Close one end of the No. 2 tube in the blowpipe flame again and while it is still hot blow carefully into the open end un-til you have a bulb about 1/2 inch ir: diameter (Fig. 12). Now let it cool.
Make a scratch with the file about inch from the bulb, break the tube at this point (Fig. 13), and smooth the rough edge.
Fig. 12. Blowing a bulb
Fig. 13. A water balloon
Fig. 14. The Balloon Sinks And Rises
Put the bulb in a tumbler of water. Does it float? If not, make another balloon with a larger bulb.
Find a large bottle made of clear glass, the neck of which will fit your solid rubber stopper.
Fill the bottle with water to overflowing, insert the balloon, and then the stopper.
Now press down hard on the stopper. Does the balloon sink in a most magical manner (Fig. 14) ?
Release the stopper. Does the balloon rise in an equally magical manner?