or by heating and drawing off the projections, and the new tube sealed
on, usually with the first method (Exercise No. 1). If the break is very
close to the stopcock, very little reheating and blowing can be done, on
account of the danger of getting the stopcock sleeve out of shape, and
the work must be heated very slowly to prevent cracking. The main
reliance is then placed on making a good joint when the tubes are
brought together, and then drawing out this joint a little, at once, to
get an even wall.

CLOSED CIRCUITS OF TUBING.

In some pieces of apparatus closed circuits of circular or rectangular
shape are required. A similar problem is involved in apparatus like the
ordinary Soxhlet extractor, where a small tube is joined to the side of
a large one, bent to form a siphon, and attached again to a continuation
of the original large tube. The difficulty in all such cases is to
provide for the contraction taking place as the last joint cools. If
part of the circuit has the shape of the letter S, or is a spiral, the
natural springiness of the glass will take care of this. If not, the
side of the circuit opposite to the joint and parallel to it must be
heated also, the two being finally heated together to the softening
point after the joint is completed, and then allowed to cool together.

To make the last joint, the rest of the tube is made in approximately
the desired form, the two pieces which are to be joined to make the last
joint being just enough out of the desired position to allow them to
pass one another. The final joint is preferably made in the middle of a
straight piece of tube, not at a tee. The two pieces which are to be
joined are bent so as to just pass each other, marked at the right point
with the glass-knife, and cut there, preferably with a small bead of hot
glass. One or both of these tubes are now warmed to the softening point
in such a place that the tubes can be made to meet properly, and the two
cut ends pressed together. They are now warmed in the flame, and joined
together, either by simultaneously warming the opposite side of the
circuit or some other suitable part, so as to allow the two ends to be
pushed together again after they are softened, or by gently touching the
places that do not unite with a hot bead of glass, and using the glass
to fill up the crack where the ends do not quite meet. Care must be
taken not to leave knots or lumps of glass in the finished joint, and
the latt