Charles's Law states that the volumes of a fixed quantity of gas at constant pressure is inversely proportional to its temperature. Charles studied the relationship between the air in the balloon in its normal state and the air in the balloon when heated. He kept all properties constant except for temperature and volume. Using mercury and warm bath water, he was able to determine that the volume of the gas increased as the temperature of the gas was increased. The formulas and final concept of Charles Law were developed by Joseph Gay-Lussac but Charles came up with most of the ides. That is why it is called Charles Law. A real world example would be taking an inflated football that was inside and then taking it outdoors on a winter day. The inflated football would shrink slightly.