Cannons, steam engines and ‘caloric’

The idea of 'heat' is an everyday phenomenon, familiar even before fire-making. Aristotle discussed it as one quality among others, such as colour or smell. Mediaeval scholars discussed ‘degrees of heat’ but only with the development of thermometers during the 17th century did it become possible to quantify the study of 'heat'. The Scottish professor Joseph Black (1728 – 99) was the first to distinguish between temperature and 'heat', or, as we would now say, energy stored thermally. 

By the 18th century it was generally thought that 'heat' was an invisible and weightless fluid, called ‘caloric’. In 1760 Black had conducted sufficient experiments to conclude that there was a different heat ‘capacity’ for each substance. In 1781 the Swedish scientist Johann Carl Wilcke independently came to the same conclusion. Black went on to measure water’s latent heats of fusion and of vaporisation.

The first person to seriously challenge the caloric idea was Benjamin Thompson, a founder the Royal Institution who in 1791 became Count Rumford. As director of the Munich arsenal, Rumford noticed that boring cannons produces a great heating effect, especially if the boring tool is dull. Rumford argued that the supply of 'heat' was limitless, showing that a boring drill would continue to boil water so long as the horses driving it kept moving. This is more easily explained by a mechanical theory of 'heat' than the caloric (fluid) theory.

But the fluid theory was still needed to explain 'heat' transfers, and so it prevailed for many decades. In France the publication of Joseph Fourier’s mathematical theory of heat conduction in 1822 did not rely on caloric theory yet Sadi Carnot’s 1824 theory of steam engines did. When explaining how heat engines did mechanical work, Carnot mistakenly assumed that caloric ('heat') is a conserved quantity.

Finally in the 1850s William Thomson (later Lord Kelvin) and Rudolf Clausius modified the Carnot theory and began to convince others that energy is conserved (not 'heat'). As kinetic theory became established, so caloric theory withered and died.