ABSTRACT At the close of the 19th century, it had become clear that determination of the free energy of chemical reactions was the key to understanding chemical affinity. Yet the available methods for obtaining free energies were unreliable and of limited applicability, and there was no known method for determination of free energies from thermal measurements. Walther Nernst's 1906 heat theorem, which later became known as the third law of thermodynamics, would prove to be the key to thermometric determinations of free energies. The paper examines the chemical significance of the third law; earlier attempts by le Chatelier, Lewis, Richards, Haber, and van't Hoff at the problem; and some later clarifications of the third law. The paper then covers the human side of the discovery of the third law, including disputes among Nernst, Lewis, and Richards.