183 
1897-98.] Dr John Shields on Palladium Hydrogen. 
It is to he noted, then, that the electromotive force of the cell 
gradually diminished on standing and approximated to zero, or at 
least the initial electromotive force. The question therefore arises, 
whether we ought to consider the electromotive force of the cell 
immediately after the charging current was interrupted, the final 
value after standing for a sufficiently long time, or any intermediate 
value, as the true electromotive force. The choice of any inter- 
mediate value would be an arbitrary affair, and there is clearly 
nothing to recommend it. The first value, too, is very liable to be 
affected by an absence of equilibrium with respect to the equal 
distribution of the hydrogen throughout the electrode. The rate 
at which hydrogen would diffuse from one surface of the electrode 
to the interior, or to the other surface, would depend, other things 
being equal, on the concentration gradient of the hydrogen in the 
plate. For a plate approaching saturation, therefore, the time 
required before equilibrium sets in might be considerable. The 
final value of the electromotive force of the cell is thus more likely 
to be the correct electromotive force, and to be free from such 
incidental sources of error, to which we will return again presently, 
provided we have proof that the relative concentrations of hydro- 
gen in the two electrodes have not undergone any alteration on 
standing. It is conceivable that equalisation of the hydrogen 
concentrations might take place, owing to bad insulation of the 
electrodes. In several cases the final concentrations were estimated 
electrolytically by determining the amount of electrolytic oxygen 
wdiich was just necessary to remove the hydrogen, and it was found 
that they had practically remained unaltered. The hydrogen con- 
tained in p after standing eighteen hours was determined in this 
way. It was found that about 1*3 c.c. hydrogen was evolved in 
the voltameter before oxygen began to appear on the electrode p. 
The amount of hydrogen existing in p must therefore have been 
equal to or probably, since slight oxidation takes place, a little less 
than 1’3 c.c. We have therefore proof that the concentrations of 
hydrogen in the electrode (3 and p were practically unaltered on 
standing eighteen hours. 
On the assumption that the hydrogen occluded by palladium is 
dissolved in the metal, and that such a cell is to be treated as a 
concentration cell, we can calculate its electromotive force from 
