In the early days of civilization, heat was a great mystery. Humans considered it an element unto itself, along with earth, air, and water. This idea persisted until the time of Socrates (c. 470-399 b.c.) and Aristotle, who considered heat to be a fundamental substance. It was not until scientists such as Galileo Galilei and, eventually, Daniel Fahrenheit (1686-1736) constructed the first thermometers that humankind began to truly understand the nature of heat.
During the eighteenth century physicists had determined how heat was transferred from a hot object to a cooler one, but had not discovered its true nature. Most scientists subscribed to caloric theory, proposed by the French scientists Antoine Lavoisier and Pierre-Simon Laplace. They suggested that heat was caused by an invisible fluid, called caloric, that was present in all things; as a substance was heated, more caloric would flow into it, and then seep out as the substance cooled. This theory was no doubt supported by the fact that metal, when heated, would expand--apparently filling with some invisible substance.
The caloric theory was widely accepted by the scientific community, since it seemed to adequately explain the nature of heat. Many eminent scientists conducted experiments that supported this theory, and even the respected mathematician Siméon-Denis Poisson developed a persuasive mathematical system to prove the existence of caloric. Still, some scientists were doubtful as to the existence of an "invisible" fluid.
Among these doubters was the American-born physicist Benjamin Rumford, who had gained international recognition for his work on gunpowder. In 1798 Rumford was overseeing the construction of a new cannon. He noticed that, as the drill bored out the shaft of the cannon, a tremendous amount of heat was released--so much so that the workers had to continuously douse the cannon with water to keep it cool. According to the caloric theory, the invisible heat-fluid was squeezed out of the cannon's brass as the drill dug into it; however, it seemed to Rumford that far too much caloric was being released--that, had it all been inside the brass to begin with, the cannon would have melted from the heat. He began to wonder if heat was actually generated by the motion of the drill against the brass, rather than released from within it.
This pondering led to the first theory of heat as a form of motion. While many scientists still subscribed to the caloric theory, Rumford 's idea soon influenced the work of Julius Mayer and James Joule. Ironically, though, the next great step in the refutation of the caloric theory came from one of its most prominent supporters, the French physicist and engineer Nicolas-Léonard Sadi Carnot.
Carnot was interested not in deriving the nature of heat but in designing a more efficient steam engine. He created a purely theoretical engine that lost no energy to friction or to the atmosphere. To his surprise, Carnot found that his engine still could not attain one hundred percent efficiency; the efficiency was dependent not on mechanical properties but upon the temperature difference between the hottest and coldest areas within the engine.
While the Carnot engine did not immediately invalidate the caloric theory, it did seem to indicate that heat was moving from one area of the engine to another. It also hinted at the interconvertibility of energy, or the notion that energy could be transformed from one form to another.
Carnot's work led directly to William Thomson 's (Lord Kelvin's) establishment of an absolute scale of temperature (with absolute zero at its lowest point) and Joule's determination of the mechanical equivalent of heat (the amount of heat necessary to raise the temperature of one gram of water by one degree celsius).
Probably the most important application of Carnot's steam engine—particularly when combined with the work of Kelvin, Joule, Mayer, and German physicist Rudolf Clausius --was the establishment of the first law of thermodynamics. This law simply states that in a closed system the total amount of energy is conserved; that is, the energy at the beginning is always equal to the energy at the end. Along the way it may be transformed into a number of different types of energy (such as kinetic, potential, heat, electricity, etc.)--however, the total energy at any point is the same. The first law is sometimes known as the law of conservation of energy; several scientists are given credit for the authorship of this law, which says that energy can be neither created nor destroyed.
The second law of thermodynamics was proposed in 1865 by Clausius. He showed that, in any system, a certain amount of energy is always transformed into unusable "waste" heat; in a closed system, he added, all of the energy would be eventually converted into heat. (It is for this reason that a perpetual motion machine is unattainable, since a certain amount of energy is lost to heat, unable to be put back into the system.) Clausius called his theory entropy.
The law of entropy has been interpreted in many ways by many different sciences; thermodynamicists claim that it means all systems move inevitably from a state of order to disorder, while environmentalists use it to prove that the degradation of the ecosystem can never be reversed--only slowed. Probably the most disturbing application of entropy is the "heat-death" theory of the universe: it states that the universe, being a closed system, contains a finite amount of energy. According to the second law, that energy can undergo a number of transformations, but will ultimately be converted into unusable heat. At that time no further transformations can take place--the universe will be dead, with all matter resting at a temperature a few degrees above absolute zero.
Cosmologists deny the inevitability of the heat-death theory. They claim that physics as we know it is not universal--that different areas of the universe may observe different laws of physics and that entropy may not be applicable throughout.
The third law of thermodynamics underlies the other two and was developed in 1906 by the German physical chemist Hermann Nernst. In its simplest form, the third law states that if two bodies are at the same temperature as a third body, then they must also be at the same temperature as each other. While this might seem rather simplistic, Nernst's law has since been used to prove the inaccessibility of absolute zero. Many experimenters have attempted to reach absolute zero (some approaching it to within one millionth of a degree) but none have succeeded in disproving the third law.
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