ught nearer to the flame, and, finally, into contact with
it, still with constant rotation and movement, so as to warm a
considerable part of the tube. When the glass has been brought fairly
into contact with the flame, it will be safe to apply the heat at the
required part only. Care must be taken in these preliminary operations
to avoid heating the more fusible glasses sufficiently to soften them.

=Methods of working with Lead and soft Soda Glass respectively.=--When
lead glass is heated in the brush flame of the ordinary Herapath
blow-pipe, or within the point of the pointed flame, it becomes
blackened on its surface, in consequence of a portion of the lead
becoming reduced to the metallic state by the reducing gases in the
flame. The same thing will happen in bending a lead glass tube if it is
made too hot in a luminous flame. A practical acquaintance with this
phenomenon may be acquired by the following experiment:--

Take a piece of lead glass tube, bring it gradually from the point of a
pointed flame to a position well within the flame, and observe what
happens. When the glass reaches the point _A_ (Fig. 3), or thereabouts,
a dark red spot will develop on the glass, the area of the spot will
increase as the glass is brought further in the direction _A_ to _B_. If
the glass be then removed from the flame and examined, it will be found
that a dark metallic stain covers the area of the dark red spot
previously observed. Repeat the experiment, but at the first appearance
of the dark spot slowly move the glass in the direction _A_ to _C_. The
spot will disappear, and, if the operation be properly performed, in its
place there will be a characteristically greenish-yellow luminous spot
of highly heated glass. In this proceeding the reduced lead of the dark
spot has been re-oxidised on passing into the hot gases, rich in oxygen,
which abound at the point of the flame. If one end of the tube has been
previously closed by a piece of cork, and if air be forced into the tube
with the mouth from the open end before the luminous spot has become
cool, the glass will expand. If the experiment be repeated several
times, with pointed flames of various sizes, the operator will quickly
learn how to apply the pointed flame to lead glass so that it may be
heated without becoming stained with reduced lead.

If the spot of reduced metal produced in the first experiment be next
brought into the oxidising flame, it also may gradually