50 



MR. 0. W. RICHARDSON ON THE IONISATION 



had been admitted to a wire previously oxidised in nitric acid. At a temperature of 

 1350 C. the ionisation increased rather less rapidly than if it were proportional to 

 the pressure up to '01 4 millim. The writer* has made experiments to see if the same 

 kind of results could be obtained by decreasing the pressure from a high initial value. 

 The first experiments were made at 900 C., and indicated that the leak with 

 40 volts consisted of two parts, one proportional to, and the other independent of, 

 the pressure. The part proportional to the pressure could be accounted for as being 

 due to ionisation by collisions, so that the nett result was a leak independent of the 

 pressure. This leak remained constant when the wire was left hot for 2f hours, 

 although some gas was given off by the wire, the pressure rising from 0'00033 to 

 G'0017 millim. This result might be reconciled with WILSON'S by supposing that the 

 gas was retained by the platinum with extreme tenacity, and that the amount evolved 

 during the 2f hours' heating was merely an insignificant fraction of what remained in 

 the wire. 



To test this supposition, an experiment was carried out at a much higher 

 temperature (1390C.), and an attempt was made to estimate the rate of evolution of 

 hydrogen by the wire from time to time from the increase in the pressure of the 

 McLeod gauge. Before commencing the experiment the wire had been heated for 

 some time in hydrogen at a pressure of 1*35 millims., so presumably equilibrium at 

 this pressure would have been approximately established. The amount of hydrogen 

 still retained under these conditions appears to be very large. The rate of increase 

 of pressure per hour after heatings for the time in hours stated is given by the 

 following numbers : 



The numbers are only approximate, as the McLeod gauge was not well adapted for 

 measuring small pressures accurately. The volume of the apparatus (pump, McLeod 

 gauge, &c.) was of the order 2000 cub. centims., that of the wire being 0'0033 cub. 

 centim. On the assumption that all the increase of pressure is due to hydrogen 

 evolved by the wire, the concentration of hydrogen in a platinum wire at 1350 C. in 

 equilibrium with hydrogen outside at a pressure not greater than 1 millim. (it may 

 have been considerably less than this) must be of the order of that corresponding to 

 a pressure of 2 x 10 4 millims. of mercury. It seems probable that most of the increase 

 of pressure is really due to hydrogen evolved from the wire and not from the walls of 



* It is only fair to state that WILSON descriljes experiments on the effect of reducing the pressure of 

 the hydrogen, which gave a much greater diminution in the leak than that oliservcd liy the writer (vule 

 H. A. WILSON, for. a/., p. 266). 



