Take a piece of No. 2 tube about 7 inches long, close one end, smooth the other, and when cool cut the tube at the middle.
Now join these two pieces as follows: Hold the ends opposite each other near the top of the lamp flame (Fig. 72),. rotate constantly, and when nearly red hot bring the ends accurately together in the flame, press together slightly, draw out slightly, and remove from the flame.
Fig. 72. Joining Two Tubes
The ends are now stuck together, but the glass is in a slight lump around the joint and if allowed to cool will crack very easily. It is necessary to work the glass back and forth to get rid of the lump and to make the glass uniform on both sides of the joint. Do this as follows : Heat one third of the joint in the blowpipe flame (Fig. 73), and when red hot blow a slight bulge. Now turn the joint one third, heat the next third red hot and blow a slight bulge. Repeat with the remaining third.
Fig. 73. Working The Joint
Now heat the first third again until it is red hot and shrinks, then blow a slight bulge again. Repeat this with the other two thirds.
Repeat this whole operation a third time and blow just enough to leave the joint the same size as the remainder of the tube or a little larger.
This heating and blowing has worked the joint back and forth until the glass is fairly uniform. It makes a strong joint.
Cut off the closed end and smooth the edge.
Repeat with a piece of No. 4 tube.
Fig. 74. Joining tubes of different sizes
Fig. 75. Making A Large Hole