Benoît Paul Émile Clapeyron (26.01.1799—28.01.1864) was a French physicist and engineer and a member of the Paris Academy of Sciences (1858). He was born in Paris. He graduated from the École Polytechnique (1818). In 1820-30 he worked in St Petersburg at the Institute of Railway Transport Engineers. After
his return to France he was professor (from 1844) at the École des Ponts et Chaussées in Paris.
His physical research concerned heat and the plasticity and equilibrium of solids. In 1834 he gave mathematical form to the ideas of N. Carnot, being the first to appreciate the great scientific importance of his work “Reflections on the Motive Power of Fire...”, which in fact contains a statement of the second law of thermodynamics. Proceeding from these ideas, he was the first to introduce a graphical method into thermodynamics, the indicator diagram, and in particular he proposed the P-V system of coordinates. In 1834 he derived the equation of state of an ideal gas, later generalised (1874) by D. I. Mendeleev (the Mendeleev–Clapeyron equation), and the equation relating the melting and boiling temperature of a substance to pressure — the Clapeyron–Clausius equation (the latter gave this equation its theoretical justification in 1851).
A corresponding member of the St Petersburg Academy of Sciences (1830).