Le Châtelier, Henri Louis (1850–1936)

French physical chemist who formulated the principle now named after him, which states that if any constraint is applied to a system in chemical equilibrium, the system tends to adjust itself to counteract or oppose the constraint.

Le Châtelier was born in Paris and studied science and engineering at the Ecole Polytechnique. He was professor of chemistry at the Ecole des Mines 1877–98, moving first to the Collège de France and then in 1908 to the Sorbonne.

In 1887, he devised a platinum–rhodium thermocouple for measuring high temperatures by making use of the Seebeck effect. Le Châtelier also made an optical pyrometer which measures temperature by comparing the light emitted by a high-temperature object with a standard light source.

Le Châtelier's principle, formulated 1884–88, is particularly relevant in predicting the effects of changes in temperature and pressure on chemical reactions. It also agreed with the new thermodynamics being worked out in the USA by Josiah Gibbs. Le Châtelier was largely responsible for making Gibbs's researches known in Europe.

In 1895 Le Châtelier put forward the idea of the oxyacetylene torch for cutting and welding steel.