84 
Proceedings of the Royal Society of Edinburgh. [Sess. 
VI. — On the Reducing Action of Electrolytic Hydrogen on 
Arsenious and Arsenic Acids when liberated from the 
Surface of Different Elements. By William Thomson, F.I.C. 
(Read June 15, 1908. MS. received July 12, 1908.) 
This research was commenced with a view to find the velocity at which 
arsenic was liberated as arseninretted hydrogen from cathodes of different 
elements : 
(a) From arsenic in the form of arsenious acid. 
(b) „ „ „ „ arsenic acid. 
The experiments were carried out in the apparatus already described by 
me* which consists of a porous pot containing 30 c.c. of dilute sulphuric 
acid (one of strong acid to six of water by volume), with the cathode, when 
that was possible, in the form of a cylinder 60 mm. long and 10 mm. 
diameter, immersed to a depth of 27 mm., giving a surface exposed to the 
electrolyte of 8*48 square centimetres, the end passing through an india-rubber 
cork fitted into an opening in the glass stopper, which was accurately ground 
into the mouth of the porous pot and made air-tight with vaseline ; the 
liberated hydrogen was passed from a glass tube fused into the same glass 
stopper, which ended in a two-way tap and T-piece. Another similar 
apparatus was placed in series with the first-mentioned, so as to liberate 
hydrogen from pure dilute sulphuric acid. This apparatus was also 
provided with a two-way tap and T -piece, so that a stream of pure hydrogen, 
dried through a calcium chloride tube and passed over a small roll of dry 
basic lead acetate paper to remove any trace of H 2 S, could be passed through 
one or other of two drawn-out hard glass tubes which were heated to 
redness near to the drawn-out ends to receive the arsenic mirrors. These 
were deposited on the drawn-out part, the diameter of which was graduated 
by inserting a wire, the one end being F6 mm., the other IT mm. diameter, 
the length between these two points being 2 mm., the tube being cooled 
from the larger diameter. This apparatus was capable of easily detecting 
0*000,000,541,3 gramme (roughly about 2000000 a gramme) of elemental 
arsenic when contained in the 30 c.c. of acid in the porous pot. This 
* Memoirs and Proceedings of the Manchester Literary and Philosophical Society, vol. xlviii. 
part iii., No. 17. 
