 As an illustration of Charles' law, we'll cool a sample of air trapped in a balloon. As we cool the air, the molecules in the air slow down. This decreases the pressure inside the balloon. The volume of the balloon contracts until the pressure inside the balloon and the outside of the balloon are equal. The volume of the gas is directly dependent upon the temperature of the gas. When we remove the balloon from the liquid nitrogen, the temperature of the gas increases. The volume of the gas expands until the pressure inside the balloon and the outside of the balloon are equal again.