ELECTRICAL AND OTHER FACTORS 249 



The fact indicating most clearly the essential nature 

 of the conditions governing the course of the reaction is 

 that the rhythm of decomposition is associated with a 

 parallel rhythm of electrical potential. The mercury 

 in contact with the H^O^ solution is always found 

 cathodal with reference to the calomel electrode, but 

 during the active phase, while O2 is being freed, it is less 

 so than during rest; i.e., the metal becomes anodal 

 relatively to the resting condition. According to 

 Antropoff's measurements, the inactive mercury is 

 about 0.12 volt more cathodal (i.e., nobler) than the 

 active. This difference resembles that between passive 

 and active iron in nitric acid solution, the inactive 

 mercury corresponding to the passive iron. A change 

 of surface-tension accompanies the change of potential, 

 the rounded convex surface of the mercury becoming 

 flatter (from decreased surface-tension) as the film^ 

 forms; this behavior is in accordance with the Lippmann- 

 Helmholtz rule of electrocapillarity, according to which 

 the surface-tension decreases with increase of the 

 potential-difference across the surface. Graphic registra- 

 tion of the curve of potential change shows that its 

 course runs closely parallel with the curve of oxygen 

 evolution as measured by a manometer. 



Close observation shows that the gas is evolved only 

 during those times when part of the mercury is film- 

 covered and part bare; further, that the evolution of gas 

 occurs chiefly near the boundary between the bright 

 and the film-covered surfaces. When a regular rhythm 

 is established, the reaction during each cycle is observed 

 to pass rapidly over the surface of the mercury in a 

 wavelike fashion. At the beginning of a cycle, when 



