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? [Illustration: FIG. 15 A BALLOON RACE ] Experiment 9. Balloon races. Make another water balloon. Put the two balloons together in the bottle filled to overflowing with water. Insert the stopper and press down hard. Do the balloons sink (Fig. 15), and does one sink more quickly than the other? Release the stopper. Do the balloons rise, and does one rise more quickly than the other? The most buoyant balloon sinks last and rises first. The “why” of it [Illustration: FIG. 16 DRAWING A THIN TUBE ] You boys who have the Gilbert set on “Hydraulic and Pneumatic Engineering” will know the “why” of the last three experiments. Any body floats in water if it is lighter than an equal volume of water, and it sinks if it is heavier than an equal volume of water. Water is practically incompressible but air is very compressible: thus when you press down on the stopper, you force water into the balloon and compress the air in it; when you release the stopper, the compressed air in the balloon expands and drives the water out. When the weight of the balloon and the weight of the water in it are together greater than the weight of water displaced by the balloon, the balloon sinks; when they are less, it rises. Experiment 10. Fun with thin tubes. Hold a piece of No. 2 tubing in the lamp flame and turn it constantly. When it is red hot and soft, =take it out of the flame= and pull your hands apart until the tube is stretched ten or twelve inches (Fig. 16). Is the tube in the shape shown in Fig. 17? [Illustration: FIG. 17 A GLASS TUBE STRETCHED ] Allow the tube to cool, break the large ends away from the thin tube, place one end of the thin tube in a glass of water, and blow into the other end to make air bubbles in the water (Fig. 18). If you can do so, it is a real tube. [Illustration: FIG. 18 AIR THROUGH TUBE ] Does the thin tube bend easily and does it spring back when released? Repeat the experiment with another piece of No. 2 tubing, but make the thin tube as long as you can. Can you blow air through the thin tube, and does it bend very easily indeed? Repeat with a piece of No. 4 tubing. These thin hairlike tubes are called “capillary” tubes, from the Latin word =capillus=, meaning a hair. Experiment 11. Magic. [Illustration: FIG. 19 WATER RUNS UPHILL ] You have always heard that water runs =downhill=, but you will now see it run =uphill= and remain there in a most =magical= manner.