Henri Louis Le Chatelier (1850-1936), the French physical chemist and metallurgist, seen here while a student at the Ecole Polytechnique. His early research was on cement; he worked also on the structure of alloys, on flames and on thermometry. In the 1880's he developed the idea known as Le Chatelier's principle: this states that if the conditions (temperature, pressure or volume) of a chemical system initially at equilibrium are changed, then the equilibrium will shift in the direction which will tend to annul the change, if possible. Although far from perfect and much critised, this principle is still used today.