1842 – Germany
‘Heat is a form of energy and energy is conserved’
In equation form ΔE = H − W where ΔE is the change in the internal energy of a system, H is heat energy received by the system and W is work done by the system.
The first law of thermodynamics is simply a restatement of the law of the conservation of energy: energy is neither created nor destroyed, but may be changed from one form to another.
Mayer and HELMHOLTZ, independent of JOULE and each other, came to similar conclusions at around the same time.