frequently of service to the glass-blower--_e.g._, when
tubes of very fine bore have to be united, it is necessary to maintain
an internal pressure slightly exceeding that of the air throughout the
operation, in order to prevent the viscous glass from running together
and closing the tube. An india-rubber blowing-ball is very convenient
for this purpose.

To seal the end of a glass tube (Fig. 8), adjust the flame so that it
will heat a zone of glass about as broad as the diameter of the tube to
be sealed (see _A_, Fig. 8). Hold the tube on each side of the point
where it is to be sealed in the manner described in the description of
bending glass tubes (p. 28). Bring the tube gradually into the flame,
and heat it with constant rotation, till the glass softens (for lead
glass the oxidising flame must be used, as has been already
explained).[6] When the glass begins to thicken, gently pull asunder the
two ends, taking care not to pull out the softened glass too much, but
to allow the sides to fall together, as shown at _A_. When this has
occurred, heat the glass at the narrow part till it melts, and pull
asunder the two ends. The closed end should present the appearance
shown at _D_. If the glass be drawn out too quickly its thickness will
be unduly reduced, and it will present the appearance shown at _B_. In
that case apply a pointed flame at _b_, and repeat the previous
operation so as to contract the tube as at _c_, taking care not to allow
the glass to become much increased nor decreased in thickness.

[6] Remember that when the lead glass is heated to the proper
temperature it will present an appearance which may be described as a
greenish phosphorescence. At higher temperatures it assumes an
orange-red appearance. If it loses its transparency and assumes a dull
appearance, it must be moved further into the oxidising parts of the
flame.

If a considerable mass of glass be left at _d_, it may be removed by
heating it to redness, touching it with the pointed end of a cold glass
tube, to which it will adhere, and by which it may be pulled away.

[Illustration: FIG. 8.]

When the end of the tube presents the appearance shown in the diagram
_D_, and the mass of glass at _d_ is small, the small lump that remains
must be removed by heating it till it softens, and _gently_ blowing with
the mouth, so as to round the end and distribute the glass more
regularly, as shown in _E_. The whole end, from the dotted line _e_,
mus