d _a2_, show the stages by which this form of internal seal
is made. For the first trials, it is well to work with fairly
thick-walled tubing, which should be cut into two pieces, each being
about eight inches long.

[Illustration: Fig. 9]

First seal the end of one tube as described on page 13, heat the sealed
end and expand to a thick walled bulb. Fuse the end of the other tube,
attach a piece of glass rod to serve as a handle, and draw out; cut off
the drawn-out portion: leaving an end like _a_.

Now heat a small spot at the end of the bulb, blow, burst out, and
remove the thin fragments of glass. Heat a zone on the other tube at the
point where the drawn-out portion commences and expand as shown by _a1_.

The next stage is to join the tubes. Heat the ragged edges of the
burst-out portion until they are thoroughly rounded. At the same time
heat the drawn-out tube to just below softening point. Then, while the
rounded edges of the burst-out portion are still soft, insert the other
tube; rotate the join in the blowpipe flame until it is quite soft, and
expand by blowing. If necessary, re-heat and expand again. The finished
seal, which should be slightly annealed by smoking in a sooty flame, is
shown by _a2_.

_A Spray Arrester._--This is made by the second method, in which the
piece of tube which projects inside the bulb is fused in position first
and the outer tube is then joined on. The various stages of making are
illustrated by _b_, _b1_ and _b2_, Fig. 9.

A bulb is blown between two tubes by the method given on page 22, the
larger tube is then cut off and the small piece of tube introduced into
the bulb after having been shaped as shown in by _b_, Fig. 9. The
opening in the bulb is sealed as shown by _b1_. The sealed part is now
heated and the bulb inclined downwards until the inner tube comes in
contact with the seal and is fused in position. This operation requires
some practice in order to prevent the inner tube either falling through
the soft glass or becoming unsymmetrical. The end of the bulb, where the
inner tube comes in contact with it, is now perforated by heating and
blowing, thus giving the form shown by _b2_, and the outer tube is
joined on. The finished spray arrester is shown by _b3_. Practice alone
will give the power to produce a symmetrical and stable piece of work.

_Two Forms of Filter Pump._--That illustrated by _d_, Fig. 9, is made by
the method explained under "An Air Trap on a Barome