f the constricted portion, which should never be
less than that in the main tube, and usually a little greater. This
operation is most successful with tubing having a relatively thin wall.

=Flanging a Tube.=--This operation produces the characteristic flange
seen on test-tubes, necks of flasks, etc., the object being twofold: to
finish the end neatly and to strengthen it so that a cork may be
inserted without breaking it. This flanging may be done in several ways.
In any case the first operation is to cut the tube to a square end, and
then heat this end so that the extreme sixteenth or eighth of an inch of
it is soft and begins to shrink. The tube is of course rotated during
this heating, which should take place in a flame of slightly greater
diameter than the tube, if possible. The flange is now produced by
expanding this softened part with some suitable tool. A cone of charcoal
has been recommended for this purpose, and works fairly well, if made so
its height is about equal to the diameter of its base. The tube is
rotated and the cone, held in the other hand, is pressed into the open
end until the flange is formed. A pyramid with eight or ten sides would
probably be better than the cone.

[Illustration: FIG. 2.--Flanging tool.]

A better flanging tool is made from a triangular piece of copper or
brass, about 1/16 inch thick, and mounted in a suitable handle. Such a
tool is shown in Fig. 2, being cut from a sheet of copper and provided
with a handle made by wrapping asbestos paper moistened with sodium
silicate solution about the shank of the tool. It is well to have
several sizes and shapes of these tools, for different sizes of tubing.
The two sizes most used will be those having about the following
dimensions: (1) _a_ = 2 inches, _b_ = 1 inch; (2) _a_ = 1 inch, _b_ = 1
inch. When the end of the tube is softened, the tool is inserted at an
angle, as indicated in Fig. 3, and pressed against the soft part, while
the tube is quickly rotated about its axis. If the flange is
insufficient the operation may be repeated. The tool should always be
warmed in the flame before use, and occasionally greased by touching it
to a piece of wax or paraffin. After the flange is complete, the end
must be heated again to the softening temperature and cooled slowly, to
prevent it from cracking.

[Illustration: FIG. 3.--Flanging a tube with flanging tool.]

[Illustration: FIG. 4.--Flanging a tube with carbon rod or wire.]

Some gla