A potentiostatic investigation of the kinetics in the initial stages of the formation of calomel upon a mercury electrode in hydrochloric acid is described. The deposition starts with the laying down of several monomolecular layers, (110) lattice planes, by the two-dimensional growth of twodimensional nuclei. This growth mechanism is treated algebraically and by a computer simulation. The slowest step in the overall mechanism is the incorporation of new material into the lattice at the edge of the growing patches for which the rate constant at the reversible potential is found to be about 0-005 mole cm-2 sec-1; the nucleation rate constant is about 1010 nuclei cm-2 sec-1. The lattice building mechanism which fits the results most closely is :

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THE FORMATION OF THE SULPHATE LAYER Three different types of sulphate deposit were formed on the lead electrodes by polarizing them at -1.902, -1.884 and -1.864V with respect to the reversible lead dioxide potential in 2 N sulphuric acid at 25" C . The current time relationships obtained 93 9 , -1.864V ,, 0.253 V.(f) sulphate formed at -1.884 V oxidized at 0.368 V in 0.02 N sulphuric acid and 0.99 M sodium sulphate.

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