together, and avoidance of hard or sudden blowing
when expanding the joint. The finished work is shown by _c_, Fig. 5.

[Illustration: Fig. 5]

To join a small glass tube to the end of a large one, the large tube
should first be sealed, a small spot on the extreme end of the seal
heated, and air pressure used to expand the heated spot as shown in _d_.
This expanded spot is then re-heated and blown out until it bursts as
shown in _e_, the thin fragments of glass are removed and the end of the
small tube turned out as shown in _f_. After this the procedure is
similar to that used in jointing two tubes of equal size.

When these two forms of joint have been mastered, a T piece will present
but little difficulty. It is made in three stages as shown in Fig. 5,
and the procedure is similar to that used in joining a large and small
tube. Care should be taken to avoid softening the top of the "T" too
much, or the glass will bend and distort the finished work; although a
slight bend can be rectified by re-heating and bending back. Local
re-heating is often useful in giving the joint its final shape.

An exhaustion branch is often made by a totally different method. This
method is shown by _g_, _h_, and _i_, Fig. 5; _g_ is the tube on which
the branch is to be made. The end of a rod of similar glass should be
heated until a mass of thoroughly liquid glass has collected, as shown,
and at the same time a spot should be heated on that part of the tube
where it is desired to make the branch. The mass of hot glass on the rod
is now brought in contact with the heated spot on the tube and expanded
by blowing as shown by _h_. The air pressure in the tube is still
maintained while the rod is drawn away as shown by _i_. This will give a
hollow branch which may be cut off at any desired point, and is then
ready for connection to the vacuum pump.

If the rod used is of a dissimilar glass, the branch should be blown
much thinner. Such a branch will often serve as a useful basis for
joining two tubes of different composition, as the ordinary type of
branch is more liable to crack when made with two glasses having
different coefficients of expansion.

_Blowing Bulbs._--A bulb may be blown on a closed tube such as that
shown by _c_, Fig. 5, by rotating it in the blowpipe flame until the end
is softened, removing it from the flame and blowing cautiously. It is
desirable to continue the rotation during blowing. In the case of a very
small tu