What scientific ideas about heat appeared in Henry Cavendish’s unpublished theory of heat?

In his unpublished theory of heat, developed in the late 1780s, Henry Cavendish proposed a framework that was both mathematical and mechanical in nature. Contrary to the material or elementary theories of heat popular at the time, Cavendish explained heat as the result of the motion of matter, consistent with his work within the framework of Newtonian mechanism.1

Key concepts within his theory included:

• Conservation of Heat: His work contained the principle of the conservation of heat, which was later understood as an instance of the conservation of energy.1
• Mechanical Equivalent of Heat: His manuscript included the concept of the mechanical equivalent of heat, although he did not use that specific label.1

Cavendish maintained a consistent objection to the idea that heat had a material basis. While he utilized the concept of latent heat in his 1783 research on the freezing point of mercury, he avoided using the term itself because he believed it implied an acceptance of the material theory of heat.1