1908-9.] On the Reducing Action of Electrolytic Hydrogen. 85 
quantity was taken as a unit, and for each experiment 5 c.c. of a solution 
containing 0 - 000,005,413 grammes per c.c. was taken in the form of : 
(a) Arsenious acid. 
( b) Arsenic „ 
This makes 50 units, each of which can be easily detected. This amount 
was deduced from another standard in which 50 c.c. of a liquid containing 
ToVo‘th of a grain per gallon produces a well-defined mirror of arsenic on 
the cooled drawn-out part of the glass tube. 
Having filled the apparatus with hydrogen by passing a current of 
3 amperes through 10 c.c. of dilute sulphuric acid in the porous pot, 5 c.c. 
of the standard solution of arsenic above mentioned is made up to 15 c.c. 
with dilute sulphuric acid and poured into the apparatus by a funnel fused 
into the glass stopper of the porous pot, stoppered by a glass rod at the 
bottom, the funnel being rinsed into the porous pot with 5 c.c. of the same 
dilute acid. After the apparatus has worked for 2 k minutes and a mirror 
has been deposited, the two-way tap is turned so that the hydrogen from 
the apparatus in series generating pure hydrogen sweeps the tube for a few 
moments. The tube is then removed and another inserted to receive a fresh 
mirror, the second mirror being produced by the two-way tap diverting the 
flow of the hydrogen containing arseniuretted hydrogen for 2\ minutes to 
the second tube, through which pure hydrogen had previously been passing, 
and which was heated to redness for the purpose of receiving the second 
mirror. In this way mirrors were received without loss of arsenic on the 
drawn-out portion of hard glass tubes, cooled at the point at which the 
mirror was to be deposited by means of cold water flowing continually over 
them. The mirrors thus received were sealed off in an atmosphere of dry 
hydrogen, and the quantity on each estimated by comparison with a series 
of standard mirrors examined and compared by a magnifying glass. 
Plotting the amounts of arsenic deposited in successive intervals of 
2\ minutes as the ordinates, and the time as abscissae, curves were obtained 
for all these experiments. It was observed that, however carefully each 
experiment was repeated, no two results agreed absolutely, and this dis- 
agreement was greater for arsenic acid than for arsenious acid. 
Thirteen different elements were tested — viz., lead, zinc, cadmium, tin, 
silver, graphite, iron, platinum, aluminium, gold, cobalt, nickel, and palladium * 
* The following metals were used in the form of cylinders : — Lead, zinc, cadmium, 
and tin. 
Iron was used in a cylindrical form made by wrapping pure iron wire on a cylinder of 
glass, giving about the same surface exposed to the electrolyte as the metallic cylinders. 
The graphite electrode was cut in the form of a rectangular block giving about the same 
