Joseph Black's Pioneering Discoveries about Heat

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Joseph Black's Pioneering Discoveries about Heat

Overview

In 1761, Joseph Black (1728-1799), an English chemist, discovered that ice, while it was in the process of melting, did not warm up until it was completely melted. He later made the same discovery about boiling water; it stayed stubbornly at 212 degrees while boiling, regardless of the amount of heat applied to the pot. These discoveries, simple though they seem, led directly to major discoveries in the science of heat transfer, called thermodynamics, and to greater efficiencies in the steam engines that powered the Industrial Revolution. These same principles continue to be taught to physics students and engineers today, and are frequently used in operating and designing air conditioning, refrigeration, jet engines, nuclear power plants, and many others.

Background

The Industrial Revolution was as much a revolution in power supplies as in manufacturing techniques, although both were vitally important. Power was originally supplied by wind, water, or muscle because no other sources existed. Unfortunately, wind power was unreliable, water power was limited to very specific areas near rivers or streams, and muscles wore out and needed to sleep (rather, the animals supplying the muscles needed sleep). So, at the turn of the eighteenth century, all the world's inventiveness was dependent on manufacturing technologies powered by sources of energy virtually unchanged in over 1,000 years.

This began to change with Thomas Savery's (c. 1650-1715) Miner's Friend, the first useful invention that used steam as the motive force. In theory, early steam engines were simple—boiling water produced steam, which takes up a great deal more volume than the water did—1,000 times more volume. As the steam formed and expanded, it could be used to push a piston, which was attached to a rod of some sort. On the other end of the rod was another piston that was designed to let water in on top of it. When the steam pushed the piston, it forced the water into a pipe that took it to the ground surface. Then the steam escaped, the piston returned to its original position, and the cycle started again.

Over time, engineers learned that keeping the water under pressure would produce higher pressure steam, which could do more work. They also learned that, at elevated pressures, higher temperatures were needed to make the steam. By trial and error, engineers and physicists developed a number of rules of thumb for making steam engines increasingly efficient and powerful, but they really didn't know how or why water boiled. Without that knowledge, they could not design an efficient steam engine except by trial and error.

In 1761, English chemist Joseph Black gathered the first bits of information that would shed light on this problem. In his laboratory, Black noticed that the temperature of melting ice stayed exactly at the same temperature from the time the ice started to melt until it was completely gone. Later, he noticed that boiling water also maintained the same temperature from the time it started boiling until the container was dry. Both of these were completely independent of the amount of heat being added to the containers of ice or water. In other words, "turning up the flame" had absolutely no effect on the temperature of melting ice or boiling water. Later work would show the converse to be true, too; freezing water and condensing steam also maintained the same temperature.

This discovery led Black to a fundamental understanding of the nature of heat and its applications to making steam. He understood that, up to a point, heat would make water (or ice) warmer, but that after a certain point was reached, heat instead went into melting (or boiling) rather than warming. This meant that two phenomena were occurring, and that each used heat in a different way. Black referred to two different types of heat that he called "sensible heat" and "latent heat."

Sensible heat is the heat that raises water temperature. Latent heat is the heat needed to change water (or any other substance) from one phase to another (that is, from solid to liquid or from liquid to vapor). So, heating a tub of water from freezing to boiling requires the addition of sensible heat. Actually boiling the water takes latent heat—in this case, the latent heat of vaporization. Later scientists realized that sensible heat goes into making the water molecules move more quickly while latent heat actually makes them move so much more quickly that they break away from the solid (in melting) or from the liquid (in boiling). They also realized that the only difference between sensible and latent heats are in their effects on the liquid.

Black made another important discovery: he found out how much heat it took to boil (or melt) a given quantity of water. This was the crucial step in learning how to make heat engines more efficient—by knowing exactly how much heat had to be added to a given quantity of water to turn it into steam, engineers could start to design engines along scientific principles rather than by hit-or-miss techniques. By so doing, they could design machines that had a specific power output, or they could begin to control the output of a given engine. In short, Black's discoveries did not cause steam engines to be constructed, but it made it possible to construct them intelligently, for specific purposes.

Impact

Black's discoveries had impacts on science, society, and European politics. Many of these impacts reverberate to the present day.

In the scientific arena, Black's discoveries formed one of the first foundation blocks of what was to become the science of thermodynamics. This field of study, elevated to an independent discipline by William Thomson, Lord Kelvin (1824-1907), has proven enormously fruitful over the intervening two centuries. Simply put, thermodynamics is as its name suggests, the study of heat in motion. However, it has expanded incredibly since its first appearance, and thermodynamic principles now encompass molecular and chemical reactions, refrigeration plants, and engines of all sorts. For example, for a chemical reaction to take place, some chemical bonds must be formed while others are broken. In some cases, heat must be added to the chemicals for this to occur, while in other cases, heat is produced by these reactions. By understanding the thermodynamics of the chemicals involved in these reactions, a chemical engineer can determine what sort of reaction vessel to use—one that will heat the chemicals or one that is refrigerated to keep them from boiling. Some of the finest minds in science for the next century devoted themselves in whole or in part to further elucidating the science of thermodynamics, including James Watt (1736-1819), Rudolf Clausius (1822-1888), Ludwig Boltzmann (1844-1906), Lord Kelvin, James Joule (1818-1889), and Sadi Carnot (1796-1832).

Similarly, electricity is produced in most parts of the world by steam turbines. Other thermodynamic principles can be used to design these power plants to be as efficient as possible, wringing every last kilowatt out of the steam used. This helps to save fuel, reduce emissions, and cut energy bills.

It is in this last area that Black's contribution was most immediately important. He was able to help engineers determine how to make a better steam engine because he helped them to learn more about what happens when water boils. Specifically, he was able to show them that neither water nor steam in contact with the water can be heated to a temperature higher than boiling until all of the water is gone. Since the amount of work that can be extracted from steam depends on its temperature, this meant that a limited amount of work could be done by any steam engine, unless a way could be found to heat the steam even further. To do this, water is boiled and the steam is drawn off into a second chamber with no water in it. In this chamber, the steam is "superheated," since it's no longer in contact with the water, raising its temperature to several hundred degrees. At this point, steam engines become much more efficient, letting the same size engine do much more work with only a minor increase in fuel used.

This translated directly into benefit to society. More efficient steam engines made railroads possible. Factories could become more efficient, mills could grind grain without water or horse power, mine shafts could remain de-watered more efficiently, ships could be propelled without dependence on the wind, and much more. The Industrial Revolution was powered by steam, and steam engines were more efficient because of Black's discoveries.

Finally, steam power had a significant impact on European politics of the late eighteenth and early nineteenth centuries. As the first nation to develop steam power, Britain enjoyed an industrial and military advantage over most of the rest of Europe for several decades. These translated into economic and political advantages, much to the dismay of Napoleon. In fact, Sadi Carnot, a great French scientist who fought under Napoleon, came to the conclusion that only Britain's supremacy in steam power had led to France's defeat in their wars because steam power helped Britain mine coal, fabricate iron, saw lumber, and weave cloth. Because of this, Britain made more arms, built more ships, fed its people and soldiers better, made more sails, and, with a stronger economy, could better afford a war. Steam power became military, economic, and political power. Britain's supremacy in these areas led her to become a great world power for over a century, and she remains a world center for finance and trade to this day.

P. ANDREW KARAM