The Flow of Heat

Overview

In 1791, the Swiss physicist Pierre Prévost (1751-1839) published a theory of heat exchanges, which described how heat is transferred from one object to another. This theory, which is still accepted today, formed a basis for other scientists who studied heat transfer. Prévost also supported the caloric theory of heat—the idea that heat is a liquid. His work helped to convince other scientists of caloric theory, and it was not until late in the 1800s that this idea was finally disproved.

Background

In the eighteenth century, certain phenomena, such as heat, light, electricity, and magnetism, were considered to be imponderable fluids. Scientists used the term imponderable to mean "weightless." Imponderable fluids were supposedly composed of weightless, invisible particles that could flow from one object to another. (It is now known that such fluids do not exist.) Scientists thought that the imponderable fluid was responsible for heat caloric, and they believed that the greater the temperature of an object, the more particles of caloric it contained. This idea is known as the caloric theory of heat.

In 1789, Antoine Lavoisier (1743-1794) published the first chemistry textbook, called Elementary Treatise on Chemistry. In this book, he included a list of all of the chemical elements known at that time. Two items Lavoisier incorrectly listed as elements were light and heat. He included them because he believed them to be imponderable fluids.

Ironically, Lavoisier had disproved the existence of one imponderable fluid, that of phlogiston. Phlogiston was supposedly responsible for combustion, or burning. Prior to his work, scientists believed that a burning object released the fluid phlogiston. Lavoisier, however, showed that combustion involved a gain of oxygen from the air rather than a loss of phlogiston. Partly because of this work, Lavoisier was highly respected by other chemists. For this reason, his belief that heat was a fluid was very influential.

One scientist who accepted the caloric theory of heat was a Swiss physicist named Pierre Prévost. Prévost was a professor of philosophy and physics at the University of Geneva, where he conducted experiments involving heat. At the time, most scientists believed that cold, like heat, was an imponderable fluid. A cold object was said to contain an abundance of "cold" particles. Prévost, however, noted that all observations concerning heat and cold could be explained by a single fluid rather than a pair of fluids. Therefore, he argued that cold was simply a loss of heat.

Suppose, for example, that a person puts his or her hand in snow. Before Prévost's work, many scientists believed that the person would feel the sensation of coldness because cold was entering his or her hand. Prévost, however, claimed that the person's hand would feel cold because of a loss of heat, not because of a gain of cold. In other words, he believed that caloric flowed from the hand to the snow, rather than that cold flowed from the snow to the hand.

Prévost also showed that all objects give off heat, regardless of their temperature. He observed that the hotter an object is, the more heat it gives off, or radiates. For this reason, he argued that caloric would always flow from a hot object to a cold one. In addition, he conducted experiments into the nature of heat radiation. For instance, he heated groups of nearly identical objects to the same initial temperature. He found that dark objects radiated more heat than light-colored objects and that rough objects radiated more heat than smooth objects.

In 1791, Prévost used the results of his experiments to develop a theory of heat exchanges. He proposed that when objects at different temperatures are placed in contact with each other, they will exchange heat by radiation until they are all at the same temperature. The objects would then remain at this temperature as long as the amount of heat they radiated equaled the amount of heat they received from their surroundings.

For instance, suppose that a spoon at room temperature is placed in a hot cup of coffee. Because the coffee is at a higher temperature, heat will flow from the coffee to the spoon. The temperature of the coffee will decrease (because it is losing heat) and the temperature of the spoon will increase (because it is gaining heat) until both the coffee and the spoon are at the same temperature. Eventually, both will fall to room temperature as they radiate heat to their cooler surroundings.

Impact

Many of Prévost's ideas are still accepted today, including his theory of heat exchanges and his observation that coldness is due to a loss of heat. However, the caloric theory of heat (the idea that heat is a fluid) has long been abandoned. Nevertheless, Prévost's observations agreed with the caloric theory, and his results were offered as proof of it at the time. Within a few years, however, a different theory of heat began to emerge. (Prévost's observations also agree with this second theory of heat, which is the one accepted today.)

In 1798, the British physicist Count Benjamin Thomas Rumford (1753-1814) proposed that heat is not a fluid, but a type of motion. He based this conclusion on his observations of cannon making in Munich, Germany. Workers used tools to bore tubes down the center of brass cannons. Rumford noted that an enormous amount of heat was given off during this process. The cannons had to be cooled with water as the boring tool pounded against the brass. According to caloric theory, this heat resulted from the release of caloric as the brass was broken down into shavings.

Rumford, however, believed differently. He developed an apparatus that could be used to measure the temperature of a cannon as it was being bored. From his measurements, he concluded that the total amount of heat given off during the boring process was more than enough to melt the cannon itself if it were not cooled with water. In other words, more caloric seemed to be released from the brass than it could have possibly contained. In fact, there seemed to be almost no limit to the amount of heat that could be produced by this process.

Rumford repeated his experiment with a boring tool that was so dull it did not produce metal shavings. According to caloric theory, the temperature of the cannon should not have changed because caloric would only have been released if the metal were being broken into smaller pieces. However, Rumford found that the cannon continued to heat up as before. (In fact, even more heat was produced). From these observations, Rumford concluded that the motion of the boring tool was being converted to heat. Therefore, heat was a form of motion.

In 1797, the English chemist Humphrey Davy (1778-1829) read Lavoisier's textbook. After learning about caloric theory, he began his own experiments with heat the following year. In one of these, a block of ice was rubbed while its surroundings were kept just below the freezing point of water. According to caloric theory, there should not have been enough heat present to melt the ice. However, Davy observed that the ice did melt. He concluded that the rubbing motion was converted to heat and that this heat was sufficient to melt the ice. (Some scientists now doubt that Davy's experiment could have worked as he described it. However, Davy was convinced and published his results.) Neither Rumford nor Davy's work persuaded most scientists to abandon the caloric theory. Instead, the conclusions drawn by Prévost and the opinions of the respected physicists who supported him seemed more likely to be true.

Then, in 1843, the English physicist James Joule (1818-1889) measured the amount of heat that was produced by a given quantity of mechanical energy (motion). At first, his work was little read, and it was not until the second half of the nineteenth century that the Scottish physicist James Clerk Maxwell (1831-1879) helped to convince the scientific community of the kinetic theory of heat. The word kinetic refers to motion; the faster an object moves, the more kinetic energy it has.

Maxwell showed that the average velocity of gas molecules increases with an increase in temperature. In other words, the greater the temperature of a gas is, the faster its molecules move. Maxwell's findings demonstrated that temperature and heat could be described by the movement of molecules, rather than an invisible, weightless fluid.

Even though the majority of scientists in the eighteenth and nineteenth centuries did not know the exact nature of heat, advances were made on Prévost's work regarding the exchange of heat between objects. For example, the French mathematician Jean Fourier (1786-1830) began to investigate the conduction of heat within an object.

He found that this type of heat transfer depended on many variables, including the difference in temperature between the warmest and coolest parts of the object and the object's shape and conductivity. (Conductivity is the ability of a material to transfer heat. For example, metal has a greater conductivity than wood.) Fourier was able to describe this process mathematically, and he published his findings in a book titled Analytic Theory of Heat in 1822. The mathematics Fourier developed for studying heat transfer were later used in studies of sound and light.

STACEY R. MURRAY