History of Heat - 19th Century

19th Century

The work of Joule and Mayer demonstrated that heat and work were equivalent forms of energy, and led to the statement of the principle of the conservation of energy by Hermann von Helmholtz in 1847. Clausius demonstrated in 1850 that caloric theory could be reconciled with kinetic theory provided that the conservation of energy was employed rather than the movement of a substance, and stated the First Law of Thermodynamics.

In 1851, William Thomson outlined the essentially modern view, as based on recent experiments by those such as James Joule on the dynamical theory of heat, that:

Heat is not a substance, but a dynamical form of mechanical effect.

On this view, he argued that we must "perceive that there must be an equivalence between mechanical work and heat, as between cause and effect.”