D O C . 2 3 1 W A R B U R G A S R E S E A R C H E R 1 9 1 main axis. Initially using gold-leaf electrodes, they obtained from a high voltage a kind of polarization that had the effect of slowly diminishing the current when a voltage was applied. When using a sodium amalgam as the electrodes, this polar- ization was eliminated. In recent years these investigations, which are of impor- tance in the study of the solid state of aggregation, have been successfully continued by Joffe. In 1890 a paper appeared by Warburg on galvanic polarization, the significance of which has perhaps not yet been fully recognized even today. Helmholtz is known to have proposed a theory for Lippman’s capillary electrome- ter based on the following idea. At the boundary surface between mercury–dilute sulphuric acid, there is an electric double layer, one of which is occupied by the metal, the other by the electrolyte. A polarization current activated by an applied voltage alters the occupation density of this double layer in such a way that in this process the surface of the metal plays the role of an insulator. The observable sur- face tension of the mercury touching the electrolytes is composed of the actual (positive) surface tension of the surface layer and the negative voltage T of the electric double layer. The total tension thus, according to him is at a maxi- mum when T, thus also the electric double layer, disappears. One would conse- quently here have a means of making the difference in electric potential between mercury and electrolyte disappear and thus of conducting absolute measurements of the metal–electrolyte differences in potential. Warburg objects that a large part of the polarization current can very well be uti- lized to discharge hydrogen cathodically and that the change in the mercury’s total surface tension could very well be based on the change in the mercury’s surface caused by the discharged hydrogen and hence on . This interpretation also leads to a theory of the nature of polarization other than the purely physical one given by Helmholtz. Warburg underpinned his standpoint in detail in a number of papers and with this analysis seems to me to have taken a path-breaking step in the field of the electrochemistry of boundary layers, which is anything but con- cluded. Two other important papers by Warburg are related to this problem, one from the year 1896 on the behavior of unpolarizable electrodes against alternating current and one (1901) on the polarization capacity of platinum. One “unpolarizable elec- trode” is, for inst., Cu upon dissolving into CuSO4. Today we would characterize such electrodes as ones in which the difference in electric potential between metal and electrolyte at any moment is determined by the metal-ion concentration at the electrode. In this case, as Warburg showed, the entire polarization is attributable to the changes in concentration produced by electrolysis at the electrodes, limited by diffusion. The phase difference between the polarization e. m. f. and the current is [p. 825] T0 T0 T + T0 T + T0