PHYSICO-CHEMICAL BASIS OF TRAXSMISSION 407 



supported in a sheet of lens i)ai)cr, is intcn)()sc(I between 

 two salt solutions, one of wliich contains a readily 

 oxidizable compound, ferrous chloride, together with an 

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

 With membranes of about sofi thickness and a P.O. of 

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

 ferric thiocyanate appears rai)idly 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 quaUtative 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-like 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 clcctro- 

 stenolysis, where also a critical P.D. is required for pro<lucinK chemical 

 effects. In the experiments of Braun (Ann. d. Physik, XLIV (1891), 

 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 AgNGj. Braun compares such chcmic;il ciTccts with Bcc- 

 querel's "electrocapillary reactions" (of. Comptcs rend us, LXW'I 

 [1873], 1037) ^^d suggests tliat tliey 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 tliere is no sejxiralion of mcUil. Acros^s 

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

 volts per millimeter in currents efTective with AgXGj. 



For observations on chemical effects at preci[)ilalion membranes 

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

 VI (1890), 71; Overbcck, Ann. d. Physik, XLII (1891), 193; SprinR- 

 mann, ibid., LI (1894), 140; Bein, Z. physik. Chew., XXVIII (1SQ9), 



439- 



