In some unpublished work the writer has shown that under certain conditions a high " contact resistance " exists between the surface of an electrode of sheet zinc and the electrolyte. The " contact resistance " between zinc and ammonium chloride is a secondary development, occurring with measurable rapidity after the immersion of the zinc in the eIectrolyte. At the instant of immersion it is very small. It may rise to values of 2oo to 4oo ohms per square inch (I25o to 2500 per square centimetre) of electrode surface. It forms the principal part of the internal resistance of dry cells at low-current drains, and, as determined by measurements upon the latter, it has a high temperature coefficient, decreasing to one-tenth of its value with a temperature rise from o ยฐ to 45 ยฐ C. This is the contact resistance of the zinc electrode which was measured by Carhardt in his resistance measurements upon the early Gassner dry cells which caused the rapid rise in his resistance curves at lowcurrent densities. With densities above certain low limiting values, it is lessened or temporarily destroyed, being practically unaffected by currents of less than I milliamt)~re per 50 square inches (315 sq. cm.) of electrode surface, even if continued for a considerable time. At higher current densities it begins to rapidly decrease, and becomes negligible at current densities of about I ampere per 5o square inches. Upon open circuit the resistance again slowly rises toward its initial value. It is destroyed or very greatly reduced by chemical treatment of the zinc surface ; e.g., by corrosion with dilute sulphuric acid.
An explanation of these phenomena is based upon the supposed existence of a hydrogen film upon the electrode surface, discharged there by the local action between the zinc and the electrolyte. This is supported by a considerable amount of experimental evidence in which it is shown that the predicted behavior of such a hydrogen film under selected conditions is in agreement with the observed behavior of the contact resistance. These facts must have a significant relation to the theories of over-voltage phenomena and kindred problems bearing on the nature of the equilibria between the electrode surfaces and the electrolyte.
Electrolytic Coating of Silvered Mirrors. AnoN. (Revue General des Sciences, vol. 27, No. 12, June 3 o, I916.)--The old method of coating mirrors by the mercury process is a long, tedious, and unwholesome operation which has been nearly universally re-415