James Clerk Maxwell

James Clerk Maxwell (1831 to 1879) is best known for unifying electricity, magnetism and light into a single theory, but in thermal physics he opened an equally consequential line of work. Around 1866 he formulated, independently of Ludwig Boltzmann, a kinetic theory of gases showing that heat and temperature are statistical properties of molecular motion rather than a substance flowing between bodies: molecules at a given temperature move at a range of speeds distributed according to a definite law, so a hotter body's molecules are only more likely, not certain, to be the faster ones. This statistical reframing did not overturn the existing laws of thermodynamics but gave them a mechanical foundation, and the distribution he derived, later completed and rigorously derived by Boltzmann, is named for them both.