One of the few physical controversies of the nineteenth century that skill smoulder is that regarding the relation between metallic contact potential and the electromotive force of any voltaic cell. While the dispute as to the physical facts themselves is now almost extinct, the dispute as to the interpretation of these facts remains. In a recent paper, which will be referred to as Part I, the author has made a first application of quantum mechanics to such problems of electrochemistry. Instead of dealing with the problems of widest significace, the spceial problem of "overpotential" was there analysed, in order to show that the new method was capable of giving definite results where other methods had been less successful. In the present paper the general question of the voltaic cell will be considered, including electrode potentials, the meaning of solution pressure, the role of contact potential, and of chemical energy. At the beginning of the century it was still uncertain whether the Volta contact potential difference between two metals was due to some intrinsic property of the metals, or to an adventitious surface layer of moisture or gas. But in 1906 Einstein proclaimed the intrinsic nature of the potential difference as being approximately equal to the difference between the photoelectric work functions characteristic of the two metals ( Φ 1 - Φ 2) = εB.