s of
tubes or in globes, in fact, in almost any position on glass apparatus.
If another tube is to be attached at the opening, it is a good plan to
proceed to this operation before the tube has cooled down.

[Illustration: FIG. 13.]

The openings obtained by the method above described are too large when
platinum wires are to be sealed into them. Suppose that it is necessary
to pierce the tube _A_ of Fig. 13 in order to insert a platinum wire at
_a_; direct the smallest pointed flame that will heat a spot of glass to
redness on the point _a_. When the glass is viscous, touch it with the
end of a platinum wire _w_, to which the glass will adhere; withdraw the
wire and the viscous glass will be drawn out into a small tube, as shown
at _B_; by breaking the end of this tube a small opening will be made.
Introduce a platinum wire into the opening, and again allow the flame to
play on the glass at that point; it will melt and close round the wire.
Before the hot glass has time to cool, blow gently into the mouth of the
tube to produce a slightly curved surface, then heat the neighbouring
parts of the tube till the glass is about to soften, and let it cool in
cotton wool. Unless this is done, I find that glass tubes into which
platinum wires have been sealed are very apt to break during or after
cooling.

To ensure that the tube shall be perfectly air-tight, a small piece of
white enamel should be attached to the glass at _a_ before sealing in
the wire.

=Uniting Pieces of Glass to Each Other, known as Welding, or
Soldering.=--The larger and more complicated pieces of glass apparatus
are usually made in separate sections, and completed by joining together
the several parts. This is therefore a very important operation, and
should be thoroughly mastered before proceeding to further work.

In order to produce secure joints, the use of tubes made of different
kinds of glass must be avoided. Soda glass may be joined securely to
soda glass, especially if the tubes belong to the same batch, and lead
glass to lead glass. But, though by special care a joint between lead
glass and soda glass, if well made, will often hold together, yet it is
never certain that it will do so.

_To join two Tubes of Equal Diameters._--Close one end of one of the
tubes with a small cork. Heat the open end of the closed tube, and
either end of the other tube in a small flame until they are almost
melted, taking care that only the ends of the tubes