129 
1906-7.], Photo-Electric Discharge from Platinum. 
The photo-electric current of 85 units at 13° C. fell to 83 units at 43° C., 
67 units at 143° C., 61 units at 258° C., and 62 units at 368° C., rising almost 
immediately to its original value at 13° C. on switching off the heating 
current. 
On a separate occasion a precisely similar result was obtained, a gradual 
fall from 85 units at 15° C. to 60 units at 370° C. being obtained. 
The much shorter time required for the platinum to attain its final 
constant sensibility at any temperature in carbon dioxide than in air was 
strongly marked at this pressure, the steady values in the above experi- 
ments being attained within a minute or two at most after increasing or 
reducing the temperature of the platinum. 
(3) Carbon Dioxide at a Pressure of 0*0035 mm. 
Observations were made in carbon dioxide at a pressure of 0‘0035 
millimetres of mercury. The potential applied and other conditions were 
precisely the same as those for the experiments in air at the same pressure. 
The results obtained were also identical with those obtained for air, a 
comparatively small heating current producing an increase in sensibility up 
to a certain value, beyond which no further increase in the heating current 
produced any effect. It is, therefore, unnecessary to go into further details 
of these series of observations. 
Experiments in Hydrogen. 
(1) Hydrogen at Atmospheric Pressure. 
The results obtained in hydrogen at atmospheric pressure are summarised 
in the following table : — 
Mean, value of 
photo-electric current 
in arbitrary units. 
Heating current 
in amperes. 
Temperature 
in °C. 
92 
0 
14° 
110 
15 
91 
120 
20 
137 
135 
25 
225 
181 
30 
340 
and after the heating current had been switched off the photo-electric 
current fell to 105 in seven minutes and to 95 in twenty minutes. 
We thus see that in hydrogen the photo-electric current increases 
steadily with the temperature from the ordinary temperature of the room — 
an absolutely different behaviour from that in either air or carbon dioxide. 
VOL. xxvii. 9 
