A scientific observation made by French scientist Jacques Charles (1746-1823) that for any gas, as long as the pressure is held constant, there is a direct correlation between temperature and volume. That is, if you take a gas, put it in a container that holds it at constant pressure, and double its temperature (measuring from absolute zero), then its volume would also double.

Usually written mathematically as:

V1 / T1=V2 / T2

The lack of a pressure component in the equation shows that as long as the pressure is held constant, the absolute pressure of the gas doesn't matter.

Charles' Law was subsumed under the Ideal Gas Law after unification with Boyle's Law and Gay-Lussac's Law. Like the Ideal Gas Law, Charles' Law deviates from reality when real gasses are measured very accurately.

Gritchka told me to mention yossarian's contention that this law is the basis for the invention of the hot air balloon. Although this fits the French obsession of hot air balloons in the 18th century, I haven't heard about it before. I guess if you also take into the account of the mass of the gas in question being constant, then as the temperature increases, the density goes down. But I think people knew warm air rises before Jacques Charles formally stated his law.