The overwhelming majority of important papers in physics are written by physicists. But the physician Julius Robert Mayer (1814-1878, see photo) did a valid theoretical calculation of the mechanical equivalent of heat just before Joule reported on his results from his well-known paddle-wheel experiments. Joule is well-known to physics people and Mayer is a relatively obscure and unknown name. Joule was part of the physics establishment; Mayer was not. Joule's paddle-wheel experiments were easy to understand, and physics people appreciated the painstaking manner in which he made corrections in his experiments. The fame of Joule is well-deserved. On the other hand, Mayer's work is not traditional and there will almost certainly never be a Mayer physics laboratory experiment. We can admire the tenacity with which Mayer pushed his priority case, but that is all.