128 
Proceedings of the Royal Society of Edinburgh. [Sess. 
been heated to 200-300° C. the sensibility always fell to this value in 
twenty-four hours, falling to 50 units in three days, and to about 36 units 
in three weeks. 
Between temperatures of 60° C. and 350° C. (the highest attainable in 
vacuo with our apparatus) the sensibility of the platinum remained always 
very nearly constant, perhaps falling at most 4 per cent, between these 
limits, though the accuracy of the method hardly justifies us stating 
definitely that such a small diminution actually occurs. Whatever the 
initial value of the sensibility of the platinum, a rise of temperature of 50° 
or 60° C. always restored it to the same constant value, and on switching off 
the heating current the sensibility always decreased regularly, falling to 
about half its maximum value in something like twenty-four hours. 
It is possible that the heating current may affect the sensibility of the 
platinum directly, quite apart from the increase of temperature produced. 
In this case it would seem that, with the piece of platinum used, a current 
of 5 amperes is as effective as one of 15 or even 20 amperes, since an 
increase from one to the other produces no change in the photo-electric 
current. We made a number of experiments on this point, but the results 
obtained were not very conclusive. 
Experiments in Carbon Dioxide. 
(1) Carbon Bioxide at Atmospheric Pressure. 
The results obtained in carbon dioxide at atmospheric pressure were 
very similar to those obtained in air at the same pressure, and are 
summarised as follows : — At 14° C. a photo-electric current of 91 units was 
obtained ; at 140° C. the value fell rapidly to 71 units, at 263° C. to 34 units, 
and at 440° C. to 23 units. Above this temperature it rose again, and a 
current was also obtained due to the temperature alone, this latter current 
reaching 15 units at 670° C. 
On stopping the heating current the sensibility was found to have 
increased, giving 190 units at the temperature of the room. The maximum 
diminution in sensibility observed is thus practically identical with that 
found in air under the same conditions, but the sensibility took up its final 
value rather more quickly than in the case of air. 
(2) Carbon Dioxide at a Pressure of 46 mms. 
The fall in the photo-electric current with temperature in carbon dioxide 
at this pressure was less marked than that in air at the same pressure and 
under precisely similar conditions. 
