Nineteenth-Century Development of the Concept of Energy

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Nineteenth-Century Development of the Concept of Energy


The concept of energy is fundamental to the understanding of all physical motion, whether in nature or derived from humanity's technologies. Nature's examples of energy are familiar enough to anyone: pounding ocean surf, volcanic eruptions, wind and electrical storms, and even the beating down of the Sun's rays. These and many other displays have intrigued and influenced humanity in its development and applications of the concept of energy in nature and in the laboratory. The theory of energy and its conservation was long in coming but has led to numerous practical technological applications.


The word energy comes from Greek, meaning work, and the concept was explored progressively by medieval thinkers (momentum), Galileo (1564-1642; force acting on a body), Isaac Newton (1642-1727; gravitational force and the laws of basic mechanics), and other seventeenth- and then eighteenth-century scientific thinkers. However, the modern conceptualization of energy, the delineation of its several forms, and the finalization of most physical laws governing it are products of the nineteenth century.

There are six types of basic energy: mechanical, heat and light (both part of radiant), chemical, electrical, and nuclear. All were identified and explored in the nineteenth century, with the last of them, nuclear, just being on the threshold of definition when the century ended. Incipient concepts of conservation of energy touched on previously would find realization through nineteenth-century scientific experimentation of the various energy forms, which proved their transformation principles of one into the other, further proving that energy can neither be created nor destroyed—that is, energy is conserved.

The term "energy" came into use in 1807, coined by an English physician and physicist named Thomas Young (1773-1829) as a definition of the ability or capacity to do work. This is still fundamental to understanding the concept of energy. Young's definition was in regard to perhaps the simplest example of energy, mechanical energy, an example being an object pushed, pulled, raised, or lowered. Work is performed on the object—or the object has realized energy—and the object goes from a state of rest to one of motion.

The cumulative knowledge in the understanding of energy began its progression with seventeenth- and eighteenth-century investigators. The term for energy at that time was "vis viva," or "life force," reflecting the idea of physical motion involved. Dutch scientist Christiaan Huygens (1629-1665), studying colliding objects, concluded that the force of the objects after the collision changed but was not lost—this was an early concept of the conservation of energy. Other steps in the theory were studying the processes of heating, cooling, and chemical change (such as combustion). These were recognized as part of physical motion, but even by the early nineteenth century the understanding was at a formative stage. It was only after the middle of the previous century that Scottish chemist Joseph Black (1728-1799) had finally distinguished between heat and temperature.

Attempts to explain energy progressed with more sophisticated observations and experiments of physical phenomena. That there was a unity to all forces began with the idea of a single energy-producing substance, first being the phlogiston theory (1697), where "phlogiston" was defined as a weightless element present in any combustible matter. This was used to describe all physical and chemical phenomena. By late in the next century phlogiston was forgotten due to Black's caloric theory, which defined heat as an "expansive fluid" in matter called "caloric," a type of weightless particle that was attracted to matter and raised its temperature—that is, imparted energy. The idea enabled Black to explain how matter could change state with his concept of "latent heat"—water containing more caloric could change to steam (heated) or ice (cooled).

The nineteenth century was a period of prevalent experimentation with supposed energy forms. The heating of gases, explaining expansion and contraction—as well as combustion (using different chemical gases)—held prime clues to the nature of energy. Electrical investigations were also much in vogue in the nineteenth century. Electrical current was thought of as a fluid. In fact, the fluid idea of force and energy—as caloric—persisted for a long time in regard to electricity and magnetism. Radiant energy from light—the Sun being the oldest recognized—was one area that the caloric theory could not interpret.

In 1798 American scientist Benjamin Thompson (also known as Count Rumford, 1753-1814) experimented with heating metal and then water by mechanical work without the use of fire. He concluded that heating was accomplished by a mechanical motion that heated the particles of the object itself. In 1824 the son of a famous French Napoleonic general, Nicolas Sardi Carnot (1796-1832), wrote a paper called "Reflections on the Motive Power of Heat" in which he discovered by simple heat engine experiments that heating had a definite direction: the imparting of energy was done from a higher temperature to a lower one. That is, heat always moved from a warmer source to a colder one—-an expression of what would be called the Second Law of Thermodynamics. But, like Rumford's experiments (forgotten for nearly 50 years), Carnot's conclusion was not appreciated for some 25 years.

During that time two other thinkers independently continued with the same research Rumford had done on the relationship of heat and work. English scientist James P. Joule (1818-1888) did extensive experiments from 1839 that proved the equivalence of mechanical work and heat. Joule was evidently the first investigator who realized that heat was a form of energy (other mechanical experiments would also reveal that frictional loss was heat and thus energy). His original experiments were in terms of electrical work (1840) producing heat. Here was yet another clue to the transformation of energy—mechanical to electrical to heat. About the same time German physician Julius R. Mayer (1814-1878) was reaching the same conclusion that there was a quantitatively fixed relation between mechanical work and heat without benefit of experiments but by deductions from physiological observations. He then provided an estimate of the equivalence by turning to data on the specific heats of gases (1842). In an 1845 paper he related his findings in relation to what was again the conclusion of the conservation of energy by saying that energy in all its forms was preserved. Mayer also provided many examples of how this could be applied. His ideas were doubted, yet he and Joule are credited with the same theory.

Both Joule and Mayer provided a mechanical, or more generally, a dynamic theory of heat, in which heat was not a special substance but the energy of motion—what would be called kinetic energy. Mayer's investigations with gas pointed again to this important medium of experiment for resolving the nature of energy by studying the relationship between various dynamic means of heating.

Two of Mayer's contemporaries were prime proponents in this quest. Rudolf Clausius (1822-1888) was a German academic theoretical physicist, and William Thomson (also known as Lord Kelvin, 1824-1907) was an academic mathematician and experimental physicist. Clausius and Thomson resolved Carnot's interpretation of the Second Law of Thermodynamics with the prevalent mechanical theory. In 1851 Thomson added a paper ("On the Dynamical Theory of Heat") that established the many applications available via the laws of thermodynamics. The next year he wrote on the concepts of available heat and the dissipation of heat in mechanical work, topics that Clausius did not pursue until after he had rendered the Second Law of Thermodynamics mathematically (1854) and reasoned through a series of papers in which he called the dissipation of heat entropy (from the Greek for "change"). He went on to further define concepts of the kinetic theory of gases already inaugurated by Joule in 1848.

During the mid-1800s, one of the most distinguished scientific minds of the century was also studying these questions of energy in relation to physical motion at the molecular level. Hermann von Helmholtz (1821-1894) was a German physiologist and physicist and the most well rounded in his research of any nineteenth-century scientist. It was Helmholtz who provided the most comprehensive treatment of the conservation of energy (he still used the term "force" from the old "living force" idea) in an 1847 paper. He rendered the theory mathematically and introduced the fundamental concept that the conservation of energy was based on only two basic physical abstractions: force and matter.

Another outstanding nineteenth-century scientist, James Clerk Maxwell (1831-1879) pursued the interrelationship of electromagnetic energies. His field theory provided the link of electromagnetic forces in space to the nature of light energy and paved the way for the research of the late century that delved into the forces and energy of the atom itself. Here, once again, the theory of the conservation of energy provided a template for future research and technology application.


The concept of energy proved to be an invaluable and adaptable foundation to explore a progressively complex physical world manifested in the phenomenon of motion. The most important guiding principle in that pursuit of energy delineation has been the theory of the conservation of energy.

The practical applications of the understanding of energy and energy conservation were already at work in the modification and improvement of steam engines and the instruments measuring the parameters of those engines. Subsequently, the need to improve the efficiency in the energy cycle with steam led to the greater efficiency of the steam turbine. Later, the chambered engine medium for providing energy advanced to the internal combustion engine in a similar progression of sought-for efficiency. This led to advances in heating, ventilating, and refrigerating systems. And power plant applications of energy to provide greater yield led to natural gas, electric, and hydroelectric systems. Finally, as the nineteenth century gave way to the twentieth, the advance of energy technology from scientific knowledge applied to modern warfare on the eve of World War I raised inevitable moral questions of the right and wrong of such applications—questions that have continued to the present day.


Further Reading

Feynman, Richard P., Robert E. Leighton, and Matthew Sands. The Feynman Lectures on Physics. 3 vols. Reading, MA: Addison-Wesley, 1963-65.

Resnick, Robert, and David Halliday. Physics for Students of Science and Engineering. 2 vols. New York: Wiley, 1960.

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