in the middle; this must be
done gradually and evenly; that is to say the tube must be rotated
during heating and held some considerable distance in front of the flame
at first; otherwise the outer surface of the glass will expand before
the interior is affected and the tube will break. From two to five
minutes, heating at a distance of about eight inches in front of the
flame will be found sufficient in most cases, and another minute should
be taken in bringing the tube into the flame. Gradual heating is
important, but even heating is still more important and this can only be
obtained by uniform and steady rotation. Until the student can rotate a
tube steadily _without thinking about it_, real progress in
glass-blowing is impossible.

When the tube is in the flame it must be held just in front of the blue
cone and rotated until the glass is soft enough to permit the ends to be
drawn apart. Continue to separate the ends and, at the same time, move
the tube very slightly along its own axis, so that the flame tends to
play a little more on the thicker part than on the drawn-out portion. If
this is done carefully, the drawn-out portion can be separated off,
leaving only a slight "bleb" on the portion it is desired to seal. This
is illustrated by _b_, Fig. 4.

To convert the seal at _b_, Fig. 4., into the ordinary form of test-tube
seal, it is only necessary to heat the "bleb" a little more strongly,
blow gently into the tube until the thick portion is slightly expanded,
re-heat the whole of the rounded end until it is beginning to collapse,
and give a final shaping by careful blowing after it has commenced to
cool. In each case the glass must be removed from the flame before
blowing. The finished seal is shown by _c_, Fig. 4. If desired, the open
end may now be finished by heating and rotating the soft glass against
the large turn-pin, as illustrated in _d_, but the turn-pin must not be
allowed to become too hot, as if this happens it will stick to the
glass. After turning out the end, the lip of glass must be heated to
redness and allowed to cool without coming in contact with anything;
otherwise it will be in a condition of strain and liable to crack
spontaneously. The finished test-tube is shown by _e_.

When it is necessary to seal a substance inside a glass tube, the bottom
of the tube is first closed, as explained above, and allowed to cool;
the substance, if a solid, is now introduced, but should not come to
wit