1903.] On the Discharge of Electricity from Hot Platinum. 273 



a long time, irregular variations in the leak occur, especially with a wire 

 which has been much heated. 



The wire disintegrates when heated at low pressures and evolves 

 gas. If the pressure is kept constant, by pumping, the apparatus of 

 course soon gets filled with the gas evolved by the wire, which is 

 mostly hydrogen. The leak measured under such circumstances is, 

 therefore, not the leak in air but that in hydrogen, which, as will be 

 shown below, is much larger than that in air, and depends on the 

 amount of hydrogen in the wire. When the wire is first heated 

 hydrogen is gradually driven out of it and so the leak falls off. On 

 standing cold the wire again absorbs some of the hydrogen, and so the 

 leak is larger again when the wire is heated. 



Long-continued heating causes the surface of the wire to become 

 covered with a network of deep cracks, and it seems probable that the 

 irregular variations in the leak which sometimes occur are connected 

 with the formation of these cracks. 



The sudden opening of a crack exposes a fresh surface of platinum, 

 and may suddenly set free some occluded hydrogen, and so produce a 

 sudden increase in the leak. It appears that the last traces of hydrogen 

 can only be got rid of with the greatest difficulty, and the wires 

 continue to evolve gas when heated in a vacuum for an extraordinarily 

 long time. 



In order to measure the leak in pure air, precautions were taken to 

 get rid of the hydrogen. The wire, after being mounted in its tube, 

 was boiled in nitric acid, and then washed with distilled water. It 

 was then heated to a high temperature in air at atmospheric pressure, 

 and then the air was pumped out until the pressure was very low, the 

 wire being kept hot all the time. Air was then let in again, and 

 pumped out, several times. This treatment entirely got rid of all the 

 irregularities in the negative leak, and greatly diminished the evolution 

 of gas by the wire. 



Cleaning the wire with nitric acid, and changing the air in this way, 

 was found to diminish the leak to something like one thousandth part 

 of its ordinary value, and very thorough cleaning of the wire with 

 nitric acid diminished the leak to about one part in 250,000. 



The presence of traces of phosphorus pentoxide was found to 

 enormously increase the negative leak, and it is known that alkali 

 salts have a similar effect. The results obtained lead to the conclusion 

 that the negative leak is due to the presence of traces of hydrogen, or 

 possibly other substances in the wire. The reasons for believing that 

 the leak is mainly due to hydrogen will be mentioned later. 



With a particular wire, treated in the way described, in air, the 

 small remaining negative leak only falls off slowly at a constant 

 temperature, and does not vary in an irregular manner, so that its 

 variation with the pressure and temperature can be measured. 



