Smooth one end of a piece of No. 2 tube and allow it to cool. Close the other end in the blowpipe flame, turn it slowly, and heat until it is very hot. Take the tube out of the flame, put the smooth end into your mouth quickly, and blow as hard as you can (Fig. 11). Do you get a fine big glass bubble which bursts with a pop?
Fig. 11. Making A Glass Bubble
If you get only a small bulb at the first trial, heat the end. and try again. Do you find that the bulb shrinks when heated but blows out again readily?
When you get a big bubble, place the bubble end of the tube on a cooling block and break all the thin glass away from the tube by striking it with the file or blowpipe. Then close the end and blow another bubble.
Repeat until you can blow bubbles easily.
Repeat with a piece of No. 4 tube.
Do you find that the thin glass of the bubbles shows colors, especially in sunlight, just as soap bubbles do? You boys who have had the Gilbert set on "Light Experiments" will know that these colors are due to "interference." The colors produced by a thin film of oil on water are also produced by "interference."