Demonstration. - Put a rubber bag toy balloon will answer - under the receiver of an air pump, having first blown a little air into it and tied the stem. Exhaust the air, and the balloon will be seen to increase in size. It will do this as long as air is pumped from the receiver.
This expansion of the air is best explained by the kinetic theory of gases : The molecules of a gas are in rapid motion in straight lines, and they continue to move in their paths until turned aside by striking either other molecules or the side of the containing vessel. When the balloon is put under
the receiver of the air pump, the number of molecules of air per cubic centimeter is the same inside it as outside of it, and the bombardment of the molecules on one side neutralizes that on the other, so the walls of the bag remain as they are. As soon as air is taken from the receiver, however, the number of molecular blows on the inside exceeds the number on the outside for the same surface, and the bag is stretched out until they are equal. This means that there are again the same number of molecules in the same space both inside and outside, or, the density is the same. If now the air is let into the receiver, the blows on the outside increase and the bag shrinks to its original size, beaten down by the impact of the molecules of the outside air. The combined effect of the impact of the molecules of air inside the bag makes up the internal pressure of the air.
