The Replacement of Caloric Theory by a Mechanical Theory of Heat
The Replacement of Caloric Theory by a Mechanical Theory of Heat
The nineteenth century witnessed a definitive resolution of questions regarding the nature of heat. The flow of heat was recognized as one way in which systems could exchange energy with their environment, and the equivalent of heat in mechanical work was determined. While heat flow into a system increased its store of energy, only a fraction of the energy could be recovered as useful work. An additional quantity, entropy, that determined the direction of heat flow was introduced and ultimately interpreted as an increase in the randomness of molecular energy. The mechanical theory of heat firmly established the atomic hypothesis—that all aspects of the behavior of matter can be explained by the interaction of component atoms and their parts—as a scientific fact. In so doing, it increased confidence that all natural phenomena could ultimately be explained on an atomistic basis, an advance that modern historians consider one of the most important in modern science, on a par with another great nineteenth-century achievement—Charles Darwin's groundbreaking theory of evolution.
At the beginning of the nineteenth century, the majority of scientific thinkers accepted some form of the caloric theory as an explanation for the phenomena associated with heat. Caloric was considered to be a massless fluid that could flow between bodies, a fluid not unlike the electric fluid proposed by Benjamin Franklin or a comparable magnetic fluid, also in vogue. In some versions of the theory, the fluid was composed of discrete particles. The great French chemist Antoine Laurent Lavoisier (1743-1794) devoted the first few sections of his definitive text, The Elements of Chemistry, to the interaction of caloric with matter. The caloric theory was in accord with a great many observations, including the existence of heat capacities that governed the exchange of heat between bodies, the latent heat absorbed by substances in phase changes, and the release of heat in chemical reactions. Although some phenomena, such as the production of heat by friction, lacked a satisfactory explanation within the theory, adherents of the theory assumed that one would eventually be found.
The caloric theory provided the basis for French engineer Sadi Carnot's essay "Reflections on the Motive Power of Heat." In this work Carnot (1796-1832) analyzed the behavior of an idealized steam engine in which, in each cycle, heat was withdrawn from a reservoir of caloric at an elevated temperature, did work by expanding inside a piston, and was discharged to a second reservoir, the exhaust, at a fixed lower temperature. Carnot showed that the amount of mechanical work that can be obtained from a given amount of heat energy by such a device is independent of the material that does the physical work. Instead, it depends only on the temperatures of the two reservoirs. The connection between heat and motion was elaborated by German physicist Julius Robert Mayer (1814-1878) in a paper published in 1842, generally considered to be the first clear statement of the principle of conservation of energy. Mayer's interest in the subject dates from his observation, made in 1840 while working as a ship's surgeon in the tropics, that the venous blood of men in the tropics was a brighter color than that in northern latitudes, suggesting that less energy was needed from the oxidation of food to maintain body temperature in warmer climates. In his 1842 paper Mayer described the interconvertibility of kinetic potential and heat energy and provided an estimate of the mechanical equivalent of heat, the amount of mechanical energy needed to produce a fixed rise in temperature in a given mass of water. This was refined in a series of experiments by the highly respected English amateur scientist James Joule (1818-1889) beginning in the following year.
With a quantitative relation between heat and mechanical energy established, the caloric theory became far less credible. German physicist Rudolf Clausius (1822-1888) noted that Carnot's analysis, which assumed that the amount of caloric extracted from the heat source equaled that delivered to the exhaust in each cycle, was incorrect, in that part of the heat energy had been converted into work. In his paper of 1850 he corrected Carnot's error by assuming that the amount of energy which could not be converted to work in normal operation of the engine would be the same as the amount that would be extracted from the exhaust in each cycle if the engine were run in reverse. He found that the minimum amount of energy per cycle discharged to the exhaust was in the same ratio to the absolute temperature of the exhaust as the amount of energy taken in during each cycle was to the absolute temperature of the hotter reservoir. In 1865 Clausius introduced the concept of entropy change, to denote the ratio of heat energy exchanged to absolute temperature, the importance of this ratio having been noticed by Lord Kelvin (1824-1907) at about the same time. Clausius noted that energetically allowed processes would occur spontaneously in nature only if the entropy of the universe as a whole were to increase. He and Kelvin were thus among the first to suggest that the final state of the universe would be one of maximum entropy.
With heat energy understood to be essentially mechanical in nature, it remained for physicists to provide an interpretation of the energy and entropy in terms of molecular properties. Joule, Scottish physicist J. C. Maxwell (1831-1879), and Austrian physicist Ludwig Boltzmann (1844-1906), who together established the distribution of molecular speeds in a gas and found that the average kinetic energy of gas molecules is directly proportional to the temperature, quickly resolved the question as to energy. Boltzmann also provided a molecular interpretation for entropy, proposing that it was proportional to the number of different ways in which the random kinetic energy could be distributed among the molecules in a system. While controversial for a time, this introduction of statistical concepts into physical theory was elaborated, especially by American physicist J. Willard Gibbs (1839-1903), into a new discipline. The statistical mechanics of Gibbs and Boltzmann, brought into its current form after the development of quantum mechanics in the twentieth century, is now universally accepted as providing the needed connection between thermal phenomena and the underlying dynamics of matter on the atomic level.
The replacement of the caloric theory by a mechanical theory of heat illustrates a number of issues in the history and philosophy of science. One important point is that theory is never fully determined by observation and experiment. Any given set of observations will be explainable by more than one theory. The caloric theory worked well enough for many aspects of the behavior of gases. Theories may be modified or discarded, however, not only when discordant observations are made but also when it becomes possible to unify the theories of phenomena previously considered unrelated. Such unifications include that of celestial and terrestrial mechanics accomplished by Isaac Newton (1642-1727) in the seventeenth century and that of optics, electricity, and magnetism accomplished by Maxwell in the nineteenth century, as well as that of heat and mechanics as discussed above.
In 1917 American biochemist and philosopher L. J. Henderson wrote that "Science owes more to the steam engine than the steam engine owes to science." The industrial revolution and the accompanying concentration of production and wealth was in large part driven by the steam engine, a device that made it possible to convert heat energy into mechanical energy, to power the looms and lathes of industrial nations. The new industries required not only mechanical energy, but also new materials and an improved understanding of the thermal properties of matter and the process of combustion. Thus, the technology of heat energy raced ahead of the basic scientific understanding of heat, but many important advances in physical theory occurred nonetheless as the result of the very practical need to be able to build bigger and more efficient heat engines.
From a philosophical and religious standpoint, the success of the mechanical theory of heat represented a redefinition of materialism. The atomic hypothesis, originally enunciated by the philosophers Democritus and Leucippus in the fifth century BC and integrated into the materialist philosophy of Epicurus, had long been considered suspect by Christian theologians who feared that it allowed no place for spirit. As the various imponderable fluids of the eighteenth century—caloric, electricity, and magnetism—were given atomistic interpretations, the Cartesian dualism of body and soul was largely abandoned within science in favor of a purely mechanistic view. At the same time, a number of modern empirically minded philosophers, including the Austrian Ernst Mach (1838-1916), were critical of the atomic theory for another reason: they opposed treating as real entities that were in principle not observable. Boltzmann, in particular, was severely criticized, a factor that may have led to his suicide in 1906.
From a purely scientific standpoint, American physicist Richard Feynman (1918-1988) has described the fact that all matter is composed of atoms as the single most important scientific discovery of our civilization. Its consequences are very far reaching, and most practicing scientists now accept it as a universally accepted truth. Thus, it is generally believed among scientists that all aspects of the behavior of matter, from the flow of heat between bodies to thought and emotion in humans, can be explained—if not predicted—by the interaction of the component atoms and their parts. The discovery that energy, although conserved, would become increasingly unavailable in time due to the increasing entropy of the universe led to some unpleasant conclusion as to the long-term fate of the universe. According to Clausius and Kelvin, the universe will inevitably evolve into a uniform thin gas of atoms at a constant temperature, and nothing within the scope of nature can be done to prevent it. While the Christian churches maintained faith in an eventual "new heaven and new earth" to be brought about by direct divine intervention, for the majority of non-believers the inevitable "heat death" of the universe added to a sense of the pointlessness of existence. More recent developments in mechanics and cosmology allow some room for alternatives to the heat death scenario, however.
DONALD R. FRANCESCHETTI
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