he flame of
a common Bunsen's burner.

When the end of a tube is to be closed with a cork or stopper, its
mouth should be expanded a little, or =bordered=. To do this, heat the
end of the tube by rotating it in the flame till it softens, then remove
it from the flame, at once introduce the charcoal cone (Fig. 5, p. 11),
and rotate it with gentle pressure against the softened glass till the
desired effect is produced. In doing this it is very important that the
end of the tube shall be uniformly heated, in order that the enlargement
shall be of regular form. If the tube cannot be sufficiently expanded at
one operation, it should be re-heated and the process repeated.

Borders, such as are seen on test-tubes, are made by pressing the
softened edge of the tube against a small iron rod. The end of the rod
should project over the softened edge of the tube at a slight angle, and
be pressed against it, passing the rod round the tube, or rotating the
tube under the rod.

=Sealing=, that is closing the ends of tubes, or other openings, in
glass apparatus.

In performing this and all the other operations of glass blowing, the
following points must be constantly kept in mind:--

(_a._) That it is rarely safe to blow glass whilst it is still in the
flame, except in certain special cases that will be mentioned
subsequently. Therefore always remove apparatus from the flame before
blowing.

(_b._) That when heating glass tubes, unless it is specially desired to
heat one portion only, the tube must be constantly rotated in the flame
to ensure that it shall be uniformly heated, and to prevent the tube or
mass of glass from assuming an irregular form.

(_c._) Always blow gently at first, and slowly increase the force
applied till you feel or see the glass giving way. It is a good plan to
force the air forward in successive short blasts rather than in one
continued stream.

(_d._) When it is necessary to force air into tubes of fine bore, such
as thermometer tubes, the mouth must not be used, for moisture is
thereby introduced into the tube, which it is very difficult to remove
again in many cases. All tubes of very small bore should be blown with
the aid of an india-rubber blowing-bottle, such as are used for
spray-producers, Galton's whistles, etc. The tube to be blown must be
securely fixed to the neck of the bottle, which is then held in one
hand, and air is forced from it into the tube as it is required. These
bottles are