Full Idea
James Joule established the equivalence of heat and mechanical energy, in the sense of showing that a specific amount of heat will always be produced by the expenditure of a given amount of energy, and vice versa.
Gist of Idea
Joule showed that energy converts to heat, and heat to energy
Source
report of James Joule (works [1870]) by David Papineau - Thinking about Consciousness App 4.2
Book Reference
Papineau,David: 'Thinking about Consciousness' [OUP 2004], p.245
A Reaction
This was a major step towards the law of conservation of energy.