be, it is sufficient to melt the end without previous sealing,
rotate it in the flame until enough glass has collected, remove from the
flame and blow while keeping the tube in rotation.

_Thermometer Bulbs._--If the thermometer is to be filled with mercury,
it is desirable to use a rubber bulb for blowing, as moisture is liable
to condense inside the tube when the mouth is used, and this moisture
will cause the mercury thread to break. In any case, a slight pressure
should be maintained inside the thermometer tube while it is in the
flame; otherwise the fine capillary tube will close and it will be very
difficult to expand the heated glass into a bulb.

_Large Bulbs._--When a large bulb is needed on a small or medium sized
tube, it is often necessary to provide more glass than would be obtained
if the bulb were blown in the ordinary way. One method is to expand the
tube in successive stages along its axis, as shown by _a_, Fig. 6. These
expanded portions are then re-heated, so that they run together into one
hollow mass from which the bulb is blown; _b_ and _c_, illustrate this.
Another method, and one which is useful for very large bulbs, is to
fuse on a length of large, thick-walled, tubing. The heat reflector,
_g_, Fig. 3, should be used, if necessary, when making large bulbs. It
consists of a sheet of asbestos mounted in a foot, and is used by being
placed close to the mass of glass on the side away from the blowpipe
flame while the glass is being heated.

[Illustration: Fig. 6]

_Bulbs of Dissimilar Glass._--These may be made by the second method
given under "Large Bulbs," but the joint should be blown as thin as
possible. Further instructions in the use of unlike glasses are given on
page 94.

_A Bulb in the Middle of a Tube._--Unless the bulb is to be quite small,
it will be necessary to join in a piece of thick glass tubing, or to
draw the thin tube out from a larger piece, thus leaving a thick mass in
the middle as shown by _d_, Fig. 6. This mass of glass should now be
rotated in the blowpipe flame until it is quite soft and on the point of
running together. Considerable practice will be necessary before the two
ends of the tube can be rotated at the same speed and without
"wobbling," but this power must be acquired. When the glass is
thoroughly hot, remove from the flame, hold in a horizontal position,
and expand by blowing. It is essential to continue the rotation while
this is done. Should one part of