History of Heat - 18th Century

18th Century

In 1761, Scottish chemist Joseph Black discovered that ice absorbs heat without changing temperature when melting. From this he concluded that the heat must have combined with the ice particles and become latent. Between 1759 and 1763 he formulated a theory of latent heat on which his scientific fame chiefly rests, and also showed that different substances have different specific heats. James Watt, who later invented the Watt engine, was Black's pupil and assistant.

The ability to use heat transfer to perform work allowed the invention and development of the steam engine by inventors such as Thomas Newcomen and James Watt. In addition, in 1797 a cannon manufacturer Sir Benjamin Thompson, Count Rumford demonstrated through the use of friction that it was possible to convert work to heat. He designed a specially shaped cannon barrel, thoroughly insulated against heat loss, then replaced the sharp boring tool with a dull drill bit, and immersed the front part of the gun in a tank of water. To the amazement of observers, he brought cold water to a boil within two and one half hours, without the use of fire.

Several theories on the nature of heat were developed. In the 17th century, Johann Becher proposed that heat was associated with an undetectable material called phlogiston which was driven out of a substance when it was burnt. This was finally refuted by Lavoisier demonstrating the importance of oxygen in burning in 1783. He proposed instead the caloric theory which saw heat as a type of weightless, invisible fluid that moved when out of equilibrium. This theory was used in 1824 by the French engineer Sadi Carnot when he published Reflections on the Motive Power of Fire. He set forth the importance of heat transfer: "production of motive power is due not to an actual consumption of caloric, but to its transportation from a warm body to a cold body, i.e. to its re-establishment of equilibrium." According to Carnot, this principle applies to any machine set in motion by heat.

Another theory was the kinetic theory of gases, the basis of which was laid out in 1738 by the Swiss physician and mathematician Daniel Bernoulli in his Hydrodynamica. In this work, Bernoulli first proposed that gases consist of great numbers of molecules moving in all directions, that their impact on a surface causes the gas pressure. The internal energy of a substance is the sum of the kinetic energy associated with each molecule, and heat transfer occurs from regions with energetic molecules, and so high internal energy, to those with less energetic molecules, and so lower internal energy.