181 
1897-98.] Dr John Shields on Palladium Hydrogen. 
serving as a zero instrument. The capillary electrometer used 
was Ostwald’s form of Lippmann’s instrument, and was sensitive to 
two or three ten-thousandths of a volt. The working Leclanche 
cells were calibrated several times daily with a normal Carhart 
Clark cell. 
In general, the potential difference between the two electrodes, 
immediately after the cell had been set up, was not zero. Before 
beginning an experiment, the electromotive force was brought as 
nearly as possible to zero either by re-igniting the electrodes, or by 
short-circuiting the cell in itself for a few days. In this way the 
electromotive force at the start could generally be reduced to a few 
thousandths of a volt. 
A small measured quantity of hydrogen was then introduced 
into one of the electrodes. With the different forms of electrode 
this produced an electromotive force varying from 075 — 083 volt. 
On now adding approximately the same quantity of hydrogen (the 
weight of the electrodes being approximately equal) to the other 
electrode, the electromotive force of the cell diminished to nearly 
zero. Successive additional quantities of hydrogen were now intro- 
duced into this electrode. In most cases this produced only slight 
alterations in the electromotive force of the cell. In general, the 
alteration was so slight that it could safely be considered as expe- 
rimental error or want of equilibrium. Where a marked differ- 
ence occurred, this generally disappeared after the cell w T as allowed 
to stand for some time (in certain cases a few days were necessary). 
It would be very tedious to catalogue the scores of measurements 
made with the different cells and different electrodes. The follow- 
ing set has therefore been selected, and may be taken as typical 
of those obtained. 
The electrodes in this case were cut from palladium foil which 
had been used in former experiments (Phil. Trans., cxci. 150), 
and which was capable of occluding 846 volumes of hydrogen 
when exposed to the gas under the proper conditions (loc. cit.). 
The dimensions of the electrodes were about 30 mm. x 19 mm. 
x 0*025 mm., and they were known as f3 and p respectively. 
(3 weighed 0*2234 g = 0*0186 c.c., whilst p weighed 0*2453 g 
= 0*0204 c.c. 
After ignition, the electrodes were placed in n - H 2 S0 4 , and 
