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Proceedings of the Royal Society of Edinburgh. [Sess. 
— and the order given above is the order in which they are capable of 
removing arsenic from solution as arseniuretted hydrogen when it exists 
in solution in the form of arsenious acid. This order, however, does not hold 
for the relative power of the different elements for removing arsenic from 
solution as arseniuretted hydrogen when it exists as arsenic acid in the 
electrolyte. 
Chapman and Law * suggest that the reducing efficiency of hydrogen 
with respect to arsenious and arsenic oxides depends largely upon the super- 
tension f of the cathode at which the hydrogen is liberated, and they 
put forward a formula to explain this, as follows : — 
M asHo^=^L- AsP h , 
“ where As represents the amount of unreduced arsenious oxide and K 
a constant factor. If the equation be disturbed by the addition of As, 
i.e. if the factor As be increased, reduction takes place or the reaction 
proceeds from right to left, and the same result is reached if P H is made 
larger. On the other hand, if P H is made smaller, As must be made 
larger to preserve the equilibrium. In other words, there is always left in 
solution after each experiment a certain residuum of unreduced arsenious 
oxide.” 
M As h 3 is a minute quantity of arseniuretted hydrogen always left in 
solution, and P H the potential of the hydrogen effecting the reduction. The 
results we have obtained contradict this hypothesis. 
The following table shows the relative velocities of thirteen different 
elements in decomposing arsenious and arsenic acids respectively, and the 
supertension of each element is also given when obtainable. 
The figures in Table I. represent units of arsenic as above described 
liberated in 25 minutes from 50 units (taken as 100 half-units) of elemental 
arsenic contained in the electrolyte (a) as arsenious acid, and ( b ) as 
arsenic acid. 
It was found that the current density had little influence on the result. 
The metals which could be easily melted were formed into cylinders 1 cm. 
surface as the cylinder, and all the others were used in the sheet form ; but it was found that 
the area of the cathode did not materially affect the result. 
* Chapman and Law on “ The Reducing Action of Hydrogen,” part ii., “ The Estimation 
of Traces of Arsenic by the Marsli-Berzelius Method and the Insensitiveness of Zinc,” Analyst , 
1906, vol. xxxi., p. 3. 
t “ Supertension ” of an electrode, according to Caspari (“ Ueberspannung,” Zeit. physikal. 
Ghem ., 1899, xxx., 89) is the excess of electromotive force necessary for the liberation of 
hydrogen at that electrode over the electromotive force required for the reversible produc- 
tion of hydrogen on a cathode of platinised platinum. 
The values of the “ excess-voltage ” or supertension for different metals are given by 
Caspari in his paper, and also in Lehfeldt’s Electrochemistry, part i., p. 176. 
