es of globes or
flasks; great care must be taken, however, especially if the walls of
the globe be thin, to secure that the tube is well attached to the mouth
of the opening when the melted ends are first brought into contact, for,
with thin glass, any hole that may be left will probably increase whilst
the joint is being blown into shape, owing to cohesion causing the glass
to gather in a thickened ring round an enlargement of the original
opening.[8]

[8] If such an opening be observed, it may usually be closed by touching
its edges with a fused point of glass at the end of a drawn out tube.

In order to unite a tube of soda glass to a tube of lead glass, the end
of the soda glass tube must be carefully covered with a layer of soft
arsenic glass.[9] This must be done so perfectly that when the ends to
be united are brought together the lead and soda glass are separated by
the enamel at every point.

[9] This can be obtained from Messrs. Powells, Whitefriars Glassworks.

_To Seal a Tube inside a Larger Tube or Bulb._--Suppose that an air-trap
(3 of Fig. 18) is to be constructed from a small bulb (_A_) blown on a
glass tube (1).

[Illustration: FIG. 18]

Either cut off the tube close to the bulb at _B_, or better, remove the
end by melting the glass and pulling it away from _B_, and then pierce
_A_ at _B_, No. 2, by heating the glass there and blowing out a small
bulb as described under Piercing.

Prepare a tube (4) drawn out at _E_ with a bulb blown at _D_. Insert _E_
into the opening _B_, press _D_ well against the mouth _B_ and slowly
rotate before the blow-pipe till _D_ adheres to _B_. Then heat and blow
the joint spot by spot as in other cases, taking care that the glass is
blown out on each side of the joint; lastly, heat the whole joint
between _aa_, and blow it into its final shape.

These joints are very apt to break after a few minutes or hours if the
glass of _D_ be much thicker than that of the bulb _A_. They should be
wrapped in cotton wool for annealing as soon as possible, as the rate at
which the tube _E_ cools is likely to be less rapid than that of the
parts of the apparatus which are more freely exposed to the air;
therefore all such internal joints require very careful annealing, and
they should always be made as thin as is consistent with the use to
which they are to be put.

Tubes may also be sealed into the ends or sides of larger tubes by
piercing them at the point at which the inserte