Manchester Mejnoirs. Vol. Ixl. (191 7) No. 9 13, 



It is evident from the above table that each group has .what 

 we may term a "typical overvoltage." Further than this, we 

 may now explain the multiple values shown by most metals as 

 merely due to change of valency. Thus, lead, which readily 

 forms two series of compounds, in which it is divalent and tetra- 

 valent respectively, can easily change its overvoltage from 0.72 

 to 0.46 volt. Almost without exception, all the multiple over- 

 voltages will be found to correspond with definite changes of 

 valency, which are indicated by the existence of compounds, in 

 which the metal in question has the required valency. Since it 

 is 'difficult, if not impossible, to conceive of valency apart from 

 co-mpounds. we are forced*, to the conclusion that ovc\r- 

 voKage is due to the presence of coinpounds of the electrode 

 with the discharged ion. or some product of the discharged ion. 



The values approaching zero, observed with many metals 

 are therefore due to the absence of these compounds, and it is 

 noteworthy that the metals which most easily show these low 

 values are the platinum metals. This is undoubtedly due to 

 the well-known reluctance of these metals to form any com- 

 pounds. The zero value may therefore be considered as typical 

 of group o, which contains the inert gases. 



The followmg table shows the distribution of the metals 

 in the groups, those which have the overvoltage of a group to 

 which they do not properly belong being placed in brackets : — 



Cathodic Overvoltages of the Periodic Groups. 



Group o. — Typical overvoltage, 0.0 volt ; (silver), (mercury), (aluminium), 

 (bismuth), (nickel), (rhodium), (palladium), (iridium), (platinum). 



Grottp I. — Typical overvoltage, 0.35 volt ; sodium, copper, silver, gold. 



Group II. — Typical overvoltage, 0.70 volt; magnesium, zinc, cadmium, mercury, 

 (carbon), (tin), (lead), (antimony), (bismuth), (manganese), (cobalt), (palladium), 

 (iridum), (platinum),- (copper). 



Group III. — Typical overvoltage, 0.50 vnlt : aluminium, thallium, (antimony), 

 (tantalum), (bismuth), (gold). 



Group IV. — Tvpical overvoltage, 0.45 vf)lt ; carbon, tin, lead, (iridium), 

 (platinum). 



Group V. — Typical overvoltage, 0.42 volt : antimony, tantalum, bismuth, 

 (chromium). 



Group VI. — Typical overvoltage, 0.32 volt ; chromium, molybdenum, tungsten, 

 (iron), (nickel). 



Group VII. — Typical overvoltage, 0.25 volt ; manganese, (iron), (nickel), (cobalt), 

 (palladium), (platinum). 



Group VIII. — Typical overvoltage, 0.18 volt; iron, nickel, rhodium, iridium, 

 platinum. 



'The only metals in this table which do not appear in their 

 proper groups are cobalt and palladium. Values for cobalt as 

 low as 0.18 volt have not been obtained. Palladium shows 

 values above and below the group value, but in the neighbour- 

 hood of the group value the overvoltage is indefinite and 

 changing rapidly. 



