He had been motivated to think of a "wave theory of heat" by analogy with the wave theory of light and some experiments by James Forbes and Macedonio Melloni on radiant heat. His statement that "... in mixed media the mean square molecular velocity is inversely proportional to the specific weight of the molecules" has been seen as the first statement of the equipartition theorem for translational motion. Waterston grasped that, while the kinetic energy of an individual molecule with velocity formula_3 is formula_4, heat energy is proportional to temperature formula_5. That insight led him to derive the ideal gas law: