sirable that
the student should learn to maintain a steady steam of air with his
mouth and, at the same time, be able to breathe. This requires a little
practice.

As a first exercise in breathing, before trying to breathe while using
the mouth blowpipe, the student should close his mouth and inflate his
cheeks with air; now, still keeping his cheeks tightly inflated, he
should attempt to breathe through the nose. At first, this may be found
rather difficult, but it becomes remarkably easy after a little
practice. When he has mastered this, the student may practise the same
operation, but with the blowpipe. It is important to bear in mind that
the cheeks, not the lungs, form the reservoir for air used in
maintaining the blowpipe flame. After a while, the student will find
that he can maintain a steady air pressure and yet breathe with complete
comfort.

In adjusting the flame, care should be taken not to blow so hard as to
produce a ragged and noisy cone of fire. A small jet, such as that
commonly used on a mouth blowpipe, will with care give a pointed and
quiet flame, having an appearance similar to that shown in the
illustration.

With a blowpipe like this, it is quite easy to seal glass tubes up to an
inch in diameter, to join tubes up to half an inch in diameter, to bend
tubes, to blow small bulbs, and to make the simpler forms of internal
seal; but the provision for condensation of moisture is not ideal, and
prolonged use of such a blowpipe also tends to produce undue fatigue.

_A Mouth Blowpipe With an Expanding Reservoir._--This form of blowpipe
can be made to give most excellent results; it is highly portable, and
does not produce nearly so much fatigue when used continuously as the
blowpipe described in the last section. Various slight modifications
have been made in its construction during the last eighty years, but
that described below will be found quite satisfactory.

The apparatus consists of a tube through which air is blown from the
mouth, a valve through which the air passes into an expanding reservoir,
and a blowpipe jet in communication with the reservoir.

In making the valve, several essentials have to be remembered; it must
allow a free passage of air into the reservoir, it must open easily, and
must close quickly. A satisfactory form of valve is that shown by _b_,
Fig. 16. The moving part consists of a light glass bulb of about
three-eights of an inch diameter and having a glass stem of rat