1908-9.] On the Reducing Action of Electrolytic Hydrogen. 87 
diameter and immersed to the depth of 2*7 cm., giving a current density 
of C 100 = 35 (i.e. the current density was at the rate of 35 amperes dis- 
tributed over 100 square centimetres of surface). When the cathode had 
to be used as a sheet, the current density was C 100 = 3'5, the surface being 
10 times greater. 
Table I. — Showing the Relative Amounts of Arsenic removed as 
Arseniuretted Hydrogen from ( a ) Arsenious Acid and (6) Arsenic 
Acid from 100 Parts introduced, in 25 Minutes, when Working 
with 3 Amperes. 
Metal. 
(a) Arsenious 
Acid. 
(6) Arsenic 
Acid. 
Supertension 
(Volts). 
Lead .... 
100 
98-5 
0-64 
Zinc .... 
100 
41 
0-70 
Cadmium 
100 
27 
0-48 
Tin 
100 
18-5 
0-53 
Silver .... 
100 
0 
0T5 
Graphite .... 
100 
4 
. . . 
Iron ..... 
93 
40-5 
0-08 
Platinum .... 
88-5 
0 
0-09 
Aluminium 
82 
9 
• • • 
Gold .... 
74 
4 
0-02 
Cobalt .... 
43-5 
0 
... 
Nickel .... 
42-5 
0 
0-21 
Palladium 
38 
0 
0-46 
It will be seen that lead is the most efficient of the elements examined, 
and it is remarkable that it liberates arsenic as arseniuretted hydrogen 
(AsH 3 ) when it exists in solution as arsenic acid almost with the same 
velocity as when it exists as arsenious acid. Lead has a high supertension, 
but not so high as zinc ; and whilst both of these metals liberate arsenic at 
about the same velocity from arsenious acid, lead liberates as AsH 3 dooths 
of the arsenic present as arsenic acid in 25 minutes, whilst zinc only 
liberates Afo^hs of the amount present in the same time. 
The next remarkable metal in the series is silver, with a very low 
supertension of 0T5, as compared with lead 0 - 64 and zinc 0'70. It liberates 
arsenic as As] 1., from arsenious acid with about the same velocity as lead 
or zinc, whilst it is powerless to liberate any arsenic when it exists in the 
form of arsenic acid under the conditions of the experiment. 
Again, palladium, which has a comparatively high supertension, is 
at the bottom of the series as regards efficiency in decomposing either 
arsenious or arsenic acid. Gold, which is at the bottom of the series 
as regards supertension, has the power of decomposing arsenious acid 
to the extent of three-quarters of the most efficient elements, and it 
