owing a little now and
again, the glass may be got even, and a new attempt may be made. It
must not be supposed that this process can be carried on indefinitely,
for the glass tends to lose its viscous properties after a time, or,
at all events, it "perishes" in some way, especially if it has been
allowed to get very thin; consequently too frequent attempts on the
same glass are unprofitable. Two or three trials are as many as it
generally pays to make. As a rule the largest possible flame may be
used with advantage in this operation.

Sec. 32. To blow a bulb in the middle of a tube, the procedure is much
like that already treated, but the manipulation is, if anything,
more difficult, for the further end of the tube must be carried and
turned as well as the end which is held to the lips.

Sec. 33. To make a side Weld.

This is by no means difficult, but is easier with lead glass than with
soda glass. The tube to which it is desired to make a side connection
having been selected, it is closed at one end by rubber tube stops, or
in any other suitable manner. The zone of the proposed connection is
noted, and the tube is brought to near softness round that circle (if
the tube is made actually soft, inconvenience will arise from the
bending, which is sure to occur). Two courses are then open to the
operator, one suitable to a thick tube, the other to a tube of
moderate thickness.

Taking the former first. Provide a piece of glass rod and warm its
end. Direct a small flame against the spot on the thick tube where
the proposed joint is to be. When the glass becomes almost
incandescent at this spot, put the end of the rod against it and draw
out a thread of glass till sufficient "metal" has been removed. Then
fuse off the thread close to the tube.

Fig. 27.

The subsequent procedure is the same as for thin tubes. In this case
heat the spot by the smallest flame available, and get the spot very
hot. Blow it out gently into a bubble, perhaps extending to a height
equal to its diameter. Then heat the top of the bubble till it is
incandescent and blow violently. This will produce an opening fringed
by glass so thin as to exhibit interference colours. Remove the filmy
part, and heat the frayed edges till they cohere and form an incipient
tube. If the flame has been of a correct size, the tube will now be
of the same diameter as the tube to be welded on, and will project
perhaps one-sixteenth of an inch fr