German physicist Hermann Helmholtz's 1882 paper "On the Thermodynamics of Chemical Processes", which showed that free energy is the measure of affinity; it is one of the founding papers of chemical thermodynamics. |
“Given the unlimited validity of Clausius' law, it would then be the value of the free energy, not that of the total energy resulting from heat production, which determines in which sense the chemical affinity can be active.”