Joule was son of a beer-grower in Manchester and could made his experiments in the laboratory in the father's manufactury. In the forties of 1800 he proved that the heat was a form of energy and also proved the equivalence of mechanical energy and heat.
First consequence: the heat was'nt a non material fluid. The concept of "caloric" was wrong.

Diagram of the most famous Joule experiment: the conversion of mechanical energy in heat.
The descent of the weight run the turbine-paddle in the water. The water temperature increases by the friction.
Comparing the mechanical work made by the descent of the weight with the temperature increase of water,
Joule proved the equivalence of mechanical energy and heat.

Fathers of
thermodynamics:

Sadi Carnot 

Lord Kelvin 

 R. Clausius

L. Boltzman