ABSTRACT
There is a tendency, particularly among physicists, to view the first law of thermodynamics as merely a consequence of the energy conservation principle of mechanics. That is to some extent a valid perspective when atomism has been firmly established and when thermodynamics is seen as the effective description of a very large number of these microscopic constituents in terms of a very few macroscopic thermodynamic degrees of freedom. Doubts about the correctness of the caloric theory had come to many minds. Though many thoughts pointing to the so called first law can be found in the literature, the works and thoughts of Robert Mayer, James Joule and Helmholtz stand out in their relative clarity. Around 1845, a few years before Clausius gave the final, and complete, formulation of the first law, Joule performed an experiment of deep significance, namely, his experiment on the so called Joule Expansion.
