The systematic study of the adsorption of organic compounds on electrodes began with the comprehensive survey of adsorption on mercury carried out by Gouy in the first decade of this century. His studies with the capillary electrometer are still useful but do not lend themselves to detailed quantitative analysis. A more detailed study of a few systems by Frumkin in his thesis (1919) led him to propose a quantitative phenomenological theory of organic adsorption (1925, 1926) at almost the same time as Stern proposed the model of the electrical double layer which remains the picture accepted in general terms today. The attempt at a molecular model made by Butler (1929) should be more satisfying but up to the pres­ ent the formidable difficulties of a molecular theory of interfacial phenomena have prevented the full interpretation of experimental results along these lines. In his work with Proskurnin (1935), Frumkin is also respon­ sible for the major experimental advance in the demonstration that reliable measurements of the capacity of an electrode-solution interface can be obtained provided that the work is carried out under conditions of scrupulous cleanliness. Even so, preCise measurements of double layer capacities were not obtained until Grahame (1941) showed how convenient and reliable the dropping mercury electrode was the for these studies. This method and the hanging drop electrode remain the preferred methods for study of adsorp­ tion on mercury. Solid electrodes present a more difficult problem.