I'KODUCEI) BY HOT PLATINUM IN DIFFERENT OASES. 



53 



appears to increase more rapidly with the temperature. The values of the negative 

 leak are bigger than those obtained at the lower pressures for the same temperature, 

 the difference being greatest at the lower temperatures. This would seem to indicate 

 that the small increase in the negative leak with pressure obtained at the lowest 

 pressure, and which it was suggested might be due to ionisation by collisions, is 

 really a genuine direct effect of the gas and becomes greatly magnified at high 

 pressures. 



The values of the energy change associated with the liberation of one gramme 

 molecule of each kind of ions at this pressure are w+ = 57x10* calories and 

 W- 5'56xlO* calories. Thus increasing the pressure of the hydrogen appears to 

 increase the work required for a positive ion to escape from the metal, whereas it 

 decreases it in the case of the negative ion. This result so far as it refers to the 

 negative ionisation has previously been obtained by H. A. WILSON.* 



VIII. 15. EXPERIMENTS WITH A PLATINUM TUBE. 



The writer has also made experiments on the change produced in the ionisation at 

 the. outside surface of a platinum tube in air when hydrogen was allowed to diffuse 

 from the inside of the tube. A brief abstract of the results obtained has already 

 been published f ; the present section gives a more detailed account of the experi- 

 ments. These platinum tube experiments, in the opinion of the writer, settle 

 decisively a number of questions which have been, or might be, raised with regard 

 to the origin of the ionisation produced by hot platinum. For instance, H. A. 

 WILSON \ has suggested that the negative ionisation produced by hot platinum in 

 ;iir is due to traces of occluded hydrogen which are retained by the wire in a very 

 persistent manner. If this were the case, the small negative leak in air would be 

 enormously increased by allowing any considerable quantity of hydrogen to diffuse 

 out of the wire from inside. As a matter of fact, when hydrogen was allowed to 

 diffuse out of the walls of the tube at a rate corresponding to 2 cub. centime, at 



* ' Phil. Trans.,' A, vol. 202, p. 269. 

 t 'Camb. Phil. Proc.,'vol. 13, p. 192. 

 \ 'Phil. Trans.,' vol. 202, p. 243. 



