n the main tube in the two crotches,
so that both tubes blow out a little and give space for the gases to
turn in, as indicated in _c_, Fig. 7, and at the same time increase the
mechanical strength of the job. On the other hand, care is taken not to
deform the main tube, and not to produce such a bulge or bulb at the
joint as will prevent the finished tube from lying flat on a table.

=Discussion.=--Most beginners tend to err in the first steps of this
operation, by blowing too hard and too long when blowing out the little
bulb. The result is a large, very thin bulb, which breaks off in such a
way as to leave a hole in the main tube, occupying nearly half the
circumference of the tube at that point, instead of the neat side tube
which they should have. It is not difficult to seal a tube on this side
tube, but it is very difficult to seal a tube into a hole in another
tube. Care should be taken here, as in the two previous exercises, that
the lump obtained at the joint when the two tubes are put together is
made as small as possible, and reduced if possible by gently drawing on
the side tube as soon as the tubes have actually joined. It is much
easier to prevent the formation of a lump at the joint than it is to
remove the lump after it is formed. The remarks previously made about
blowing quickly after removing the work from the flame apply here with
especial force. A "tee" tube, from its very nature, is exposed to a good
many strains, so care must be taken that the walls of the joint are of
uniform thickness with the rest of the tube.

The beginner will find it easiest to make this tube out of two pieces of
the same tube, about 1/4 inch in diameter. Larger or smaller tubing is
usually more difficult. If tubing much more than 1/4 inch is used, the
whole joint, including part of the main tube, must be heated nearly to
the softening point at the close of the operation, and well annealed, as
described in Chapter 1 (page 3) or it will be almost certain to crack.
In the larger sizes of tube it will be necessary to heat the whole
circumference of the main tube frequently during the operation, to
prevent it from cracking.

In sealing a small tube on the side of a large one, it is usually
advisable, after warming the spot where the joint is to be made, to
attach a small drop of glass to the tube at that point, and direct the
flame upon that, thus supplying at the same time both a definite point
to be heated and an extra supply