z 19). The silver iou concciitration nround the colloidal silver iodide pnrticles is coutinunlly chnnging. Tliese pnrticles dissolve inore or less in sodium thiosulplinte niid the silver ion concentriitioii on their surfnce is cortscqucntly higher than in thc solution. The silver iodide surfnce is therefore relntivclv redox-positive. Hence reduction stnrts on the silver iodido surfnce, which becomes relnt ively rctlos-ncgntive, becnusc the silver ion concentrntion on the silver iodide surfnce decrcnses and tlic tliiosulphntc conccntrnt.iou increases. A s tlie thiosulplinte concentrntion increnses, bornever, pilver iodide will ngnin he disolred nnd tlic redox potential on the cilrer iodide surfnce will become niore retlos-positive ittiti1 it once litore renrlics the criticnl point nt which retliiction begins. The tlissolred silver iodide is nlways regencrnted a t sonic other point in the solution by the iilknli iodide formed from the silver iodide :IIIL~ thiosulphnte, so thnt it is nlnnys present tliongh continunlly changing (reduced and reprccipitntcd).
The cencriil fnrictiori of the critnlysts discussed nbovo is tliereforo n focussiiig or lacnl concentrntion of tlie electron current on tho aitulyst, siinilnr to tlint on exposed fiilrcr hnlide crystals, where the rloctrons from the silver iodide are focussed on tho relntively redoxpo.sitire ccntree (cf. the Gurney-Nott electron trnp theory of the