Walther Nernst was the first to recognize the principle that underlies the third law. From published experimental results, Nernst inferred a postulate known as the Nernst heat theorem. The experimental results that inspired Nernst were measurements of enthalpy and Gibbs free energy differences, $${\Delta }_rH$$ and $${\Delta }_rG$$, for particular reactions at a series of temperatures. (We define $${\Delta }_rH^o$$ in Section 8.6. We define $${\Delta }_rH$$ the same way, except that the reactants and products are not all in their standard states. Likewise, $${\Delta }_rG$$ and $${\Delta }_rS$$ are differences between Gibbs free energies and entropies of reactants and products. We give a more precise definition for $${\Delta }_rG^o$$ in Section 13.2.) As the temperature decreased to a low value, the values of $${\Delta }_rH$$ and $${\Delta }_rG$$ converged. Since $$T{\Delta }_rS = {\Delta }_rH-{\Delta }_rG$$, this observation was consistent with the fact that the temperature was going to zero. However, Nernst concluded that the temperature factor in $${T\Delta }_rS$$ was not, by itself, adequate to explain the observed dependence of $${\Delta }_rH-{\Delta }_rG$$ on temperature. He inferred that the entropy change for these reactions decreased to zero as the temperature decreased to absolute zero and postulated that this observation would prove to be generally valid. The Nernst heat theorem asserts that the entropy change for any reaction of pure crystalline substances goes to zero as the temperature goes to zero.