1908-9.] On the Reducing Action of Electrolytic Hydrogen. 89 
does decompose some arsenic acid, whilst palladium is powerless to decom- 
pose any. 
Iron, in this respect, is a remarkable metal, with the lowest but one 
supertension ; it decomposes arsenious acid nearly as rapidly as lead, and 
it stands second (next to lead) in its power of decomposing arsenic acid. 
These experiments show, I think, that the power of any element to 
decompose arsenious and arsenic acid is a peculiarity of the individual 
metal, and appears to be independent of its supertension. 
The diagram No. 1 shows the various elements arranged as abscissae 
in their relative supertension values, whilst the ordinates show the amount 
of arsenic liberated from each in 25 minutes. 
Table II. gives the results of the experiments made with the different 
elements as electrodes to discover the velocities at which the cathodes 
liberate the arsenic in periods of 2J minutes. 
If we take the first five elements, it will be seen that the average 
velocities with which they decomposed arsenious acid into arseniuretted 
hydrogen are : — 
Table II. — “Units” of Arsenic evolved as AsH 3 . 
Intervals in Minutes . 
Oi. 
* J 2 
5 
71 
1 2 
10 
121 
15 
17J 
20 
221 
25 
Totals. 
Lead .... 
9 
13 
8-5 
7 
5-5 
4 
2-25 
1*5 
1 
0-5 
52*25 
Zinc .... 
8-5 
14*5 
7-5 
6 
4 
3 
2-5 
1-75 
2 
1 
50-75 
Cadmium . 
8*5 
12*5 
8*5 
6*5 
5 
3 5 
2-5 
2*25 
1*5 
1-25 
52-0 
Tin .... 
9 
12*5 
8 
6-5 
4*25 
3-5 
2-75 
2 
U75 
1-25 
51-5 
Silver 
6 
12 
10-7 
7 3 
5 
3*8 
2*7 
1-3 
•8 
•3 
49-9 
It will be seen that the first figure is always lower than the others ; 
this was due to residual hydrogen in the apparatus, although it was made 
as small as possible, the total volume being 20 c.c. of air-space in tubes and 
apparatus with 30 c.c. of electrolyte (3 amperes liberate 21 c.c. per minute). 
These results are averages of two or more experiments. Allowing for 
experimental error, a constant is obtained for these first five metals for a 
unimolecular velocity reaction, and this is one of the few electro-chemical 
reactions in which such velocities can be measured. 
If a is the original concentration of the arsenious acid, and x the 
quantity transformed at the time t , the rate of transformation at that time 
will be, according to the unimolecular formula, 
dx v / \ 
— = Jc(a-x), 
where dx represents the very small quantity transformed in the very small 
