Background to the first law

The first law of thermodynamics is essentially an extension of the principle of the conservation of energy to include systems in which there is flow of heat. Historically, it marks the recognition of heat as a form of energy.

The work which led up to this is well known. There were two rival theories of the nature of heat. According to the caloric theory, heat, or caloric, was an indestructible fluid which permeated matter and flowed from hot bodies to colder ones. According to the molecular motion theory, heat was associated with rapid vibrations of the molecules of which matter was composed. Of the two, the caloric theory had the greater support until the middle of the last century, although some of the most significant experiments were done much earlier.

In 1761 Black had studied the melting of ice. He noted that the temperature of a pail of ice-cold water placed in a warm room rose quite quickly, whereas, if the pail contained ice, the temperature remained constant for many hours while the ice melted. If caloric flowed into the pail from the surroundings when it contained ice-cold water, it must also do so when it contained ice. Therefore, he argued, ice-cold water must contain more caloric than ice. In 1799 Davy showed that both wax and ice could be made to melt by rubbing two pieces together.