130 Proceedings of the Royal Society of Edinburgh. [Sess. 
(2) Hydrogen at a Pressure of 49 mms. 
The platinum, as in all the experiments at high pressures, was charged 
to 200 volts negative, while the value of the arbitrary unit in which the 
photo-electric currents were measured was the same as in air and carbon 
dioxide at the same pressure. Owing to the high conductivity for heat of 
hydrogen, it was found impossible to raise the temperature of the foil above 
200° C., a heating current of 20 amperes being required to give this 
temperature. 
The results obtained are given in the following table : — 
Mean value of 
photo-electric current 
in arbitrary units. 
Heating current 
in amperes. 
Temperature 
in °C. 
85 
0 
14° 
91 
5 
30 
98 
10 
62 
100 
15 
111 
105 
20 
196 
On switching off the heating current the photo-electric current fell to 
96 in ten minutes, and to 88 some twenty hours later. 
(3) Hydrogen at very Low Pressures. 
The influence of temperature on the photo-electric currents in hydrogen 
at a pressure of 0*0035 mm. is similar to that in air and carbon dioxide — 
viz., a small increase of heating current, and therefore of temperature, 
produces an increase in the sensibility up to a maximum value, after which 
the heating current may be increased so as to raise the temperature up to 
at least 370° C. without producing any further increase in the sensibility. 
This maximum sensibility appears to be at least 30 per cent, lower in 
hydrogen than in either of the other gases, whilst the change in sensibility 
produced is much less marked, the photo-electric current of 45 units at 
14° C. (after the platinum had been left unheated for three days) rising only 
to 70 units at 370° C., this same ratio of increase being obtained on other 
occasions under the same conditions. 
It is important for the success of these experiments that the vessel shall 
have been standing exhausted to the low pressure for some days before the 
observations are taken, in order that the quantity of hydrogen occluded in 
the platinum may have reached a constant value. 
We noticed that when the apparatus, which had been standing filled 
with hydrogen at a high pressure for nearly a week, during which time the 
