Tafel in 1905 observed that the hydrogen overvoltage could be expressed as a function of the current density by the equation i = ke - b V , and that the constant b was approximately equal to F/2RT, where R is the gas constant and F the electrochemical equivalent. Bowden has recently shown that the dependence of the constant b on the temperature required by the relation b = F/2RT holds for the liberation at the anode and cathode respectively of both oxygen and hydrogen. In a recent paper Gurney has examined the quantum mechanics of the transfer of electrons between a metal and ions in the solution and has shown that a relation of this kind, with the observed factor, may arise from the conditions of this transfer. He gives as the necessary conditions for the transfer of electrons from the electrode to positive ions in the solution, or from negative ions to the electrode as E + > Φ + V§, E_ < Φ + V, respectively, where Φ is the thermionic work function of the metal and E + , E_ the neutralisation potentials of the ions. In the extended form of the theory these are defined as E + = I + — W + — R + , E_ = I_ + W_ + R_, where I + , I_ are the ionisation potentials of the ions, W + , W_ the hydration energies, and R + , R_ quantities representing the repulsive potential energy between the solvent and the ion at the instant after its neutralisation. By integrating the probabilities of transfer between ions for which these conditions are satisfied he obtains, on certain assumptions, the expression i = k 'Te ±FV/2RT where the positive (negative) sign applies to the discharge of negative (positive) ions.