French mathematician, physicist, and inventor who, with Nicolas Robert, was the first to ascend in a hydrogen balloon (1783). Around 1787 he developed Charles's law concerning the thermal expansion of gases.
Charles' Law: a statement that the volume occupied by a fixed amount of gas is directly proportional to its absolute temperature, if the pressure remains constant. This empirical relation was formulated by the French physicist J.-A.-C. Charles about 1787 and later by Joseph Gay-Lussac. It is a special case of the general gas law and can be derived from the kinetic theory of gases under the assumption of a perfect (ideal) gas. Measurements show that at constant pressure the thermal expansion of real gases, at sufficiently low pressure and high temperature, conforms closely to Charles's law.