1831-1879 Maxwell, James Clerk
Framed the kinetic theory of heat, arguing that matter was comprised of molecules and that heat was the motion of these molecules, and so thermodynamic properties like temperature and pressure could be explained mechanically. That led to statistical mechanics. Maxwell also showed that electricity and magnetism could be united into a single theory of electromagnetism and went on to show that light was an electromagnetic wave, thereby bringing optics under the umbrella of electromagnetism. This meant that mechanics, gravitation, and thermodynamics were one unified branch and electricity, magnetism, and optics were another.
His work on thermodynamics led him to devise the thought experiment that came to be known as Maxwell’s Demon, where the second law of thermodynamics is apparently violated by an imaginary being capable of sorting particles by energy.