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Proceedings of the Royal Society of Edinburgh. [Sess. 
up to a certain maximum value, beyond which no further increase in the 
heating current produced any further appreciable result, the sensibility 
remaining constant for a range of temperature from 60° C. to at least 350° C. 
Between these limits we can then say that the specific photo-electric dis- 
charge from platinum is independent of the temperature. 
On stopping the heating current the sensibility of the platinum slowly 
fell, decreasing to about half its value in twenty-four hours. The rate of 
decrease was proved to be in no way influenced by allowing ultra-violet 
light to illuminate the surface of the platinum. 
Whether or no the passage of heating current through the platinum, 
apart from the rise in temperature produced, is a factor bringing about the 
increase in the sensibility of the platinum to its maximum value, cannot be 
definitely stated. We hope, however, at an early date to decide this point, 
an investigation to that end being in progress. 
The apparatus required in the above investigation has, in part, been 
provided out of a grant made by the Carnegie Trustees, to whom our best 
thanks are due. 
Our thanks must also be expressed to Professor Baily for granting us the 
very frequent use of certain machines in the alternating current laboratory. 
{Issued, separately May 31 , 1907 .) 
