Joseph Louis Gay-Lussac was a French chemist from the late 18th/early 19th century. He worked with gasses and discovered what has become known as Gay-Lussac's Law. It states that if a gaseous chemical reaction (gaseous reactants combine to form gaseous products) is being done at a constant temperature and pressure, the ratio of the volumes of the products and reactants are simple whole numbers. One example Gay-Lussac worked with was using oxygen and hydrogen to produce gaseous water.
2H2 + O2 --> 2H2O
He saw that 2 volumes of hydrogen gas combine with one volume of oxygen gas to produce 2 volumes of water vapor. In essence he validated the balanced chemical equation above.
Gay-Lussac also did other work with gasses in conjunction with other chemists. He first published what is called Charles's Law based on the work of fellow French chemist Jacques Charles which states that the volume of a gas is directly proportional to its temperature (if pressure and mass are kept constant). Gay-Lussac's name is also associated with Admonton's Law which states that the pressure of a gas is directly proportional to its temperature (if mass and volume remain constant). These two laws in conjunction with Boyle's Law formed the basis for the universal gas law PV=nRT.