Mayer, Julius Robert von (1814–1878)

German physicist who in 1842 anticipated James Joule in deriving the mechanical equivalent of heat, and Hermann von Helmholtz in the principle of conservation of energy.

In 1845 Mayer extended the principle to show that living things are powered solely by physical processes utilizing solar energy and not by any kind of innate vital force. He described the energy conversions that take place in living organisms, realizing that plants convert the Sun's energy into food that is consumed by animals to provide a source of energy to power their muscles and provide body heat.

Mayer was born in Heilbronn and studied at Tübingen. In 1840 he took a position as a ship's physician and sailed to the East Indies for a year. He then settled in his native city and built up a medical practice. Despairing because others were making the same discoveries independently and gaining priority, Mayer tried to kill himself in 1850, but achieved recognition before his death.

During his 1840 voyage, Mayer found that the venous blood of European sailors was much redder in the tropics than at home. He put this down to a greater concentration of oxygen in the blood caused by the body using less oxygen, since less heat was required in the tropics to keep the body warm. From this, Mayer made a conceptual leap to the idea that work such as muscular force, heat such as body heat, and other forms of energy such as chemical energy produced by the oxidation of food in the body are all interconvertible. The amount of work or heat produced by the body must be balanced by the oxidation of a certain amount of food, and therefore work or energy is not created but only transformed from one form to another.

In 1842 Mayer stated the equivalence of heat and work more definitely. He published a theoretical attempt to determine the mechanical equivalent of heat from the heat produced when air is compressed. His result was inaccurate but the principle of the conservation of energy was demonstrated for the first time.