Julius Robert von Mayer

Dates: 1814–1878.

Summary: Julius Robert von Mayer is considered one of the scholars who discovered the energy conservation law, a foundational principle of thermodynamics, by identifying the equivalency of heat and work.

Julius Robert von Mayer, a German physician, wanted to explain physiological phenomena such as body temperature, respiration, metabolism, and movement without assuming that there was a special life force or that life was directly dependent on divine action. He looked for an answer that agreed with physics.

In the early 1840s, the concept of energy did not exist. Light, electricity, magnetism, heat, and motion were considered to be different forces. Mayer decided that it was first necessary to clarify what the relationship between the different natural forces was, before he could explain the phenomena of life. He began in his first essay of 1842, “Remarks on the Forces of Inorganic Nature,” with a philosophical premise: Cause is consistent with effect. Every change has a cause, and nothing can emerge from nothing—that is, without a preceding state. Applied to force, this meant that no force could be created out of nothing; every force that appeared had to have come from somewhere. Correspondingly, every force that disappeared had to have become something else. Force could not be created or destroyed, since that would contradict the philosophical causality principle. Because experiments by Michael Faraday, Hans Christian Ørsted, and other scientists had shown that there was a relationship between the disappearance of one force and the appearance of another, Mayer postulated that all forces were expressions of a single force (now called energy).

Although there were numerous examples of one force seeming to call forth another, such as the steam engine, batteries, and electromagnetism, scientists had no mathematical means to relate these interactions to one another. Mayer’s most significant contribution to the development of thermodynamics (the study of the transfer of heat energy) was to establish a general formula that allowed scientists to compare the different forces quantitatively. He did this by making them all comparable to a determined amount of heat. This formula was called the mechanical equivalent of heat.

Mayer expressed the mechanical equivalent of heat through a thought experiment: He asked how far a weight would have to fall to raise the temperature of an amount of water of equal weight from 0 to 1 degree Celsius through the heat generated by its impact. This would indicate how much mechanical (lifting) work had been done to raise the weight to that height. Mayer calculated that a weight of 1 kilogram would need to be lifted, and fall from, a height of 1,197 feet (365 meters) to raise the temperature of 1 kilogram of water 1 degree Celsius. The mechanical equivalent of heat was, according to Mayer, 365 kilogram-meters.

This thought experiment quantified the relationship between motion and heat. Because every other force could also be transformed into heat, it meant that all forces could be indirectly compared with one another. The mechanical equivalent of heat was a general formula that could be used independently of a specific transformation to calculate quantitative relationships between energy transformations.

Because change in nature could be reduced to these transformations, the mechanical equivalent allowed scientists to compare very different kinds of natural processes. When Mayer returned to his original interest in living organisms in 1845, he used the mechanical equivalent of heat to estimate the relationship between the chemical energy of foodstuffs and body temperature and motion in horses and humans. He also compared the efficiency of living beings, viewed as transformation machines, with the efficiency of a steam engine, which turned the chemical energy of coal into heat and motion. Mayer later expanded his comparisons to encompass all natural processes, calculating the equivalent of the sunlight reaching the Earth into horsepower and describing the heat equivalent of the motion of a meteorite. The mechanical equivalent of heat made it theoretically possible to compare every process in the world with any other.

While Mayer was developing his ideas in Germany, the British scientist James Joule used a similar thought experiment to come to a slightly different and more accurate equivalent. As a working physician, Mayer was, during this period (the mid-19th century) a scientific outsider, and he argued from philosophy, not physics. As a result, his essays, mode of presentation, and philosophical argumentation did not conform to scientific conventions, and his writings were ignored by physicists. Joule was therefore given the credit for formulating the mechanical equivalent of heat. In 1847, Hermann von Helmholtz abstracted from the mechanical equivalent of heat the concept of the conservation of force as a general law of physics.

Lack of recognition may have contributed to Mayer’s mental breakdown in the early 1850s, but in the 1860s, his early writings were rediscovered and reassessed by some German and British scientists. They disputed the priority given to Joule as the discoverer of the mechanical equivalent of heat and fought to have Mayer’s work acknowledged for its important contributions to the development of the energy conservation law. Fortunately, Mayer lived to see his accomplishments acknowledged.

Bibliography

Caneva, Kenneth. Robert Mayer and the Conservation of Energy, Princeton, NJ: Princeton University Press, 1993.

Heimann, P. M. “Mayer’s Concept of ‘Force’: The ‘Axis’ of New Science of Physics.” Historical Studies in the Physical Sciences 7 (1976).

Turner, R. Steven. “Mayer, Julius Robert.” In Dictionary of Scientific Biography, edited by Charles Gillispie, et al. Vol. 9. New York: Charles Scribner’s Sons, 1981.

Youmanns, Edward Livingston. The Correlation and Conservation of Forces: A Series of Expositions. New York: Appleton, 1865.