This report presents two demonstration methods to show physicochemical behavior of gases using a transparent tube about 1 m long and 3-4 cm in diameter. One is for recognizing the presence of air by observing the sharp difference in velocities of a ball falling through the tube when one or both ends are closed, and when both ends are open, provided that the ball fits around the tube closely with a small gap. The other is for enhancing the understanding of the effects of adiabatic compression and expansion of gas. The instrument is like a big popgun except for the use of the stoppers made of wet paper. Compression makes the inside of the tube warmer and full of vapor from the stoppers. Blasting the front stopper off makes fog around the frond end of the tube. By covering the end of the tube with one's palm and pushing again the stopper on the other side, one's palm can feel the air's temperature increase and the fog disappears in the tube. This process can be repeated again.
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