The principle of the conservation of energy was among the most important developments of nineteenth-century physics, and Robert Mayer, a physician from a small city in Germany, was one of its codiscoverers. As ship's doctor on a voyage to the Dutch East Indies in 1840, Mayer noticed that the venous blood he let from a European seaman was lighter than he expected. This observation set off a train of reflections that led him first to conclude that there...