular to the body, and brought into
the position shown in the figure during bending, by which means it will
be found easy to secure a good result. Lead glass tubes must be removed
from the flame before they become hot enough to undergo reduction. If
they should become blackened, however, the stain may be removed by
re-heating in the oxidising flame (see p. 18).

When a very sharp bend is to be made, it is sometimes best to heat a
narrow zone of the glass rather highly in the blow-pipe flame, and to
blow the bend into shape at the moment of bending it, as previously
described, one end having been closed for that purpose beforehand. Lead
glass should be heated for this purpose in the oxidising flame (pp. 17
to 22).

The processes of bending large tubes, making U-tubes and spiral tubes,
are more difficult operations, and will be explained in Chap. IV.

=Rounding and Bordering the Ends of Tubes.=--After cutting a piece of
glass tube in two pieces, the sharp edges left at its ends should be
rounded by holding them in a flame for a few moments till the glass
begins to melt. The oxidising point of a pointed flame may be used for
both kinds of glass. The flame will be coloured yellow by soda glass at
the moment of melting. This indication of the condition of soda glass
should be noted, for it serves as a criterion of the condition of the
glass. The ends of soda glass tubes may also be rounded in t