PHYSICO-CHEMICAL BASIS OF TRANSMISSION 407 



supported in a sheet of lens paper, is interposed between 

 two salt solutions, one of which contains a readily 

 oxidizable compound, ferrous chloride, together with an 

 indicator (KCNS) to show the formation of ferric ions. 

 With membranes of about sojJ- thickness and a P.D. of 

 about II volts between the two faces, the red color of 

 ferric thiocyanate appears rapidly at the surface facing 

 the cathode; i.e., where the positive stream of the 

 current passes from the membrane to the solution. The 

 surface of the membrane under these conditions acts 

 (in the quahtative sense) like the surface of a platinum 

 anode. When the direction of the current is reversed, 

 ferric ions are reduced to the ferrous state, as shown by 

 the gradual disappearance of the color. In order to 

 obtain these electrolysis-Kke effects, a certain minimal 

 P.D. (i.e., steepness of gradient) across the membrane is 

 required;^ for example, with a current giving 9 volts 



* There is an interesting analogy here with the conditions of electro- 

 stenolysis, where also a critical P.D. is required for producing chemical 

 effects. In the experiments of Braim (Ann. d. Physik, XLIV [189 1], 

 N.F., 473) thin sheets of mica were used (ca 8o/x thick) in which fissures 

 were cut; metallic silver separates out rapidly at the borders of the fissure 

 when a strong current is passed through such a sheet separating two 

 solutions of AgN03. Braun compares such chemical effects with Bec- 

 querel's " electrocapUlary reactions" (cf. Comptes rendus, LXXVI 

 [1873], 1037) and suggests that they may have biological significance; 

 he regards a narrow split or fissure in a thin layer of insulating material 

 as acting essentially like an electrode; a certain critical intensity of 

 current is required, -below which there is no separation of metal. Across 

 the fissure there is a steep fall of potential, which he estimates at 700-900 

 volts per millimeter in currents effective with AgNOs. 



For observations on chemical effects at precipitation membranes 

 through which electric currents are passed cf. Ostwald, Z. physik. Chem., 

 VI (1890), 71; Overbeck, Ann. d. Physik, XLII (189 1), 193; Spring- 

 mann, ibid., LI (1894), 140; Bein, Z. physik. Chem., XXVIII (1899), 

 439- 



