large tubes, however, because it leaves a very large lump of
glass which may crack on cooling or reheating.
Practice the following method of
closing a large tube; first
FIG. 31 CLOSING A LARGE TUBE
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with a piece of No. 4 tube, then with a piece of No. 6: Attach a working
handle to the end to be closed, heat the tube ½ inch from the end
in the blowpipe flame, turn constantly, and when soft pull apart until
the tube has
FIG. 32 MAKING A SUBMARINE
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the shape 1, Fig. 31. Heat, turn, and pull the end away to
leave the tube as in 2. Heat the end and blow out until it has
the shape 3. The end is now closed and the glass has about the
same thickness as the remainder of the tube.
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