nd pulling the ends apart with a slight inclination away from the
scratch. In other words, combine a small bending moment with a
considerable tensional stress. It is important to learn to do this
properly. If the proportions are not well observed, the tube will
break with difficulty, and the section will not be perpendicular to
the main length. If the knife is in good order it will make a fine
deep scratch--the feel of the glass under the knife will enable the
operator to decide when the scratch is made. The operation of cutting
large tubes will be treated further on. The two halves of the tube
being held one in each hand, and one tube closed at one end, the
extremities to be united will be warmed, and then put in the flame as
before.

Fig. 16.

There are many ways of proceeding--perhaps the easiest is as follows.
As soon as the glass shows signs of melting at the ends--and care
should be taken that much more is not heated--take both bits out of
the flame. Stop rotating for a moment, and resting the arms carefully
on the edge of the table, raise the tubes above the flame and bring
the ends swiftly and accurately together. This is a case of "sudden
death no second attempt at making the ends meet can be allowed; if
the tubes join in any other than a perfectly exact manner a kink more
or less objectionable will result. In practice the operator will
learn to bring the ends together, commencing at one point; i.e. the
axes of the tubes will be inclined at first, so as to cause adherence
at one spot only. If this is not quite "fair", then less damage is
done in moving one tube slightly up or down to get the contact exact.
The tubes will then be closed upon one another as if they were hinged
at the joint. This must be done lightly, yet sufficiently, to ensure
that the glass is actually in contact all round.

Having gone so far, replace the tubes--now one--in the flame, and
carefully rotating the glass, raise the temperature higher than in the
operation just described, in fact the higher the temperature, short of
burning the glass, the better. Take the tube out of the flame and
blow into the open end, turning constantly as before. One puff is
enough. Then turn and pull the glass apart till it is of the same
diameter and thickness throughout, and feel that it is straight as
before.

Though it is in general of high importance that the joint should be
well heated, the beginner will probably find that he "ties